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As plates have different thicknesses, and because they sag, it is necessary to focus the imaging system on the emulsion and to continually vary the z-axis position as a function of {x,y} position on the plate. This is done by first determining the vertical position of the emulsion for the plate about to be measured at a small number of locations over the plate. A low-order polynomial surface is then fitted to that data, and the coefficients of the polynomial surface are loaded into the transputer control system. When scanning, the transputer control software uses those coefficients to calculate the z-axis position required to maintain focus, and to update the z-axis position accordingly: because of limitations on the rate of data transfer along the Leitz RS232 control line, these updates are only made every 2.5mm of x-axis movement.

The initial determination of the height of the emulsion is carried out by taking data with the table static, at a series of different z-axis positions. Those data are analysed by measuring the sum of the absolute value of the difference between adjacent pixel values, for all pixels within some specified range: this is a measure of the apparent granularity of the emulsion, and should be a maximum when the emulsion is in focus. The depth of field of the imaging system is about micron: c.f. the amount of plate sag of about 300 micron.

Mon Aug 5 15:56:21 BST 1996