JANUARY 1938 5

THE USE OF ULTRAVIOLET RADIATION IN INDUSTRIALLUMINESCENCE nESEARCH

by A. VAN WIJK.

In the case of -many manufactured products abnormalities can easily be recognized bytheir luminescence. Because of this the observation ofluminescence has provided valuableas a check in many industries such as the textile industry, thepaper industry, the paintindustry, the rubber industry, food industries and laundries: The ultra violet radiation' ofmercury lamps is used for exciting luminesoence, the visible rays being absorbed by afilter. Two pieces of apparatus for luminescence analysis are described and several examplesof their application are given.

Luminescence

The impression of colour which a mixed radiationmakes on the eye depends upon the relative intensityof the wave lengths in the visible region occurring'in that radiation. When a "white" mixed radiation'falls on a substance, the reflected radiation may becoloured, that is, in the reflected light the relativedistribution of the rays is modified in the visibleregion.In many cas~s the observed change may be

adequately explained by noting that each of theincident visible wave lengths is weakened in thereflection by a specifiç amount depending on' the.wave length and the substance, so that certainwave length regions make a relatively smallercontribution to. the total light reflected. >

The influence on the radiation by the substanceirradiated is, however, not always confined to amore or less weakened reflection of each of theincident wave Iengths _individually, With an ÏJ;l-. cident radiation of suitable wave 'length manysubstances have the property of emitting a portionof the absorbed energy with an altered, gene:t:allylonger wave length 1). If an appreciable length oftime elapses between, the absorption of energy andits radiation, one usually speaks of phosphorescence, .otherwise of fluorescence. The two phenomenacollectively are called luminescence .. ,'The, :phe-,nomenon of luminescence may lead to, differencesbetween the mixed radiation incident on .a sub-stance and that reflected from it, which are essen-

, tially unlike the differences which occur in the. ah-sence of Iuminescence. When this property is pres-ent wave lengths may occur which are missingin the spectral composition of the incident r'adiation ..The fact that colour changes due to luminescence, are not usually observed in daily life, is not due to

1) This law, called Stokes' law, is related to the fact thatthe energy of one light quantum is smaller at longer wavelengths. After losing a portion of its energy in reflection,a light quantum can change to a longer wave length, butnot to a shorter one. .

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the fact that substances which exhibit the, phe-nomenon are rare. On the contrary, a great numberof substances in our surroundings have this propertywhen they are irradiated with' suitable wave.lengths.

Nor does the cause of this failure in obser-. vation lie as a rule in the 'lack .. of these wayelengths in the light used for .irradiatjçn, but ,in the. '.;r~, .., ol-" •

domination of light reflected nOJ:wally, that . is,without change in wave length. T~i~ 'is especiallytrue when' "white" Iight such as daylight or electriclight is used. If the incident mixture of radiationis not white but coloured, the conditions for ob-servation of colour components due to luminescenceare more. favourable, provided th~,luminescencegives wave lengths which are mis,siil'g:Îinthe incidentlight.

Luniinescence is, however, most striking when amixture is, use'd for irradiation. which contains novisible liglit. Normal reflection then gives no visiblecomponent, so' that .there is:no 'obstacle to the ob-ser~ation .of any Iuminescence resulting from the

, irradiation with, i~visible rays: For the purposeof ~;'citing' IUillin~sce~cein this )Vay only radiationwith. a wave length shorter, than that of visiblelight may be used, since the wave length increasesupon. Inminescence. The first to he considered is,therefore; ultra violet light, which continues thespectrum at the short waye end of the visibleregion, Further the. ability .of X-rays to causeluminescènce in' substances is ,quite well known.Frequent use is made of this property in renderingX-rays visible, and in reinforcing the photo-graphic action of X-rays 2). The use of X-raysfor testing materials by luminescence, however,is out of the question for economic and otherreasons (danger) .

The character of the luminescence, that isthe composition of the luminescence light, and

2) See Philips techno Rev. 2, 314, 1937.

