No detector is perfect, there are several known imperfections. With the goal to have with constant dose a uniform image most of the imperfections can be corrected.
Let us start with three examples:
1. Capacity of photodiode discharges even without light due to the not perfect TFT switch
This discharge is measured with Dark-Images with X-Ray OFF and substracted as Offset-Image from every image in production (pixel by pixel)
2. Efficiency of photodiodes are different (e.g. by deviation in size)
Gain for each pixel is measured with test-images; gain Factors are calculated for each pixel (Gain Images); in production each pixel is corrected with this gain images
Hint: The non-uniformity of the dose from the X-Ray tube is corrected with the same procedure ...
3. Single Photodiodes which are out of specification (Bad Pixel) due to an imperfection in the TFT switch or in the photodiode
These pixel are filtered out with test images and are substituted with the neighbourhood pixel in production.
Hint: Two or more connected bad pixels are called a cluster. Pixels are called connected if they are connected by a side or a corner (eight-neighborhood
possibilities).
Pixels which do not have five or more good neighborhood pixels are called cluster kernel pixel (CKP).
For more details see E2597 page 4.