US2582964A - Photoelectric sound reproduction - Google Patents

Description

Jan. 22, 1952 l.. E. CLARK 2,582,964

PHOTOELECTRIC SOUND REPRODUCTION Filed March 8, 1949 E Mcm/m arf 6745 Pff-$501915' ZZV m 102 M e Patented Jan. 22, 1952' l PHo'roELEc'rmc soUND REPRODUCTION Lauriston E. Clark, Los Angeles, Calif., assignor to Technicolor Motion Picture Corporation, Hollywood, Calif., a corporation of Maine Application March 8, 1949, Serial No. 80,153

claims. l

It is desirable in the field of color cinematography to record sound by way of colored tracks formed by conventional methods similar to those used for printing the picture records which the sound tracks are intended to accompany. The coloring matter of such sound tracks may consist of mechanically or chemically applied colored pigment, or of the material in which the pictures are preferably formed at the present time namely dyes mechanically transferred by imbibition printing from a record matrix to a blank film, or dyes photographically developed within the emulsion by now conventional methods. or dyes initially incorporated in the emulsion and subsequently partly destroyed to form a record.

.- Sound tracks formed in coloring matter which is by necessity primarily selected on the basis of suitability for picture recording, have certain disadvantages. One of these is that such tracks cannot be properly reproduced with the aid of the cesium-oxygen-silver photocells customarily used in motion picture theatre equipment, because the dyes in question are highly transmissive in the infrared spectral range to which these cells are sensitive, so that it is dimcult to obtain signals that are initially strongly accentuated by differential absorption of cell affecting light. Photocells which are not sensitive to the infrared range, or generally speaking photocells which are mainly sensitive to a spectral range within which the dye in question strongly absorbs, have heretofore not been used because of the lack of correspondence of sensitivity and transmission ranges of photocells and dyes respectively.

The problem is further complicated by the requirement that, if at all possible. only a single dye should be used for the sound track in order to avoid complications which obviate the advantages inherent in using dye sound tracks. This requirement excludes matching of track and phototube by way of modifying the dye absorption. For example the single subtractive dye of magenta hue which transmits blue and red light and is used for reproducing the green color aspect picture record, has an absorption range which fairly well agrees with the sensitivity range of a now available cesium-antimony phototube (essentially insensitive to infrared light) when illuminated by tungsten light. In actual practice however this combination has not proved to be satisfactory. The quality of reproduction by such an arrangement is below that of a fine grained silver sound track, particularly 'with regard to the signal-to-nolse ratio which depends to a considerable degree upon the effective density of the opaque sections of the track which is in the case of a sound track and photocell combination of this type lower than desirable. As will be explained more in detail below' when describing a specific embodiment of the invention utilizing a cesium-antimony cell with a magenta sound track. the absorption range ofv a colored sound track is apt to be narrower than the sensitivity range of the phototube used for scanning that track. This causes loss of effective density due to the fact that the track passes light of the spectral range adjacent to the primary effective range of the tube which is in effect analogous to the above-mentioned drawback resulting from the infrared transmission of most dyes available for colored sound tracks.

This discrepancy between dye (or other coloring matter) absorption and phototube sensitivity cannot be corrected by modifying the dye, for reasons indicated above nor can it be corrected by interposing a conventional lter between light source and tube with the idea of eliminating the undesirable border absorption ranges of the dye. because the loss of total output would be so great as to require additional amplification in each theatre sound equipment whether or not primarily intended for use with such tracks, which is impractical for obvious reasons. It is wellknown that the sensitivity of a photocell can be increased by increasing the pressure of the inert gas filling the tube; this however is impractical as applied to the above suggested association of the tube with a conventional filter, because the increased pressure renders the cell unstable when it is over-illuminated as for example by the operator using a trouble lamp for examining the equipment while the photocell polarizing voltage is applied. The cell would pass excessive current during such examinations which soon results in destruction of the photosensitive surface.

