Light threshold contrast

PSvcHomrsicAL (eye-brain) RESPONSE TOINCOMIN6 LIGHT "THRESHOLDS OF PERCEPTION FOR CONTRAST COLOR CHANGE -SENSITIVITY TO SIZE,PATTERN DISTRIBUTION OF COLOR. -SUBJECTIVE JUDGMENT OF PERCEIVED IMAGES... 

Threshold contrast is illustrated in Fig. 10-2. / is the intensity of light received by the eye from the object, and / + A/ represents the intensity coming from the surroundings. The threshold contrast can be as low as 0.018-0.03 and the object can still be perceptible. Other factors, such as the physical size of the visual image on the retina of the eye and the brain s response to the color of the object, influence the perception of contrast. 

Fig. 10-2. Threshold contrast in distinguishing an object from its surroundings. The eye responds to an increment in light intensity by increasing the number of signals (pulses) sent to the brain. The detection of threshold contrast involves the ability to discriminate between the target (1) and the brighter background (/ + A/). Source Gregory, R. L., Eye and Brain "The Psychology of Seeing." Weidenfeld and Nicolson, London, 1977.

Figure 3.8 Floes of silica viewed against a black background with overhead lighting. Excellent contrast is achieved between the image and the background, making thresholding a good choice for image segmentation in this case.

The molecular assays in Clk"mAic2As bom fide rhythms with a predominant effect on circadian rhythm amplitude and no more than a modest effect on phase or period. With circadian per and tim enhancers, we observed reduced enhancer activity and a reduced cycling amplitude in a Clk" background, consistent with the role of Clk in regulating these enhancers. Nonetheless, the phase of oscillating bioluminescence is similar to that of wild-type flies. The presence of molecular rhythms contrasts with the absence of detectable behavioural rhythms. We favour the notion that this reflects a level or amplitude reduction below a critical threshold for behavioural rhythmicity. The absence of anticipation of light—dark transitions makes it very unlikely that an effect restricted to the lateral neurons — the absence of the neuropeptide PDF, for example — is primarily responsible for the behavioural phenotypes. This is also because LD behavioural rhythms are largely normal in flies devoid of PDF or the pacemaker lateral neurons (Renn et al 1999). However, we cannot exclude the possibility of selective effects of Clk" on other behaviourally relevant neurons. 

When the helix is completely unwound a clear state is produced as a consequence of the formation of a pseudonematic phase with no helical structure from the corresponding chiral nematic phase. The analyser then absorbs linearly polarised light. Therefore, black information can be displayed at the appropriate pixels against a white background to produce an image with positive contrast. The threshold voltage for the unwinding of the helix is dependent on a number of physical parameters ... 

Visibility requires that the contrast of an object exceeds a critical value called the contrast threshold. The contrast threshold of an object is not constant for all images but varies with image brightness. In bright light, the threshold can be as low as about 3% while in dim light the threshold is greater than 200%. 

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