We have developed a mobile vision assistive device based on a head mounted display (HMD) with a video camera,
which provides image magnification and contrast enhancement for patients with central field loss (CFL). Because the
exposure level of the video camera is usually adjusted according to the overall luminance of the scene, the contrast of
sub-images (to be magnified) may be low. We found that at high magnification levels, conventional histogram
enhancement methods frequently result in over- or under-enhancement due to irregular histogram distribution of subimages.
Furthermore, the histogram range of the sub-images may change dramatically when the camera moves, which
may cause flickering. A piece-wise histogram stretching method based on a center emphasized histogram is proposed
and evaluated by observers. The center emphasized histogram minimizes the histogram fluctuation due to image changes
near the image boundary when the camera moves slightly, which therefore reduces flickering after enhancement. A
piece-wise histogram stretching function is implemented by including a gain turnaround point to deal with very low
contrast images and reduce the possibility of over enhancement. Six normally sighted subjects and a CFL patient were
tested for their preference of images enhanced by the conventional and proposed methods as well as the original images.
All subjects preferred the proposed enhancement method over the conventional method.