27
5.9 Gamma Correction
5.9.1 Definition:
Gamma is a measure of the linearity of the camera’s response to light.
The CCD is inherently a linear device. The output signal is directly proportional to the scene
llumination (or exposure). Doubling the exposure, will double the output signal.
The phosphors that are used to make monitors are non-linear; typically, the phosphors have less
brightness response for dark signals and more brightness response for bright signals. This
characteristic is shown in Figure 5-9 as a concave exponential brightness curve in response to a
linear camera signal. Most phosphors have a
γ
>1; a typical value is 2.2; see section 5.5.2 for a
definition).
To compensate for this, the opposite kind of non-linearity is introduced in the output video of
the camera. The Video Processor adds gain for dark signals and reduces the gain for bright
signals in such a way that the overall system (camera and monitor) produce a linear effect !
This non-linearity is called Gamma (
γ
) correction.
A gamma setting of 1.0 represents a linear camera response to scene illumination - if the
camera is connected to an Image Processor for measurement applications, this is usually
desired. However, as seen in the above figure, if the gamma setting of the camera is 1.0 , then
the monitor has a non-linear, concave, exponential response to scene illumination. Details in the
darker part of an image will not show up with as much contrast as details in the brighter part of
an image.
A gamma setting of 0.45 in the camera results in a non-linear, convex, exponential camera
response to scene illumination (not appropriate for measurement applications). However, due to
the monitor’s concave, exponential characteristic this leads to a linear monitor response to scene
illumination.
5.9.2 Gamma Equations:
Mathematically, gamma correction is represented in the following manner:
Monitor Brightness 2
Monitor Brightness 1
M
Camera signal 2
Camera signal 1
=
×
γ
Gamma Correction
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Scene Ilumination (lux)
Normalized Monitor Brightness
Gamma = 0.45 curve
Monitor Response (camera gamma=0.45)
Monitor Response (camera gamma =1.0)
Gamma = 0.6 Curve
Monitor Response (camera gamma = 0.6)
Figure 5-12: Gamma Correction Curves
Summary of Contents for DigitEyes Series
Page 14: ...9 Figure 4 3 TC 245 Gate Level Drawing Texas Instruments 1994 ...
Page 61: ...56 12 Appendix D Camera Mechanical Drawings Figure 12 1 Camera Mechanical Drawings ...
Page 63: ...58 Figure 13 2 Camera Noise Spectrum Min Gain Bandwidth 10kHz to 4 2MHz ...
Page 64: ...59 Figure 13 3 Camera Noise Spectrum Max Gain Bandwidth 100kHz to 4 2Mhz ...
Page 65: ...60 Figure 13 4 Camera Noise Spectrum Max Gain Bandwidth 10kHz to full ...