Generally when talking about the EM spectrum, at least in ham circles, "below" means longer wavelength, lower frequency. "Above" means shorter wavelength, higher frequency. In this case the "standard safety color" for the Allied Worlds is smack in the middle of the terahertz range, which is above microwaves but below infrared.
My original idea was that the yinrih could perceive all non-ionizing radiation from just above direct current all the way up to the high end of soft UV rays. Now I'm thinking that's too Mary-Sue-ish (if there is such a word), and I might narrow that down to wavelengths shorter than the aperture of their eyelids, so a little over a centimeter, or however big dog eyes are. The reason for this is that if you have light incident on a grill with openings of X diameter, wavelengths longer than X will not pass through. That's why microwave oven windows have a metal mesh. The waves are about 13 cm long, and the holes in the mesh are much smaller than that, so it lets visible light through but blocks the microwaves. In the yihnrih's case, the diameter of their open eyelids will dictate what wavelengths can fall on the nantenna patches that serve as their eyes.
Even this is a bit of a handwave, as the nantennas covering these eye patches should only efficiently couple with wavelengths about equal to four times their length. Each nantenna is as a quarter-wave dipole sitting on a shared ground plane. These nantennas would presumably be optimized to receive the peak wavelength radiated by Focus after passing through Yih's atmosphere. Since yih is more-or-less Earth-like, that would mean a wavelength of about 555 nanometers. The effective passband would be wider, of course, with the convention for measuring the bandwidth of a passband being when the signal drops 3 dB below the maximum.
This isn't even getting into the fact that I'm not sure if an array of nanoantennas like this could form an image. I'm sort of treating it like a really dense compound eye or a bundle of very very fine optical fibers.
While I'm on this ocular tangent, yinrih think human eyes are gross--slimy goo-filled orbs covered in mucus sliding around in our skulls. But we are the first and only other sophonts they've found after nearly 100 millennia of searching, so their joy at finally being able to fulfill the Great Commandment helps them power through the initial revulsion. There's lots of other stuff they find off-putting about us, like our lack of fur, our larger size, and the smell of our sweat. Humans, however, think yinrih are adorable li'l monkey foxes.