6 PHILIPS TECHNICAL REVIEW Vol. 3, No. 1

even the very occurrence of the phenomenon uponirradiation with certain wave lengths, are specificproperties of the substance irradiated, and as suchare useful in the recognition and examination ofthat substance. In many cases the result of themethod of manufacture or of a preliminary treat-ment manifests itself in the luminescence, Thevalue of luminescence observation for materialtesting and process control is heing recognizedmore and more in the most diverse cases in allkinds of industries.

Since the observation is very rapid and does notinjure the substance examined or interfere withthe process of manufacture, and furthermore sinceonly very small quantities of the substance arenecessary, while the results obtained in manycases are remarkably good, it is not surprising thatluminescence analysis, as the process is nowcalled, is steadily gaining ground.

Ultra violet sources of radiation

An important condition for the general appli-cation of luminescence analysis in industry is theexistence of a simple apparatus, suitable for in-dustrial purposes, which supplies a sufficientlyintense ultra violet radiation with the exclusionof visible light. In the following we shall give adescription of two such pieces of apparatus eachof which has its part.icular range of application.In both cases the necessary ultra violet light is

supplied by quartz mercury lamps. The visiblelight emitted by these lamps is absorbed by asuitable filter. The filter is made of a special kindof glass "blackened" with nickel oxide.

Table I gives the transmission of two types ofglass coloured with NiO for a number of mercurylines. In both cases the transmission for visiblelight is practically zero (except for the extremered, where they both transmit again; since mercurylamps give only very little red radiation, thistransmission of the red is usually of no greatimportance) .

Table I.

Transmission of two types of "NiO"-glass. The objects testedwere bulbs of equal thickness.

Je (Á) Transmission In %1 Á = 0.1 mf1. Type 1 Type 2

4047 2 23655 71 653342 68 273130 43 33022 282967 10

Fig. 1. The mercury lamp 80 watts, 3000 lumens.

The maximum of transmission for both kinds ofglass lies at about 365 mu, where the strongestline of the whole mercury spectrum is situated.On the other hand the two glasses show a greatdifference at shorter wave lengths. With type 1 thetransmission of the wave lengths 313 mfJ. and297 mu is still considerable; with type 2 the trans-mission is here practically zero. This has beenbrought about by an extra addition to the moltenglass in order to prevent the occurrence of con-junctivitis. Conjunctivitis (snow blindness) is apainful inflamation of the conjunctiva of theeye which can be caused by ultra violet radiation(mountain sunlight). All ultra violet light, however,is not equally active in causing this complaint;the shorter the wave length the greater the action 3).

3) See Philips techno Rev. 2, 21, 1937.

JANUARY 1938 ULTRAVIOLET RADIATION IN LUMINESCENCE RESEARCH 7

Radiation with a wave length greater than about313 mfL no longer causes conjunctivitis even inlarge doses; it is, however, effective in causingluminescence, so that without important sacrificesin the desired activity of the radiation, the safetyof the user may be insured.The most obvious method of obtaining a filtered

heam is to build the lamp into a cabinet and toprovide the cabinet with an opening which maybe closed by a window of the filter glass. It is clear,however, that in this way only part of the radiationemitted hy the lamp is available for use, unless thedimensions of the windoware made very great.To this there are technical objections such asrisk of cracking of the glass which has to be fairlythick. In the Philips apparatus a different methodhas been chosen which makes the total radiationof the lamp available by surrounding the lamp withan outer bulb of the filter glass.As a source of ultra violet radiation the high

pressure mercury lamp "Philora" 80 watt (fig. 1)and the "Biosol" lamp (fig. 2) are used. Both ofthese lamps have been described in this periodical 4,).The high pressure mercury lamp of very smalldimensions is always surrounded by an outer bulL.By making this bulb of the glass containing nickeloxide of type 2 above, a luminescence 80 watt lampgiving 3000 lumens is obtained which may be connect-ed directly to the 220 V alternating current mainsvia a small choke coil. By placing the lamp in areflector made of material suitable for the desiredapplication, the greatest possible efficiency canbe obtained. Aluminium is a very suitable materialfor the reflector. Table II gives the intensity of theradiation emitted at a distance of 40 cm from thelamp in a direction perpendicular to the radia-tion element. The input to the lamp is 80 watts.

