A History of Light & Colour Measurement: Science in the by Sean F. Johnston

Light

By Sean F. Johnston

It is a heritage of the hidden workings of actual technological know-how - a technical endeavour embedded in a social context. It argues that this 'undisciplined' topic, straddling academia, trade and legislation, could be general not just of twentieth century technology, yet of its destiny.

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London 90 90, 293, 437; Herschel 1800 'Experiments on the refrangibility of the invisible rays of the sun' Phil. Trans. R. Soc. London 90 284; Herschel 1800 'Investigation of the powers of the prismatic colours to heat and illuminate objects: with remarks, that prove the different refrangibility of radiant heat' Phil. Trans. R. Soc. London 90 255. 39 For later variants, see Cartwright C H and Strong J 1938 'Vacuum thermopiles and the measurement of radiant energy' in Strong J 1938 Procedures in Experimental Physics (New Jersey).

Early photographers were concerned with the effect of light on the photographic plate rather than with the intensity itself. The two were not synonymous. A correctly exposed plate was the goal of the photographic method, and light intensity was merely one of the factors that could affect the result. Instead of a fundamental interest in light, the photographer had an interest merely in its control as an exposing agent. The control of light was straightforward, in principle, for most photographic work: the intensity could be varied over wide limits simply by altering the aperture of the camera lens.

He reasoned that while the eye can determine the brightest point of a pattern with relative accuracy, it can judge the dimmest even more surely (the eye, once dark adapted with the iris fully dilated, cannot 'accommodate' any further to weak lighting). In a thresholding or extinction technique, the observer compares the intensity to a minimum detectable level. The intensity is reduced by some means until it is below the threshold of visual detection. The amount of reduction required is then a measure of the relative brightness.

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