Imaging systems are compared to the human eye in terms of acquisition, spectral sensitivity, transmission, and display. Although the performance of imaging systems is generally of a high standard, there is still room for improvement. At the acquisition stage, the use of flash, or infra-red lighting, results in images with some unnatural features, and there is a general absence of systems that give monochrome images at very low light levels, as are provided by the rod system in the eye. The overlapping nature of the spectral sensitivities of the cones results in unwanted cone stimulations which reduce reproduction gamuts; in printing, extra colorants, such as orange, green, and violet, can be used to extend the gamut and reduce metamerism. Commercially available imaging systems incorporate spectral sensitivities that do not usually exactly match a set of color matching functions, which is a requirement for special applications where high colour accuracy is important. The transmission of image signals in broadcast television makes use of the important luminance/chrominance principle, but full benefit is not achieved because of gamma correction, and only one system makes use of the reduced resolution of the yellowness-blueness channel of the eye. Successful bit reduction in digital images is achieved by taking advantage of the reduced contrast sensitivity of the eye at high spatial frequencies and other effects, but a reduction in the consequent artefacts is desirable. In the visual system, the display is in the cortex, which has an enormous ability to interpret the retinal signals so as to recognise objects, including very efficient compensation for changes in illumination level and color; some improvements in the similar compensation provided in imaging systems are desirable, together with increases in the dynamic ranges available, especially in display devices. Automatic image-enhancement adjustments can be included when making images, but there is room for more sophisticated techniques that avoid impairing some types of scene. A low-cost image-display device for the mass market that is more convenient than the cathode-ray tube remains an important challenge.
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