This lamp is suitable for use in cases where smallsamples are to be examined one at a time.

For industrial purposes where larger surfaces

Table n,Intensity of the radiation of the HPW-80 Watt lamp at adistance of 40 cm in a direction perpendicular to the radiatingelement.

Je (A)1A = 0.1mu

Intensity in ergs/sec/cm2

4047365533423130

1310506032

4) The mercury lamp 80 watts, 3000 lumens, Philips technoRev. 1, 129, 1936; the "Biosol" lamp, Philips techno Rev.2, 18, 1937.

must be intensely irradiated it is better to use the"Biosol" lamp which gives a higher intensity andhas a reflector specially designed to give a widebeam. The "Biosol"lamp, type A, is intendedfor use with a tubular filter which absorbs ultraviolet radiation ofwave lengths shorter than 280 mu,

When the lamp is to be used for luminescenceanalysis this filter must be replaced by a simi-larly-shaped filter of NiO glass of type 2. Theluminescence lamp so obtained is about 3 times as

Fig. 2. Philips "Biosol" apparatus type A.

8 PHILlPS TECHNICAL REVIEW Vol. 3, No. 1

strong as the 80 watt lamp. The chromium-platedreflector increases the intensity in thé workingdirection by an additional factor of about 2.5. Ata distance of 50 cm the dimensions of the crosssection of the beam are about 50· 80 cmê,

The higher intensity of the "Biosol" lamp necess-itates a higher consumption of energy, namely250 watts. The lamp HPW-i25 occupies a positionbetween the "Bios ol" A with a NiO filter and the80 watt lamp. This lamp is similar to the latterbut it has an input of 125 watts.

AppJications

Several examples' of the application of theselamps are given in the following, from which itDlay be seen that the possibility of irradiating largeareas is of practical importance.

Inspection of eggs: As was shown by van Oyenand Molanus the age of hens' eggs may be as-certained at a glance by means of the fluorescenceof the shell. A fresh egg fluoresces red and an oldegg blue: For checking large numbers of eggs,for instance at an 'egg market, luminescenceanalysis is the best method; it, is however, necessarythat whole racks can be inspected at once.

Control of laundry: If hard water is used whichcontains lime, spots of insoluble calcium soap,appearin the material. When th~se spots are freshthey are invisible under normal lighting conditions,while under the luminescence lamp they glow witha bluish brightness. For the inspection of largequantities of laundry the irradiation of large sur-faces is desirable.

Invisible marking of linen: A clever application.ofthe phenomenon of luminesce~cè is the following.Instead of disfiguring numbers or other visiblemarks, the laundry stamps the incoming goods with'a number in invisible ink of a kind ~hich luminescesstrongly. The ink is fast to washing, so that afterbeing washed the goods may be sorted in the usualway by the light of a _luminescence lamp.

Control of coloured textiles: Another interestingapplication ofluminescenee investigation is suppliedby the textile industry. The printing of materialsis often done with' soluble colourless rednetionproducts of dyes, the leuco-bases, Only in thefollowing step, oxidation, is the insoluble adsorbeddye itself obtained. Printing faults can, in general, beseen only after the oxidation; they can then, however,no longer be corrected. Practically all the leuco-dyesfluoresce strongly upon irradiation with ultra violetlight, so that the printed' m-aterial may' easily beinspected before the oxidation process with the 'help of the luminescence lamp. .In t1!§ case also

the observation of a large surface is desirable; inmany cases, however, the simpler lamp is suf-ficient, since the luminescing powcr of the Ieuco-dyes may be very strong, and an extremely strongirradiation is then not required to make themvisible.

It is impossible in the space of this article togive a description of the observations which canbe made with apparatus for luminescence analysison the diverse substances which enter into allkinds of industries. Such a description could notbe given without reservations, moreover, becausethe observations are liable to be found incorrectin certain points upon closer examination. Theextremely sensitive method of luminescence analy-sis has, in common with other sensitive methods,the property of being easily disturbed. In manycases the luminescing power of a given substanceis due only to extremely -small quantities of im-purities; or such impurities :.;naymodify the lumi-nescence, Sometimes .such impurities will beessential for the use to which the substance isput, sometimes they are completely useless. In thelatter case it would of course be incorrect to con-demn the substance on the basis of the presenceor absence of luminescence, or of luminescence ofa changed character.As examples óf industries in which experience .

has shown the value of luminescence analysis inskilled hands, the following may be mentioned:textile industry, paper industry, paint industry,rubber industry, food industry, laundries.

