Light detecting devices: their use for colour measurement and image capture

Tim Dawson

INTRODUCTION The overall principles of the instrumental capture of colour data for measurement or recording purposes have been described by a number of authors [l-41. The 1998 SDC review covering image capture concentrated mainly on silver halide photographic systems 151, at a time when the widespread public adoption of digital photography (both video and particularly still photography) was a relatively recent development. A later review of these aspects has been produced in the USA IS ] .

At the heart of every modern device used either to measure colour or to record coloured images, lie light- detecting devices most of which are nowadays based on silicon chip technology. Depending on the particular type of device, exposure to light results in a change in the cell’s resistance, or the generation of voltage, current or electrical charge. A photovoltaic effect was first recorded in 1887 by Hertz during experiments with spark gap transmitters. The full elucidation of the phenomenon was only achieved during the next two decades through the individual research studies of Thomson, Lenard and Milliken. and particularly through the development of the quantum theory of light, originated by Planck and elaborated by Einstein. who explained how photons impinging on certain metal surfaces can result in the release of electrons [7].

EARLY PHOTOELECTRIC DEVICES Selenium photocells The first practical device to be made was composed of a selenium-coated iron disc (the cathode) with a very thin, transparent layer of gold or metal oxide (e.g. cadmium oxide) acting as the anode (Figure 1).

n

J L v B A

A Transparent electrode B Contact ring (negative) C Selenium coating D Steel support plate (Anode) E Protective back-coating

Figure 1 Seleniuni photocell

This type of photocell (called a barrier layer cell) requires no external battery and was widely used i n photographic exposure meters and in early electrical colorimeters, as the spectral response curve of a selenium cell is similar to the luminous efficiency function curve of the human eye. i.e. the CIE Y colour matching function. I f the resistance of the external circuit connected to the cell is sufficiently low [cu. 100 ohm or less), the output current from a selenium cell is directly proportional to the intensity of the light falling upon i t . Selenium cells are also used for a wide variety of other applications such as automatic light switches. flash actuators, smoke detectors and aligning/edge guide mechanisms and were used as the light sensing dnti:c:tors for early television experiments.

Vacuum photocells At the end of the 19th century, when Thomson and Lenard were experimenting with the effect of light on certain metal surfaces (the cathode) contained in evacuated glass tubes, it was found that the ejected electrons could lie accelerated towards another plate (the anode) maintained at a higher potential and the resultant current flowing i n the circuit measured. The relationship between both the intensity and the wavelength of the incident radiatioii from an arc lamp on the current produced was derived. Later, researchers developed efficient and stable cathode surfaces and in particular the high vacuum, silvericaesium oxide, glass photocell was produced in various forms and used especially for the sound reproduction system for cinema films introduced by RCA (the Radio Corporation o f America) in 1928 and also in some early spectrophotonietric devices.

A major improvement in sensitivity for the basic: vacuiiii photocell was achieved in the 1940s with the developincnt of the photomultiplier tube (PMT) which provides lo\\, noise, ultra-fast response and extremely high sensitivity. When light impinges on the photocathode the ejcctetl electrons pass between focusing electrodes called dynodes and are accelerated towards a succession of anodes, each of which is at an increasing voltage, the final anode h i n g at around 1 kV (Figure 2). At each of these dynodes more electrons are emitted thus greatly amplifying even the smallest signal. Very many varieties of photomultiplier are produced but principally they are of the end-oii or side-window type and usually have from 9 to 12 dynode sections tightly packed in a relatively small glass envelope. As a result of the ‘cascade’ magnifying effect on the original signal very high degrees of amplification (10fi-107) of low level light signals are possible.

Many photocathode coating materials are used including

72 0 Het: Prog. Cob., 34 (2004)

A B / /

\ C

A Photocathode coating B Focusing electrode C Initial emission D Anode (output)

\ D

Figure 2 Photomultiplier cell (end-window type)

silvericaesium oxide, gallium arsenide, antimony/caesium and multi-alkali (Na/K/Sb/Cs) which are sensitive inter alin not only to visible light but also in the ultraviolet (UV) and infrared wavebands. Depending on the required application, the window material may be of borosilicate glass, synthetic silica or even magnesium fluoride. For reliable operation, the voltage supply to the dynodes has to be very well regulated because any slight variations are greatly exaggerated due to the high amplification of the device. Despite the high sensitivity of PMTs their quantum efficiency (the efficiency of conversion of photons impinging on the photocathode compared to the generation of electrons) is relatively low (10-25%).

Photomultipliers have found very many uses, not only as detectors in various types of spectrophotometer and because of their extremely high sensitivity. They are also used for the detection of low light level events such as exist in many astronomical observations [a]. for analytical measurements involving chemi- and bio-luminescence [9] and, in particular, for scintillation counters for the detection of radiation, often for medical purposes [lo].

Television camera tubes Many early experiments with television utilised various electromechanical scanning systems, such as the Nipkow perforated disc used in conjunction with a simple selenium photocell. and a similar combination was employed in the 1930s for experimental BBC television (TV) transmissions with the John Logie Baird system. All these were of low definition and brightness and by 1938 electronic scanning systems had won the day.

Various vacuum tube devices for picture capture were developed such as the Farnsworth image dissector, the Zworykin/RCA Iconoscope, and finally the Orthicon camera was introduced commercially in 1939 [ I l l . This tube was to reign supreme in capturing TV images even after the advent of the improved Philips Plunibicon camera in the 1960s and only relatively recently have the smaller charge-coupled device (CCD) cameras come into their own for both TV and personal camcorders.

The Iconoscope was essentially a glass envelope with a photocathode screen at one end onto which the illuminated scene was optically focussed. The photocathode was coated with a mosaic of granules of photoemissive material which acquired a charge proportional to the luminosity of the light falling on i t . The capacitative charge from each location in the mosaic was then read sequentially by a scanning

beam generated from an electron gun and focusing coil arrangement to one side of the main camera assembly tube. The scanning beam thus produced a modulated analogue voltage signal at the base of the photocathode. This camera required high levels of illumination owing to its low overall sensitivity (light at a minimum level of 1000 lux). This problem was overcome in the Image Orthicon (RCA) camera which scanned the photocathode screen image with a beam from an in-line electron 'gun' assembly mounted at the rear of the camera tube. These tubes had much improved light sensitivity [minimum 200 lux) as they incorporated a five-stage photomultiplier assembly. By comparison, modern CCD TV cameras have sensitivities down to 1 lux or even less.