It is the principal object of the present invention to provide a method of reproducing colored sound tracks with a quality comparable to that furnished by fine grained silver tracks with equipment that is of standard type so far as the practical operation of the sound reproducing components is concerned, that is without in any way changing the shape of the phototube, without changing the illuminant, and without, adding components such as a separate filter or additional amplifying equipment not used in standard apparatus. Another object is to provide a phototube of conventional configuration which closely adjacent the first rangefsuch as the n blue and yellow border ranges), by enveloping the cathode in a vessel which absorbs these adjacent or border ranges, and by projecting the track on the cathode with light containing the above-mentioned ranges (such as tungsten light) whereby the tube can be safely handled when intensely illuminated. In a preferred embodiment the tube is filled with an inert gas at comparatively high pressure for the purpose of increasing the sensitivity of the cathode, to render the phototube sensitivity approximately equal to that of a tube having a similar cathode in an essentially clear vessel filled with gas at the standard pressure, thus preserving the sensitivity of such tubes with clear vessel and standard pressure. In another aspect. the invention provides a gas filled phototube for use in sound reproduction from a colored sound track (for. example in green absorbing dye) which tube comprises a cathode sensitive to a light range absorbed by the sound track and in addition to adjacent ranges (for example towards the blue and yellow) this cathode being coated with appropriate material (for example a cesium-antimony alloy if the dye is a-bsorptive of the green spectral range). and the envelope of the tube being colored to absorb the adjacent ranges being for example of glass containing substances (for example suitable metal compounds) rendering` it absorptive in this manner or being covered with a filter layer absorptive of these adjacent ranges (for example a lacquer layer containing suitable coloring matter); in a preferred embodiment the envelope is filled with an inert gas (such as argon if a cesium-antimony cathode is used) at a pressure rendering the tube approximately as sensitive as an otherwise similar tube having an essentially clear vessel filled with gas of lower pressure.

These and other objects, aspects. and features of practical construction will appear in the following description of two typical practical embodiments illustrating the novel characteristics of my invention. This description refers to a drawing in which Fig. 1 is a diagram illustrating the phototube sensitivity and dye absorption characteristics present in a system according to the invention;

Fig. 2 is a diagrammatic elevation, with part of the envelope broken away. of a-phototube according to the invention; and

Fig. 3 is a diagram illustrating a sound detecting arrangement according to the invention.

In Fig. l. A is the sensitivitycurve of a cesiumantimony phototube of the type commercially known as 1F37 cell. Curve B indicates the light absorption of a conventional magenta (minus green) dye used for making motion picture positives by dye imbibition from wash-.off gelatin reliefs. It will be noted that the dye transmits considerable light in the wave length ranges 400 to 460 and 600 to 700 millimicrons, adjacent the region of maximum sensitivity of the phototube cathode at approximately 520 millimicrons. This would ordinarily lower the capacity of the sound track in its high density portion thus causing impairment of the quality of sound reproduction particularly as regards the signal-to-noise ratio. This loss of effective density is caused by light of wave lengths 40G-600 and 600-700 passing through the magenta dye with relative ease. These wave lengths are emitted by the tungsten filament lamps customarily used for sound track scanning. y

In accordance with the invention the glass envelope of the phototube is made of green glass or covered with a green filter layer of the proper characteristics in order to modify the effective sensitivity of the tube as indicated at C of Fig. l. Curve C indicates that a phototube of this design responds practically only to a narrow region around the above-mentioned maximum cathode sensitivity.

If the glass envelope itself is used as a filter it may be made of the soft glass customarily used for machine manufacture of bulbs of this type, containing about 14 per cent NazO and 5 to 6 per cent CaO, to which have been added, according to the invention, copper and chromium oxides in amounts up to about 1 per cent. This admixture limits the effective transmission of the glass essentially to the region from 520 to 540 millimicrons, with a fairly sharp cut on both sides of this region, thus filtering out the light transmitted by the magenta sound track dye in the above-mentioned ranges adjacent to the region of maximum effectivity.

If it is desired to coat the phototube instead of using a colored glass envelope, the tube is sprayed with or dipped into a nitro cellulose lacquer solution containing for example l per cent each by weight of the dyes Guinea Green B (C1666) and Quinoline Yellow Spirit Soluble (C1800).

Fig. 2 indicates this construction `of the envelope I0 of a tube P and also tie construction of the cathode Il, so far as essential for purpose of the present invention, namely a' cathode coated with an alloy of antimony and cesium. A method of producing such an alloy layer is for example described in U. S. Patent No. 2,122,860, comprising the steps of evaporating antimony on the effective portion of the cathode, treating this antimony layer with oxygen, and then eve;L orating cesium thereon. Subsequent heat treatment of the layers causes the metals to form a light sensitive alloy responding with electron emission principally to radiation with the blue and green regions of the spectrum. As mentioned above, this sensitivity i's indicated at A of Fig. l which curve illustrates this sensitivity with regard to illumination by tungsten light customarily used in motion picture sound detecting apparatus.

