-rvMnKoCFcxzfAodzDpSyg. Making Telecoms Work Higher Contrast MTF Highest Perceived Increased MTF Resolution Lower brightness CTF Large Features Spatial Frequency Small Features Figure 9.1 Contrast threshold function and MTFA. Reproduced by permission of Planar Systems Inc. (Clarity Visual Specialist Display Technologies). 6 method is to use a method known as the modulation transfer function area. This is illustrated in Figure 9.1. The quality of a video stream is partly a function of the frame rate and colour depth, but also a function of the contrast ratio, resolution and brightness of the display. Contrast ratio defines the dynamic range of the display (the ratio of the brightest white the system can generate divided by the darkest black), resolution defines the display's ability to resolve fine detail expressed as the number of horizontal and vertical pixels. Brightness is, well, brightness, measured in foot-Lamberts or candelas per square metre. Brightness captures attention, contrast conveys information. To be meaningful, brightness and contrast need to be characterised across the range of spatial frequencies being displayed. Spatial frequency is the ratio of large features to small features - the smaller the features in an image, the higher the spatial frequency. The overall number of pixels in the display determines the limiting resolution. The modulation transfer function is way of comparing contrast to spatial frequency. As features get smaller, the contrast ratio will reduce. However, this measure does not take into account the limitations of human vision. We need a minimum contrast for an image to become distinguishable and this is measured using a contrast-threshold function. Adding together the modulation transfer function of the imaging system and the contrast threshold function of human vision yields a crossover point that determines the highest per- ceptible resolution. Increasing the brightness does not make much difference to the maximum perceived resolu- tion, whereas increasing the contrast ra