What’s in a Name? The Optical Words We Use Every Day

What’s in a name? More to the point, what’s in a name for optical terms?

Specifically, the words we use every day as lens designers. Have you ever stopped to wonder where they came from? It would be a shame to ignore the reasons behind the words and terms we use every day, because the etymology of optical terminology is genuinely fascinating. These aren’t arbitrary labels someone invented at a committee meeting. They’re words with history, with roots in Latin and Greek, with little stories baked into them.

I noticed this a few years ago while writing one of my long pieces, the Ultimate Guide to Lens Design Forms. It’s a massive piece of text and images, so I take breaks in the form of “Coffee Breaks” to explore terms and tidbits, usually from the history of lens design. Somewhere in the middle of writing those asides, I started paying attention to the bones of words, the pieces they’re built from. And once my curiosity got the better of me, I couldn’t stop. I started looking at the words I use every day in optics and thinking: wait, where did that come from?

Turns out, our field is full of these.

1. Lens

We’ll start with the most obvious one.

The word lens comes from the Latin lens, which means lentil. As in, the legume. The little flat round thing you put in soup. I had some the other day. It’s a good source of protein.

Early lenses were named after lentils because they looked like them: biconvex, roughly circular, flattened in the middle. Someone held up a piece of shaped glass, looked at a lentil, and said: yes, that. And the name stuck. For hundreds of years. I wonder what would have happened if, on a whim, we’d used the chickpea instead.

I find this enormously charming. We design highly sophisticated optical systems for satellites, lithography machines, and augmented reality headsets, and the fundamental building block of our work is named after a salad ingredient.

2. Focus

You knew this word was going to come up early. The Latin focus means hearth, or fireplace.

Think about that for a moment. The point where rays converge, where the image forms, was named after the place where fire gathers, back in ancient Rome. It’s a beautiful metaphor. A hearth is where heat concentrates, where energy collects, where things come together. It’s also the center of a home, the point everything else organizes around.

Johannes Kepler is usually credited with first using focus in the optical sense, in his Astronomiae Pars Optica in 1604. He was describing the point where a lens brings sunlight to a convergence, and reached for the word that meant the place where fire comes together, as a metaphor. I think he made the right call.

Every time we talk about back focal distance, front focal length, focal plane, or depth of focus, we’re using a word that once meant fireplace. I’m going to think about that every time I open Zemax from now on.

3. Mirror

From Latin mirari, meaning “to wonder at” or “to admire.” So if you’re staring at a mirror, you are admiring yourself in an admiring device. Got it.

The root mirari also gives us mirus, meaning wonderful or remarkable, and is the root of the word miracle. Interestingly, mirror and smile are distant linguistic relatives. Try to picture the alternative: someone gazing at a polished flat rock, or at their own reflection in a pool of water. Not easy to replicate on demand. By the 13th century, flat shiny mirrors were an extreme luxury item, affordable only to royalty. The object was wanted not just for what it did, but for the emotional reaction it caused.

In optics, mirrors are used to steer laser beams in the most advanced lithography machines, as wavelengths have gotten shorter and shorter to accommodate the Rayleigh-Sommerfeld diffraction integral. Mirrors are also used in space telescopes such as the Hubble Space Telescope and the James Webb Space Telescope, where the surface roughness of these mirrors averages 2 to 4 nm RMS. That’s akin to the size of a golf ball, if the mirror itself were the size of a continent.

4. Aperture

From Latin apertura, meaning opening.

From the same root as aperire, to open or to uncover, which is also where we get the word aperitif, the drink you have before dinner to “open” the appetite. So next time someone asks why you’re having a pre-dinner drink, you can tell them you’re doing aperture research, and enjoy the blank stares that follow.

In optics, of course, aperture is exactly what it says: the opening through which light passes. Nothing metaphorical here, nothing poetic. Just a very accurate Latin description of a hole. Sometimes the best name is the obvious one, and every imaging system has to have one.

5. Spectrum

Latin spectrum, meaning appearance, image, or apparition.

Newton used this word when he described what happened when sunlight passed through a prism and spread into its component colors. He saw something that appeared: a vision, a display, a spectacle. The word spectre (as in ghost) comes from the same root. So does spectacle, and spectacular, and inspect.

I stopped and thought about this word for a while. A spectrum is light made visible to the human eye in a way it normally isn’t: revealed, made to appear. An apparition of color. Newton’s use of the word is more poetic than it might first seem. Might there be an intersection of art and science hiding inside optical terms generally? I think so.

6. Aberration

From Latin aberrare, to wander away from, to stray.

Ab (away from) plus errare (to wander, to err). The same root gives us error, erratic, and, yes, errant, as in a knight-errant, someone wandering without a fixed path.

An aberration, in optics, is a ray that has wandered away from where it should be. It has erred. It has gone off the path. I’ve always thought this was a deeply satisfying word for what it describes, because anyone who has stared at a spot diagram full of aberrated rays knows exactly what wandering looks like.

