COLOR PHOTOGRAIPH\- OF LVMISESCESCE BY J . >I. LOHR
I t was pointed out in a preliminary paper' that successful use had been made of color photography t o determine the quality of the visible light emitted during cathodoluminescence. and t h a t in the preliminary experiments, photographs of the light from highly luminescent minerals such as willemite (yellon--green) and fluorite (deep blue), and from a red fluorescent sulphide had been obtained. This paper deals irith the photographing of the much fainter cathodoluminescence of salts of the alkaline halides and some other salts, and of the flames of various substances.
Cathodolumineseence The cathodoluminescence was excited by exposing to cathode rays substances contained in an 8 inch by I ' inch cathode tube consisting of two halves connected by a ground joint and a mercury seal. The lower half contained the substance and the upper half the ring anode and disc cathode. The tube was mounted on a wooden framenork in such a m y t h a t the body of the tube proper was inclined about 30' from the vertical, thus allowing the side arm of the cathode tube. n-hich is joined to its lower half. to rest in a horizontal position. A ground-glass hollow stopper leading from this horizontal arm was fused l o a glass tube irhich led to the .i-acuum pump. -4 small glass pan or spoon mounted on a glass rod support about 3 inches high and tipped slightly in front from the horizontal was placed in the tube for the purpose of holding the substances. -4 small n-ooden box painted black and with part of the bottom removed was placed dotvn over the cathode tube and held securely in position a t the bottom. The front of the box v a s beveled so that the cover or door rested in a slanting position and parallel in front to the en-
' Farnau a n d Lohr 20,
13:
(1912).
,
Eighth I n t e r n a t i o n a l Congrcss of Alpplied Chernistr!.
J . -14.Lohr closed cathode tube. This door, save for a small rourld hole opposite the spoon holding the substance to be used under the cathode rays, was covered with lead to cut off the X-rays. The light emitted by the action of the cathode rays on the substance under consideration passed through the small opening in the front of this closed box and through a condensing lens to a mirror placed a t such an angle that the rays striking i t were reflected don-nnard in a vertical path so as to strike the color plate placed in a horizontal position. The color plate in this position was out of the range of any X-rays which might come through the opening in the box. This simple mechanical arrangement was very satisfactory and rendered easy the introduction of neiv substances into the tube. The vacuum was maintained by means of a two-cylinder Geryk oil pump and a Gaede rotary mercury pump, connected in series. For substances very highly luminescent, such as willemite, the vacuum from the Geryk pump was sufficient, b u t for the much fainter light of most of the salts used in this n-ork, the addition of the Gaede pump was necessary. Tl\’hen using these pumps in series, the Geryk pump was first used t o obtain a vacuum of about 2 0 mm of mercury, a t which point the Gaede pump was started and both were run together, the Gaede pump serving to evacuate the tube, and itself in turn being evacuated by the Geryk pump Motion to the two pumps was supplied by a small motor placed between them. -In induction coil furnished electrical energy. It \\ a5 connected to the I I O volt circuit through a bank of lamps placed under a box lined with asbestos, and the terminals of the coil were connected to the terminals of the cathode tube through rubber stoppers fastened in the box covering. and serving as insulators from the lead covering of the box. The whole apparatus was set up in a dark room \\hich acted as a camera. Dufay color plates were used the light from most of the substances was rather faint, the rays had to be condensed to a circular image of less than an inch in diameter. For this reason it was not necessary to use the plates of the
Color Photography o j Luwinesceizce
677
