Digital image capture — Filmless camera technology and techniques

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  • Digital image capture - Filmless technology and techniques

    camera

    A Kazhuciunas

    Department of Colour and Polymer Chemist ,ry, University of Leeds, Leeds> LS2 9]T, United Kingdom

    Keywords Cr!a!ge-CoL~p!ec! sen,icunducbl de'riLL, L.orr!p!c!rT!entary metal oxide somicondiidol, Jdotosensm; a'ra- iog'e to digital converte ~. rroi!e fdrgi'rg, onotodJo6e.

    l lazlauciunas

    Department o f. CoJour and Poi},mer Chemist!-} UniversiT}, of Leeds, leeds, iS? 9Jh United Kirigdom

    Tel: +44 (0)! I 3 243 175! EmaJi: aig)q

  • In t roduct ion Although electronic imaging is perceived to be an invention of the modern age, its origins date back to the 1930s, when allegedly the very first colour photo graph to be electronically transmitted was a studio portrait of the legenda U silent movie star of that age, Rudolph Valentino2 The image was taken on the set of the film Monsteux gess and three separations of the image (ie red, yellow and blue) were sent from Chica- go to New York using the telephone lines of the Bell Telephone Company, Once transmitted, a three-colour reproduction was generated,

    The general notion of utilising a charge coupled semiconductor device (CCD) as the image capture component lbr a solid state camera was the brainchild of William Boyle and George Smith in 1972, who were employed by Bell Sys terns in the USA, 4 As recognition for the invention and development of the CCD, a contribution that has had a major impact on image creation and utilisa- tion, Boyle and Smith were awarded the Edwdn H Land Medal by the Societ 7 of Imaging Science and Technology in 2001 .s

    Filmless canera technology took the best part of a decade to emerge, "when Sony introduced their MA\qCA still cam era to the world at Plhotokina in Ger many in 1981. ~ Although this camera utilised the CCD device as the image sensor, it was not a digital camera, and used the same techno[o~' being utiiised for non-digital motion picture video cameras. The first professional digital single lens reflex camera (DSLR) was ulti- mately launched by Kodak in 1991, namely the DCS IOOF

    Sensor techno logy

    Charge coup led semiconductor (CCD)

    The charge coupled semiconductor device has bee:n the dorninant imaging sensor since the introduction of solid- state still cameras in the 1980s, Vvhen image quality is measured with respect to quanttan effidency and noise levels alone, the CCD could not be surpassed, These sensor ~-pes tend to be utilised within areas where image capture of the highest quality is demanded (eg medical, scientific and industrial).

    The CCD consists of a molecular layer of dielectric silicon dioxide deposited onto a p b'pe silicon substrate. When light enters the silicon substrate at individual photosites, it provides sufficient ener~,

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    Figure 1: The basic working principle of photosites on a CCD sensor

    Table 1: Peffom|ance comparison between CCD arid CMOS image sensor technolo- gies

    Performance Oharqe coup led Oomplementary metal oxide ~emiconductor device (OCD) semiconductor (CMOS)

    Respor~sivity Moderate Oyna~~ic ~ange High Uni%mii}, High Speed Moderate "o high Biasing Muiripie U!]JLOl-lrl shuttering Fast, commor-

    to release negatively charged electrons from the silicon atoms in the substrate. By applying a positive electrical field, the negative charges can be drained to the substrate. Each photosite has an electri col contact, referred to as a gate, ~d when a voltage is applied to this gate, an area of silicon becomes receptive to the freed electrons, These negatively charged electrons are then stored in pro- portion to the intensity of light encoun- tered. The charge on the photosite is then moved onto a shift register and read. Finally, the variable analogue si@tal is sampled and quantised as a discrete series of steps in the form of a grey scale via an analogue to digital (A,9 convert er. Figure 1 illustrates, in simple temps, the basic ,a.orking principle of photosites on a CCD sensor.

    Complementary metal oxide semiconductor (CMOS)

    Cornplementa U metal oxide semicon ductor circuits were invented by Dank Wanlass of Fairchild Semiconductor, ~ v~ith the first CMOS integrated circuits being introduced by RCA, Not tmlike the CCD, the CMOS imaging sensor also converts light :into an electrical charge

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    and then processes it into electronic Big :rials. However; mflike the CCD sensog each pixel is independent of its neigh bouL resulting in random access to each individual pixel by means of an XY address. There is also present an ampli tier at each of the pixel positions consist ing of three transistors utilising C_,MOS technology, This amplification at the pixel level results in a ntm~ber of key advantages over conventional CCD technology, namely lower power con- sumption at the chip level and built-in analogue-to-digital conversion, Table 1 Ibatures a performance comparison between conventional CCD and CMOS image sensors.

    kk[)ifilm super CCD

    The initial version of the Fujifilm Super CCD was introduced in 19997 with the present-day fourth generation version now being incorporated into cun'ent ~ujifilm cameras, including their flagslhip digital single lens reflex (DSLR) model, the $3 Pro Fujifilm, having overcome the limitations associated with the convert tional CCD rectangular array by re designing both the shape and the layout of the photo@odes in their proprieta U

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    Figure 2: A comparison of the octagonal geometry of the Super CCD and the rectangular geometry of the conventional CCD design

    Figure 3: Diagram illustrating the similarity between the working principle oF the Super CCD and modern-day film

    Super CCD. A compmison between the octagonal geometry of the Super CCD and the conventional CCD design is illustrated in Figure 2,

    A key advantage of the octagonal geom- etry is that it enables microscopic elec- tronic contact areas to be greatly reduced, and as a consequence, results in individual photosensors being greater in size than would be possible in a con ventional CCD rectangular geometry design. The key benefit of the Super CCD design is a marked reduction in the level of noise.

