Glossary of Optics Words

This glossary is written simply, with as little confusing jargon as possible, to aid the layman's understanding. If there are more words you need explained here, please drop us a note and we'll add them!

Abbe Number

Typically pronounced "abbey" (like where monks live). Also called "V-number".

​This value describes how differently a transparent material will bend different colors of light that pass through it, aka the amount of dispersion. High Abbe numbers represent materials that have low dispersion. Lower Abbe numbers represent higher dispersion, or materials that bend red light a lot differently than than they bend blue light (and all the colors in between).

So, if you want to make a prism that separates white light into the many colors it's made of, you're better off creating that prism with a low Abbe number material. Usually, though, high dispersion is not a desirable characteristic when designing an optical system - especially in imaging optics. If Abbe number is an important factor in your system, usually you'd be looking for materials with a high Abbe number, or low dispersion.

BRDF

BRDF stands for "Bidirectional Reflectance Distribution Function". It is a mathematical description of a surface finish, or rather, how light reflects off a surface.
If a surface is pretty evenly rough, you can usually get a BRDF measurement of it and create an optical model from that. Every optical simulation software has its own method of importing and formatting these models and usually, you'll have to have a custom model made for each optical software being used and for each material. Once you have a working model in your optical software, you can simulate the surface texture in your simulations, which is often super important.

CCT - Correlated Color Temperature

This is a description of light color in units of Kelvin. Lower Kelvin means yellower or more red/orangey color. Higher Kelvin means bluer color. See longer article on CCT HERE.

Diffuse

This term can refer to the surface finish of a material. If it's not specular (see below), it's probably diffuse. Diffuse means a rougher surface than, say, a mirror and a rough surface means that light coming it to hit that surface will encounter lots of different incident angles. So then, when it bounces off the surface, it will be leaving at all sorts of angles.

If you think about a small, red laser beam hitting a specular mirror surface, that beam is still going to be a small dot of light when it leaves. On the other hand, if your small, red laser beam hits a diffuse surface like sand paper, that small red dot is going to spread out into a bigger blob. The light rays going onto the sandpaper would be closely similar, and the light rays leaving the sandpaper would be shooting in all sorts of directions.

Diffuser

Diffuser usually refers to a sheet of clear, diffusing material. As the term "diffuse" would suggest, a diffuser makes angles of light shoot out in a bunch of random directions. Another way of saying this is that it "scatters" light. Diffusers are often sheets of clear material, but they can also be custom created by adding a textured surface to an already existing part.

If the a lit appearance has lots of unevenness -- dark and bright parts next to eachother, or color variation -- a diffuser can help even that out. Or, if you are using the diffuser in an illumination application and you want to light up a bigger area in front of your lighting fixture (larger beam angle), diffusers can do that, too.

For many commercial off-the-shelf diffusing materials, there are mathematical models that can be imported into optical simulation software. So, if you want to add a diffuser to your hardware, you can see the effect it would have in an optical simulation. This can be done before you select and buy the diffuser you want (there are many to choose from) and build a prototype.

Flux Capacitor

We are sad to report this is not actually a thing.

Fresnel Lens

This word is French and gets mispronounced a lot. FYI: it's pronounced "freh-nell" -- the "s" is silent.
​
This is a lens that gets cut into segments and smushed down to a thinner lens. These were famously used in lighthouses that needed super heavy, gargantuan lenses to shine big lights far out into foggy waters. By going to a Fresnel lens shape instead, lighthouse lenses could be made more thinly and of lighter weight.

Imaging Optics

Imaging optics is a category encompassing most optical systems that produce an image. Cameras, microscopes, telescopes, etc. all get dumped in the imaging optics bucket.

Index of Refraction

In optical equations, this shows up as "n". Different materials usually have different indices of refraction. As a light ray goes from one material to another, the amount the light ray bends will be determined by how different the "n" of the first material is from the "n" of the second material. If you don't want the light to bend as much, you'd look for materials that have similar or matching indices of refraction.

Lambertian

If you heard this word, it could have been in regard to either a light source (Lambertian emitter) or a reflective surface (Lambertian reflectance). In either case, Lambertian means light rays are going evenly in all directions (kind of). So no matter if your eye is at 0 degrees to the thing, or 80 degrees to the thing, it looks equally bright.

What we perceive as brightness has to do with both a.) how much light gets to our eyes and b.) how much surface area we see. With Lambertian things, although the surface area your eye sees changes as you change your viewing angle, the amount of light you see changes too, in a proportional way.

The true definition has to do trigonometry and with the dividing an amount of light by the surface area it comes from. We won't get into that here, though, because this description is for laymen, and you frankly probably don't need to care that much. If you do care that much, look up Lambert's cosine law for further reading.

A white sheet of matte (not shiny) paper is a good way of visualizing Lambertian scatter. For being an everyday object, it's pretty close to a material with a truly Lambertian reflectance! Blocks of barium sulfate are frequently used for calibrating lab equipment because its reflectance is close to perfectly Lambertian, plus it reflects a super high percentage of light that hits it (usually at least 97-98% depending on wavelength, surface finish, etc.).

Truly Lambertian emitters are harder to find an everyday approximation of. One of these would give off light the same way a white piece of paper reflects light. If you look at the white part of some lit McDonald's signs, this might be as close as we can get. The white letters would be about the same brightness whether you were in front of the sign looking at it, or standing to the right side of it, or the left side of it, or below it.

