Method for determination of magnification in a line scan camera

摘要

The magnification M_ref for a line scan camera can be found by exploiting a difference in the way M_ref affects the notion of 'focus' in the x and y directions. M_ref enters into the calculations for selecting z while focusing in the y direction, but not in x. A thin opaque calibration target is provided at a convenient height in z called the reference plane, and has straight edges aligned parallel to the x and y directions. To find M_ref the line scan camera forms images of the calibration target over a range of trial z values known to include the height of the calibration target. An arbitrary and possibly incorrect trial value M_i of M_ref is then assumed and many slices of the calibration target are calculated. Within these slices the edge parallel to the x direction will be sharply defined for some z_x, while at some other z_y the edge parallel to the y direction will be sharply defined. Make a note of e_i=z_y. Repeat these steps for some number different trial M_i that cover the plausible range of M_ref. Now fit a curve (e.g., a quadratic) to the data set {(e_i, M_i)}, and find the y-intercept (where e=0). The associated value of M is the magnification M_ref in the reference plane containing the calibration target, and we found it without knowing the actual length of any part of the calibration target.