Gamma Correction - Smartek UCC User Manual

User Manual - twentynine

8.2.5 Gamma Correction

Gamma is an important characteristic of digital imaging systems, as it translates between the light sensitivity
of the human eye and thus of the image sensor. Generally it has to be distinguished that there are basically
two definitions of gamma. The first one is called Gamma Correction , the second one Gamma Adjustment .
The Gamma Adjustment assumes that the sensor's gamma is 1.0 and comes into consideration when
displaying an image on a display device. It is used to encode linear luminance or RGB values to match the
non-linear characteristics of display devices.
Depending on the characteristics of the sensor and also influenced by gain or black level, the gamma output
of the camera is not ideally 1.0. If only the Gamma Adjustment is applied to the image, the real gamma may
represent a combination of the encoding gamma and the sensor's gamma. The consequence of this effect
is that the brightness levels of the image outputted on the display are distorted.
In situations where the gamma of the sensor is not 1.0, the Gamma Correction can be used to linearize the
non-linear sensor's characteristics to match a linear gamma of 1.0. For this purpose a well calibrated gray
scale is usually used to determine the Gamma Correction values. The gamma value can be applied using
the ImageProcAPI .
The term Gamma Correction will be used throughout this document and depends on the context, it can be
understood as either Gamma Correction or Gamma Adjustment .
Gamma Correction Workflow
The workflow of correcting gamma is illustrated in Figure 104. First an image of a known and calibrated
object like a Color Checker chart will be captured. Based on this the gamma of the sensor can be
determined, a sample of a sensor gamma curve is shown in the first block of Figure 104 (from left to right).
After applying the Gamma Correction value the brightness levels should match the values known from the
calibration chart and represent a Gamma of 1.0.
If it is intended to display the image to the human eye the gamma of the display device should be known.
Based on the device's gamma the Gamma Adjustment process could be started, encoding linear luminance
to match the non-linear characteristics of the display device; a common display gamma value is 2.2. After
gamma adjustment the displayed image appears luminance correct on the display device.
Gamma
Gamma Adjustment
Correction
Sensor Gamma Gamma 1.0 Adjusted Gamma Display Gamma Gamma 1.0
Figure 104: Gamma correction workflow
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