Bit Depth is the number of bins into which the camera’s full dynamic range is divided. For example, a 12-bit camera has 212 bins, or 4096 bits. A higher bit depth provides smaller increments and therefore higher resolution of the detected light level. Calculations for data analysis are always performed at maximum bit depth in order to see the smallest changes in the signal.
With some 4D systems, the included 4Sight Analysis Software has a Bits to Show setting in the Camera Settings dialog box (this is the window in which the operator selects the camera Gain and Exposure for a measurement). This setting alters the Bit Depth that is displayed on-screen, to make the data more easily discernible. It does not, however, affect the calculations.
Consider an example in which a 4D interferometer is being used in a high-vibration environment. An extremely short Exposure time will mitigate the effects of the vibration but will also decrease the integrated signal of the incident light. Consequently, a higher gain value would normally be selected in order to decrease the exposure time yet still maintain enough image brightness to provide sufficient signal to make the measurement.
Sometimes, however, even with the Gain set to maximum it may still be impossible to obtain sufficient image brightness to align the system or acquire good quality data. In this case, changing the Bits to Show value from 12 to 10 will truncate the top 2 bits (which, in this case, would be all black anyway) and promotes the remaining 10 bits to display full scale. Doing so will effectively increase the displayed light level by a factor of 4, making it easier to see the fringe pattern and align the test configuration. Changing the bit level from 12 to 8 can also be done and would increase the displayed signal level by a factor of 8. Neither selection, however, would affect the measurement analysis, which is always completed at the maximum available bit depth.
