Luminance Lookup Table - Basler GigE VISION CAMERAS User Manual

Standard Features

10.5 Luminance Lookup Table

The type of electronics used on the camera allow the camera's sensor to acquire pixel values at a
12 bit depth. Normally, when a camera is set for a 12 bit pixel data format, the camera uses the
actual 12 bit pixel values reported by the sensor.
The luminance lookup table feature lets you create a custom 12 bit to 12 bit lookup table that maps
the actual 12 bit values output from the sensor to substitute 12 bit values of your choice. When the
lookup table is enabled, the camera will replace the actual pixel values output from the sensor with
the substitute values from the table.
The lookup table has 4096 indexed locations with a 12 bit value stored at each index. The values
stored in the table are used like this:
When the sensor reports that a pixel has an actual 12 bit value of 0, the substitute 12 bit value
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stored at index 0 will replace the actual pixel value.
The numbers stored at indices 1 through 7 are not used.

When the sensor reports that a pixel has an actual 12 bit value of 8, the substitute 12 bit value

stored at index 8 will replace the actual pixel value.
The numbers stored at indices 9 through 15 are not used.

When the sensor reports that a pixel has an actual 12 bit value of 16, the substitute 12 bit value

stored at index 16 will replace the actual pixel value.
The numbers stored at indices 17 through 23 are not used.

When the sensor reports that a pixel has an actual 12 bit value of 24, the substitute 12 bit value

stored at index 24 will replace the actual pixel value.
And so on.
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As you can see, the table does not include a defined 12 bit substitute value for every actual pixel
value that the sensor can report. If the sensor reports an actual pixel value that is between two
values that have a defined substitute, the camera performs a straight line interpolation to determine
the substitute value that it should use. For example, assume that the sensor reports an actual pixel
value of 12. In this case, the camera would perform a straight line interpolation between the
substitute values at index 8 and index 16 in the table. The result of the interpolation would be used
by the camera as the substitute.
Another thing to keep in mind about the table is that index 4088 is the last index that will have a
defined substitute value associated with it (the values at indices 4089 through 4095 are not used.)
If the sensor reports an actual value greater than 4088, the camera will not be able to perform an
interpolation. In cases where the sensor reports an actual value greater than 4088, the camera
simply uses the 12 bit substitute value from index 4088 in the table.
The advantage of the luminance lookup table feature is that it lets a user customize the response
curve of the camera. The graphs below represent the contents of two typical lookup tables. The first
graph is for a lookup table where the values are arranged so that the output of the camera increases
linearly as the actual sensor output increases. The second graph is for a lookup table where the
values are arranged so that the camera output increases quickly as the actual sensor output moves
from 0 through 2048 and increases gradually as the actual sensor output moves from 2049 through
4096.
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