Glare and Visibility
One of the most important and least explored phases of lighting is that complex and indefinite one which is known as glare. We commonly separate its causes into such components as excessive brightness of the light-source, excessive brightness-contrasts, and excessive quantity of light. Glare is generally considered, rather indefinitely, to result in impaired vision, decreased safety to the individual, visual and even bodily discomfort, injury to the visual organs, and even detriment to the health of the individual. Cause and effect are not always clearly differentiated. Definitions are not standardized and, of course, they cannot be until we have analyzed the causes and effects of glare.
That the problems before us in this field are complex and difficult of solution is evident if for not other reason than by the lack of analyses of glare into its components and the lack of data pertaining to these components. It seems advisable to simplify and clarify our discussion of glare by differentiating between glare and visibility by confining the term glare solely to us and the term visibility primarily to the object we wish to discriminate. In other words, an observer may experience discomfort due to a distribution of brightness which decreases the visibility of an object at which he is looking. We may also have decreased visibility due to a veiling-brightness without any discomfort to the observer. We may also have discomfort to the observer without any effect on visibility because the observer may not be in the act of discriminating anything. The terminology used here may not always be consistent because, for the sake of being understood, some of the older terms will be used notwithstanding their unsatisfactoriness.
The sections which follow are the briefest condensations of extensive investigations which have been prosecuted for the past two year in the Lighting Research Laboratory at Nela Park and which are still in progress. Sufficient space has not been provided for the inclusion of the numerous tables of actual data, therefore, only glimpses of some of the procedures and of the mean results are presented. Throughout these investigations two observers - a man and a woman - have faithfully subjected themselves to many trying ordeals. Their results were confirmed at strategic points in each test by a number of other observers. Therefore, it is felt that the results presented herewith are approximately the average of a number of observers whom we have no reason to believe are abnormal in their reactions to the causes involved. Many experiments with so-called colored lights have been carried on in these investigations, but they are not included here. In all cases in this paper the spectral character of the light is that obtained by modern high-wattage, gas-filled, tungsten-filament lamps.