Sensitometry

thumb|350px|right|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. Sensitometry is the scientific study of light-sensitive materials, especially photographic film. The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions. They determined how the density of silver produced varied with the amount of light received, and the method and time of development.

Sensitometry is the scientific study of light-sensitive materials, especially photographic film. The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions. They determined how the density of silver produced varied with the amount of light received, and the method and time of development. contrast ratio =   O max. / O min. As we have already seen, opacity is not easily measured with standard photographic equipment—but the logarithm of opacity is continually measured since, in fact, it is the unit of image saturation known as density. Since density is a logarithm we must take the ratio of the anti-logarithms of the maximum and minimum densities in the image in order to arrive at the contrast ratio. This may be written so: contrast ratio = antilog ( D max. — D min. ) If this is applied to the well-known B.B.C. Test Card 'C', we find that, in the positive film version of the card, the maximum density is 2.0 whilst the minimum density is 0.3. Therefore the contrast ratio is as follows: contrast ratio = antilog (2.0 — 0.3)                       = antilog (1.7)                       = 50 Therefore contrast ratio = 50 : 1 (50 to 1). When applied to the negative film version of the same test card, the maximum density is 1.30 although the minimum density remains at 0.30. The contrast ratio of the negative is therefore as follows: contrast ratio = antilog (1.3-0.3)                       = antilog (1.0)                       = 10 Therefore contrast ratio = 10: 1 (10 to 1). Figure 1 illustrates the several ways in which the viewer may receive monochrome television pictures. 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. Since the cathode-ray tube in the domestic receiver has an effective gamma value of 2.5, the final screen picture will be at a gamma of 1.0—equal to the original scene. Film is used to supplement television programmes in two ways ; either originating as a telerecording or as a motion picture film. In any event it must pass through film processing and possibly printing equipment before reaching the telecine machine and, in all cases, the overall gamma for the entire film-using system must be 1.0 so that, for example, sections of film may be inter-cut with live transmissions. One example of this is, of course, the many sections of television newsfilm material rapidly intercut with live announcements by the newsreader. The telerecording film chain can be arranged to produce a direct negative-image film recording, a direct positive-image film recording, or a positive print can be made from the negative. 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. The display cathode-ray tube. The negative and positive film processing. The telecine transmitting machine. In the remaining case the gamma of the film printing machine and also of the positive film processing must also be accommodated. When motion picture films are made for television purposes the conditions shown at the foot of Figure 1 will apply. Here it is possible to transmit the negative film image directly by phase or contrast inversion, or to make a positive film copy and to transmit this instead, in either case the gamma of the films plus the telecine equipment must result in a product-gamma of unity. There are several ways of displaying the picture which is to be telerecorded; there are several types of film on which to make the recording; there are various types of telerecording cameras, some of which record a so-called suppressed-field image, whilst others record full information; finally, there are various types of telecine equipment, such as vidicon or flying-spot image transducers. 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. However, some idea of the variations which may be encountered is gained from the table on Figure 2. In system 'A' a telerecording negative is printed before final transmission and, by some standards, the recording amplifier gamma is high, the display tube and film print gammas are low and the final telecine gamma correction is somewhat high. By comparison, system 'C' employs a much lower recording amplifier gamma, higher display tube and print film gamma values, and a relatively lower telecine gamma correction.

See also

(Sorbonne University).]] Film speed Densitometry Callier Effect Leon Warnerke, inventor of the first practical sensitometer in 1880 Hurter and Driffield Josef Maria Eder – Eder–Hecht neutral wedge sensitometer Julius Scheiner – Scheiner sensitometer Emanuel Goldberg - Goldberg mirror, Goldberg wedge, Goldberg gamma rule, contributions to sensitometry Spectral sensitivity Zone system

References

External links

Basic Sensitometry And Characteristics of Film (Kodak undated)* [https://archive.org/details/memorialvolumeco00hurtiala A memorial volume containing an account of the photographic researches of Ferdinand Hurter & Vero C. Driffield ; being a reprint of their published papers, together with a history of their early work and a bibliography of later work on the same subject. (Royal Photographic Society of Great Britain 1920)

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