Sensitometry

The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions.

[1][2] They determined how the density of silver produced varied with the amount of light received, and the method and time of development.

This naturally causes an increase in the speed of image movement and raises the frequency of sound reproduction by approximately 4 per cent.

Five types of film image are acceptable for television transmission: (1) conventional motion picture camera negatives, (2) conventional motion picture laboratory positive prints derived from (1), (3) telerecordings made by filming a cathode-ray tube display to produce a negative image, (4) telerecordings as in (3) but arranged to produce a direct positive image on the original telerecording camera film, (5) motion picture laboratory prints made from (3).

This facility may also be employed during live studio transmissions, for special trick effects, and is not confined only to film work.

Since black spots are far less noticeable to the viewer, this is one strong reason for transmitting positive film images whenever possible.

Telecine equipment scans the pictorial image information and creates an electrical version of the picture in terms of a television signal.

This signal is eventually converted back into a recognisable picture when, at suitably modified strength, it energises the phosphor in the cathode-ray tube of the domestic receiver.

At the top of the diagram we see that an original scene is fed from the television camera during a live transmission via a video transmitter having a gamma value of 0.4.

In the first two cases we have the following four units in which local gamma or effective image contrast may be adjusted: The recording channel amplifier.

When motion picture films are made for television purposes the conditions shown at the foot of Figure 1 will apply.

It is quite impossible to discuss all the various techniques and fundamental principles of television equipment in a book of this nature; for similar reasons, it is not possible to quote one fixes set of gamma and density values which, one achieved, would satisfy each stage of the various combinations of equipment involved in the basic methods outlined in Figure 1.

Page 10 of Raymond Davis Jr. and F. M. Walters, Jr., Scientific Papers of the Bureau of Standards , No. 439 (Part of Vol. 18) "Sensitometry of Photographic Emulsions and a Survey of the Characteristics of Plates and Films of American Manufacture," 1922. The next page starts with the H & D quote: "In a theoretically perfect negative, the amounts of silver deposited in the various parts are proportional to the logarithms of the intensities of light proceeding from the corresponding parts of the object." The assumption here, based on empirical observations, is that the "amount of silver" is proportional to the optical density.
Fig.1. Monochrome Telefilm Transmission.
Fig. 2. Combinations of gamma values in film chain.