Camera Calibration Technical Report:
OD Comparison

OD Comparison

In order to determine the repeatability of OD measurements made by different cameras, the OD of a set of neutral density filters was measured using the Pulnix and Sony cameras. Each camera, was calibrated by the described procedure. Scaling was selected so that an OD of 1.0 was represented by a pixel value of 200.

A blank area of an arbitrary slide was selected, and after setting the lamp brightness to avoid saturation, a bright field was captured. Each neutral density filter was inserted in the illumination path, an image captured then corrected to optical density with reference to the bright field image and appropriate calibration table. The OD of the filters were recorded as the mean pixel value, after rescaling (ie division by 200) and are shown in table 1. Note, the actual values measured depend on the accuracy of the 0.1OD filter used in the calibration process.

 

Table 1: OD of neutral density filters measured with the Pulnix and Sony cameras. The filters used were Kodak Wratten gelatin neutral density filters with nominal densities as shown.

Nominal	Pulnix	Sony
0.1	0.100	0.096
0.2	0.213	0.206
0.3	0.310	0.308
0.4	0.465	0.461
0.5	0.502	0.514
0.6	0.671	0.662
0.7	0.777	0.786
0.8	0.831	0.845
0.9	0.902	0.957
1.0	1.003	1.141

The correlation coefficient between the two Pulnix and Sony readings is 0.9950. Notice that it is the two higher OD readings which differ the most (with these points excluded, the correlation is 0.9996). We believe this is because the Sony camera has a near linear response and therefore has low resolution of high optical densities. This is especially apparent for 8-bit digitisation. The pulnix camera has $\gamma < 1.0$(ie it is more sensitive at low intensity levels) giving it greater discrimination for higher OD measurements.