Thus if the distance to the source is doubled, the illuminance reduces to one quarter of its original value. This law is only strictly accurate for infinitesimally small light sources, but is approximately true if the maximum dimension of the light source is less than one-fifth of the distance from the light source to the point of interest. Simultaneous with assuring that sufficient quantity of light is present, the designer will carefully consider issues of lighting quality. This includes aesthetic concerns, emotional and psychological issues, and human factors such as the direction from which the light arrives at the tasks and the viewers. If this last item is overlooked, glare can result.
Glare is brightness in excess of the level to which the eye is adapted.
The difference between daytime and nighttime driving helps to illustrate. When driving at night, the headlight of an oncoming car can cause dangerous glare problems. However, if the same headlights are viewed in the daytime it may be difficult to even discern if they are on. Excessive brightness in the visual field can make one uncomfortable. In order to design office lighting that will not cause discomfort, the visual comfort probability, or VCP is calculated.