On the admissible brightness of lighting fixtures

Abstract

Since higher levels of illumination can now be applied, the distribution of brightness in the visual field asks for special attention. Most studies on this subject are based on the classic formula of Holladay concerning comfortable lighting. In the experiments of Holladay the criterion was taken from the feeling of comfort or discomfort during the first seconds after the momentary shock experienced by an observer who is suddenly exposed to a glaring source. In our opinion it is questionable if such a criterion applies to the feeling of comfort or discomfort of a person who is present and working in a lighted interior for a long time. Therefore we made investigations on a rather small scale on the admissible brightness of lighting fixtures under circumstances as usual for interior lighting. A laboratory model of lighting fixtures was observed in a room, the surrounding brightness of which could be changed from 4.10-5 sb to 5.10-3 sb. The fixture was represented by a rectangular diffusing surface, the dimensions of which could be varied. The observations were made at various distances. From this resulted a solid angle subtended at the eye by the source of 10-5 to 10-1 steradians, whereas the angle at which the source was located above the line of vision varied from 10 to 40 degrees. By twelve observers the brightness of the fixture was adjusted to values that they considered to be the limit above which an increase would cause a transition from comfort to discomfort. The results of these investigations have been compared with the values that can be determined from the work of other investigators. Although there is agreement in some parts of the investigated regions, the deviations between our results and the values that can be computed from other work are considerable. It appeared that the influence of elongated sources on the feeling of comfort or discomfort is quite different from what is found in case of rectangular or circular sources. Tests of limited scope proved that in certain cases discomfort decreases with increasing length of the source when observed crosswise. Similar indications were obtained from observations on an interrupted linear source. These results cannot be understood from Harrison’s glare rating method.