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Unformatted text preview: r any other
relevant physical magnitude. In practice, the relationship between a pixel value and
the corresponding physical quantity is usually complex and almost always non-linear.
The exposure function specifying the
relationship between the logarithmic light
intensity B and the resulting film density D is
almost linear within a certain range. The slope
of the linear part is traditionally referred to as
the “gamma” value of the photographic
material. The term was adopted later to
characterize other devices such as the
cathode ray tube (CRT), that has a nonlinear
relation between the amplitude (voltage) of
the video signal and the emitted light. To
compensate for this non-linearity a gamma
correction can be applied to the video signal
before visualizing it with a CRT. 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 26 Gamma correction (2)
The gamma correction is based on the gamma function. Where ° is called the gamma value. If a is limited to the
interval [0 1], then – independent of ° – the value of the
gamma function also stays within [0 1] and the function
always runs through the points (0,0) and (1,1). Depending
on the value of °, the function can imitated both logarithmic
and exponential types of function. Because of its monotony
properties the function can easily be inverted that again
leads to a gamma function with the new gamma value 1/°: Device ° 2.5 camera light 1.8 to 2.8 Receiver (TV, …) The transfer characteristic of a device with gamma
value ° is compensated for by a gamma correction with
1/°. The resulting signal b is proportional to the
original light intensity B. CRT, LCD 1/1.956=0.5
1 camera gamma
signal where s denotes the output signal of a certain
device (e.g. a camera).
19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 27 Gamma correction (3)
Gamma correction denotes a simple point operation to compensate for the transfer
characteristics of different input and output devices and to map them to a unified
intensity space (“cali...
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This note was uploaded on 07/09/2009 for the course MEDIT 1 taught by Professor Pierreelschbinger during the Spring '09 term at Carinthia University of Applied Sciences.
- Spring '09