Other noises associated with CCDs are sensitivity variations from photosite-to-photosite on the CCD detector or across the detector. Modern CCDs are uniform to better than 1% between neighboring photosites and uniform to better than 10% across the entire surface. Also, any sensitivity variation from pixel to pixel can be removed through flat fielding. For the polarization camera used in the ISCORE instrument, the flat fielding file has already been generated by the camera manufacturer and is built into the software package that runs the polarization camera and automatically corrects all images for flat fielding. The final processed image would then be obtained from subtracting the dark image from the raw image and dividing by the flat field image [(raw image—dark image)/flat fielding or (raw image/flat fielding—dark image/flat fielding)]. Here, through a crude experiment, we have explored the reliability of the manufacturer provided flat fielding file built into the operating software of the camera. To test this concept, we first illuminated the CCD in the polarization camera with white light that was unfocused, unpolarized, and passed through two diffusers and 10 images were captured with the built-in flat fielding corrector in the software switched off and we will call the average of these 10 images as , where is the integration time that allowed to record digital counts in majority of the pixels. Then we captured three dark images that matched the integration time with the white-light lamp and the built-in flat fielding corrector in the software switched off and we will call the average of these dark images as . Then the gain (G) is obtained from , where is the average of the difference . Next, an image was taken by the polarization camera with the built-in flat fielding corrector in the software switched off and we call this , where is the integration time. Again, three dark images were taken with an integration time and the built-in flat fielding corrector in the software switched off and the average of these three we will call . Now the same image and the dark images were taken with an integration time of , but this time with the built-in flat fielding corrector in the software switched on and we will call these and , respectively. Now, Figs. 4(a)–4(c), show the following: , , and the difference between the two, respectively. The bottom image confirms that the majority of the differences are located close to the zero value as seen from the color coded bar that interprets the data and confirms that the laboratory generated flat fielding file does not qualitatively or quantitatively alter the final image from using the flat fielding file built-in to the operating software of the camera. However, individual pixels could degrade with time and optical systems in the ISCORE instrument can introduce sensitivity variation in the CCD, for example, vignetting. To account for these sensitivity variations, we will generate our own flat fielding file at the time and location of our experiment by taking multiple images through pointing the ISCORE instrument toward cloudless parts of the blue sky. The straight forward way to perform the comparison shown in Fig. 4 is to compare the flat fielding file created in the laboratory with the flat fielding file embedded in the camera software and both these were created without any lenses attached to the polarization camera. Unfortunately, we do not have access to the flat fielding file embedded in the camera software.