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Chapter 13
array of very tiny single sensors each assessing the brightness of the equivalent of the dot earlier described as making up a printed image in this book. In this application each tiny segment of the image is known as a pixel. The greater number of pixels each color image is broken up into, the finer will be the resultant definition of the image. The principle is exactly the same as that described in Chapter 10. In paper printing terms we define resolution as the number of dots per inch (DPI) but in electronic imaging we refer to the pixels on the chip, i.e. for the most common HD format 1920 ⫻ 1080 – the vertical number of pixels and then the horizontal number.
13.4 The Sensor Chip The sensor chip itself is a printed circuit board (PCB) divided into a grid with horizontal and vertical squares relating to the number of pixels chosen by the camera manufacturer, most often for HD 1920 pixels across the chip and 1080 from top to bottom. The whole of each square of the grid cannot be made sensitive for various reasons, including the need for wiring space. Each sensor square therefore comes out slightly smaller than the grid size and the actual sensitive spot is smaller still. Figure 13.1 shows a grid layout in plan and Figure 13.2 a cross-section where the relatively small size of the sensitive area relative to the grid pattern can clearly be seen. The biggest problem with this manner of construction is that only a small percentage of the light energy available for the whole of the grid area will reach the sensitive areas. This leads
Figure 13.1: Pixel receptors
Discrete pixel
Sensitive area
PCB
Figure 13.2: Pixel array showing area covered by the sensitive units