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. [1] [2] They determined how the density of silver produced varied with the amount of light received, and the method and time of development.
Plots of film density (log of opacity) versus the log of exposure are called characteristic curves, [3] Hurter–Driffield curves, [4] H–D curves, [4] HD curves, [5] H & D curves, [6] D–logE curves, [7] or D–logH curves. [8] At moderate exposures, the overall shape is typically a bit like an "S" slanted so that its base and top are horizontal. There is usually a central region of the HD curve which approximates to a straight line, called the "linear" or "straight-line" portion; the slope of this region is called the gamma. The low end is called the "toe", and at the top, the curve rounds over to form the "shoulder". At extremely high exposures, the density may come back down, an effect known as solarisation.
Different commercial film materials cover a gamma range from about 0.5 to about 5. Often it is not the original film that one views but a second or later generation. In these cases the end-to-end gamma is approximately the product of the separate gammas. Photographic paper prints have end-to-end gammas generally somewhat over 1. Projection transparencies for dark surround viewing have end-to-end gamma approximately 1.5. A full set of HD curves for a film shows how these vary with developer type and time. [3]
Conventional 35 mm. and 16 mm. motion picture films are widely used to supplement television programmes. They carry images which are visually similar to those used in the cinema. Continuous-tone images are derived from conventional motion picture cameras, whilst images built up in the form of line structures are derived from telerecordings. To synthesise a moving picture these films are projected at the rate of 25 frames per second—the television picture frequency in Great Britain instead of 24 frames per second as in the motion picture industry. In America the television picture frequency is 30 frames per second and this raises considerable problems when conventional motion pictures which have been shot for the cinema at 24 frames per second are to be televised.
Although films originally made for television in Great Britain (whether by telerecording or by conventional cinematography) will be photographed at 25 frames per second, films exposed for cinema exhibition at 24 frames per second are also transmitted for television at 25 frames per second. This naturally causes an increase in the speed of image movement and raises the frequency of sound reproduction by approximately 4 per cent. (this results in the pitch of musical notes rising by something less than a semi-tone and is acceptable to all but the most critical ear).
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).
Gamma-control amplifiers in television transmission equipment are capable of inverting the phase or contrast relationship of the signal—in practice this means that an incoming negative image can be electronically converted eventually to appear as a positive image displayed by the television receiver. This facility may also be employed during live studio transmissions, for special trick effects, and is not confined only to film work. Because of this it is not necessary to make prints from motion picture negatives before they can be utilised in television programmes although, for several reasons connected with programme acquisition and distribution, it often happens that positive film images are used. Furthermore, the presence of any dirt or dust on the film will appear as a white spot when negative is transmitted, but as a black spot if a positive film is transmitted. Since black spots are far less noticeable to the viewer, this is one strong reason for transmitting positive film images whenever possible.
In television the original image passes through many stages before finally emerging as a recognisable picture but, in all cases, the film is ultimately projected via a telecine machine—this is basically a special form of film projector in conjunction with a television camera. 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.
Apart from the widely employed factors such as log-exposure, density, opacity and transmission, sensitometric control of film for television transmission is also particularly concerned with contrast ratios. The definition of contrast ratio is therefore re-stated as follows : 'The ratio between the opacities of the darkest and lightest points in the film image', thus:
contrast ratio = Omax. / Omin.
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 ( Dmax.— Dmin. )
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.
Film stock is an analog medium that is used for recording motion pictures or animation. It is recorded on by a movie camera, developed, edited, and projected onto a screen using a movie projector. It is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast and resolution of the film. The emulsion will gradually darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. Instead, a very short exposure to the image formed by a camera lens is used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. This creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. In addition to visible light, all films are sensitive to X-rays and high-energy particles. Most are at least slightly sensitive to invisible ultraviolet (UV) light. Some special-purpose films are sensitive into the infrared (IR) region of the spectrum.
Keykode is an Eastman Kodak Company advancement on edge numbers, which are letters, numbers and symbols placed at regular intervals along the edge of 35 mm and 16 mm film to allow for frame-by-frame specific identification. It was introduced in 1990.
