Validity of Environmental Brightness Estimation by Virtual Luminance Distribution Method

Our recent studies have shown that the main factor for determining of envionmental brightness is the perceived amount of light filling a space. Therefore, to estimate the environmental brightness from the physical properties of a space, we have to compute the physical quantity that corresponds to the perceived amount of light filling the space. We propose using the virtual luminance distribution methods (VLDM) to estimate environmental brightness. Because our past results did not show that VLDM is more effective than other environmental brightness indices, we performed a new experiment. In the experiment, the subjects evaluate enviromntal brightness of the inside of test boxes with variant reflectance of the walls and variant lighting patterns by using the magnitude estimation method. The brightness estimated using VLDM correlated well with that evaluated by the subjects. However, the vertical illuminance at the point of the subjects' eyes correlated poorly the estimated brightness. This demonstrates that VLDM is valid for brightness estimation. We performed another experiment in order to investigate the difference between using VLDM and the reference matching method (RMM) for brightness evaluation. The results show that it is possible to estimate the brightness evaluation using RMM by considering photometry and brightness unlinearity. This suggests there is a similarity between the psychological process of brightness evaluation using RMM and the computation process of brightness estimation using VLDM.