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12 5 0 0
Temporal frequency (Hz)
50 100
Figure 3.16 Temporal contrast sensitivity (according to de Lange, 1958)
Critical Flicker Frequency (CFF). This frequency tends to increase with retinal eccen-
tricity. That is why a television screen can create a flickering sensation in peripheral
The variations in thresholds of contrast with respect to temporal frequency are
described using a temporal contrast sensitivity function (FSC t ) as per the average level
of luminance expressed in Troland. On the curve shown here, temporal frequency of
modulations is shown in absciss (logarithmic units) and contrast sensitivity given in
percentage of contrast (starting from the top) is shown in ordinate (logarithmic units).
The four curves correspond to different intensities, 10000 td, 100 td, 10 td and 1 td
from right to left (Figure 3.16). Visual acuity
Visual acuity is not homogenous in the entire field of vision. For an emmetropic eye
(normal), monocular acuity is very high for a cone centred on a 2 angle since the
distribution of retinal cones is restricted to the fovea of the eye. The minimum value
of the angle from which the two points are seen separately depends on the stimulus
A strip of light on a black background: 30'' angle;
Two points of light on a black background: 1' angle;
Two dark points on a light background: 2' angle;
As we can see, the average value (1') is in accordance with the density of photoreceptors
in the fovea (refer to the explanation on retina). An angle of 1minute corresponds to the
vision of two points separated by 0.1mm for a distance of 35 cmbetween the image and
the eyes. To follow these characteristics, a 25 cmwide screen positioned at this distance
should display 2500 pixels horizontally. We can thus note that the image definition of
even good-quality computer screens (1280 pixels) needs to be improved. These numeric
values are established for a screen giving a narrow field of vision (40 degrees). If we
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