Speigle J M, Brainard D H
Department of Psychology, University of California, Santa Barbara 93106, USA.
J Opt Soc Am A Opt Image Sci Vis. 1996 Mar;13(3):436-51. doi: 10.1364/josaa.13.000436.
Color constancy is often modeled on the assumption that color appearance in natural scenes is a function of the visual system's estimates of surface reflectance. Some stimuli, however, do not look like illuminated surfaces. Instead, they appear to be self-luminous. We hypothesized that the appearance of luminosity occurs when the visual system estimates a reflectance spectrum that is outside the gamut of physically realizable surfaces. To test this idea, we measured luminosity thresholds as a function of stimulus chromaticity and illuminant spectral power distribution. Observers adjusted the luminance of a test patch until it just appeared self-luminous. The test patch was spot illuminated by a computer-controlled projection colorimeter viewed in an experimental room lit diffusely by computer-controlled theater lamps. Luminosity thresholds were determined for a number of test patch chromaticities under five experimental illuminants. The luminosity thresholds define a surface in color space. The shape of this surface depends on the illuminant. We were able to describe much of the luminosity threshold variation with a simple model whose parameters define an equivalent illuminant. In the context of our model, the equivalent illuminant may be interpreted as the illuminant perceived by the observer. As part of our model calculations we generalized the classic notion of optimal stimuli by incorporating linear-model constraints. Given the equivalent illuminant, the model predicts that a patch will appear self-luminous when it is not consistent with any physically realizable surface seen under that illuminant. In addition, we show that much of the variation of the equivalent illuminant with the physical illuminant can be modeled with a simple linearity principle. The fact that our model provides a good account of our data extends the physics-based approach to judgements of self-luminosity. This in turn might be taken as support for the notion that the visual system has internalized the physics of reflectance.
颜色恒常性通常基于这样一种假设来建模，即自然场景中的颜色外观是视觉系统对表面反射率估计的函数。然而，一些刺激物看起来并不像被照亮的表面。相反，它们似乎是自发光的。我们假设，当视觉系统估计出的反射光谱超出物理上可实现的表面色域时，就会出现发光现象。为了验证这一想法，我们测量了作为刺激色度和光源光谱功率分布函数的发光阈值。观察者调整测试色块的亮度，直到它刚刚呈现出自发光状态。测试色块由计算机控制的投影色度计进行点照明，该色度计置于一个由计算机控制的剧场灯漫射照明的实验室内。在五种实验光源下，针对多个测试色块色度确定了发光阈值。这些发光阈值在颜色空间中定义了一个表面。这个表面的形状取决于光源。我们能够用一个简单模型描述大部分发光阈值变化，该模型的参数定义了一个等效光源。在我们的模型背景下，等效光源可以解释为观察者感知到的光源。作为我们模型计算的一部分，我们通过纳入线性模型约束，推广了最优刺激的经典概念。给定等效光源，该模型预测，当一个色块与在该光源下看到的任何物理上可实现的表面不一致时，它将呈现出自发光状态。此外，我们表明，等效光源随物理光源的大部分变化可以用一个简单的线性原理来建模。我们的模型能够很好地解释我们的数据这一事实，扩展了基于物理学的自发光判断方法。这反过来可能被视为支持视觉系统已经内化了反射率物理学这一观点的证据。
