Flash photography of cats produces glowing eyes. This is familiar enough that most people do not find it strange. I started thinking about it after photographing our cats at dusk — through the doorway; they are indoor cats now, for health reasons — and seeing green from one angle and gold from another.
The eye contains two optical structures relevant to dim light: a pupil whose area can vary unusually far, and a reflective layer behind the retina. They solve different problems. The evidence for each also has a different shape.
The Slit as a Variable Aperture
The pupil controls retinal illumination approximately in proportion to its area, subject to the optics and transmission of the rest of the eye. Banks and colleagues reported that the domestic cat’s vertical pupil changes area by a factor of about 135 between constricted and dilated states. Their comparison for the human circular pupil was about 15, not an exact constant for every eye.
Thus the supported comparison is simply
$$\frac{A_{\max}}{A_{\min}}\approx135$$for the cat measurements cited by Banks et al. It should not be reconstructed from an unsourced rectangle with invented pupil dimensions, and it is not the entire dynamic range of vision. Photoreceptor adaptation and retinal processing remain essential across changing light levels.
The slit also changes image quality anisotropically. A narrow aperture increases depth of field in the direction across its narrow dimension while leaving a different blur cue in the perpendicular direction. This motivates a functional question: why are vertical pupils common in some predators but not others?
What the Comparative Study Found
Banks et al. classified 214 terrestrial species and found a strong association between pupil shape, activity pattern, and foraging ecology. Vertically elongated pupils were common among ambush predators active by day and night; the association was strongest for predators with eyes close to the ground. Horizontally elongated pupils were common among laterally eyed prey animals.
Their optical models supply plausible functions. In a forward-facing predator, a vertical pupil can preserve sharp vertical contours for stereopsis while allowing horizontal defocus to carry distance information. In grazing prey, a horizontal pupil supports a panoramic view near the ground; the studied animals counter-rotate their eyes as the head lowers.
This is comparative and model-based evidence, not a recording of the historical selection path of the domestic cat. The 42-centimetre shoulder-height split in the paper is a statistical feature of the analysed species, not a biological law. Lions, house cats, snakes, and geckos do not receive their pupils from a single rule with no exceptions.
The Reflector Behind the Retina
Many nocturnal and crepuscular vertebrates have a tapetum lucidum behind the retina. Light not absorbed on its first passage can be reflected through the photoreceptor layer again. That second passage can increase the chance of absorption, though it can also trade spatial precision against sensitivity.
Comparative anatomy shows that tapeta are not one universal material. The cat has a cellular tapetum containing ordered intracellular rodlets. Older chemical and microscopy studies identify riboflavin-associated structures in the feline tapetum; other animals use different cellular or crystalline arrangements. Ollivier et al. review this diversity across selected species.
That organisation makes interference and scattering relevant, but it does not license a single perfect-stack calculation for every cat. Domain orientation, pigment, absorption, disorder, incidence angle, wavelength, age, and individual anatomy affect the measured reflection. Classic in-situ measurements of the cat tapetum found a wavelength-dependent spectrum rather than an ideal mirror.
What the Thin-Film Equation Can Show
For a plane-parallel layer of refractive index $n$ and thickness $d$, a common normal-incidence interference condition is
$$2nd=m\lambda,$$subject to the phase shifts at the interfaces. A periodic stack can reinforce a band of wavelengths when the optical thicknesses and interface phases align. This is the same family of physics used in dielectric mirrors.
The equation explains why subwavelength periodic structure can select colour and why changing the incidence angle can change a reflected spectrum. It does not, by itself, establish the layer thickness, refractive index, number of coherent layers, or peak wavelength of a cat tapetum. Those are measurements, not values to choose until green appears from the arithmetic.
For the same reason, the ideal lossless-stack expression
$$R=\left(\frac{1-(n_2/n_1)^{2N}}{1+(n_2/n_1)^{2N}}\right)^2$$is useful as an optical limiting model but not as a feline reflectance result. Substituting assumed indices and $N$ can make $R$ approach one because the model contains no absorption, scattering, or disorder. The previous version of this article reported that limit as though a cat eye had been measured. It had not.
A Second Pass, Without Invented Efficiency
If a photon has probability $\eta$ of absorption on one pass and the tapetum returns a fraction $R$ of the unabsorbed light through equivalent tissue, a minimal two-pass model gives
$$\eta_{\mathrm{two\ pass}}=\eta+(1-\eta)R\eta.$$The formula states the mechanism cleanly. It is conditional on independence and equivalent passes, and it needs measured $R$ and $\eta$ before yielding a cat-specific number. Values such as 25% absorption, 98% reflectance, a 1.7-fold gain, or an absolute lux threshold were not supported by the cited sources and have therefore been removed.
Eyeshine also should not be reduced to one Bragg angle. The observed colour can depend on the tapetal spectrum, illumination and viewing geometry, ocular pigmentation, and camera response. My green-to-gold photograph is an observation; a complete spectral explanation would require measurements from that eye and setup.
Percy Shaw’s Different Cat’s Eye
Percy Shaw applied in 1934 for British patent GB436,290 concerning a reflecting road stud. The later Catseye name makes the biological comparison explicit, although familiar stories about the exact moment of inspiration are difficult to verify from the patent itself.
The road device and the eye do not use the same reflector. A road stud uses a lens-and-reflector geometry to return incident headlamp light toward its source; modern retroreflectors may instead use corner cubes. For three ideal mutually perpendicular reflections, the outgoing direction is reversed:
$$\hat v_{\mathrm{out}}=-\hat v_{\mathrm{in}}.$$The feline eye obtains return light through the combined optics of the eye and the tapetum. The engineering device borrowed the name and function, not a stack of feline cells.
What Remains
The robust numerical surprise is the pupil: about a 135-fold area change in the measurements cited by Banks et al. The robust anatomical surprise is the reflector: an ordered cellular structure gives transmitted photons another pass through the retina. Between those observations and a species-wide claim of near-perfect, green-tuned reflectance lies the work of optical measurement.
When our cats’ eyes shine at dusk, I am seeing returned light from that optical system. The colour is real. Its explanation is not a licence to tune the parameters after the photograph.
Literature checked through 2026-07-11.
References
Banks, M. S., Sprague, W. W., Schmoll, J., Parnell, J. A. Q., & Love, G. D. (2015). Why do animal eyes have pupils of different shapes? Science Advances, 1(7), e1500391. https://doi.org/10.1126/sciadv.1500391
Ollivier, F. J., Samuelson, D. A., Brooks, D. E., Lewis, P. A., Kallberg, M. E., & Komáromy, A. M. (2004). Comparative morphology of the tapetum lucidum (among selected species). Veterinary Ophthalmology, 7(1), 11–22. https://doi.org/10.1111/j.1463-5224.2004.00318.x
Weale, R. A. (1953). The spectral reflectivity of the cat’s tapetum measured in situ. The Journal of Physiology, 119(1), 30–42.
Born, M., & Wolf, E. (1999). Principles of Optics (7th ed.). Cambridge University Press.
Shaw, P. (1934). Improvements in Studs for Roads and like Surfaces. British patent GB436,290, application filed 3 April 1934.
Changelog
- 2025-12-15: Corrected the adoption date of Percy Shaw’s road Catseyes and removed an unverified reference.
- 2026-07-11: Retained the sourced 135-fold pupil result, bounded the ecology inference, and replaced unsupported cat-specific reflectance, sensitivity, and eyeshine calculations with explicitly ideal optical models.