PRNU (Photo Response Non-Uniformity)
Photo Response Non-Uniformity, or PRNU for short, is one source of pattern noise in digital cameras. Like DSNU, it is seen as the variation in pixel responsivity over the CCD. However, while DSNU occurs as a variation in pixel responsivity when the CCD is not illuminated, PRNU is the pixel variation under illumination.
To characterize the PRNU, we use the camera to take multiple images of a uniform scene, produced by the Optoliner. We kept the illumination level fixed at 3.00 candelas since the brighter light is more easily detected by the camera, and we also checked to ensure that the camera is focused before taking the pictures. We took 100 exposures each for three exposure times: 1/10, 1/4 and 1/2.5.
The calculation of the PRNU is as follows:
Obtain the average image over the 100 images taken:
Subtract the DSNU image from this average image to eliminate the contribution from the DSNU.
Obtain the spatial variance of the pixel values over the entire CCD
Divide the spatial variance by the average image from (ii) to obtain the PRNU as a percentage of the actual pixel values.
Repeat the calculations for the different exposure times to compare the PRNU.
We expect the PRNU to increase with increasing illumination, since increasing the illumination level will enhance the difference in the photo-response of the pixels across the image and lead to a higher PRNU. In our measurements, since the maximum value of the Optilinear device is around 4 candelas, and increasing the illumination level increases the non-uniformity of the illumination produced by the Optilinear, we chose to increase the exposure times to mimic the effect of increasing illumination levels.
We plotted the graph of the PRNU values obtained for each of the three color channels across the different exposure times, and the results are shown below. We can see that the relationship between the PRNU and the exposure time is approximately linear, which corresponds to what we would expect. We also see that the green channel has the highest PRNU, followed by the red and then blue channel respectively. This corresponds to our other measurements of the parameters of the CCD, which consistently showed the green channel to be the noisiest channel among the three.
However, for the blue channel, the PRNU at low exposure values is actually much higher than that at higher exposures. This could be due to other sources of noise in the CCD: the blue channel has the lowest responsivity among the three color channels, and at low levels of illumination/exposure, the contribution from other noise sources will be more significant, affecting the value of PRNU obtained.
Figure 1: Graph of PRNU across exposure times
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