Another characteristic of HVS is that it tends to “overshoot” around image edges (boundaries of regions having different intensity). As a result, regions of constant intensity, which are close to edges, appear to have varying intensity. Such an example is shown in Figure (2.5). The stripes appear to have varying intensity along the horizontal dimension, whereas their intensity is constant. This effect is called Mach band effect. It indicates that the human eye is sensitive to edge information and that it has high-pass characteristics.

Figure (2.5a)

 

Figure( 2.5b)	Figure( 2.5c)

 Figure (2.5). The Mach-band effect:

(a) Vertical stripes having constant illumination;

(b) Actual image intensity profile;

(c) Perceived image intensity profile.

 Spatial Frequency Sensitivity

 If the constant intensity (brightness) I₀ is replaced by a sinusoidal grating with increasing spatial frequency (Figure2.6a), it is possible to determine the spatial frequency sensitivity. The result is shown in Figure (2.6a, 2.6b).

Figure 2.6(a) Figure 2.6(b) shows Sinusoidal test grating ; spatial frequency sensitivity

To translate these data into common terms, consider an “ideal” computer monitor at a viewing distance of 50 cm. The spatial frequency that will give maximum response is at 10 cycles per degree. (See figure above) The one degree at 50 cm translates to 50 tan (1 deg.)  =0.87 cm on the computer screen. Thus the spatial frequency of maximum response f_max= =10 cycles/0.87 cm=11.46 cycles/cm at this viewing distance. Translating this into a general formula gives:

where d=viewing distance measured in cm.