A new physical photometer has been designed to measure the spatially weighted average equivalent luminance of all visible areas of the total visual field, relative to the average luminance of a task background subtending two degrees in diameter. The spatial weighting follows traditional practice in general. The glare evaluation meter (gem) uses two parallel optical systems with identical components. The left system collects the average luminance of the task backgroundat 2 degrees. The right system has a modified disability glare lensmounted in front of the objective lens. The photometer measures l and l sub v and then computes the glare contrast factor (gcf). All three values are output to a backlit 3 1/2 digit lcd meter calibrated in cd/sq m. The fixed-focus, hand-held gem is battery operated. Calibration of the meter is performed by using a variable level (up to 17, 000 cd/sq m) source of known solid angle and rotating the meter togenerate various off-axis angles. The 2-degree task background luminance can also be varied. The operation of the gem is described and sample values of gcf are given for sample glare situations. The new meter will allow dynamic and static conditions to be measured. Sample values of gcf are given for different glare conditions and with different values of the individual disability glare factor. The readaptation correction caused by continuous exposure to the glare source will be discussed for a sample glare situation involving the headlights of an oncoming vehicle. This paper appears in transportation research record no. 1316, Visibility for roadways, airways, and seaways1991: proceedings of a conference, july 25-26, 1990, washington, d.C.
Abstract