As already mentioned, the tube sensitivity is considerably decreased by the filter so that, withfrom 0.1 to millimeters of mercury column, as compared with less than 0.1 millimeter of conventional tubes of this type. As an approximate rule, the sensitivity of a cell with integral filter envelope according to the invention will be increased to the level of an otherwise similar tube with clear envelope,` that is according to present practice to a sensitivity not exceeding 150 microamperes per lumen.

Fig. 3 indicates a conventional sound gate incorporating the present invention. The film F has a picture area p upon which picture records R can be printed for example by imbibition techniques. these picture records including a green absorbing magenta dye. The sound track s is printed in the same magenta dye whose absorption is indicated at B of Fig. 1. The film F is threaded through a conventional sound gate G which comprises an optical system O-including a slit S, a tungsten lamp L and a phototube P of the type described above with reference to Fig. 2. f

It will now be evident that the method and arrangement according to the invention correct the deficiency in phototube sensitivity due to the overlap, adjacent to the regions of maximum sensitivity and absorption, of the sensitivity and transmission ranges of tube and sound track respectively. The sensitivity of the system as a whole can be restored to the conventional level by increasing the gas pressure, and the tube of the new system is permanently protected against excess illumination to`the same extent as conventional units by the lter layer which has thus the double function of correcting the phototube sensitivity and of preserving theoperativeness of the tube. It should be noted in this context that the intrinsic sensitivity of the cesium-antimony surface is high and that the secondary emission has been kept low in the conventional photocell of this type by limiting the gas pressure to about one-tenth of the permissible or otherwise preferable value, for the purpose of protecting the cathode against excess illumination. The invention utilizes this fact that the gas pressure can be increased from 8- to 12 times above the level of the normal tube if this protection is instead provided by way of the filter envelope according to the invention. Thus, the phototube of a system according to the invention is not exposed to invjury by excessive current destroying the sensitive cathode surface upon strong illumination with a flash light or trouble lamp, while it is suiiiciently sensitive without additional ampliiication equipment, in spite of the reduced transparency of its envelope.

It will be evident that thev principle upon which the above described systems is based can be applied to photocells and coloring matters which have specific sensitivity and transmission characteristics diierent from those above described, so long as these characteristics have the general 'relation herein dealt with.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modiiications and equivalents which fall within the scope or the appended claims.

I claim:

1. A phototube for use in sound reproduction from a colored track which absorbs a selected intermediate range of the visible spectrum, comprising a cathode sensitive to said range and to two adjacent ranges, and an envelope for said cathode colored to absorb said adjacent ranges.

2. A gas filled phototube for use in sound reproduction from a colored track which absorbs a selected intermediate range of the visible spectrum, comprising a cathode sensitive to said range and to two adjacent ranges, an envelope for said l cathode colored to absorb said adjacent ranges, and within said envelope an inert gas at a pressure rendering the tube approximately as sensitive as an otherwise similar tube of standard sensitivity having an essentially clear vessel and iilled with lgas at lower pressure.

3. A phototube for use in sound reproduction from a colored track in green absorbing dye comprising a cathode sensitive to the green range absorbed by the dye and to adjacent ranges towards the blue and yellow, and an envelope for said cathode colored to absorb said adjacent blue and yellow ranges.

4. A gas filled phototube for use in sound reproduction from a colored track comprising a.

cathode coated with a cesium-antimony alloy, a glass envelope for said cathode containing cop per and chromium oxides rendering said envelope absorbent of blue and yellow light and within said envelope argon at a pressure of from 0.1 to 10 mm. mercury column.

5. A gas filled phototube for use in sound re,

, production from a colored track comprising a cathode coated with a cesium-antimony alloy, a glass envelope for said cathode covered with a layer of nitro cellulose lacquer containing Guinea Green B and Quinoline Yellow Spirit Soluble, and within saidenvelope argon at a pressure of from 0.1 to 10mm. mercury column.

Y LAURISTON E. CLARK.

REFERENCES CITED The `following references are oi.' record in the file of this patent:

773,398 France 1---- Sept. 3, 1934

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