Joseph A. Marquisee Case Institute of Technology Cleveland,
Ohio
Color Photography of Spectra
T h e emission spectra of excited atoms and molecules are one of nature's most beautiful and inspiring spectacles. In the multitude of colored spectral lines nature reveals to us some of her most intimate secrets. Unfortunately, many students of science do not have the good fortune to see a spectrum first hand. Most of the spectral photographs made each day are in black and white, and understandably so, since most of the lines used in analytical work are in the ultraviolet region of the spectrum where prism dispersion is greatest. The pictures of spectra reproduced in most books leave much to be desired, the plates being so small and poorly reproduced that most of the detail, color and immediacv of the real s ~ e c t r are a lost. The advent of the new high-speed color films which can be processed by the user has made color photography of spectra a practicality. Using a relatively inexpensive spectrograph, students with no previous experience a t color photography can turn out beautiful full color transparencies of spectra, which can be viewed by a group, in less than two hours. The choice of spectrographic instrument is dictated hy several factors, the
580 / Journal o f Chemical Education
most important of which are cost and the instrument's ability to accept standard 35-millimeter color film. Most prism spectrographs are unsuit,able since they have inherently poor dispersion in the visible, and they dictate the use of glass plates which are not generally available for color photography. A medium sized grating instrument such as the Bausch B. Lomb 1.5 meter Stigmatic Instrument is well suited to color photography of spectra. The visible portion of the spectrum is spread out to a length of about 9 in. which is entirely adequate to show good separation of major spectral detail. Because the grating spectrograph uses 12-in. strips of film it would be impossible to have them processed commercially due to the difficulty in handling such short lengths of film on a production basis. This makes it essential that the user process his own spectral films. Of the various color films on the market, the new high speed films with ASA exposure ratings of 100 or 160 are most suitable for spectral photography. The film may be purchased in standard 36-exposure cassettes or, if much work is anticipated, in more economical 50- or 100-ft. rolls. The exposed strips of film may be proc-
essed conveniently in an inexpensive 16-ounce plastic reel-type developing t,ank. Kits of chemicals which will process four .?&exposure rolls of color film are available from most photographic dealers for under two dollars. One 36-exposure roll of film will supply enough film for 4 or 5 loadings of the spectrograph film holder. The finished films are ready t,o view about 1% hours aft>er processing is begun. Technique
A tungst,en lamp, t,he kind. used for microscope il111minat,ion, plared about one foot from the slit and defocused so t,hat t,he filament image is not visible, makes a good source for a series of trial exposures. A wide range of expos-we times will show a t a glance t,he effects on color rendit,ion of under and overexposure. A test exposure which shows a wider band of yellow on t,he film than t,he eye sees on a piece of t,ranslucei~tpaper placed over the focal plane is overexposed. The results of tests run on a number of different sources are given in Table 1. I t should he noted that for a given experiment,al setup these exposure t,imes may have to he modified to suit existing conditions. Since a grating inst,rument forms a number of spectral systems (orders) which overlap one another, it is advisable to use an ultraviolet absorbing filter to remove any overlapping UV lines which might confuse or degrade the images. This precaution is especially important in the photography of quartz mercury arc or carbon arc spectra which are rich in ultraviolet wavelengths. Any filter which cuts off below 3600 Angstroms is suit,able. The emission spectra of gases are ohtained from capillary-type high voltage discharge t,uhes. Tuhes filled with various gases can he ohtained commercially from a number of sources.' I t is also possihle to const,ruat your own tuhes, provided you have some glass blowing skill, wire leads, a good vacuum system and a supply of the necei.sary gases. The tubes are huilt, evacuated, haked out and are slowly filled while connect,ed to a high voltage transformer. After a
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numher of tries, the gas pressure which gives the best discharge can be determined and the t,uhe sealed off and ',pulled" from the vacuum system. It is also possible t,hat a local neon sign shop could be persuaded to make (and possibly donate) some small disrharge tuhes. While gas discharge tubes may be operated by touching a spark coil t,o one end and grounding the other, a. much more satisfactory method is to run them fiom a. high voltage, low current t,ransformer capable of delivering about 4000 v. Used neon sign transformers, which are ideal for t,he job, can be obtained from sign shops for a few dollars. With t,he transformer primary connected to a variable autotransformer the secondary voltage may be adjusted for optimum light output. The voltage should be adjusted to the minimum value that will maintain a stable discharge. Excess voltage will only shorten tube life due to electrode material s p u e t,ering onto the glass with resultant loss in light intensity. Ordinary spark plug wire is quite satisfactory for making ~onnect~ions between the lamp and the high voltaae transformer. All exoosed connections should with plastic~ elect,rical~ ~ ~ 427 W,~ 42nd i street, ~ ~ ~ be covered ~ ~ ~ rubher , ~tub:lng and i tape to redure the possibility of elertriral shock.
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Toble 1.
Light source Tnngstcn lamp (100 wat,t) Fluorescent lamp Mercury are (quartz) Neon tuhe Helium Nitrogen Carbon dioxide Hvdroeen L