In the 1950s, experiments were being carried out by RCA with colour transmissions based on the use of three Orthicon TV cameras, the light passing through a system of dichroic mirrors (replaced later by prism optics) and thence to the three camera tubes. The RCA TK41 model was introduced commercially in 1954 and became the standard colour camera for the next 15 years. The whole camera assembly was 1.5 m long, weighed about 150 kg and including the ancillary circuits required 3.2 kW of power! The RGB colour signals, line scan and synchronisation pulses were converted to a composite broadcast signal to the NTSC (National Television System Committee) specification (for many years jokingly said by American technicians to mean 'never twice the same colour'). The poor colour reproduction was a result of the particular method used to encode the chrominance and luminance signals so that transmissions could still be viewed on older monochrome receivers.

In due course other colour broadcast systems were developed, still using the three tube/dichroic mirror principle (Figure 3 ) , but incorporating superior colour encoding methods. Thus the French SECAM (systeme electronique couleur avec memoire) was adopted in France, eastern Europe and Africa, and the German PAL (phase alternating lines) system elsewhere in Europe including the UK. It was not until about 1990 that most broadcast colour cameras changed to CCD types.

I t is interesting to note that even as late as 1969 colour cameras were still very heavy and for the Apollo-11 moon mission they had to revert to an earlier technology. In the 1950s, CBS (Cable Broadcasting Service) investigated frame sequential scanning systems related to those used

sync pulse

* n Lens

- Normal mirror 0 Dichroic mirror p-"d RGB

Luminance

Figure 3 Colour signal separation i n a three-tube colour caniern svstem

by John Logie Baird and involving synchronised spinning colour filters. Using the same principle they later produced a colour camera weighing only 2.5 kg for the NASA space capsule.

MODERN PHOTOSENSORS The major developments in silicon ch ip technology during the past three decades also heralded a new era of photoelectronic devices including photodiodes, complementary metal oxide semiconductor (CMOS) cells and above all CCDs [12]. The advent of these has had a marked effect on a number of technologies including spectrophotometry, biomedical instrumentation, astronomy, digital communication (fibre-optic) systems and , in particular, image capture (digital photography and video for both personal and commercial applications)

Photodiodes In general all modern photosensitive devices are based on fabricated chips which incorporate one or more semiconductor p-n junctions. The fundamental operation of all semiconductor diodes depends on the interface between its hole-rich (electron deficient) p-type junction, and the electron-rich n-type regions, and the intermediate depletion region. The most common semiconductor material used is highly purified silicon or gallium arsenide (GaAs) although for special applications indium antimonide and arsenides, lead selenide. gallium nitride and phosphide and even silicon carbide (for high dose U V monitoring) have been used. These materials absorb light over a range of wavelengths (e.g. 250-1100 nm for silicon and 800-2000 nm for GaAs).

Silicon photodiodes were first produced in the 1960s but were not widely used commercially until the early 1980s. The general structure of a photodiode cell is shown in Figure 4. The use of such cells for solar energy conversion has received much publicity as an ecological alternative to burning fossil fuels but, although their efficiency has been greatly improved in recent years [13], their use is largely confined to trickle charging of batteries (e.g. to power

A Metal oxide B Metal contact C p+ region D Intrinsic (absoption) layer E n+ region F Metal contact G n region

Figure 4 Photodiode cell

equipment in remote areas, be it on a Highland road, a lighted buoy at sea or a satellite in space).

The two main types of photodiode are the PIN (p- intrinsic-n) and avalanche photodiodes 1141. The latter has the advantage of greater signal amplification, being in effect the solid state equivalent of the photomultiplier tube and particularly suitable for fibre-optic coupling and low light applications where the extremely high gain of a PMT is not required. The past decade has seen enormous development effort in large area avalanche photodiode (LAAPD) technology with applications as diverse as nuclear physics and missile guidance systems to retinal mapping with a scanning laser ophthalmoscope. Avalanche photodiodes achieve their modest amplification factor (8-20 fold) owing to the use of a special, neutron-bombarded silicon which allows the application of a high reverse bias voltage (up to 2 kV/cm) without electrical breakdown occurring, thus creating a strong electron-acceleration lieltl [13,15]. Photodiodes give a linear response (output current against illuminance) with 80-90% quantum efficiency (QE). the output photocurrent being directly related to the QE [lS], making them particularly suitable for quantitative light measurements. Their speed of response depends on a number of factors but can be as short as a few nanoseconds. Photodiodes are available as single cells, segmented cells and as both linear and matrix arrays.

Complementary metal oxide semiconductors For many applications, arrays of photosensing elements are required (eg. a linear array for a spectrophotometer and a matrix array for a camera), and since the individual cells of the array generally require a mixture of digital and analogue processing, it is logical that this can be achieved using an integrated, single chip circuit. One well established method is to use CMOS fabrication technology [17]. CMOS camera cells may be of two types, namely APS (active pixel sensor) or ACS (active column sensor), with the latter being preferred because the inbuilt amplifier transistors all give exactly the same boost to the output current whereas there can be a degree of mismatch in APS devices resulting in a lack of uniformity in the captured image.

Presently CMOS multi-pixel arrays are available with pixel sizes down to 4-5 pm and, using 0 . 1 8 pni chip fabrication lines, it is planned to reduce this to 3-4 pin. System-on-chip sensors that include full-colour processing, image compression and interface logic are now being developed [18] for use in mobile phone applications because of the low power requirements of this technology compared with the CCDs described in the next section. A further advantage is that cells in a CMOS array may be individually matrix-addressed at very high speeds [19,2Ol in a manner similar to that used to address active matrix TFT LCDs (thin film transistor liquid crystal displays) 1211, in contrast to the slower serial, row-by-row, readout of CCDs. CMOS devices do suffer from higher noise generation than CCDs [22], bul this can be minimised by electronic compensation. This is achieved by subtracting the pixel values with the camera shutter closed from the exposed values for each frame.

In each CMOS cell appreciable space is occupied by the individual amplifier/switching transistors so these devices usually have a lower light-gathering capacity compared to

CCDs. Thus, whilst the so-called ‘ f i l l factor’ for CCDs can be almost loo%, that for CMOS devices may be 70% or less. This is also the case for so-called ‘interline transfer’ CCDs in which part of the pixel area is occupied by download electronics which enables the video-like display on the LCD viewfinders found on most consumer cameras. More concentrated light capture can be achieved by fabricating a layer of micro lenses above the cells 1231, a system which has found application in both CMOS and CCD camera arrays.

