Study on Effect of Headlight Source Spectrum on Apparent Brightness of Visual Targets

Abstract

In this study, comparative experiments were conducted on the apparent brightness of visual targets illuminated by three types of headlight sources (halogen, HID, and LED), and the results were analyzed in terms of characteristics of visual perception. Those visual targets (with visual size of about 1 degree) were used on an actual road surface and a grey cotton cloth. To compare the apparent brightness under different light sources, we used the matching stimuli presented on the CRT display as a reference. The subjects adjusted the gradation of the matching stimuli until they perceived the brightness of the matching stimuli and that of the sample as equal. Under different light sources, luminance was set to 13 cd/m² for the road surface sample and 9.6 cd/m² for the grey cotton cloth. Atotal of 40 subjects aged between 25 and 65, including three women, participated in the experiment. Their average age was 48, and four of them wore glasses. The results indicate that the apparent brightness of road surface differed among the light sources even at the same level of luminance. This demonstrates that the LED light source had higher brightness efficiency than the HID and halogen light sources (meaning that the same brightness can be obtained from the LED light source moreover at a lower level of luminance). Furthermore, such perception changed with observer age, the results for the grey cotton cloth were similar mostly. Instead of the spectral luminous efficiency, V(λ), used for the present photometric system, a new spectral sensitivity, Vk(λ), which takes account of the spectral sensitivity of the short-wave sensitive cones associated with photopic vision, was used for the analysis, and it could accurately explain the experimental results. This suggests that the apparent brightness of white light can be quantitatively compared and evaluated based on the practical effect of luminance of brightness calculated using the spectral sensitivity that takes into consideration the contribution of the short-wave sensitive cones.