Camera Calibration Technical Report:
Calibration

Calibration

To use the calibration method outlined above requires the determination of the graph of G(I) against G(I'). This can be obtained using a single neutral density filter (possibly unknown value), plus any microscope slide with a reasonable variation of densities.

We assume that each of the elements of the array has a transfer function of the form

g_i(I) = a_i + f(b_i I),

where i is an element index and each may have a different dark-field value, a_i and a different shading, b_i. Variation in shading coefficient b_i arises from non-even illumination and from differences in individual CCD array elements for example their respective quantum efficiencies. The procedure to generate the points, (G(I), G(I')), is:

1.

digitise an entire dark field image by switching the microscope lamp off;

2.

select a field on a slide that exhibits a reasonable range of densities and adjust the lamp level so that the brighter parts of the field almost saturate the digitised image;

3.

digitise the field twice, without moving the slide, first without and then with the neutral density filter in the illumination path, ¹ typically before the condenser. Subtract the dark-field image from each of these new images; 

4.

accumulate the values (G(I), G(I')) by scanning through the two images, corresponding pixels from the two images are the required value pairs, G(I') is the value from the image with the neutral density filter and G(I) is the value from the image without the filter ²;

5.

inspect the range of values extracted from the images; if parts of the response range are missing then adjust the lamp voltage and repeat from 3;

6.

establish the values $G({\rho}_0 + i {\delta \rho})$;

7.

select a starting value G₀ near the bright end of the range to correspond to the arbitrary offset density ${\rho}_0$ and tabulate the function $\rho(G)$ by the method outlined earlier.