Charge-coupled devices CCDs were invented by Willard Boyle and George Smith (Bell Labs) in 1970 whilst seeking a new type of data storage device 1241, but commercialisation of the invention took another five years. The advent of these devices revolutionised the availability todav of high resolution, relatively low cost digital cameras whilst allowing great technological strides in image scanning, photometry, astronomy etc. [see the following sections on their uses) 1251.

When light falls on a CCD array the cells acquire a capacitative charge corresponding to the intensity of light impinging on each part of the cell surface. The quantum efficiency of this conversion in CCDs is high (70-85%). After a specific time interval the individual charge data is read, serially, row by row with a shift register system. The pixels of a CCD are controlled by electrodes (gates) on the chip surface and by applying varying forward and negative [reset) biasing voltages the charges acquired can be moved across each cell and then read out (from left to right in Figure 5) 126,271.

Gate, Oxide 0.1 pm thick p-type silcon substrate

Figure 5 CCD system

CCD arrays can now be made with very small pixels (say 7 x 7 pm) and in a matrix corresponding typically. for high definition cameras, to around six million pixels ( e g 2832 x 2128). In general one can say that 3 megapixel cameras can yield good quality images up to a 150 x 100 mm print size and cameras with 5-6 megapixel images are in general accepted for commercial prints alongside conventional film photographs 1281.

Image intensifiers The development of early versions of these devices took place during World War I1 as a means of detecting troop and vehicle movements in the dark. The simplest form consisted of a cylindrical glass, high vacuum tube with one flat face

being a translucent, multi-alkali type of photocathode and the opposing face a luminescent screen coated with a zinc sulphide phosphor, giving a green image. A lens focused the required image area onto the photocathode and a high voltage (3-5 kV) was applied between electrodes on the two faces so as to accelerate the electrons emitted. A lens eyepiece was used to view the final recreated image (see Figure 9a). For portable use as a night-view monocular by the military, the required high voltage drive had to be obtained by the use of a voltaic pile since semiconductor voltage multipliers were not available at that time.

In general, the various forms of night viewing equipment initially produced worked in conjunction with an infrared illumination beam but later more sensitive devices could be used where there was natural illumination such as starlight. Unfortunately the early devices were somewhat unreliable and image brightness was gradually lost due to screen ‘burn-in’. In the intervening 50 years this type of device has been greatly refined and has found many new uses. I t is perhaps interesting to note that without such appliances a s night vision binoculars 1291 many attacks during the Gulf and Iraq wars would never have been carried out at night. Among many present day uses are security surveillance, night navigation, monitoring nocturnal animals and law enforcement and for many of these applications digital CCD video cameras with an additional image intensifier are now available.

APPLICATIONS OF PHOTOSENSORS Light and exposure meters Photographic analogue exposure meters of various types have been available for many decades. The earliest models were based on selenium, then cadmium sulphide cells and in more recent times on silicon based photodiodes. Despite the advent of automatic exposure cameras (whether conventional film or digital photography is involved) many professional photographers continue to use incident light [lux) meters, and a measure of judgement tempered by experience. One can measure either the light falling on the subject, using a meter fitted with a hemispherical light-diffusing dome, or the light reflected from the subject to the camera either with a directional lens cover over the detectors or directly from the image itself in a reflex camera.

The rule of thumb for converting from incident light to reflected light is to take 17-18% of the former to equal the latter. Nearly all consumer cameras have built-in exposure control with a directional reflected light measuring device. Other types of meter available include spot meters, which measure light reflected from an object within a very narrow field of view, and colour temperature meters to assess sources of illumination [ e g for filming) which allows the selection of appropriate colour compensation filters to balance the film type with the particular illuminant. Digital cameras can allow for the three main types of illuminant. such as fluorescent, incandescent and daylight, by electronic correction software. When using lux meters great care is needed when comparing results from light sources having similar levels of illuminance but very different spectral power distributions 1301.

8 Hor. Prog. Color.. 34 (2004) 75

Early spectrophotometry Some 50 years ago, simple electrical absorption (optical transmission) photometers usually employed either one or two selenium cells and were used for transmission colorimetry in association with a selection of colour filters 1311. Such devices were typified by the Hilger Spekker (Figure 61, a rugged double beam device which the author well remembers being the workhorse colour-measuring unit for all quantitative dyeing experiments in the ICI laboratories. The layout of the instrument shows the whole system being based on a compensating arrangement of two beams from a 100 W projection lamp falling ultimately on two photocells as part of a balanced electrical circuit connected to a sensitive spot galvanometer. Optical density values were obtained by direct reading from the logarithmic scale on the aperture control cam. To improve sensitivity, the unit was usually provided with up to eight medium- or narrow-band filters.

A,O Selenium photocell B,D.I,K,N Lens C,M Colour filter L Cuvette J Optical density cam G.F Lamp and heat filrer

Figure 6 Spekker abscirptiorneler (Hilger)

Where it was necessary to obtain more precise spectral data (e.g. for estimating relative strength of a batch of dye against the ‘standard’ by taking optical density measurements at the peak absorption wavelength) a more sophisticated photometer device, such as the Hilger Uvispek or the Unicam SP500 (Figure 7) was used. These were single beam instruments utilising two vacuum photocells (one covering 200-650 and the other 600-1000 nm) and an accurately controllable quartz glass prism and slit system as the monochromator, but even these instruments were very tedious to use if a ful l spectral curve was to be obtained. To obtain complete spectral transmission curves the ultimate degree of electromechanical control that was available at that time was represented by instruments such as the GI.: recording spectrophotometer whose output appeared on a pen chart. Maintenance of such a machine was frequent and costly.

By the time that recipe match prediction systems were introduced in the mid 1960s a number of manually operated, 3- or 4-filter, reflectance colorimoters were available but in time it became clear that for more reliable results the patterns to be matched required measuring with a spectrophotometer capable of giving a minimum of a 16-point input (say at 25 nm intervals or less). Today measurement at 5 or 10 nni intervals is the common practice.

The fundamentals of instrumental colour measuri:ment were well summarised by Chong 111 some years ago in a review which is equally relevant today, as have the basics of the calibration of colour measuring instruments [321.

