Image Enhancement

Abstract

When using displays outdoors, the readability of the content shown can degrade at large extent by reflections of bright ambient light, like on a sunny day outdoors. Technically speaking, the display’s luminance and the reflected luminance of ambient light superimpose in the eyes of an observer. Therefore, display quality parameters (contrast ratio, grayscale, and colour reproduction) are perceived significantly lower as indoors or the values provided in data sheets, which are measured under dark room conditions (no ambient light). Additionally, the effect of superposition is degrading the “darker” (lower gray levels) relatively more than the “brighter” content on displays.

This affects, besides mobile personal devices, many applications like displays in cars and e-signage (such as information terminals, signs, kiosk applications). There are many possibilities to enhance the image quality of a display for bright ambient light: high luminance of the display, reflection reduction, blocking ambient light by shields, or raise lower gray levels.

The latter method, which is called “image enhancement,” is presented here. Its advantage is that the costs are significantly lower compared to the other improvement methods as being only software based. Generally speaking, the original gamma value (typical γ = 2.2) is lowered so that the value of a low gray level, e.g., 50 (of 256 for 8 bit grayscale depth) is raised to, e.g., 70. There exist several methods, which are presented and discussed here. They differ basically in terms how the gray levels are modified and the artifacts which might occur. An essential point is that the colour of the original image should be kept as best as possible. However, image enhancement has its limits when, e.g., the luminance of the display is too low.