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Measurement Good Practice Guide No. 39 Figure 24: Image distortion. 4.4 LENS SELECTION As mentioned previously there are two standard types of lens mounts for lenses and CCD cameras, namely C and CS mounts. It is worth noting that the mounts have different flange back lengths, that is, the minimum distance between the lens and the sensor. In the C mount this distance is approximately 17.5 mm and in the CS mount, it is approximately 12.5 mm. From this, it can be seen that a CS lens cannot be used with a C mount camera, as the lens cannot be positioned close enough to the camera. However, C lenses may be used with CS mount cameras if a 5 mm extension ring is used. Figure 25: Selecting the correct lens/CCD sensor format. Lenses and cameras are supplied in a standard formats. The lenses and CCD arrays are described in imperial sizes as 1”, 2/3”, 1/2” or 1/3”. These sizes do not actually correspond to the actual sensor size on the CCD camera but are there to match the camera to the correct lens format. Figure 25 shows how the format describes the lens/camera 41
Measurement Good Practice Guide No. 39 association. It can be seen that by using unmatched cameras and lenses, the sensor area may not be fully illuminated. 4.5 COMMERCIAL LENSES 4.5.1 Standard lenses These are designed for low-resolution vision system applications and are generally supplied in 1/2” or 2/3” format. The minimum object distance is typically 200 mm or more. Distortion at the lens edges may be as much as 3%, with a MTF of 30 lines per mm in that region. Their use should be limited to presence verification applications only. 4.5.2 High resolution lenses Made with precision-coated optics, these lenses are suitable for use with small pixel CCD cameras. Distortion values are typically less than 1% and resolutions in excess of 125 lines per mm are available. Various lenses are available with minimum object distances from 1 mm to 1200 mm, in the 1/2” or 2/3” format. 4.5.3 Macro lenses Vision systems use Macro lenses where small fields of view are required. Image distortion is generally less than 0.1%, with resolution as high as 200 lines per mm. On-axis illumination fittings are common and magnification up to 12x is available. 4.5.4 Telecentric lenses This is, perhaps, the most widely used vision system lens for dimensional measurement. It is different to other lenses in that, within a particular range, known as the telecentric range, variations in the height of features on an object do not cause a change in the image size. Therefore, the image does not suffer perspective distortions. In addition, image distortions are low, in the range of 0.1- 0.9%. The lens also has constant magnification and a large depth of field within the telecentric range. The drawback to the use of telecentric lenses is that the object size cannot be greater than the diameter of the lens. 5 CAMERAS A CCD is the most common type of camera used with vision systems. Instead of using film, a CCD sensor positioned behind the lens converts light intensity into electronic signals that can then be transmitted directly to a computer. The sensor consists of a discrete number of light sensitive points, called pixels, formed in a 2D grid. Advances in manufacturing technology have reduced the size of microchips and therefore CCD cameras are becoming increasingly smaller and faster. This has also led to the 42
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