Modern spectrophotometers The last 25 years has seen a revolution in the accuracy and the degree of sophistication available in both transmission and reflectance spectrophotometers, the operation of which is now largely automated, with overall control of input and output data via a PC interface. The introduction of ‘c:olour by numbers’ systems (software for instrumental dye recipe

P

A Reflecting mirror 6 Reflecting mirror C Ouartz prism D Bilateral curved slits E Ouartzdisc F Light source G Quartzlens H Ouartz test cells I Reflecting mirror J Filter slide K Test cell slide L Blue PE cell M PE tube slide N Dark current slide 0 Red PE cell P To amplifer

Figure 7 SP500 spectrophotorneter (Cambridge Instruments)

7ti 0 H w . Prog. Color.. 34 (2004)

prediction, colour matching, sorting and approval) has particularly driven the development of partially-automated reflection spectrophotometers such as the Color-Eye 7000 (GretagMacbeth) (Figure 8 ) , Color i5 (GretagMacbeth) and Spectraflash 600 (Datacolor International). These utilise diffuse illumination from a pulsed xenon light source with the aid of a specially designed integrating sphere (331.

been suitable, for example, for measuring the individual colours in textile prints or of three dimensional samples such as foodstuffs or pharmaceutical products. There was therefore a growing interest in camera-based colorimetric systems which could potentially yield CIE tristimulus values for each pixel location (361. Modern digital video colour cameras are capable of being characterised and therefore used for colorimetry 137,381 and the University of Derby have collaborated with VeriVide to introduce the DigiEye camera which is capable of yielding colorimetric data at pixel level.

The products to he measured are placed in a white, uniformly illuminated drawer and scanned using a characterised colour CCD camera. This device can be used to obtain fine detail colour information by capturing two dimensional images of objects of any shape. Examples for which this device has been used a re the colour measurement of individual tufts in a patterned carpet, pearlescent and metallic automotive finishes, colour control data for medical prostheses and as a more reliable method for assessing colour fastness tests on textiles 139). Taking the use of colour cameras as colorimeters a stage further, workers at the Oak Ridge National Laboratory have shown i t is possible to measure complex printed patterns and determine colour variations during the on- line manufacturing process 1401, as was proved feasible for plain shade dyeings more than thirty years ago. F

A Integrating sphere assembly B Optical bench C Xenon flash lamp D High voltage pulse drive E Holographic diffraction gratings F 40-Element photodiode array G Multiplexing interface H Spectro microprocessor I External computer and keyboard

Figure 8 spectrophotorneter (Gretaghlacbeth Color-Eye 7000)

Block-layout of a modern benchtop reflection

Spectral diffraction is achieved by means of holographic diffraction gratings using either a concave grating and planar mirror arrangement (the preferred option) or vice versa. which requires additional optical elements. Almost instantaneous detection of reflection data for the full range of wavelengths is achieved by means of a linear, high resolution, photodiode- or CCD-array. Integrated software packages are available from many suppliers which allow for immediate computer readout of colour data or colour differences in all recognised colorimetric units as well as the full reflectance spectra.

Miniaturisation technology of both sensor and data processing chips has also led to the development of many hand-held spectrophotometers which, despite their small size accommodate an integrating sphere, a rnicrodiffraction grating/CCD array such as that made by STEAGmicroParts (341 and all the electronics for data processing and on- screen or printed output. Because of their small size some of these instruments can be utilised not only for rapid reflectance measurements but also for colour calibration of both ink-jet printers and on-screen colour displays 135).

Although modern reflectance spectrophotometers can acquire colour data for fairly small areas they have not

Radiometers and colour analysers These devices are usually based on the use of special, precalibrated photodetectors (photodiode arrays have now largely replaced photomultipliers) and vary from sophisticated instruments, such as the Bentham TP300 telespectroradiometer, which can measure the complete spectral power distribution of a light source 1411, to simpler devices using multi-channel detectors from which the CIE coordinates of a colour displayed on a monitor/screen can he computed. Both types are of particular value for the standardisation of monitor displays for which purpose low cost instruments are now available and typified by the ColourVision Spyder which incorporates seven filtered photodiode elements and one neutral luminance sensor

A variety of inputs a re possible, depending upon the particular instrument, which may view the source from a distance as with the Bentham teleradiometer and even from space, as in the MISK (multi-angle imaging spectroradiometer) satellite, via a fibre-optic cable coupling or by placing the instrument directly on the surface to be measured. Spectroradiometers find many and varied applications such as the monitoring of UV sources for medical use 1431, water sterilisation lamps (particularly for aquaria] (441 and. in combination with special fluorescent dyes and developers, the detection of fatigue cracks in metal structures 1451. In all applications where UV overdosing presents a hazard (e.g. when used medically to treat skin disorders such as psoriasis or even for personal tanning) it is essential that the monitoring instruments are frequently calibrated 1281.

1421.

Image intensifiers and converters The basic structure of an image intensifier is shown in

0 H e ! : Prog. Color.. 34 (2004) 77

Figure 9a and continual improvements to the design have resulted in greatly improved performance and durability. For example much higher image resolution was achieved by having the two faces of the intensifier cell close together or by making both faces concave and adding a focusing electrode within the cell. In the so-called ‘second generation’ intensifiers a special microchannel plate is interposed between the photocathode and the screen. This consists of one or two layers of fine (10 pm diameter) microcapilleries across which a high potential (6 kV) is applied (Figure gb) (46).

(a)

Luminescent screen

Photocathode

(b)

Photocathode

Light (IR)

Microchannel plates (V-stack)

Luminescent screen

Figure 9 iii ic:ruc:han iiel plates

linage intensifiers: ( a ) basic s t ruc ture ; ( b ) with

The microchannel plates produce a greatly magnified stream of electrons and indeed a similar microchannel s t ruc ture has been used to make a very compact photomultiplier device per se. The microchannel capillaries are slightly angled so as to maximise electron collisions within the walls from which additional electrons are then emitted. Thus whereas the original units gave a light amplification of some hundreds of lumens per lumen (Im/lm) second generation types increased the gain to between 10‘‘ to 10’ Im/lm. In recent years ‘third generation’ intensifiers incorporate gallium arsenide in the photocathode which has further increased the sensitivity approximately fourfold.

These devices may be used to sense either infrared radiation (e.g. from the heat radiated by a person or a vehicle) or that reflected from objects which are at ambient temperature but reflect illumination from, say, starlight or

from an infrared beam, such as an incandescent lamp with a filter or a bank of infrared-emitting LEDs (light emitting diodes) (471. Strictly speaking a unit used for the former purpose is an image converter and i f used in conjunction with an illuminant is an image intensifier.

