Absorption spectrum of melanin

In the near infrared wavelengths of light used for iris recognition (700nm - 900nm), iris pigmentation and apparent colour in the visible band (400nm - 700nm) play almost no role. The peak absorption of human melanin pigment occurs around 335nm, and absorption is almost completely attenuated for wavelengths longer than 700nm. The reflectance of the iris is quite constant over the 700nm - 900nm band, although the iris looks much brighter in video images acquired in the longer wavelengths simply because the sclera and skin are much darker (less reflective) in those wavelengths, and so the camera gain setting tends to increase. Pigmentations in the iris itself, including freckles and the blotches sometimes caused by drug treatments for glaucoma (latanoprost and other prostoglandin analogues), are largely invisible in the near infrared wavelengths used for iris recognition and thus do not affect the IrisCode.

Data sources:

"The spectroscopy of human melanin pigmentation," by N. Kollias. In: Melanin: Its Role in Human Photoprotection, pp. 31 - 38. Valdenmar Publishing Co. (1995).

"Optical properties of human sclera, and their consequences for transscleral laser applications," by A. Vogel, C. Dlugos, and R. Nuffer, Lasers in Surgery and Medicine 11(4), pp. 331 - 340 (1991).

"The incidence and time-course of latanoprost-induced iridial pigmentation as a function of eye color," by P. Wistrand, J. Stjernschantz, and K. Olsson, Survey of Ophthalmology 41(S2), pp. S129 - S138 (1997).

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