On Light

Light is the only thing the eye was built for. Everything else — the eyelid, the cornea, the lens, the intricate hydraulics of aqueous humour — exists in service of a single transaction: photons strike photoreceptors, chemistry becomes electricity, electricity becomes experience. It is, when you reduce it far enough, a very old trick. Organisms have been doing some version of it for five hundred million years. The earliest light-sensitive structures were probably simple patches of photoreceptive cells — barely more than a chemical alarm that told the organism which way the sun was. The vertebrate eye, with its lens and focused image, arrived much later. Evolution kept refining the same basic idea for half a billion years, which is either reassuring or unsettling depending on your mood.

What I find strange, still, after years of practice, is how little of this transaction is visible to the person experiencing it. You do not feel your photoreceptors bleaching. You do not notice the signal travelling from retina to lateral geniculate nucleus to primary visual cortex. You see a chair, a face, a sky the colour of old pewter. The machinery is perfectly hidden behind the result.

What painters knew first

Long before there was a science of vision, painters were conducting experiments on it. They noticed things that took physiologists centuries to confirm: that shadows are not simply the absence of light but are tinged with colour; that the same grey looks different against white than against black; that the eye is drawn to contrast before it is drawn to anything else. Simultaneous contrast — the phenomenon by which a neutral grey appears lighter or darker depending on its surround — was described by Michel Eugène Chevreul in 1839, but painters had been exploiting it intuitively for much longer. Delacroix kept notes on it. The Impressionists built an entire movement around it.

This is not a coincidence. Painters are, among other things, professional observers of light — people who have trained themselves to see what the eye actually receives rather than what the brain has decided is there. The brain is an aggressive editor. It normalises colour, fills in gaps, suppresses flicker. Painters learn, slowly and with difficulty, to undo some of that editing. To see the raw material rather than the finished interpretation.

The ophthalmologist learns something similar, but from the other end. You study the machinery that produces the experience, until the experience itself starts to look different.

The quality of light

There is a moment, in the early morning, when the light through a north-facing window is as close to neutral as natural light gets — cool, flat, shadowless. Artists have prized north light for exactly this reason: it shows colour without distortion and changes slowly enough to work by.

Clinically, I think about light quality too, though in a different register. The wavelength of your examination light affects what you can see. The slit lamp — that elegant instrument, essentially a microscope crossed with a searchlight — can be adjusted in angle, width, and filter to reveal different structures. A narrow beam catches corneal haze that a broad beam misses. The red-free filter makes blood vessels on the retina stand out like cracks in old porcelain. Red-free light, which blocks the red end of the spectrum, makes haemoglobin appear nearly black against the pale background of the retinal nerve fibre layer. This is useful for detecting haemorrhages and for assessing the nerve fibre layer itself, which thins in glaucoma often before other signs appear. The technique is old but the principle is exact.

You are always choosing what kind of light to look with. The choice shapes what you find.

On darkness

The eye adapts to darkness with a kind of patience that feels almost moral. In full darkness, after about thirty minutes, your rod photoreceptors are operating at something close to their theoretical maximum sensitivity — capable, in principle, of detecting a single photon. Dark adaptation proceeds in two phases: a rapid initial phase driven by cone photoreceptors, followed by a slower rod-driven phase that takes twenty to thirty minutes to complete. This is why stepping out of a bright room into darkness is disorienting for only a few minutes, while achieving full night vision takes much longer. Sailors on night watch used to wear red goggles below deck to preserve their dark adaptation. Red light does not bleach rhodopsin.

I think about this when patients complain of difficulty driving at night — one of the earliest symptoms of several retinal conditions. The system that evolved to find a path through darkness is the same system that fails first when the retina is under stress. It is both the most sensitive part of the visual apparatus and the most fragile.

Light, it turns out, is easy. Darkness is the hard part.

Yours truly,
Coditis