The use of image intensifiers is not confined to night vision applications but has also aided medical diagnosis particularly using short wave radiation (soft X-rays). Although standard X-ray film is still well established, i t is now possible to obtain digital moving images (for example of a flexing joint or a beating heart) using a special image intensifier tube in conjunction with a CCD camera 1481. Inside the input screen of this type of intensifier is a very thin layer of aluminium which has a coating of scintillator material. This was originally based on silver-doped zinc: cadmium sulphide (ZnCdS:Ag) but may now be replaced by a coating of specially aligned microcrystals of caesium iodide (CsI:Na), which has a much higher X-ray absorption efficiency. Light produced at the input screen is convorted into electrons at the photocathode and these are accelerated and focused by internal electrodes at a very high potential (25-35 kV). The output screen consists of another thin film of aluminium coated with a phosphor. The P20 phosphor (ZnCdS:Ag) is commonly used but Siemens have recently achieved improved performance by use of the P43 phosphor (Gd,O,S:Tb) [49).

Photodetectors in astronomy Almost every type of photon-detecting device has at some time been used for photometry or to aid visual observations in astronomy 1501 and gradually to replace photographic film as the recording medium [51]. CCDs were first used i n astronomy in 1976 and were subsequently widely adopted for this purpose, for example in the Hubble telescope, Voyager mission, etc. Digital cameras can be some hundred times more sensitive that film 1521. Special CCD cameras, such as the thin back-illuminated type (in which light enters through a transparent rear window; the reverse of the normal CCD situation) have now been developed for astrophysical applications at the visible and infrared wavelengths (300-1100 nm) [53]. As the detection of extremely low light levels is often involved i t is essential that electronic and thermal noise is minimised so that detectors a re usually cryogenically cooletl with liquid nitrogen. More recently, certain photodetectors based on mercury alloys with cadmium and tellurium have shown promise of being usable at higher temperatures, say with simple thermo-electric and Peltier-effect coolers (54).

Fibre optic communication systems Some 30 years ago it became evident that faster, higher capacity telephone communication systems would be required in the future. Waveguide systems built into rigid tubes which could be coupled together were researched but their lack of flexibilty was a severe limitation. I t was only when it became possible (around 1985) to manufacture special high purity glass fibre cables that local area and long distance optical communication networks for computing, telephony and TV transmission began to be adopted worldwide. As an example of the ubiquity of such cable networks there is a present plan to extend a fibreoptic cable for 2000 km across the Antarctic, one of

the few places on earth where there is no reliable satellite communication.

Fibre optic cables have many advantages over copper wire conductors including:

They are lightweight and as individual fibres have a small diameter (50-100 pm), which means large bundles of fibres can be accommodated in each flexible cable. Such cables do not create, and are immune from, external electromagnetic fields and extremely difficult to tap into, thus having a very high degree of security. The transmission losses are very low (say 0.2 decibels/ km, much less than that of conductor wires) but signals still need amplifying at intervals over long distances. Higher data transmission rates are possible compared to wire conductor communications (3 .2 versus 0.8 gigabitsh]).

In its simplest form an optical cable consists of a core of special silicon/gernianium glass surrounded by a glass skin cladding having a lower refractive index and, finally, a plastic outer protective ‘buffer’ cladding. The cable and presentation of the light beam (its frequency and so-called mode) are selected so that a very high proportion of the light is continually internally reflected at the core/sheath boundary 155).

For short distance local area networks, multi-mode glass fibres with a core of about 50-100 pm diameter, or even polymer fibres, are used and the transmitters are usually special LEDs using red or infrared light (650, 850 or 1300 nm wavelength) , whi l s t for long d i s t ance transmissions fine, single mode fibres (about 10 pm diameter) and laser diodes (1550-1800 nm) are employed. With either type of transmission the receivers are based on photodiodes (silicon up to 1100 nni and gallium arsenide up to 2000 nm). Signal attenuation occurs over long distances so that i t is necessary to have amplifiers. known as optical regenerators. at intervals because signal loss can range from more than 50% to as little as 10% per km depending on the fibre optic material, the transmission mode and the frequency of light used ISS].

COLOUR IMAGE CAPTURE DEVICES Still cameras The rapid development of CCD arrays during the past decade has seen a corresponding major expansion in the sales of digital cameras which are making major inroads into the non-professional, film-based photographic market 161. A basic digital camera consists of a suitable lens, an automatic exposure system controlling a mechanical or electronic shutter, a CCD or CMOS array on a chip, a small LCD viewing screen and hardwarehoftware for control and downloading of image data, which is initially stored 011 a flash memory chip or removable compact flash memory card in a compressed (usually ]PEG) form. For non- professional use, better quality digital cameras would also normally be fitted with some type of automatic focus device based 011 an infrared photodiode transmitter and receiver system [ 57). Professional photographers traditionally prefer

sophisticated cameras such as the Nikon or Hasselblad fitted with mechanical shutters and high quality, exchangeable lenses, but suitably mounted, ‘clip-on’ CCD backplates. which are very expensive, are now available even for such cameras. I n addition for professional use it is preferable to store the picture data i n an uncompressed format such as KAW or TIFF 1581.

Consumer CCD cameras tend not to have mechanical shutters but usually employ the CCD array output to control the exposure time electronically. Initially the picture resolution of digital cameras was low [say VGA (video graphics array) format 640 x 480 or 0.3 megapixels] but there has been a rapid improvement so that 3-6 megapixel cameras are now conimonly available.

At the other end of the scale there are many inexpensive minicameras, such as those incorporated into modern mobile phones and the toy makers Mattel have even brought out a Rarbie doll digital camera, albeit with a poor definition of 320 x 240. I n addition to being able to produce good definition prints, using a photographic quality ink-jet printer for example, modern digital cameras can also capture short bursts of video at up to 30 frames per second.

