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The eye plays an important role in our life, not only for seeing objects in the surrounding world, but also for reading letters, viewing paintings, photographs, films, etc. The visual acuity of the eye is generally regarded as the most important factor for the ability of the eye for seeing objects. The acuity of the eye is usually measured by acuity tests where single black letters on a white background have to be recognized, or where the minimum visible separation is measured of black rings with a small interrupted part (Landolt rings). These tests are used for decisions about the use of certain types of eye glasses, but give no information about several other factors that also play a role in the properties of the human visual system. Objects can generally be better distinguished from each other or from their background, if the difference in luminance or color is large. Of these two factors, luminance plays the most important role. The content of this book will, therefore, concentrate on luminance, and color will be left out of consideration. In practice, it appears that it is not the absolute difference in luminance that is important, but the relative difference. This relative difference can be expressed by the ratio between two luminance values, which is called contrast ratio, or by the difference between two luminance values divided by the sum of them, which is simply called contrast. Both are dimensionless quantities. Objects that have only a small contrast with respect to their background are difficult to observe. The eye is more sensitive for the observation of objects, if the required amount of contrast is lower. The reciprocal of the minimum contrast required for detection is called contrast sensitivity.
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