Round Robin Test Appendix B: Two Suggested Methods to Minimize the Effects of Reflected UV
1. Detector mask method
In this method, a black cardboard or wooden mask is placed at a distance about D/2 from the lamp, where the size and the positioning of the mask casts a complete shadow over the detector. The mask should be of a size that completely blocks the direct rays from the lamp, but not much larger.
In this case the irradiance reading Erefl from the detector represents only UV reflected from the floor, ceiling and walls. Erefl should be subtracted from the overall irradiance reading and should represent less than 1% of the total irradiance at the detector.
2. Two-chamber method
UV can reach the UV sensor by reflection from walls, the floor and the ceiling. This reflected UV must be avoided or subtracted from the detector signal in order to get proper irradiance values. A two-sector approach can be used for this purpose.
In this method, the test chamber is divided into two light-tight sectors, with the divider between the two sectors at least 35 cm from the center of the UV lamp. The lamp and detector should be mounted at least 25 cm (preferably about 1 m) from the floor and preferably about 1 m from the wall behind the lamp.
A rectangular hole 3 cm longer than the arc length and 2 cm wider than the width of the lamp should be cut in the divider, so the UV sensor can “see” the entire arc length of the lamp through the hole. See Figure 3 for a possible setup.
Irradiance measurement procedure that minimizes the effect of reflection
When the two-sector chamber approach is used, the procedure is the same as that described in the main body of this protocol. In addition, a measurement should be made with the hole between the two chambers covered with black cardboard. The radiometer signal in that case should be virtually zero.