The CCD or CMOS array of cells is normally configured as a mosaic of red, green and blue (KGB) pixels, called a colour filter array (CFA) corresponding to a pattern of tiny filter elements over the surface of each group of pixels. These are commonly arranged in a so called Bayer pattern (devised by Bryce Bayer of Kodak some 30 years ago) there being two G for every one R or B pixel (Figure 10) 1591. Less commonly, some cameras use a four filter CFA. consisting of cyan, green, magenta and yellow pixels in equal numbers. Pixel size can vary considerably, depending on the end use of the array but typically the cells may be from 7 to 15 pni wide. with a black surround border of about 1.5 pm to prevent c:olour cross-talk between pixels.

a Green Blue

Figure 10 caniere

KGB arrangement of pixel eleinents in a digilal

0 Rri: Prog. Color.. 34 (2004) 79

The colour data of a captured image is not directly downloaded to the computer as the camera’s onboard processor performs a so-called de-mosaicing calculation which, for each position, for example, of a G pixel, interpolates the Rand B values from those of neighbouring R and B pixels, and so on, for the R and B pixel positions. There are many different mathematical methods (some of which are quite complex algorithms) for performing these calculations and these are discussed further in the section on image data handling.

The individual cells in a typical CCD array respond linearly with the intensity of light falling upon them [SO], but beyond a certain intensity there is no further increase in their response, in which respect the dynamic response range of a conventional CCD camera is inferior to that of photographic film. I t is claimed that this disadvantage has been largely overcome by Fuji’s development of the super-pixel, SR dual-cell camera. Fuji initially marketed conventional CCD cameras but then introduced novel CCD arrays consisting of diagonal rows of octagonal cells, claimed to achieve improved sensitivity and a better signal-to-noise ratio. In 2003 they announced a further improvement [Sl]. In these later devices each pixel element consists of a normal sensitivity ‘S’ cell adjacent to a smaller, lower sensitivity ‘R’ cell which in effect extends the dynamic response of the camera, making the overall performance more akin to that of normal film. They claim that these two types of cell respond in an analogous manner to that of the rods and cones of the human eye and like the smaller and larger silver halide crystals in a photographic emulsion. Both pairs of cell are exposed simultaneously but the data from them are downloaded sequentially. The practical result is a digital camera with improved retention of details in highlight areas.

All the CCD cameras so far described are based on individual R, G and B pixels but the latest breakthrough is the production of CMOS arrays in which each pixel element records all three colour responses in the same cell. Thus a camera based on the Foveon chip could theoretically capture practically all the light passing through the lens whilst a normal CCD camera captures in effect only a third. Figure 11 shows the principle of the device, consisting of three layers which respond according to the wavelength of the incident light as a result of the different depths into the silicon to which light penetrates [SZ].

This advance has been made possible by the availability of National Semiconductor’s 0.18 pm CMOS fabrication technology which has produced a 70 million transistor, 22 mm square, chip which could be used in still, video or hybrid cameras. This versatility results from the fact that because each cell in the matrix can be addressed individually, it is possible to choose to use groups of cells to form each pixel. It is therefore possible to change at will from still pictures with the high resolution required for enlargement prints to the high frame rate but lower resolution needed for video.

Digital photography continues to make inroads into the use of conventional film, even for professional use and Kodak, who already have a strong presence in the new technology, has announced a limited withdrawal from film photography.

Figure 11 Foveon X3 camera cell

Video cameras Analogue video CCD cameras , commonly called camcorders, have been available for many years and the VHSC format, using 8 mm magnetic oxide tape as the recording medium. is well established i n Europe for consumer use. There is inherent noise i n this type of recording system, so the picture quality is not particularly good and deteriorates badly each time a copy is made. Digital recording on the other hand not only yields better quality pictures initially but electronic editing is facilitated and there is no loss of quality during re-copying. Digital video cameras are now used extensively for closed circuit (CCTV) security surveillance, for a number of website applications and increasingly for consumer use.

In addition many industrial applications, such as pick and place assembly, are now being automated with the aid of camera vision systems. These often operate at very high speeds which has been made possible by a combination of modern computers linked to the cameras by fibre- optic cables and using, for example, the software protocol Firewire (IEEE-1394) serial link, with data transfer rates as high as 3.2 gigabitsh 1631.

Since 1990 broadcast TV has also been based on CCD cameras for both analogue and, more recently, digital transmission systems. These are essentially of two types viz. those using a prism system to split the incoming light beam into its RGB components, which then fall on three separate CCD arrays, and those using a single CCD colour filter array. The latter method has the advantage of both simplicity and optical compatibility with existing 35 nini cinematography lenses [64].

For professional use cine cameras are made to feel as similar as possible to a conventional one, thus they have (unlike camcorders) a reflex optical viewfinder and use special CCD arrays conforming to various motion picture film aperture formats, such as widescreen. Cinemascope. Super 35 or Palomar. These CCD arrays vary in size from 17 mm with three arrays of 2 megapixels used by Panavision to 34 mm with a single 8 megapixel array in the Palomar camera [64]. An experimental camera with even higher resolution (three 8 megapixel arrays) has been reported

The advent of digital video cameras has already revolutionised the production and editing processes for TV and could do so for motion picture films. Thus i t becomes possible for a film production team to view the results shortly after shooting a scene and take immediate remedial action where necessary. This would eliminate

1651.

80 0 Rev. Prog. Color.. 34 (2004)

the traditional viewing of ‘the dailies’ or ‘rushes’ late each evening when the film had been processed. Editing, which can now be achieved totally computationally, is similarly greatly facilitated. Ultimately one can envision a digital cinema film distribution system which is entirely electronic but at the moment quality improvements are still required. To enable digital film distribution systems the size of a complete film file needs compressing to 50 gigabyte or less, but image quality for very large screen projection demands better compression techniques than that offered by the various MPEG (moving picture experts group) systems (based on 24-bit RGB colour) at present in use for video reproduction. Recent announcements suggest that these criteria can now be met I661 and, whilst films recorded on digital versatile discs (DVDs) are commonplace for home viewing digital cinema (either on a TV or digital projector), for larger audiences is developing rapidly particularly in the USA. The camera system used to obtain such films incorporates data processing such as the analogue to digital conversion (ADC), colour interpolation/colour space conversion and storage.

Image capture using both still and video cameras is being increasingly used for professional audiences, ranging from simple business videoconferencing to educational and consultative uses particularly in medicine, which may even involve remote patient diagnosis [671 and ‘telepathology’ 1681. Progressive scan cameras, which capture data images at higher resolution with conventional video format, are particularly suited for such end uses [69].

