Just Noticeable Differences (JNDs) | JND

Image display consistency is a major issue that could hamper the successful 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 defined “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 small differences in 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. The resulting curve is called the Grayscale Standard Display Function (GSDF) which results in a perceptual linearization.

Using this standard, monitors and printers can be calibrated so that images can be represented on different monitors and printers ina consistent manner.