When a test patch is locally illuminated by, for example, a slide projector, so that its luminance is much higher than those of other objects in a scene illuminated with a main lighting source, the appearance of the patch is perceived as unnaturally bright and the situation is expressed as the appearance being out of the Recognized Visual Space of Illumination (RVSI) of the scene. If the illuminance of the scene is gradually increased, the appearance of the patch shifts into the RVSI and becomes natural. The shifting limit was obtained for two different patches, N4.0 and 5YR4/8 placed among objects in a room simulating a study, when the illuminance of the room was increased. Determining the limit was quite easy even for naive subjects. It was suggested that the limit was reached when the subjective lightness of the test patch became nearly 100, when the subjective lightness was calculated by the CIE L* in which Y0 is replaced by the illuminance of the brightest object in the scene.
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