regular lantern size, 3 1 X A”, b u t each plate was cut into four parts, and a single exposure taken on each part thus obtained. (Smaller plates of about I ~ 4’,’ x 2” may be gotten by order from Paris. This would be preferable, as the plates must be cut in the dark, and in spite of the greatest precaution, small particles of glass quite frequently stick to the emulsion of the plate and cut it, thus rendering it useless for a perfect slide. i The materials used under the cathods rays were c. P. salts. dried in the oven a t the proper temperatures or, in some cases, fused. They consisted of sodium bromide, chloride and iodide, potassium bromide, chloride and iodide. anhydrous cadmium sulphate. cuprous iodide and mercurous chloride. The sodium iodide vas made anhydrous by crystallizing from alcohol and drying Jvith ether. The times of exposure to obtain the best photographs i-arietl somewhat ftor each substance, being any~vherefrom three to ten minutes, and ivere determined by preliminary trial. This \vas usually done by taking for each substance a nuniber of exposures side by side on a single plate, varying the time and developing, after n-hich it was determined just ivhat time of exposure gax-e the best results. The stronger blue light from potassium bromide, pot ium chloride and sodium bromide was easily photographed, requiring about fii-e minutes‘ exposure. The plates were also very sensitive to the green from potassium iodide but only slightly sensitive to the paler blues from the faint luminescence of sodium iodide and sodium chloride. For the last two, ten minutes seemed t o give the best results, but with such a long exposure of the salts to the effect of the cathode rays. the color of the rays seemed to be obscured by a dark coating over the salt, presumably a decomposition product. Exposures of various lengths were made, but it ivas impossible to obtain a correct color representation on the plate. It was also very difficult t o obtain a correct image of the yellow from cadmium sulphate. In every case, after developing the plate, the photograph had a slight greenish tint. The color plates Irere very
678
J . M. Lohr
sensitive, however, to the beautiful orange from mercurous chloride, but an exposure of more than 10 to 15 seconds resulted in the salt becoming covered with a dark coating, presumably an emulsion of mercury and mercurous chloride, which obscured the color rays. This was avoided by placing the plate holder in permanent position and taking single exposures of ten seconds each, on the same part of the plate, after which the plate holder was closed, the current stopped, and fresh salt introduced into the tube. This was a long, tedious process, requiring time for a vacuum after each introduction of salt, but tlvelii-e such exposures on the same plate produced very good results.
Flame Luminescence This work was done in the hood of one of the laboratories made totally dark. The substances were usually burned in the flame of a Bunsen burner, behind a screen of black paper, containing a small opening just above the substance to be burned. The plate holder u-as held in a vertical position by a clamp fastened to an iron stand, and a condensing lens was interposed. The substances were held in different ways over the Bunsen burner depending upon the nature of the substance. I t was not difficult to obtain characteristic flames of the different substances, but considerable trouble was encountered in producing flames of perfectly pure color unmixed with that of the Bunsen flame itself, or that of the oxidized substance already formed by the burning. The flames of sodium, potassium, lithium, arsenic, copper, and boric acid were photographed. For sodium and potassium, the best results were obtained by placing the carbonates in a small boat-shaped basket of platinum wire, held by an iron stand just below the opening in the screen in front of it. By so placing the burner that the flame would split on the edges of the basket ton-ard the screen, the half of the flame facing the screen would shoot upward giving a sheet of the characteristic colored flame. sufficiently broad to cover the range of the circular opening in the screen.