Light Guide

A light guide is a little more complicated and a lot harder to design than a "light pipe" (see below). Both these structures bring light from an LED in one place to an observer's view in another place. The difference is, with a light guide, you're usually trying to spread the light out in a larger pattern than you would with a light pipe going to a tiny indicator button. Fancy DRL's (or daytime running lights) on cars and trucks would be one place you might find these. Commercial displays can be another, or anywhere you want an even glow of light across a larger-than-button sized surface.

Like light pipes, light guides can (and should) be designed with optical simulation software. Light guides are a lot harder to design well! Often the optical engineer introduces a bunch of carefully calculated slivers of light leakage  along the light guide. These small features need to be balanced with the larger curves of the overall shape to keep the appearance evenly lit. Like a light pipe, light guides are also usually molded from a clear plastic.

Light Pipe

Usually made of a clear plastic material, light pipes "pipe" light from a source (usually an LED) to a visible part of hardware. You'll find these behind indicator lights on hardware. Often the outside part of your hardware where you need an indicator light is far away from the circuit board where your LED is mounted, and you need to get the light from the board to the viewer outside.

Light pipes can be designed and simulated in optical software to find the optimal shape that brings the most light to the outside. They can be "tested out" in the software before anything is ever prototyped in real life. With this optical engineering you save time and money in HW R&D stages!

Matte

Matte means not shiny. It's a rough surface compared to specular mirrors. You won't be able to see your image in a matte surface. Matte things would also be said to have diffuse reflectance.

MTF

Aka “Modulation Transfer Function”

MTF is a term describing how well an imaging system can reproduce contrast as details in the image get smaller and smaller (as far as you, the layman, is concerned). It’s part of a more comprehensive description of imaging quality called “OTF” (optical transfer function) -- see below. In a chart of MTF, typically, the higher the line rides on the Y-axis, the better the MTF.

See longer article for more information: HERE.

Nonimaging Optics

Nonimaging optics describes all the other optical systems out there that aren't cameras, microscopes, telescopes, etc. Things like lasers, optical sensors, illumination or lighting, fiber optics, and solar collectors all fit in the nonimaging optics category.

OTF

Aka “Optical Transfer Function”

OTF describes how well an imaging system creates an image. Part of OTF includes MTF, dealing with contrast and resolution (see MTF definition above). MTF answers the question of: how black is black and how white is white in small details on an image? Another part of OTF deals with how an imaging system messes with phase in the “PTF” or phase transfer function. If phase gets messed up, an image can show a shift in the position of patterns it takes an image of.

See longer article for more information: HERE.

Photometric

Photometric is a descriptor for measurements involved in photometry. When you're dealing with light, you'll either be looking at photometric or radiometric (see below) measurements.

There are two different measurements because the way people see light is a lot different than how machines see light. Our brains weight colors differently. The same amount of light from different wavelengths can look like they have totally different brightness. Our eyeballs love the color green so compared to red light, we make green light look brighter. To get a red light and green light on a Christmas tree to look the same brightness to our eyeballs, we have to crank the red light up.

To account for how our eyeballs change things, we change measurement machines to work the same way as our eyeballs. When we do that, we're going from radiometric measurements to photometric measurements.

Put another way: if the red and green Christmas lights looked the same to photometric equipment (and our eyeballs), the radiometric equipment would tell you the red light was much brighter than the green.

All the unit names for photometric measurements get changed, too. When you're dealing with photometry, the most common units you'll use are luminous flux measured in lumens, and luminous intensity measured in candela.

So, if you're working on something where it matters how a human eye is going to see it, you want to make sure you're working in photometric units.

Polarization

Polarization is a way of describing how light travels through space as a wave. Light waves can travel in circles or lines, to the right or to the left . . . Or, light can be composed of a bunch of different kinds of polarization, in which case the light is said to be "unpolarized" or "partially polarized".

Polarizing filters or polarizers can be put in a light path to only let light of a specific polarization through. The classic example of these filters is polarized sunglasses that block out glare from sun that bounces off shiny objects. The glare from sunlight that bounces off would be mostly a different polarization than what the sunglasses let pass through to your eyes.

Radiometric

As mentioned under "photometric" (above), there are 2 different ways of measuring light. One is how a human observer sees light, and the other is how a machine would detect light if it wasn't messed with. Radiometric deals with the measurements of light you take where you don't care how a human would see it.

So, for example, any sort of infrared (IR) measurements are going to be radiometric, because humans can't see IR light! 

Most sensors, high-powered lasers where you're melting metal, any other wavelengths outside the visible spectrum are usually measured with radiometric units. For those, you'll be using radiant flux in Watts and radiant intensity in Watts per steradian.

Reflex

Reflex is the term used to describe the optical structures on those plastic, colored reflectors you see on cars and bicycles. Yeah, that has a name! And you know it now.

Specular

Specular is just a fancy way of saying shiny. It means a surface is not rough, but smooth and reflective like a mirror.

A higher level description is that the light that hits a specular surface doesn't reflect in a bunch of directions; light bounces off a surface (reflected ray) at the same angle as it hits the surface (incident angle). Of course, perfectly specular surfaces don't typically exist in reality . . . but they're often fine approximations in simulations (depending on application).

Stray Light

Stray light is a problem that pops up in all sorts of optical systems. This is any light that goes where you don't want it to.

​This could cause glare in illumination systems, or it can degrade image quality in imaging systems. Stray laser light can cause some serious damage depending on how powerful the laser is! In automotive lighting, stray light can cause headlamps to fail government inspections. In sensor systems, sometimes stray light can come from something totally outside the optical system you design -- like ambient light coming through a window.

​Optical simulations can be used to figure out where offending stray light rays come from and bounce to. Simulations can also be used to figure out the best fix for stray light issues -- trying out different solutions before anything is even  prototyped.