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of the pixel.

       1.1.1. Landolt-C test

       In the Landolt-C test, test symbols are judged by the subject under test. In accordance with this test the smallest discernable detail is defined as the visual angle of the gap size of the Landolt C symbol at threshold size and is expressed in arcmin. The threshold size corresponds to the size at which the subject judges the orientation correctly in 75 per cent of the trials. The smallest discernable detail is determined in a test involving a human observer. A test chart containing test symbols is placed in front of the camera and the observer judges the orientation of test symbols from the monitor. From the threshold gap size of the Landolt C test symbol d (m) and the distance between the test pattern and the camera D (m) the smallest discernable detail ωc (arcmin) is calculated as follows:

       1.1.2. TOD test

       The Landolt C test can be used to determine the smallest discernable detail of the camera-monitor system. However, for sensor systems it is more suitable to use the TOD (Triangle Orientation Discrimination) method which is similar to the Landolt C method, but involves equilateral triangular test patterns. The Triangle Orientation Discrimination method is described in detail by Bijl & Valeton (1999), who provide practical guidelines on how to perform a TOD measurement. In the method, triangular test patterns (see Figure 1) are viewed through the viewing system under test. Each triangle can have one out of four possible orientations (apex up, left, right or down) and the observer indicates/guesses for each triangle its orientation. When this procedure is repeated for many (randomly oriented) triangles of different sizes the fraction of correct responses can be plotted (see Figure 2), and increases with test pattern size. The threshold is defined as the point at which the fraction correct crosses the 0.75 level and can be obtained by fitting a smooth function through the data (see Bijl & Valeton, 1999). Critical perception is reached when the critical object diameter equals twotimes the width of the triangle at threshold size. The smallest discernable detail (ωc) is equal to 0.25 times the width of the triangle at threshold size. This means that, from the threshold triangle width w (in m) and the distance between test pattern and the camera D (in m) the smallest discernable detail ωc (in arcmin) is calculated as follows:

Figure 1
       Triangular test patterns used in the Triangle Orientation Discrimination (TOD) method

Figure 2
       Typical relationship between the size of the triangle and the fraction of correct responses.

       1.2. Determination of the critical viewing distance of the monitor

        For a monitor having certain dimensions and properties, the distance to the monitor can