Projection engineering (Sept 1929-Nov 1930)

Page SO Projection Engineering, September, 1929 llllllllllllllUllllllllill'IIIITIIillllillllil Fig. 2. Visibility and tungsten energy 3000CK: (A) relative energy of radiation from tungsten at 3000° K; (B) visibility of radiation (relative brightnesses for equal energies). 500 fcDO WAVE LENGTH (tt\M) installed by the Electrical Research Products Incorporated, is of the potassium gas-filled type. The other, used in the equipment installed by the Radio Corporation of America, is of the caesium type. In Fig. 1 are shown the spectral sentitivity curves of these two cells, curve A being that for a potassium and curve B that for a caesium cell. The ordinates of these curves are proportional to the photoelectric currents generated when excited by equal amounts of energy of the wavelength as indicated by the abscissa values. The proportionality constant used in plotting these curves is not the same for the two cells ; hence these curves cannot be interpreted as indicative of the relative total sensitivities of the two cells. They do show, however, the way in which sensitivity varies with wavelength, and this is the information in which we are particularly interested at present. The monochromatic radiation used in the determination of these sensitivity functions was of high spectral purity, being obtained by using two monochromatic illuminators operated in tandem so as to effectively eliminate all scattered radiation. The photoelectric current generated was measured with a high-sensitivity galvanometer. The amount of energy incident upon the photoelectric cell was measured by means of the thermopile and high-sensitivity galvanometer. Since the thermoelectric current is directly proportional to the energy incident upon the thermopile (regardless of wavelength) it follows that the sensitivity of the photoelectric cell, defined in terms of the photoelectric current per unit of energy, is directly proportional to the ratio of the photoelectric current to the thermoelectric current, Pc/Tc Every precaution was taken to eliminate all possible errors and it is felt that the curves shown in Fig. 1 represent with high precision the sensitivity of the cells in question. The author is indebted to Dr. Otto Sandvik of these laboratories for these data. The curves in Fig. 1 show the relative magnitude of the photoelectric currents resulting from the action of equal amounts of energy of different wavelengths. In practice, the photoelectric cell is excited by an incan descent tungsten lamp which does not emit equal amounts of energy at all wavelengths. To obtain the effective spectral response curve it is necessary to know the spectral distribution of energy in the radiation emitted by the incandescent tungsten lamp. This depends upon the temperature at which the filament is operated. In commercial sound reproducing installations this is approximately 3000°K. In Fig. 2, curve A shows the relative intensity of the radiation ejinitted at different wavelengths for this source. It will be noted that relatively little energy is emitted in the short wavelength region to which the photoelectric cells are most sensitive, while relatively large amounts of radiation are emitted at longer wavelengths. In Fig. 3 are shown the effective spectral response curves for each of the two cells when used with a tungsten lamp operating at 3000°K. The ordinates of these curves are determined by multiplying, at each wavelength, the ordinate of the sensitivity curve (see Fig. 1) by that of the tungsten energy curve, Fig. 2. It will be noted that the response curve of the potassium cell (A, Fig. 3) has a relatively high sharp maximum at wavelength 425 m/*. It decreases rapidly for both longer and shorter wavelengths, reaching a value of 10 per cent of the maximum at 490 mu., on the one hand and 340 mu. (estimated) on the other. The effective response curve for the caesium cell is shown in Fig. 3, curve B, and is of a broad flat type having a maximum at 420 mu.. For longer wavelengths the response decreases gradually reaching a value which is 10 per cent of the maximum at approximately 750 ma. The response at 700 mu., the long wavelength limit of the visible spectrum, is 35 per cent of that at the maximum. It will be noted that the maximum of response is at practically the same wavelength for these two cells, although the caesium cell has a much broader spectral sensitivity than the potassium cell. It is evident from a consideration of these response curves that any coloring material which absorbs strongly in the region between 400 and 500 mu. will have a relatively high density if measured in terms of this photoelectric cell and a tungsten lamp. These wavelengths impinging on the retina give rise to the colors described qualitatively as violets and blues, and if these wavelengths are absorbed from white light the remainder produces a yellow color. Yellow dyes in general therefore have high photoelectric densities. This is true qualitatively for both cells although it applies with much greater force in case of the potassium cell which has a relatively narrow sensitivity band in the short-wave region. As a result of the difference in shape of the response curves, certain colors, such as yellows, give relatively lower photoelectric densities when measured with the caesium cell than when this quantity is determined by means of the potassium cell. Color Sensitivity of the Eye The eye is a receptor of the synthetic type and does not analyze a heterogeneous radiation into its component parts. The sensation arising from the impingement of heterogeneous radiation on the retina has a single hue characteristic, and identical sensations of hue may be excited by heterogenous radiations differing very widely in actual spectral compositions as determined spectrophotometrically. It is evident, therefore, that there is a possibility of obtaining a desired color by several different types of spectral absorption curves. Since the radiation required to actuate the photoelectric cell is localized in a very definite wavelength region, it follows that the course to be pursued in the solution of the problem in hand is to select absorbing materials which most efficiently transmit these wavelengths and at the same time most completely absorb those wavelengths which, when subtracted from white light, operate most efficiently toward the production of a color having the desired hue and saturation characteristics. In order to proceed most directly and logically in this direction, knowledge of the visibility of radiation is of considerable importance. This knowledge is of assistance in deciding just what particular type of selective absorption will most efficiently produce a desired color and, at the same time, most efficiently transmit those wavelengths which are required to excite a photoelectric cell. Curve B in Fig. 2 shows this visibility function, the ordinates being proportional to the magnitude of the visual sensation produced by the action on the retina of equal intensity of radiation of the various wavelengths, as indicated by the abscissa values. By judicious choice of dyes and dye mixtures which give spectral absorptions correctly adjusted with respect to the photoelectric response and to the retinal sensitivity, it has been found possible to produce a series of colors having hues distributed throughout the entire hue scale and at the same time having relatively low densities as measured with either the