Image scanners One of the most important methods of acquiring design images is by the use of a colour scanner. Flatbed scanners are commonly used for artwork sizes up to A3, whilst larger items are usually processed on rotary drum scanners. Designs and specific technologies vary considerably but the artwork to be scanned is placed face downwards onto the upper glass plate of the device and illuminated by ‘white’ light usually from a cold cathode fluorescent tube. To scan the whole area, the lamp and two mirror strips are swept across the area by a stepper motor, the reflected light being focused onto one or more photosensor (usually CCD) arrays. Early scanners were slow three-pass devices, i.e. they scanned the image sequentially with an RGB filter in the light path. Many present day scanners employ a single pass using either a system of dichroic or ordinary mirrors and a three-strip photodetector array with at least 2500 sensors, each strip having an R, G or B filter i f plain mirrors are used (Figure 12). Another approach is for the scanner light source to consist of three R, G and B LEDs which flash sequent ia I ly.

The maximum true definition that the scanner can attain depends on the number of photodiodes in the array and the minimum step of the stepper drive. For example a typical scanner with a true 300 x 600 dpi resolution and a stepper motor moving in steps of 0.0033 inches will have 5100 sensors in each horizontal row. Higher resolutions claimed for some scanners relate to mathematically interpolated, rather than measured, values. Most coloured images required for textile use will be scanned in 24-bit colour (8 bits each for R, G and B) and although some scanners acquire the

R

lamp

w Artwork

Figure 12 Single pass flatbed scanner

data as 32 or 36 bitdpixel i t is unlikely that this affects the perceptual result 1351. Rotary scanners can usually accommodate artwork up to A0 size and have a fixed, three sensor, KGB scanning head in which the photomultiplier or photodiode acquires the reflected light values as the drum rotates whilst the head is transported across the width. Very high definition scanning (> 10 000 dpi) is possible. Scanners are normally controlled by computer to which they are attached by either a parallel or, more commonly, a fast serial link such as USB (universal serial bus), SCSI (small computer system interface) or FireWire (IEEE- 1394 standard), which is the fastest and is therefore to be preferred when scanning large images at high resolution.

I f colour film positive or negative transparencies need to be scanned one can use a simple mirror adapter on a flatbed scanner (701 but, since any image on 35 mm and particularly 16 mm film clearly needs to be greatly magnified, the scan definition of most A4lA3 scanners (say around 2000 dpi) is not high enough for this application and accordingly special slide scanning devices (5000-6000 dpi) are marketed. Great care is needed to ensure freedom from dust etc. owing to the very high magnification subsequently required. As with flatbed scanners film transparency scanners may employ cold cathode fluorescent tubes, but LEDs are more commonly used.

Irrespective of the type or make of scanner, the industry has adopted a common communication protocol known as ‘TWAIN’ which is not, surprisingly, an acronym. This was devised in the early 1990s by a n industry-wide working group called the TWAIN Coalition [71] and the adoption of the standard now provides a consistent integration of image data between all input devices and software applications. After the initial capture of an image by a scanner in TWAIN format, the user usually stores the data in one of the other recognised file formats, such as BMP. TIFF, JPEG etc. 1351, although some more recent alternative compression systems have been reviewed 1721.

Film to digital video transfer (telecine scanners) Until the 1990s when CCD cameras became adopted for TV. many programmes were pre-recorded on film, which nowadays is used only for broadcasting films, commercials

0 R ~ K Prog. Cdor.. 34 (2004) 81

and occasionally documentaries. For home consumption, there has long been a need to transfer films to various magnetic storage media for home viewing (both in analogue VHS and more recently in digital format) or optically on CD/DVD (for music videos). In the 1970s a modern version of the 'flying spot' scanning technique was developed for this purpose. The scanner consisted of a projection CRT on the screen of which a small extremely bright spot was scanned as a picture raster. Using a suitable optical system an image of the flying spot was projected, frame by frame, onto the film being scanned. The transmitted beam was split into its RGB components by three dichroic mirrors and the light intensity detected by three photocells (Figure 13) .

Sweep sweep control driver Film

R

G

B Photocells Dichroic

mirrors

Figure 13 Flying spot f i lm scanner

For professional use i t is the original negative film which is copied, not a positive print, because this gives the greatest latitude in colour editing 1731 despite problems in attaining a meaningful quantitative assessment of comparative colour values 174).

A more direct way of transferring film to videotape became available with the advent of video cameras and this involves synchronising the frames of a projected film [normally 24 frames per second (fps)] with the frame rate of the camera (25 fps). A special telecine machine is used for the projection and in the case of the USA (TV frame rate 30 fps) a more complex frame scanning sequence is used to attain synchronism. Telecine scanners can also be used to digitise film into which special effects are to he added. For this application CCD arrays are specially designed as very good pixel-to-pixel uniformity and high signal to noise ratios are essential (751, and are typified by the Philips DVS 'Shadow' system 1761 which can provide both standard 625 line TV video as well as 1920 x 1080 high definition TV (HDTV) output. The unit employs diffuse film illumination to minimise defects from surface scratches, a three colour prism beam splitter and three separate 2048 pixel linear CCD arrays.

COLOUR AND IMAGE DATA HANDLING AND TRANSMISSION Collaborative design networking is a common feature in many production fields, including the textile industry,

where colour, pattern design and surface appearance can all play a part in acceptability criteria and may therefore need to be accurately measured. interpreted, transmitted and re-interpreted by the receiver 177,781. In some respects i t seems that the widespread adoption of instrumental colour measurement has tended to eliniinate many of the former communication problems that often existed between colourists and designers and has aided joint decision taking by creative and commercial management.

At every stage of image capture and subsequent data handling there is a complex series of computational operations, the smooth functioning of which is just as important a s that of the optoelectronic hardware that initially acquired the data. There are a number of books covering the principles of digital image processing and management 179-811.

Data handling by the capture device As has already been illustrated, at the time that an image is captured by any device, a great deal of internal data handling takes place in order to translate analogue voltages/ currents into digital representations, often for millions of pixel locations. The complexity of colour image capture is particularly true of digital cameras, initially in the downloading of the captured raw data, but also i n the subsequent interpolation of the 'missing' KGB values for each pixel location in the Bayer colour filter patterns. Tht: necessary computation niay be handled partly onhoartl the CCDKMOS array chip, partly by a microprocessor i n the camera and finally by a host PC or workstation onto which the image is finally downloaded.

