Brightness indention: A novel compression mechanism in the luminance-brightness mapping

In a previous study, Schouten and Blommaert (1995) explicitly showed that brightness constancy implies a substantial compression in the luminance-brightness mapping. Here it is argued that additional compression mechanisms are required for scenes with large luminance ranges. To trace these, a series of experiments was conducted which involved expansion of the luminance range. We used a simple spatial configuration consisting of a disk on a contourless homogeneous surround (Ganzfeld). The contrast between the disk and Ganzfeld acted as the independent variable, while the size of the disk was parametrically varied. Sequential dichoptical brightness matches with a reference were carried out for both the disk and the surround. The results reveal a compression mechanism which we termbrightness indention. This indention, which has not previously been reported in the literature, only occurs if the Ganzfeld is less luminous than the disk, and it takes the form of a brightness decrease of the immediate surroundings of the disk.     

Visual metrics: discriminative power through flexibility

An important stage in visual processing is the quantification of optical attributes of the outside world. We argue that the metrics used for this quantification are flexible, and that this flexibility is exploited to optimise the discriminative power of the metrics. We derive mathematical expressions for such optimal metrics and show that they exhibit properties resembling well-known visual phenomena. To conclude, we discuss some of the implications of flexible metrics for visual identification.     

Letter recognition at low contrast levels: effects of letter size

Contrast variation was used to measure recognition thresholds for lowercase letters, with the aim of obtaining a better understanding of the role that early stages of visual processing play in letter recognition. Frequency-of-recognition curves were measured for alphabets of different letter size. Since variation of the adaptational state of the eye changes the characteristics of primary visual processing in a quantifiable way, recognition thresholds were measured both at a high (150 cd m-2) and at a low (0.9 cd m-2) adaptation level. Thresholds decreased as letter size increased, in a way comparable with data on visual acuity. At the lower adaptation level, recognition thresholds became higher, which is also in accordance with visual acuity data. Furthermore, the slopes of the frequency-of-recognition curves for alphabets as a function of log contrast decreased with decreasing letter size. It is argued that this is mainly caused by an increasing dispersion of internal representations of individual letters on the internal psychological scale as letter size decreases.     

Brightness constancy in a Ganzfeld environment

In this paper, experiments are described in which brightness constancy was studied in a Ganzfeld environment. Luminance variation by means of neutral density filters was applied to stimuli consisting of a Ganzfeld with superimposed disks. To this end, a special-purpose apparatus was constructed. Sequential dichoptical brightness matches with a reference stimulus were carried out for the disks as well as the homogeneous surround. The results of these measurements indicate that (1) besides a clear tendency toward brightness constancy, small but systematic effects of the average luminance level are present and (2) the brightness of the Ganzfeld is hardly affected by the presence of the disks. Finally, it is shown that the experimental results can be modeled adequately in terms of a concept that involves an accumulation of contrast information.