Visiometrics HD Analyzer OQAS, Руководство пользователя

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HD Analyzer
™ Chapter 1: Introduction
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1.1.3 What is MTF?
MTF = Modulation Transfer Function
The MTF of an optical system is a function that allows for the evaluation of the
degree of detail that the system is able to distinguish, or similarly, it evaluates the
relation between the contrast of the real scene and the contrast of the image that
forms the system. Our eye is an optical system, so it also has an associated MTF,
and it indicates how much the contrast decreases in the image formed in the retina
with respect to the real scene.
In any optical system, the contrast reduction is greater for high spatial frequencies
(fine details and contours in the image). Therefore, in our case, of the following bar
codes, the codes on the right are perceived as grayer (lower contrast) than those
on the left, despite them all having the same definition. This agrees with the fact that
the more details and contours (greater spatial frequency) there are in the real
scene , the more the contrast between those details and contours is reduced in the
image formed in our retina.
Figure 2. Bar patterns with different spatial frequencies
Therefore, MTF is a function that relates the spatial frequencies, measured in cycles
per degree, with the decrease in contrast obtained for each of them in the image of
the system, in our case, the image formed in the retina.
In the following figure we have symbolized, with a key, a distance that we will
assimilate to a degree. We can observe that the number of complete space cycles
(pattern formed by one black and one white stripe) that fall in a degree, varies
according to the frequency. For the first bar code, the spatial frequency is 1, while
for the following it is 2, 4, 8 and 16, respectively. In fact, the higher the spatial
frequency, the lower the contrast in the image formed in the retina.