We spend most of our careers trying to stop rays from wandering. We are, in a sense, shepherds of light.

7. Refraction and Diffraction

Both from Latin roots involving breaking.

Refraction comes from refringere, to break back. When light crosses a boundary between two media, it bends. The Romans would have said it breaks. Diffraction comes from diffringere, to break apart, to shatter. When light encounters an edge or an aperture comparable in size to its wavelength, it spreads. Shatters, you might say.

What I find interesting here is how physical these words are. The Romans didn’t have wave equations. They didn’t have Snell’s Law. But they had a good intuition for what was happening, something bending, something breaking, and they named it accordingly. Two thousand years later, we’re still using their vocabulary.

8. Telephoto and Retrofocus

Telephoto is Greek: tele (far) plus phos/photo (light). Far light. A lens that brings distant things close.

Retrofocus is the conceptual reverse: a design where the rear principal plane sits in front of the front element, allowing a long back focal distance with a short focal length. The name was actually a brand name originally, coined by Pierre Angénieux in the 1950s for his retrofocus wide-angle design. Like Kodak and a few other names we’ll get to another time, it was too useful a descriptor not to become the generic term.

9. “A”

Yes, just a simple “a.” From Greek: a- (without).

The “a” shows up all over the place in optics: achromatic, apochromatic, aspherical, asymmetric, aplanatic, afocal, athermal, astigmatism, and my favourite, anastigmat.

Achromatic means, literally, without color: a lens corrected so that two wavelengths come to the same focus, reducing chromatic aberration. Apochromatic goes further, apo- (away from) plus chroma, a lens corrected for three wavelengths, with the secondary spectrum also reduced.

The Leica APO-Summicron-M 50mm f/2 ASPH, one of the most celebrated lenses of the modern era, has apo right there in the name, because Peter Karbe and his team earned the right to put it there. When you see APO on a lens, you’re reading Greek.

Astigmatism is an aberration where the focus on the tangential plane and the sagittal plane land in different places. It’s a common defect in human vision too, which is why we have astigmatism-corrected lenses for glasses. In lens design, we want to correct this aberration.

The first lens to correct astigmatism, along with spherical aberration and coma, was called the anastigmat. I love this name. It’s literally a double negative. Ana also means without. An anastigmat is a non-astigmatic lens, which is a non-non-stigmatic lens. (Couldn’t call it a “Stigmat,” I suppose.)

10. Pencil of Rays

I saved this one for last, for obvious reasons.

Pencil of Rays describes a bundle of light lines that all share a common vertex, or that pass through the same point.

The word pencil traces back to the Latin penicillus, meaning “little tail,” or more specifically, a fine painter’s brush made from camel hair. (The same root, incidentally, gives us penicillin. Alexander Fleming named the antibiotic after the brush-like shape of the mold. So in a roundabout way, this lens design blog shares etymology with a life-saving drug. Go figure.)

In the early Renaissance, mathematicians and draftsmen began using pencil to describe a collection of straight lines or visual rays converging from an object to a painter’s eye. The image was apt: those fine bristles at the tip of a brush, gathering to a sharp point. In 1639, the French mathematician Girard Desargues formalized the concept in projective geometry, describing a group of lines intersecting at a single point as an ordonnance de lignes, an order of lines. When those ideas were translated and expanded, the visual analogy of converging lines won out, and “pencil” stuck.

By 1675, a pencil of rays was being described as a double cone of light joined at its base: either converging from a wide area down to a focused image point, or diverging outward from a single point source. Two cones, tip to tip. A shape any optical designer would recognize immediately.

I named this website Pencil of Rays because that image captures something I genuinely love about geometrical optics. The elegance of rays converging to a point. The idea that light, properly disciplined, can be made to gather, just like the bristles of a fine brush, just like the lines on a draftsman’s page. There’s a directness to it that I find satisfying. No wave equations. No quantum mechanics. Just rays, converging, doing what we ask them to do.

Also, it was available as a domain name. But mostly the other thing.

The Bigger Picture

Here’s what I notice looking at this list.

The classical optical terms, lens, focus, mirror, aperture, spectrum, aberration, refraction, pencil of rays, were all named by people trying to describe what they saw happening in front of them. They reached for existing words that already meant something, words with physical or metaphorical weight. A lentil. A hearth. A source of wonder. An opening. An apparition. A wandering. A painter’s brush.

They weren’t naming abstractions. They were naming behaviors. And because they chose well, those names have lasted for centuries, through the invention of photography, the laser, the CCD, the smartphone camera, and the ASPH surface. We still use them every day.

There’s a lesson in there somewhere about good naming. Name the thing you see, not the thing you theorize. And name it in a way that someone who has never seen it will understand immediately.

A hearth. A lentil. A little tail.

Not bad for a few thousand years of work.