and would thus produce a circular image on the photographic plate Both the sodium and potassium flames n-ere easy t o photograph, but di tierent exposures raiigirig from tn-o to ten minutes produced difi'erent intensities of the image on the plate, all of u-liich n-ere readily distinguishable ]\-hen reproduced from the lantern. The same method \vas used for lithium, b u t it n-as almost impossible to obtain the characteristic bright red flame in \-olume, purity, and for a length of time sufficient for a successful exposure. The yellon-ish green flame of boric acid v a s obtained in a manner similar to that used for the above-mentioned salts The playing of the Bunsen flame over metallic copper gives a beautiful, clear green flame to n-hich the color plate is x-ery sensitive, but after about a minute of burning, the green flame becomes diluted by yellon-, probably caused by the oxidized copper. This was avoided by causing the flame just to graze the folded edge of a piece of copper gauze placed By this method the clear green a t an angle of about 45'. copper flame could be produced for a time sufficiently long for an exposure. In all of this flame work the best results vere obtained by using a fish-tail attachment t o the burner, with the slit made wider and shorter so as to produce a flame having a cross section almost square. effort n-as made to photograph the bluish Irhite zinc flame, but it is extremely difficult to obtain such a flame of sufficient volume and without interference flames from the burner. The rapid formation of zinc oxide a t the surface of the molten zinc also tends to interfere with the formation of a flame suitable for photographing. Photographs of mercury vapor were also taken, but they were difficult to recognize when thrown on the screen from the lantern on account of the similarity to white light. The plates after being developed were made up into lantern slides. B u t before preparing the slides, the plates after thorough drying n-ere tested out in the lantern, and some
680
J . -1I. Lolii
interesting liniitatioiis of the Uufay plates for lantern slides were observed. In practically every case, the colors blue, green, etc., as exhibited 011 the plates, \vhen left in the lantern continuously for about five minutes, \vould be replaced by \.ariegated colors of different hues. Ji-hen, hon.e\-er, the plates \vere allon-ed to remain in the lanterri for spaces of one-half to one minute, \vithdra\viiig aiid allo\viiig to cool each time, they could he thus successively used in the lantern :til indefinite number of time.: Ivithout change of color. To test the ei'lect of heat aiid light separately, a plate beariiix the image of blue light from the cathode excitation of potassium bromide \vas placed in a dark o\-en a t 70" C aiid found to change color \Tithin three to five minutes. ;inother plate was placed 011 the \\-indo\\.ledge of the laboratory iii the direct afternoon sunlight and \\-as found to change color in several hours. - 1 1 ~ 0 several plates of potassium bromide \\ere allowed to lie in the laboratory o n a table exposed to daylight but not to sunlight. and \\-ere foiind t o change color after several \veeks. The cause of this changing of color or " f a d ing" under the influelice of heat may be d u e to the deconiposition of the dyes 011 the plate. hiit is more likely due to the unequal expansion aiid contraction of the gelatine film on the plate. The slide consists of the complete rulings of iiik or dye, patches of ivhich are masked by opaque silver contained in a superposed gelatine film. The heat of the lantern very likely causes unequal expansion and contraction of the gelatine film owing to the probable presence of moisture, thus causitig the patches of silver to move out of position \\-ith respect to the color rulings arid thus expose other colors. I t is not evident just why daylight should produce such changes of color. The plates \\-ere made up into lantern slides of size j' ," by 4". The smaller-sized plates already mentioned bearing the images \\-ere inlaid in the center of a cardboard of the size 3 ' . *'' x 4" and of the same thickness as the glass plate. This \vas then covered O T ~ the emulsion side by black paper. n i t h a circular opening just over the image, and next to this
Color Plzotograplq o j Lmn'rzesccvzce
68 I
\vas placed a cover glass. O n the other side of the cardboard bearing the glass plate J\'as placed thin black cardboard also hax-ing a circular opening just over the image and the rvhole \vas bound around the edges by tape. To compare the gradation of colors, the three plates from sodium salts i\-ere made up iiito one, as \\-ere those of the potnssium salt, thiis producing rather effective slides.
Summary Successful use is made of color-photography to determine the quality of the visible light emitted during cathode luminescence and also the quality of light emitted by the flames of various substances. The Dufay plates are not particdarly sensitive t o the Yery faint cathodoluniinesceiice of some of the haloKen salts. In the use of the Lhfay color plates for lantern slides, the intense heat and light o f the lantern place ;I limitaticin on the length of time t h a t the slide should be left in the lantern. This \\-ark was taken u p a t Prof. Bancroft's suggestion. and I desire to thank hiin and also lfr. E. F. Farnau for many helpful suggestions. I also ivish to thank Dr. R . P. .lnderson for the use of the spectroscopic laboratory and dark rooni. (
ori/c/l J.'?iiwrsI'tj~,.i,,pi.. 1 9 1 2
