Moulden B, Kingdom F, Gatley L F
Department of Psychology, University of Reading, Whiteknights, Berks, UK.
Perception. 1990;19(1):79-101. doi: 10.1068/p190079.
Michelson's contrast, C, is an excellent metric for contrast in images with periodic luminance profiles, such as gratings, but is not suitable for images consisting of isolated stimulus elements, eg single bars; other metrics have been devised for such stimuli. But what metric should be used for random-dot images such as are commonly used in stereograms and kinematograms? Previously the standard deviation (SD) of the luminances (equivalent to the root mean square, RMS, of the amplitudes) has been taken as a measure of contrast, but on little more than intuitive grounds. The validity of this speculative usage is tested. Experiments are described in which a wide range of random-dot images of various compositions was used and the adapting power of these images measured. This was taken as an index of their visual effectiveness. The contrast and contrast-reducing effects of the stimuli were expressed in terms of six candidate metrics, including SD, to discover which would give the most lawful description of the experimental data. The usefulness and generality of the SD measure were confirmed. The effects of mean luminance were also measured and a general expression that would take them into account was derived. Finally, on the basis of computational modelling in which spatial filters with properties approximating those of retinal ganglion cells were used, a possible theoretical account for the success of the SD metric is offered.
迈克尔逊对比度C，对于具有周期性亮度分布的图像（如光栅）的对比度而言是一种出色的度量标准，但不适用于由孤立刺激元素（例如单条线）组成的图像；针对此类刺激设计了其他度量标准。但是对于随机点图像（如立体图和运动图中常用的那种）应该使用什么度量标准呢？以前，亮度的标准差（SD）（等同于振幅的均方根，RMS）被用作对比度的度量，但依据仅仅是直观感受。对这种推测性用法的有效性进行了测试。描述了一些实验，其中使用了各种组成的广泛随机点图像，并测量了这些图像的适应能力。这被用作它们视觉效果的指标。刺激的对比度和对比度降低效果用包括SD在内的六个候选度量标准来表示，以发现哪个能对实验数据给出最合理的描述。证实了SD度量的有用性和通用性。还测量了平均亮度的影响，并推导了一个将它们考虑在内的通用表达式。最后，基于使用具有近似视网膜神经节细胞特性的空间滤波器的计算模型，为SD度量标准的成功提供了一种可能的理论解释。
