Camera Calibration Technical Report:
Image OD Conversion

Image OD Conversion

The example slide used is a coronal section of a rat brain immunostained for tyrosine hydroxylase. The method we have detailed will work with any slide but the procedure is most efficiently undertaken using a slide with a wide range of densities. The two cameras were calibrated using a neutral density filter of OD 0.1 in each case. The example images cover approximately identical slide locations and the light level was adjusted to give visually acceptable uncorrected images in each case. Dark- and bright-field images are not shown but were obtained seperately for the two cameras by the methods described above.

In figure 4.1 we show two sets of curves. Figure 4.1(a) show the measured bright-dark curves for each camera. The Sony camera exhibits nearly linear response for the whole intensity range but with a lower sensitivity than the Pulnix. The Pulnix camera was adjusted to have a $\gamma$-value of less than one and results in a non-linear response curve. The density calibration curves calculated from these bright-dark values are shown in figure 4.1(b).

Figures 4 and 5 show the result of using these calibration curves on the raw uncorrected images. In each case the grey-level histogram of the image is shown in the inset graph. The histograms for the two uncorrected images show how different the responses of each camera are with the Pulnix exhibiting greater sensitivity at the low-light end of the range (note the histograms are drawn with increasing grey-value corresponding to increasing density). The corresponding histograms for the corrected images are nearly identical with small differences arising from different noise characteristics of the cameras and the fact that the two images are not exactly the same position or orientation.

calib-exp6in3inCalibration curves, (a) bright-dark curves for each camera, and (b) corresponding density calibration curves.

  

Figure 4: Uncorrected images and grey-level histograms (inset) for (a) Pulnix camera, and (b) Sony camera. The mean, standard deviation and peak position (as marked by the arrow) are (131.0, 49.9, 142) for the Pulnix and (95.8, 34.8, 103) for the Sony.

  

Figure 5: OD corrected images and grey-level histograms (inset) for (a) Pulnix camera, and (b) Sony camera. The mean, standard deviation and peak positions are now (31.9, 28.2, 25) and (32.8, 28.9, 26) respectively.