    A major hindrance associated with the octagonal geometry, however, is that images are required to be generated in standard fom~ats such as JPEC (joint photographic expert group) and TIFF

    (tagged image file forma0, and these require the intage to be formed of pixels within a rectangular geomet G, The pro- prietary design of the Super CCD cir- cumnavigates this requirement by using interpolation to achieve a rectangular geomet U double the size of the octago- nal geometry of the sensor, This results in the Super CCD having a resolution that is twice that of the actual nmnber of photosensors present on the imaging chip. The general consensus of opinion from within the imaging cornmuni%, is that the octagonal geomet U design has only improved resolution on vertical and horizontal lines and not on diagonal lines.

    The design of the current fourth-genera- tion Super CCD lends itself to working

    along the lines of modem-day photo- graphic film, The photosensitive compo- nent of film (ie silver halide emulsion) contains large surface area grains specif- ically sensitive to low light levels, togeth er ~ith small surface area grains that are specifically sensitive to high light levels. The current Super CCD mimics film by :mixing low sensiHvib, pixeh, referred to as R pixels, together with high sensitivity pixels, referred to as S pixels. When an :image is being recorded, the high sensi- tivity pixels are rapidly saturated by the incoming light, whereas the low sensitiv- it? pixels achieve saturation at a much reduced rate, Thus, whereas the S-pixels become quickly saturated in the high- light area of an image, the R-pixels con- tinue to gather light ~ithout achieving over saturation. B~, means of combining both sets of image detail, the principal gain is a wider dynamic range, which leads to highlight and shadow detail, :more natural sMn tones, richer tonality, and better texture rendering in both black emd white fabrics. Figure 3 demon strates the similarity between the work ing principle of the Super CCD and modem-day photographic film,

    Foveon X3 CMOS full-colour ~maging sensor

    This proprietary device, ~ which is a lay- ered silicon sensor fabricated on a stan- dard CMOS processing line, is the first full-colour imaging sensor to be incorpo- rated into commercially-available DSLRs (Sigma SD9 and Sigma SD10), although other research groups are ago working towards this end. !~ ~4

    The essential working principle of the sensor relies upon the fact that different wavelengths of light penetrate to differ ent depths ~adthin the silicon surface, However, although the sensor is por- trayed as absorbing only blue light at the upper surface, green light within the central region and red light within the lower region, this is an over simplifica- tion, Figure 4 illustrates the basic work- ing principle of the sensor, whereby three separate PN (positive negative) junctions are incorporated at specific depths within @e silicon surface and u@ised to separate the electron hole pairs that are generated by the silicon. [n reality, a mixture of colours is detected by each sensor layer and algorithms are applied during the processing to corn pensate for the overlap of the spectral sensitivity bands,

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    Figure 4: Diagram [[lustratirlg the working principle of the Foveon tull colour imaging sol/sot

    the analogue-to-digital (A/D) converter. It should be stated at this juncture that for CMOS sensors, the process is achieved within the architecture of the sensor itself.

    The maximurn number of grey levels supported by most image handling applications on a computer workstation is 256, and eight binary digits (bits) are required to enumerate 256 grey levels. floweret, the majority of digital camera analogue-to-digital converters tend to convert into I0, 12 or 14 grey levels, This capability; reDrred to as 'supersam- piing', has the desired effect of improv- ing image quality,, Pixels in an RCB image require art eight-bit number for each of the three colour charnels, offer ing a gamut of nearly I 7 million colours ,fie 256 x 256 x 256). The resulting 24 bit file occupies three times the space of

    Digital camera classification/colom' capture and reproduction methodology When light is converted into electrical potential during image capture by CCD or CMOS imaging sensors, no recogni tion is extended to the colour of the light. Thus, without modification, all CCD and CMOS sensors, with the exception of the Foveon CMOS type, are essentially monochrome devices.

    In essence, there are six distinct design t)ges of image-capture methodology associated ~r the digital caraera?

    One-shot single matrix This b,pe of design (see Figure 5), which is commonly found in all point and shoot and high end DSLR cameras, involves a colour filter array being into grated into a monochrome CCD/CMOS chip using photolithographic techniques to pattern the red, green and blue dye mordar~ts. Normally, the Bayer ~ che- quer-board pattern is used, where there are twice as many green elements com- pared with the red and blue elements, This design feature is included so that the sensor can mimic the hmnan visual system, which is more sensitive to high spacial frequencies in luminance, which is composed primarily of green light, than in chrominance. ~r ~t~e Bayer colour filter array, or 'rr~ask', thus enables con trol of the incoming light using the addi tire principle of colour reproduction,

    The interesting feature of this pa.rt[cular method of image capture and reproduc-- tion is that it mimics the design features of the many regular patterned screen--

    plates invented during the early days of additive colour photography.~r Although almost all one shot single matrix digital cameras use the RCB (red, green, blue} additive principle to achieve co]our imaging, Kodak have utilised the sub tractive method to record digital images (ie using cyan, magenta ax~d yellow (CMY) filtration), Such an example is the professional grade DCS 620X DSLR. Fob lowing initial negative image capture in CM~ the data are then converted to a positive RGB image to maintain compat- ibility with standard digital architecture.

    '1o create a digital image, the analogue charge generated by CCD elements m~dergoes quantisation, whereby each step is assigned a unique bina U number, which represents a specific tone or grey level. The process itself is tmdertaken by

    @e monochrome image. At 24 bits per pixel, digital files represent images pho torealisticall2/~ and if a file is greater than 24 bits per pixel, a high percentage of software will utflise the extra information by determin...