Schematic examples of non-uniformities in the sensor reaction to a uniform radiance in the focal camera plane. (a) and (c) are pixels in the border of the sensor, while (b) is at its center. In yellow: the optical paths from the observed radiance to the (a), (b) and (c) pixels. Optical vignetting reasons: (a) receives less light than (b) because light is partially occluded at (e); pixel irradiance differ between (a) and (b) because of different optical paths (e.g., (f) and (g)) from observed radiance to pixels. Vignetting due to defects: less light reaches the sensor at (c) than elsewhere due to dirt at (d).