The visible features of an iris arise in the trabeculum, a meshwork of connective tissue that displays arching ligaments, crypts, contraction furrows, a corona and pupillary frill, coloration, and sometimes freckles. The striated anterior layer covering the trabecular meshwork creates the predominant texture seen with visible light (see examples here), but all of these sources of radial and angular variation taken together constitute a distinctive "fingerprint" that can be imaged from some distance. Further properties of the iris that enhance its suitability for use in high confidence identification systems include: (i) its inherent isolation and protection from the external environment; (ii) the impossibility of surgically modifying it without unacceptable risk to vision; and (iii) its physiological response to light, which provides one of several natural tests against artifice.
A property the iris shares with fingerprints is the random
morphogenesis of its minutiae. Because there is no genetic penetrance
in the expression of this organ beyond its anatomical form, physiology,
colour and general appearance, the iris texture itself is stochastic
or possibly chaotic. Since its detailed morphogenesis depends on
initial conditions in the embryonic mesoderm and ectoderm from which
it develops, the phenotypic
expression even of two irises with the same genetic genotype
(as in identical twins, or the pair possessed by one individual)
have uncorrelated minutiae.
In these respects the uniqueness of every iris parallels the
uniqueness of every fingerprint, common genotype or not.
But the iris enjoys further practical advantages over fingerprints
and other biometrics for purposes of automatic identification,
including: (iv) the ease of registering its image at some distance
from a Subject without physical contact, unintrusively and
perhaps inconspicuously; (v) its intrinsic
polar geometry, which imparts a natural coordinate system and an
origin of coordinates; and (vi) the high level of randomness in its
pattern, creating inter-Subject variability spanning about
250 degrees-of-freedom, and an entropy (information density)
of about 3.2 bits per square-millimeter of iris tissue.
The human iris begins to form during the third month of gestation.
The structures creating its distinctive pattern
are complete by the eighth month of gestation, but
pigmentation continues into the first years after birth. The layers
of the iris have both ectodermal and mesodermal embryological origin,
consisting of (from back to front): a darkly pigmented epithelium;
pupillary dilator and sphincter muscles; heavily vascularized stroma
(connective tissue of interlacing ligaments containing melanocytes);
and an anterior layer of chromataphores and melanocytes with a genetically
determined density of
pigment granules. The combined effect
is a visible pattern displaying various distinctive features such as
arching ligaments, crypts, furrows, ridges, and a zigzag collarette.
Iris colour is determined mainly by the density of the stroma
and its melanin content, with blue irises resulting from an absence of
pigment: longer wavelengths differentially penetrate while shorter
wavelengths are reflected and scattered, a phenomenon resembling that
which makes the sky blue. The heritability and ethnographic diversity of
iris colour have long been studied, but until the present research,
little attention had been paid
to the achromatic pattern complexity and textural variability of the iris