In addition there is room for application in goodsinspection, the detection of all kinds of falsificationsas of checques, postage stamps, banknotes, paint-ings, etc.It may be mentioned that in many cases the use

of an extra filter between the eye of the observerand the substance being examined may provevaluable. This filter serves to absorb the ultraviolet radiation reflected from the substance, aswell perhaps as the mercury line with a wavelength of 405 mfL which is transmitted by NiOglass.It is incorrect to say that ultra violet radiation

is entirely unobservable by' the eye. Long ultraviolet wave lengths give a certain impression, if theradiation is sufficiently intense. With respect tothe limiting wave length there are individual dif-ferences, but the mercury line of 365 mfL usedchiefly to excite luminescence is observable bypractically everyone.'!,The light impreesion obtained upon lookingdirectly 'into the luminescence lamp may he as-

JANUARY.1938 ULTRAVIOLET RADIATION IN .LUMINESCENCE RESEARCH -

crihed chiefly to the ultra violet radiation, withthe exception of a small amount of red. By in-troducing a filter as described above, which absorbsradiation with a wave length shorter than about410 mu, but transmits longer wave lengths, a truerluminescence colour is observed. Many non-luminescing materi~ls give the illusion of greyishblue luminescence without the filter. The filter canbe best introduced directly before the eyes in theform of spectacles of a special glass. Noviol-O orNoviol-A glass of the Corning Glass Works issuitable for this purpose.

The luminescence apparatus may be' employednot only for analysis but also for obtaining speciallighting effects in shop windows or on the stage.In such cases, paints or textiles, of which there aremany, are chosen for their strong luminescence.Another important application of luminescence-

lamps may' be in the making visible of objects(they may he .painted with a luminescent paint)by means of a source of invisible light. For examplein cases where there is danger of glare from sourcesof visible light, this application deserves seriousconsideration.

r r:-', .,...... _- _..,.

"PHILITE" AS A STRUCTURAL MATERIAL

by L. L. C. POLIS.

Several kinds of "Philite" are discussed with special emphasis upon their application intechnology, In the first place their physical properties are described, and the condltlonsare discussed which must he fulfilled if the suhstance is to be used as a structural material,while finally various possibilities for tbe application of this material are indicated.

Introduetion

In September 1936 a survey was given in thisperiodical !) of the most important kinds of"Philite", their properties and several possibilitiesof their application, Before this material can, beP":lt into practical use it must become as familiarto those using it as the materials which are nowcommon, such as metals, wood, stone, cement,etc. For this purpose it is at first necessary that thedifferent mechanical, .electrical, chemical and ther-mal properties of "Philite" become generally known.

A table, accompanying this article, has' been com-piled in which' the most important of these valuesare given. At the same time it may be seen fromthe table that very many different kinds of "Philite"are made, and that their physical and chemicalproperties are so widely divergent that it' willusually not be difficult to find among all the 'dif-ferent kinds a material which satisfies the require-ments in any given case.

1) R. Houwink, Properties and applications of artificialresin products, Philips techno Rev. 1, 257, 1936.

Since "Philite" must be pressed in a mould, itis possible to manufacture many kinds of articlesin this way. If the shape of the article to be madecannot be pressed directly, or if it is one whichwould demand too great a complexity of themould, "Philite" will be found to have the addi-tional advantage of being easily worked.

Construction with "Philite"

Because of the high cost of the necessary mould,"Philite" is especially suitable for articles whichhave to be made in large numbers. Nevertheless,the financial returns on the mould, even from asmall number of articles, rises with the increasein complexity of the product, that is with the in-crease in' the cost of making the same product inanother way, for example by machining or casting.

For complicated small articles (up to about 100mm in diameter) (fig. 1) the manufacture of 500pieces Illay be enough to make a mould pay foritself.

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