Iris vs. retinal scan
Nicholas Millichamp, BVSc, PhD, Dipl. DACVO, Dipl. ECVO, says, "You can get some variations in iris pigmentation over time.
Since the iris scan photographic image is obtained via infrared light, perhaps you might get more evidence of vascular changes
than you would with a normal light photograph where you're just going to see pigmentation."
Otherwise, he notes, corpora nigra, which project from the upper part of the iris, may vary in shape and size. Use of the
iris scan "is an interesting thought, because in other species, such as cattle and dogs, retinal photography is used for identification,"
Millichamp says. "For dogs, there's a lot of variation in pigmentation in irises between breeds, but in other respects, the
structures would be similar [to horses]." There's probably a lot more variation in iris pigmentation in dogs, and some horses
have much reduced pigmentation.
"It depends what they're looking at—whether it's the vasculature or differences in the folded appearance on the front of the
iris—and that's pretty distinctive in horses, which you don't see quite as much of in dogs," says Millichamp. "I guess it
depends on exactly what's being picked up with the infrared."
It also depends on what one is going to photograph—the vasculature of the iris, the pigment pattern on the iris surface, or
both. "We use infrared photography for horses to look at eyes that have corneal opacities. It allows you to photograph the
anterior chamber and the iris to identify corneal edema," says Millichamp. "There is certainly a fairly distinctive appearance
to the iris. I think a good part of that is going to be influenced by the vasculature, because it is such a vascular structure.
Most of the heat that's generated—the infrared part of the spectrum that's emitted—is due to the blood vessels that are a
higher temperature than the anterior chamber. The scan is probably picking up the iris vessels as much as anything. It must
You can't identify horses based on the retinal vasculature, Millichamp continues, because the vessels are too small. In other
species, the retinal vessels are much larger, and that's what people tend to use for imaging and identification purposes.
"I would think in the iris, which is a densely vascularized structure, that would be the thing that would give most of the
pattern—far more so than pigmentation, which can change significantly over time," he says.
The camera system
The iris scan system registers both the left and right eye because each is different in individual horses, and also in twins
and clones. An infrared picture is taken of each eye. "Accuracy is greater than 99.9 percent—far more so than the human fingerprint,"
says Stewart. As a side note, it's possible to modify a lip tattoo or a microchip, even when it's still implanted in the animal.
The iris scan camera consists of two LED illuminators within the infrared spectrum and a lens. Below the lens system is a
handle with a trigger used to gather several images and to automatically feed data to the attached computer, which then constructs
the algorithm. Once triggered, the camera essentially takes a very short video, 24 pictures to the shutter-click, which the
software automatically analyzes. The software sorts the images and selects the best one for the algorithm. Once done, it converts
the algorithm data to a 512-byte code and then to a 15-digit number.
The camera was developed by a company that also developed the color television (RCA Laboratories, which is now Sarnoff Corporation).
It's been responsible for refining the algorithms Stewart and his colleagues developed. The LED light and lens system of the
battery-operated camera has been continuously improved to its current state of development. The current prototype is being
confirmed and tweaked.