Digital camera sensors are sensitive to wavelengths ranging from the ultraviolet (200-400nm) to the near-infrared
(700-100nm) bands. This range is, however, reduced because the aim of photographic cameras is to capture and
reproduce the visible spectrum (400-700nm) only. Ultraviolet radiation is filtered out by the optical elements of
the camera, while a specifically designed "hot-mirror" is placed in front of the sensor to prevent near-infrared
contamination of the visible image.
We propose that near-infrared data can actually prove remarkably useful in colour constancy, to estimate the
incident illumination as well as providing to detect the location of different illuminants in a multiply lit scene.
Looking at common illuminants spectral power distribution show that very strong differences exist between
the near-infrared and visible bands, e.g., incandescent illumination peaks in the near-infrared while fluorescent
sources are mostly confined to the visible band.
We show that illuminants can be estimated by simply looking at the ratios of two images: a standard RGB
image and a near-infrared only image. As the differences between illuminants are amplified in the near-infrared,
this estimation proves to be more reliable than using only the visible band. Furthermore, in most multiple
illumination situations one of the light will be predominantly near-infrared emitting (e.g., flash, incandescent)
while the other will be mostly visible emitting (e.g., fluorescent, skylight). Using near-infrared and RGB image
ratios allow us to accurately pinpoint the location of diverse illuminant and recover a lighting map.