Lighting Research & Technology

A scale of subjective brightness is proposed in terms of which the changes in sensitivity of the eye can be recorded. The unit of subjective brightness is called a “brill,” and it is suggested that the scale may also prove satisfactory as a means of specifying contrast. An experiment is described which enables the changes taking place during dark adaptation to be appreciated in a very striking manner and by means of which the time required to adapt down to low levels of adaptation other than complete dark adaptation can be recorded. The effect of area on visibility is briefly discussed and the action of a glare source in depressing the sensitivity of the eye and delaying its recovery is described. A short discussion of the extent to which contrast is affected by a reduction both of illumination and of area follows, after which a new test for defective night vision is described and some preliminary results reported. In the final section a brief description of a portable binocular matcher that can be used as a “brillmeter” to measure subjective brightness is given.

Part I describes the construction and calibration of a laboratory apparatus which combines light beams transmitted through colour filters in such proportions as to reproduce the intensity and energy distribution of all the coloured light signals generally used in practice.

Part 2 discusses the technique of signal colour recognition and gives the results of over 40,000 observations in the form of charts on which are drawn contours of the frequency of recognition of the simple colours under different conditions of illumination, range of possible colours, etc. The significance of these contours is briefly discussed, with a view to stating physical limits within which signal colours should lie.

Present-day problems in illuminating engineering call for the measurement of very low values of both illumination and brightness. Such measurements are difficult, both on account of the reduced sensitivity of the eye to brightness differences and because of the operation of the Purkinje effect. The need is generally for measurements which will represent what is experienced under practical conditions, and so coloured light is measured by visual comparison with white and the results are termed values of “apparent illumination” or “apparent brightness.” The particular features required in illumination gauges and illuminometers suitable for measuring low values of illumination are described, with special reference to the appropriate British Standard Specifications. For certain purposes a portable standard of brightness is convenient, especially where the use of a gauge is awkward or impossible. In the study of luminescent materials a special type of brightness gauge is used, capable of measuring apparent brightness down to 0.0001 foot-lambert.