It is known qualitatively that when the luminance of a particular surface of an object is gradually increased by spot lighting, the color changes but the appearance remains natural keeping still the object color mode. For a further increase of the luminance, the surface begins to appear unnatural as an object placed in the room. This situation was expressed as that the luminance of the surface became too high so that the surface appearance went beyound the border of the recognized visual space of illumination, RVSI to present the light source color appearance. This paper quantitatively determined the border luminance for 39 color charts for two different illuminance of the room, 5 and 50 lx. The border luminance was found hight for yellow color charts while it was low for red color charts. An equation to calculate the border luminance from the room illuminance was proposed. To find whether the brightness of the color charts determines the border, thebrightness matching was conducted for all the color charts against an achromatic reference chart of N7. It was found that the border luminance was mainy determined by the brightness but there was the secondary factor. It was unnatural appearance of the test charts other than the brightness such as dazzling, transparent or fluorescent. These appearances caused the border luminance to reduce. The spectral distribution measurements of the test charts at their borders and of the objects in the observer's room are also reported. (1).
