As the di�erences between illuminants are amplied 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.
1.Nevertheless, looking at the available data one observes that almost no surfaces have a lower NIR reflectance than a visible one, with the notable exception of water. Combining these phenomena with the NIR relative colorant transparency, one immediately notices a useful feature of NIR for illuminant estimation: if the NIR image intensity is lower than the RGB one, illumination is likely to be the preponderant factor. Examples of colorant transparency and vegetation enhanced reflectivity are shown in Fig. 2
2.The main advantage of NIR is that for common multiple lighting conditions, it has very large response variation with respect to the type of incident light: incandescent light bulbs have their emission peak in the NIR, while scattered skylight (which is the colour of outdoor shadows) and fluorescent lighting have virtually no emission in that part of the spectrum. This actually enables us to distinguish between diff�erent light bulbs that have an identical white point, but di�fferent metameric properties.
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