Magazine article American Cinematographer

Talking Technically

Magazine article American Cinematographer

Talking Technically

Article excerpt

SCREEN ILLUMINATION

Screen illumination is a subject very dear to the heart of many a lighting cameraman. How often has one seen prints which look beautiful in a laboratory viewing theatre, but are dim and dark or, conversely, over-lit and washed out in the local cinema? It is in all our interests to try to ensure that screens are properly and sufficiently illuminated, and if we don't take an interest and where necessary complain, who will?

Correct screen illumination is laid down in a number of British and International Standards, viz:

BS1404:1961, ISO2895. Screen luminance for the projection of 35mm film on matte and directional screens.

BS2954:1958, ISO2910-Recommendation for screen luminance for the projection of 16mm film.

BS2964:1958, ISO2895-screen luminance in cinematograph laboratory and studio review rooms.

BS4564:1970, ISO2910-screen luminance for the projection of 70mm film on directional screen.

An allied Standard is 885382:1976 cinematograph screens.

These lay down the following conditions and method for measuring screen luminance:

The projector shall be running under normal operating conditions, with the lens or lenses focussed on the film plane with no film in the gate.

The light source and the optical system shall be aligned so that the area maximum luminance is at the centre of the screen. . .

See that the lighting in the auditorium shall be that normally used when the film is being projected.

The screen luminance shall be measured with a photometer having an acceptance angle not greater than 20°. . .

The luminance shall be measured from any seat in the auditorium at a height of 3ft. 6in. above the floor.

They state:

The luminance of the centre shall be not less than 8 foot-Lamberts and not more than 16 foot-Lamberts (27-55cdm^sup 2^).

The luminance of each side measured on the horizontal axis shall lie between the following values:

(i) For non-anamorphotic projection from 60% to 85%, and preferably as near as practicable to 70%, of the measured luminance of the centre.

(ii) For anamorphotic projection, from 50% to 75% of the measured luminance at the centre.

BS5550:Subsection 7.2.4:1978 "Screen luminance for the projection of 70mm film on directional screens". This Standard has been metricated and states that the luminance shall be equivalent to 80 + 20 - 15 candelas per square metre (23.4 + 5.8 - 4.4 foot-Lamberts) which means that it is expected that a 70mm screen shall be considerably brighter than a 35 or 16mm.

BS2954:1958-screen luminance for cinematograph laboratory and studio review rooms, states that for 35mm projection the illumination shall be 11 ±1 footLamberts (37.5 ± 3.5 cd/m^sup 2^) and for 16mm shall be 10 ± 1 foot-Lamberts (34.4 ± 3.5 cd/sq. metre).

The Focal Press Encyclopaedia of Cinematography in an entry written by Bernard Happe gives the following definitions for the various light measuring units, viz:

ILLUMINATION

The illumination of a surface may be regarded as the incident light coming from a source of given power at a given distance: the illumination from the source of one candela at a distance of 1 ft. is known as a foot candle or one lumen per square foot, and both terms are employed in the measurement of illumination for photography. The metric unit is one lumen per square metre (or metre-candle), termed the lux.

1 ft.-candle = 1 lumen per square foot = 10. …

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