MaxADU, ElectronsPerADU, and FullWellCapacity in the Camera Interface

The MaxADU, ElectronsPerADU, and FullWellCapacity are characteristic properties which describe the camera sensor and its analog-to-digital converter (ADC). The last two are essential parameters for scientific imaging.

Values Must Reflect Current Camera Modes

The primary application use case for these properties is (1) Establish the operating modes, (2) Acquire the image, and (3) Query the interface to determine these properties. However, these properties must be available before the first image is acquired, in other words, immediately on startup (with default values). Also, after making a change to relevant properties such as Binx, BinY, Gain, Offset, or ReadoutMode, or via the camera’s setup dialog, the mode change(s) must be immediately reflected in these properties

Purpose of MaxADU

MaxADU is provided for imaging applications so they may know the maximum range of ADU values to expect from a camera in ImageArray and therefore may establish their display scaling, etc.

Meaning of MaxADU

MaxADU is a charactristic of the camera, not of the current image (if any). It must be the maximum ADU value that can ever be output by the camera sensor and its analog-to-digital convereter (ADC) in its current operating modes. Usually this would correspond to the current bit depth as 2^bitdepth - 1. However if, by design, a camera is limited in its current modes to an ADU value that is significantly lower than 2^bitdepth - 1, it must report the maximum possible pixel value (the upper limit).

For example if the camera’s ADC is 12 bit, but by design, and in the camera’s current modes, the sensor can only ever produce a 12-bit digitized pixel value of 3800 (a significantly lower value), then MaxADU must report 3800, and not 2^12 - 1 = 4095.

Inflation of Data Values - Upscaling to Higher Bit depth

This specification strongly discourages inflating the pixel values coming from the analog to digital converter (ADC) to create the illusion that the camera is capable of a higher bit depth than it actually is. This includes not interpolating between real ADU values to smooth the inflated data between inflated values. ADUs should increase and decrease in steps of 1. Cameras that inflate values are essentiallly useless for science.