Perceptual Linearization

Image display consistency is a major issue that hampers the implementation of PACS using softcopy viewing. The key to preserving the image quality of a display is to calibrate the monitor at regular intervals. There is no ideal display characteristic curve for mapping digital driving levels into luminance values. However there is a gold standard that could be adhered to.
This characteristic curve is generated by mapping just noticeable differences (JNDs) onto the light output (luminance) of a monitor or onto the optical density of a film.

The human eye-brain detection system does not work linearly. It is much less sensitive in the black than in the white area. This means that all the black values need not be mapped into luminance as displayed on the monitor.
Hence the input values (i.e. JNDs) are squeezed into fewer luminance output steps in the black area, while in the white area they are stretched. This method is called perceptual linearization.

The resulting curve is called the Grayscale Standard Display Function (GSDF) and is a part of the DICOM Standard. When monitors are calibrated to the DICOM GSDF Standard, for a given digital input, even monitors of different types and different brightness levels will produce the same perceptual response in the observer.