Optical veiling glare limitations to in-camera scene radiance measurements

Multiple exposure techniques improve the quantisation of pixel values in digital images and are claimed to extend the dynamic range of captured radiances. The combination of underexposed, normal, and overexposed images quantises more accurately images falling on the sensor. However, scene-image dynamic range on that sensor is controlled by veiling glare of the lens, light reflected inside the camera body, and light reflected off the sensor. The point-spread function of all cameras falls off very quickly with distance. This rapid decrease is offset by the millions of image pixels contributing small, but significant, glare to all other pixels. We made telephotometer measurements from test scenes to accurately document the radiances in scenes. We used a variety of high dynamic range (HDR) calibration algorithms and evaluated their accuracy. In the worst-case scene, with a small opaque center and a large-high-radiance surround (ISO 9358:1994 standard for veiling glare measurements), camera veiling glare limits are reported to vary from 1% to 8% of the maximum radiance. That is, a film(sensor)-plane image dynamic range of only ~25:1. The presented data show that veiling glare is an image-dependent limit to in-camera radiance measurement of real-life scenes and must be taken into account in HDR natural images.