What colors can horses see?

Carroll and colleagues (2001) measured the cones of domestic horses with electroretinogram flicker photometry. The short-wave cone peaks at about 428 nm and the long-wave cone (the paper calls it middle-to-long-wave) at about 539 nm, which gives horses the basis for dichromatic color vision. Human eyes have three cone types, peaking at about 420, 534 and 563 nm (Bowmaker and Dartnall, 1980).

Colors that differ for us mainly in how red or green they are can give a horse the same signal; blue against yellow stays clear. The table runs a few jump-pole colors against green turf through the same model as the simulator and gives the color difference (ΔE 2000) for a person and for the horse model: the smaller the number, the harder the pair is to tell apart. On the scale we use across this site, below 3 is a close match, 6 or more is a visibly different color, and 10 or more is clearly different. The swatches are illustrative colors, not measured paint or turf.

The simulator is built from the horse's own cone peaks (428 and 539 nm). For each color it estimates how strongly it would excite each of the two cones, using the pigment template of Govardovskii and colleagues (2000) and daylight (CIE D65), and shows a color that excites them in the same way. The cone signals weigh the light by its energy; a cone counts photons, and weighting by photon count instead would shift some shown colors slightly, by up to about 5 of 255 in one color channel by our check. The shown colors lie on a yellow–blue axis plus lightness, the same plane that Viénot, Brettel and Mollon (1999) use for people with two cones, so colors that give the horse the same pair of cone signals come out identical. It is an approximation: the eye's lens is left out, each color's spectrum is a guess from its screen color, and the picture is blurred to the animal's visual acuity, only slightly for the horse, as explained below. Everything runs on your device: the camera picture, photos and videos are not uploaded.

Timney and Keil (1992) trained three horses to press their noses against one of two panels for a food reward and measured the finest detail they could resolve. The horses varied; the best reached 23.3 cycles per degree. A 2025 review by Roth and McGreevy puts the average of the three near 20/30 on the eye chart, against 20/20 for a person with normal vision. We read only the summary (abstract) of the Timney and Keil paper and the full review.

The simulator uses 23.3 cycles per degree, so the blur is slight. With the long side of the picture taken as about 70°, its width (σ of the bell-shaped curve) is about two thirds of a pixel on a photo of 2,560 pixels, the largest size here, about a third of a pixel on the camera picture of about 1,280 pixels, and about one pixel on a 4K video. Pictures under about 960 pixels on the long side are not blurred: there the blur would be under a quarter of a pixel.

How the blur is made: the simulator follows AcuityView, the method of Caves and Johnsen (2018). Fine detail is faded along a bell-shaped curve that leaves about 3% of the contrast at the animal's acuity limit, and coarse shapes and colors stay. A picture does not say how wide the scene was, so the simulator assumes that the long side of the picture covers about 70°, the view of a typical phone's main camera (the iPhone 16 main camera is 26 mm in 35 mm terms, about 69° across). If you zoomed in or cropped, the scene was narrower and the animal would see it blurrier than shown. The blur stands for the sharpest part of the eye only; the rest of the animal's view is not modelled.