Many algorithms have been devised for the interpolation of RGB data for Bayer pattern pixel locations [82l. The simplest approach is to use a bilinear averaging system, for example to consider pairs of values from the eight nearest pixels to supply the two missing R , C or B values at any pixel location. Certain algorithms can yield artefact valuris leading to chromatic mottles or splotches in regions of high frequency luminance variations, a factor being used to promote the advantages of the new Foveon caniwa in which every pixel captures R,G and B values. Apart from image content, much depends on the camera optics, pixel density of the CCD array and the actual mosaicing pattern involved, but in particular it has been reported that de- mosaicing algorithms which also have a sharpening effect on the image are more perceptually acceptable (831. When equally sharp images were compared i t was found that tho Freeman bilinear algorithm 184) and certain colour vector filtering methods I85,86] performed well. Other algorithms have been devised to overcome a problem of pixels reaching saturation values (i.e. a digital value of 255) when the true colour at that pixel is actually more saturated 1871; a basic problem with standard CCD cameras, with the possible exception of the Fuji superpixel system.

Just as with colour printers there are limitations in the gamut of colours that can be captured by scanners and cameras, but whereas the characterisation of the gamuts achievable from output devices is well established, there have been problems in developing a universal gamut mapping algorithm for input devices. Morovic et ( I / . , on behalf of the CIE Technical Committee TC8-03 [as], have published a survey of the fundamentals involved [89) and

more recently have proposed a method based on simulating the responses of the input media to given spectral power distributions and relating these responses to a set of spectra that cover the majority of all possibilities [86].

When an image is captured by a device such as a scanner or camera i t is often essential that the data represents as closely as possible that of the original colours. The KGB values captured are device-dependent which requires each machine to be characterised and this is best achieved by colorimetric measurement of images acquired from a standard colour test card such as the GretagMacbeth ColorChecker or the Kodak (ANSI) IT8.7/2 card. Then, using suitable software, conversion of the raw data into CIE values ( e g XYZ, L*a*b*) in a device-independent colour space such as sRGB can be achieved [78]. Methods for standardising cameras and scanners in this manner and the necessary mathematical transforms from KGB to XYZ values are well established [87.88]. Farrell has suggested means by which the tristiniulus values may be converted into device-independent spectral responses of the surfaces and illuminants in a captured image 1891.

In many cases colour ranges and multicolour designs may need to be viewed on monitors by suppliers, retailers or customers where decisions are made whilst relying on the accuracy of the on-screen representations [90.91]. In such circumstances it is therefore essential to employ the characterisation methods that are also now available for both CRT and LCD displays [19]. Digital video photometers based on CCD camera arrays are said to be ideal for display characterisation including assessment of luminance and contrast, flicker and jitter measurenients 1921.

Data transmission I n the modern commercial environment there is an increasing need for fast information gathering and transmission, both internally within a business and externally with suppliers and customers alike. To facilitate this there needs to be a rapid and reliable (error free) transmission/receiving mechanism and to assist the free interchange of data, the system should be ‘open’, i.e. non-proprietary [93]. The fastest telecommunications systems, whether for local area networks within the office or externally, use fibre-optic cable interconnects and speed is particularly necessary where very large data files are involved. Examples of the large data files which may need to be transmitted are: - large, high definition, 24-bit colour images, particularly

i f transmitted in an unconipressed (e.g. BMP or TIFF) form colorimetric measurement data transmitted in, for example, XML (extensible markup language) code which can accommodate much suppor t ing so- called nietadata; examples are GretagMacbeth’s CxF language for colour data communication (941 and the SDC’s proposed ‘Digital information for textile communication’ system [93] colour management data relating to the interconversion between various colour spaces or colour acquisition and display systems ( e g ICC profiles [95]); this itself can be part of the metadata transmitted in an XML- based system.

-

-

Particular problems can arise when transmitting colour images over the internet, where it is desirable that the view on a monitor, as seen by the recipient, is as close as possible to that originally transmitted (e.g. for a commercial website selling coloured textile garments) [96]. Depending on the communications service provider there may be limitations in that there are less than 255 ’web-safe’ colours where the transmitted and received RGB values will always correspond, but there still remains a problem of characterising both displays. Some basic colour calibration systems use filters which can be placed on the monitor screen and, using software provided, adjustments to the display can be made [97].

An even more critical application concerns internet access to high resolution images of works of art in international galleries [96,98]. An interesting aspect of archiving is the selection of a storage medium that one might expect to be still readable 50 years hence, and this poses the question that even if the data does not become degraded in that time, will the mechanism that reads the data still be available beyond the confines of a museum? In the last 50 years we have seen the demise of punched paper tape, optical and some types of magnetic tape as well as three. five-and-three-quarter. and eight inch floppy disks, not to mention continual changes in computers and their operating systems [99]. Today the capacity and use of solid state storage devices advances rapidly and optical disc technology is also changing almost year on year. Much care will clearly need to be exercised in system and equipment selection to ensure years of patient labour does not become valueless to the next generation.

CONCLUSIONS The past few decades have seen enormous progress in the development, manufacture and application of light sensing devices, many of which have resulted from advances in silicon chip fabrication technology. In particular these advances have had a major impact in the photographic and film industries, defence systems and in the introduction of increasingly sophisticated and versatile spectrophotometers and radiometers. The sensitivity and reproducibility of response of the various light-sensing devices now in use has been achieved through continual refinement but, with the increasing use of colorimetric data as a basis for agreed customer specifications. it is essential to ensure that any instruments used have been calibrated against internationally recognised measurement scales and that they are traceable to national measurement standards. In the UK this is usually attained through the services of the NPL (National Physics Laboratory) or other UKAS (United Kingdom Accreditation Service) accredited laboratories [loo].

The ‘digital revolution’ looks set to continue to extend its influence, in both the manufacturing and leisure industries and also in the field of home entertainment. In the long term the development of a cheap, high efficiency solar cell array still represents the most likely solution to the provision of a freely available, non-polluting energy source. Ongoing research and development for light-sensing devices will undoubtedly continue to result not only in stepwise

0 f?f?v. Prog. cob, 34 (2004) 83

improvements but novel applications should emerge to enhance still further colour applications.

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After working in various service and marketing departments at the former ICI Dyestuffs Division for 30 years, Dr Dawson formed a consultancy partnership which carried out research and machinery development in the field of digital printing at UMIS'I: He is the author of many papers, particularly in the field of textile printing and latterly on digital colour applications, which has been recognised by the SDC by the award of the Centenary Medal and three silver medals from other professional bodies. He is also a long-standing member of the SDC Colour Measurement Committee.