35 mm film is a film gauge used in filmmaking, and the film standard. In motion pictures that record on film, 35 mm is the most commonly used gauge. The name of the gauge is not a direct measurement, and refers to the nominal width of the 35 mm format photographic film, which consists of strips 1.377 ± 0.001 inches (34.976 ± 0.025 mm) wide. The standard image exposure length on 35 mm for movies is four perforations per frame along both edges, which results in 16 frames per foot of film.
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
Kinescope, shortened to kine, also known as telerecording in Britain, is a recording of a television programme on motion picture film, directly through a lens focused on the screen of a video monitor. The process was pioneered during the 1940s for the preservation, re-broadcasting and sale of television programmes before the introduction of quadruplex videotape, which from 1956 eventually superseded the use of kinescopes for all of these purposes. Kinescopes were the only practical way to preserve live television broadcasts prior to videotape.
In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens F-number, and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system introduced in the mid-1970s. A closely related ISO system is used to describe the relationship between exposure and output image lightness in digital cameras. Prior to ISO gaining traction the best known systems were ASA in the U.S. and DIN in Europe.
Telecine is the process of transferring film into video and is performed in a color suite. The term is also used to refer to the equipment used in this post-production process.
Color grading is a post-production process common to filmmaking and video editing of altering the appearance of an image for presentation in different environments on different devices. Various attributes of an image such as contrast, color, saturation, detail, black level, and white balance may be enhanced whether for motion pictures, videos, or still images. Color grading and color correction are often used synonymously as terms for this process and can include the generation of artistic color effects through creative blending and compositing of different layer masks of the source image. Color grading is generally now performed in a digital process either in a controlled environment such as a color suite, and is usually done in a dim or dark environment.
In the processing of photographic films, plates or papers, the photographic developer is one or more chemicals that convert the latent image to a visible image. Developing agents achieve this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black when in the form of fine particles. The conversion occurs within the gelatine matrix. The special feature of photography is that the developer acts more quickly on those particles of silver halide that have been exposed to light. When left in developer, all the silver halides will eventually be reduced and turn black. Generally, the longer a developer is allowed to work, the darker the image.
Negative pulldown is the manner in which an image is exposed on a film stock, described by the number of film perforations spanned by an individual frame. It can also describe whether the image captured on the negative is oriented horizontally or vertically. Changing the number of exposed perforations allows a cinematographer to change both the aspect ratio of the image and the size of the area on the film stock that the image occupies.
Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as photographic paper or photographic film, due to exposure to light.
A cue mark, also known as a cue dot, a cue blip, a changeover cue or simply a cue, is a visual indicator used with motion picture film prints, usually placed in the upper right corner of a film frame. Cue dots are also used as a visual form of signalling on television broadcasts.
In the theory of photography, tone reproduction is the mapping of scene luminance and color to print reflectance or display luminance, with the aim of subjectively "properly" reproducing brightness and "brightness differences".
Three-two pull down is a term used in filmmaking and television production for the post-production process of transferring film to video.
A film chain or film island is a television – professional video camera with one or more projectors aligned into the photographic lens of the camera. With two or more projectors a system of front-surface mirrors that can pop-up are used in a multiplexer. These mirrors switch different projectors into the camera lens. The camera could be fed live to air for broadcasting through a vision mixer or recorded to a VTR for post-production or later broadcast. In most TV use this has been replaced by a telecine.
Television standards conversion is the process of changing a television transmission or recording from one video system to another. Converting video between different numbers of lines, frame rates, and color models in video pictures is a complex technical problem. However, the international exchange of television programming makes standards conversion necessary so that video may be viewed in another nation with a differing standard. Typically video is fed into video standards converter which produces a copy according to a different video standard. One of the most common conversions is between the NTSC and PAL standards.
A color suite is the control room for color grading video in a post-production environment.
Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film.
Test film are rolls or loops or slides of photographic film used for testing the quality of equipment. Equipment to be tested could include: telecine, motion picture film scanner, Movie projectors, Image scanners, film-out gear, Film recorders and Film scanners.
d-log-h-curve density.
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