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Sensitometry
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Sensitometry
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.
Plots of film density (log of opacity) versus the log of exposure are called characteristic curves, Hurter–Driffield curves, H–D curves, HD curves, H & D curves, D–logE curves, or D–logH curves. 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.
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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.
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Sensitometry
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.
Plots of film density (log of opacity) versus the log of exposure are called characteristic curves, Hurter–Driffield curves, H–D curves, HD curves, H & D curves, D–logE curves, or D–logH curves. 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.
Source:
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.
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