Journal of the Society of Motion Picture Engineers (1930-1949)

704 LEE December light, it pays you to condense it into the smallest possible spot and use the smallest possible surface which can just be covered by that spot. •» . , MR. MAURER: Does that not imply that in the case of the lead-sulfide cell it becomes almost imperative to have the projector so designed that the light flux on the cell changes in intensity only and not in area, in order to get undistorted response? MR. LEE: That would depend on whether the sensitivity, that is, conductivity versus light flux, is constant over the area or not. If it is constant over the area, then I think it would still hold true that the output would be dependent only upon the average intensity. Even if the size of the spot varied, if the sensitivity were uniform over the surface, I think you would still get linear response. MR. MAURER: The second point relates to your frequency-characteristic measurements. The optical systems that are generally used in 16-mm projectors are made up entirely of simple glass lenses unconnected in any way for chromatic aberration. Did you refocus the optical system when you started to use the leadsulfide cells? I think the shift from the range normally used to the infrared where these cells respond would be significant. MR. LEE : Yes, we shifted the focus, I think with a 7000-cycle track. DR. JOHN G. FRAYNE: In view of the wide disparity in response and in frequency characteristic of these cells, why is anybody interested in using them at this time? •>MR. LEE: It is difficult to answer that question in a very straightforward way. I think one reason for the wicje. disparity in results which have been obtained, quite frankly, is that a very large number, of small manufacturers entered this field practically as soon as the lead-sulfide cells were announced. It is possible at the present time to obtain from some manufacturers cells which are quite consistent so far as signal sensitivity is concerned at least. So far as frequency response goes, I do not know. DR. FRAYNE : So the reason is mainly economic then, is that right? MR. LEE: Frankly, I do not know. I have been somewhat puzzled myself that the cell has not received, wider acceptance than it has. MR. E. W. D'ARCY: 'there is a very practical reason behind using the cells. We have been experimenting with them now for almost two and one-half years, and we secure with no trouble whatever a 60-decibel signal-to-noise level with them. That is the most practical reason we use them. In addition to that, with the sulfide cell we are able to terminate the cell itself into a load resistance of somewhere around 200,000 ohms, which just about eliminates cable problems and eliminates the necessity of transformers or preamplifiers for normal coupling arrangements. That sets aside the economies of the thing almost completely. QUESTION:,. Would you care to comment upon your experience as far as variability from cell to cell is concerned? MR. LEE: They are exceptionally variable from manufacturer to manufacturer. There is a great deal of magic in making the cells. I think there are sappfes who aper^uite, reliable, sand. from, whom cell? .with quite jeproducible characteristics can b& -obtained.. .^, ••,_ : •.-.•. ;:-.. . ..' . . MR. R. T. VAN NIMAN: The.SMPE Sound Committee has a -Phototube Subcommittee-, of which I happen to be the Chairman, and we have been in vesti