CCD_Rasnik: Hints for designing a new mask.

Choosing a basic building block (or B3) depends on a number of requirements. It should be big enough to contain a sufficient amount of codebits to encode the complete mask. The it must be small enough to ensure that a little more than one blocksize of the mask is seen by the camera. Or more if redundancy is required.The size of the dots (or fields) depends on the camera used. The size of the pixels of the CCD sensor play an important role. The image of the mask on the camera will not be completely sharp due to diffraction. In the data generated by the framegrabber, the dots must big enough to be recognized. Recognizing flat tops for black and white in the data all over the view seems good practice. On the other hand, smaller dots will generate more edges to do a precise fit on. So the choice of the size of the mask pixels depends on the quality and aperture of the optics, but also on the camera-framegrabber combination. It is good practice to be able to recognize a codeline twice or more, to compensate for dust, damage etc..
When the orientation of the mask in respect to the camera is relatively fixed the width to height ratio of a B3 could be about 4/3 for a standard camera. When also the projection is very sharp, also the fields in the B3 could have this ratio.A square B3, with square fields, is mirror symmetrical over the line X=Y. This allows to make contact copies of the mother mask, without the need to rotate the origin of the copy.
It is good practice to choose an odd numbers of dots for both horizontal and vertical B3 size. In that case the resulting pattern of the coded locations are inverted every next B3. This results in a more stable average illumination of the camera when scanning over the mask. Also completely black or white codelines are avoided. This is illustrated below. The mask sizes 150x150 dots. The B3 is 6x6. The 'cameraview' shows a little more than two B3s in both directions. Two views (next to the mask) show a clear difference in dot density. The view next to them is a sample of the same size of a mask with B3s of 7x7.

A 150x150 mask with B3s with an odd number of fields in X and Y direction gives a more stable average illumination of the camera when scanning over the mask. Also completely black or white codelines are avoided. This mask is shown here.
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august 1995