The Cine Technician (1935-1937)

February, 1936 The Journal of the Association of Cine-Techniciaiis 77 tained from the use of such an instrument is then known as the time-scale gamma. The intensity in a time-scale sensitometer is regulated with respect to the speed of the emulsion being tested, so as to cover the range of density required, and the maximum exposure time in the scale is of the order of one second. We have already seen that the exposure time in sound recording is very much smaller than this — about :•, ,107777 of a second as a rule. The effect of this wide difference will now be examined. It is usually assumed for practical photographic purposes that exposure may be defined as the product of intensity and time, and that, provided the product remains the same, they can be varied reciprocally and still produce the same effect. This is known as the "Reciprocity Law" and is not strictly true. For a given emulsion and development conditions, there is an optimum value of intensity, at which a given density is produced with a minimum of exposure ; that is a minimum value of (Intensity x Time). On either side of this value the exposure must be increased to produce the same density. This gives the practical effect of a light valve gamma, which is lower than the sensitometer gamma, in the ratio of approximately 1-05 to 1. W'e come now to the printing process, which is by contact, in a continuous rotary printer. Under these conditions the exposure of the positive is dependent upon the diffuse transmission of the negative in contact with it. The effective positive gamma is, therefore, the relation between positive diffuse density and negative diffuse density. In considering the relation between effective and measured positive gamma, two factors have to be taken into account. The first is colour coefficient of the negative, and the second is again reciprocity law failure. The relation between time scale and real positive gamma is usually of the order of 1-1, so a positive development of by y2-0 will have an effective value of 1-8 about. There is another contrast factor in the chain to \)v considered, and this is concerned with projection. In a projector optical system the film is illuminated by a lens focussing a slit on the film. The emergent light is collected by a window in the photo-cell mount, which restricts the direction of the effective rays to a comparatively narrowsolid angle from the slit image. Some light is diffused by the silver grains in the image and thrown outside the effective area ; transmission is thus reduced and effective density increased. The denser the image the greater the scattering, because there are more grains to give this effect, and so the greater the density increase so caused. The result is an effective increase in contrast, or gamma, which amounts to a factor of about 1 -4. We can, therefore, multiply the effective gamma ol 1-8, obtained by diffuse reading, by 1-4, giving 2-5. The desired result is that the amount of light collected by the photocell in the reproducer shall be proportional to the negative exposure. A mathematical analysis shows that this result is achieved when the product of effective negative and positive gammas and projection factor equals unity. Taking the approximate relations between effective and measured values already given, we then have yn yp 105 11 1-4 = 1 yn X yp = -83 where the gamma values are those given by measurement on the time scale. FOR YOUR TITLES AND INSERTS STUDIO FILM LABORATORIES LTD. TITLE MAKERS TO THE BRITISH FILM INDUSTRY Al TOMATIC NEGATIVE AND POSITIVE DEVELOPMENT SENSITOMETRIC CONTROL SPECIAL PROCESSING & OPTICAL PRINTING TRICK PHOTOGRAPHY 80-82 W ARDOUR ST. LONDON, W.l GERHARD 1365-66