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Contrast, threshold

Objects close to us are easily perceived, but as we attempt to detect objects farther and farther away from us, the contrast between the object and the background decreases. The lowest limit of contrast for human observers is called the threshold contrast and is important because this value influences the maximum distance at which we can see various objects. [Pg.136]

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. [Pg.138]

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. 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.
The parameter C is sometimes called the threshold contrast or psychophysical constant because It depends on human perception. Based on data averaged over responses of a group of individuals, its value is usually taken to be 0,02 ... [Pg.149]

The lowest visually perceptible brightness contrast is called the liminal contrast or threshold contrast. The threshold contrast has been the object of considerable interest since it determines the maximum distances at which various components of a scene can be discerned. Laboratory experiments indicate that for most daylight viewing conditions, contrast ratios as low as 0.018-0.03 are perceptible. Topical observers can detect a 0.02 or greater contrast between large, dark objects and the horizon sky. A threshold contrast value of 2% (Cv = 0.02) is usually employed for visual range calculations. [Pg.704]

Equation (15.35) can be evaluated at the distance at which a black object has a standard 0.02 contrast ratio against a white background. When the contrast in (15.35) becomes the threshold contrast, the distance becomes the visual range. If Cy = 0.02, then... [Pg.705]

Fig. 3.3. CNR and threshold contrast for 0.1mm details using three human readers. The fitted curve is in the form of (3.1) along with 95% confidence lim its. Also shown are the minimum and achievable standards for threshold contrast in the European protocol (Young et al. 2006)... Fig. 3.3. CNR and threshold contrast for 0.1mm details using three human readers. The fitted curve is in the form of (3.1) along with 95% confidence lim its. Also shown are the minimum and achievable standards for threshold contrast in the European protocol (Young et al. 2006)...
FIGURE 4 Typical modulation transfer function (MTF) and threshold contrast plots. [Pg.162]

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. [Pg.314]

Generalizations. Several generalizations can be made regarding taste (16,26). A substance must be in water solution, eg, the Hquid bathing the tongue (sahva), to have taste. Water solubiUty is the first requirement of the taste stimulus (12). The typical stimuli are concentrated aqueous solution in contrast with the Hpid-soluble substances which act as stimuli for olfaction (22). Many taste substances are hydrophilic, nonvolatile molecules (15). Taste detection thresholds for lipophilic molecules tend to be lower than those of their hydrophilic counterparts (16). [Pg.11]

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... [Pg.137]

In contrast to single-crystal work, a fiber-diffraction pattern contains much fewer reflections going up to about 3 A resolution. This is a major drawback and it arises either as a result of accidental overlap of reflections that have the same / value and the same Bragg angle 0, or because of systematic superposition of hkl and its counterparts (-h-kl, h-kl, and -hkl, as in an orthorhombic system, for example). Sometimes, two or more adjacent reflections might be too close to separate analytically. Under such circumstances, these reflections have to be considered individually in structure-factor calculation and compounded properly for comparison with the observed composite reflection. Unobserved reflections that are too weak to see are assigned threshold values, based on the lowest measured intensities. Nevertheless, the number of available X-ray data is far fewer than the number of atomic coordinates in a repeat of the helix. Thus, X-ray data alone is inadequate to solve a fiber structure. [Pg.318]

The mechanical concepts of stress are outlined in Fig. 1, with the axes reversed from that employed by mechanical engineers. The three salient features of a stress-strain response curve are shown in Fig. la. Initial increases in stress cause small strains but beyond a threshold, the yield stress, increasing stress causes ever increasing strains until the ultimate stress, at which point fracture occurs. The concept of the yield stress is more clearly realised when material is subjected to a stress and then relaxed to zero stress (Fig. Ih). In this case a strain is developed but is reversed perfectly - elastically - to zero strain at zero stress. In contrast, when the applied stress exceeds the yield stress (Fig. Ic) and the stress relaxes to zero, the strain does not return to zero. The material has irreversibly -plastically - extended. The extent of this plastic strain defines the residual strain. [Pg.11]


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See also in sourсe #XX -- [ Pg.136 , Pg.137 ]




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