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How to Read LED Datasheets: the Parts Optical Engineers Care About for Illumination/Lighting Design

Simple breakdown of LED Datasheet terms important for LED lighting design.

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0:01 | Part 1

• color: warm white, cool white, neutral white

• color temperature aka CCT aka K aka Kelvin

• flux values aka lumens

2:52  Part 2

• Why don’t you get lumens at 85° for all vendors?

• test current and forward voltage

• efficiency in lumens/Watt

5:45 | Part 3

• cautions about sorting LEDs by efficiency

• CRI (color rendering index) 🌈

• metamerism 🧡❤️♥️/♥️♥️♥️

8:33 | Part 4

• more on CRI 🌈and when it’s important for machine vision

• maximum current ⚡️rating and how NOT to use it

11:27 | Part 5

•Viewing Angle – is it what you assumed it is⁉️⁉️  Probably not! Plus —

•Beam Angle 📐

•FWHM

•Lambertian Distributions

•flux 🚿

•forward voltage ⚡️

•forward current

•maximum temperature 🤒

21:02 | Part 6

•Binning! 🪣🪣🪣🪣🪣

•Flux and chromaticity 🌈 bins, and

•Do white LEDs emit UV light? 🤔

30:30 | Part 7

•Do you know LEDs warm up 🔥when they’re on?

•Do you know what happens 🙀when they warm up?

40:05 | Part 8 

•Why size matters 🐘

•Most important data for optical engineers that’s not on the datasheet! 😲 — ray files !

We got enough questions on optical engineering education that we decided to do a whole thing on it. And here it is!

Last year, our Director of Optical Engineering, Erin M. McDermott, published an article and video on SolidSmack about the basics of what optical engineering is (since so many don’t know).​

Since then we’ve been getting pinged, not just by the intended audience of other hardware engineering disciplines, but by students who want to jump into optics and photonics careers.

The latest is a student named Jung-Mundi, who on LinkedIn goes by Hexa Koo. (Huge apologies for both the pronunciation of these names and the ignorance about this individual’s gender!) Jung-Mundi gave permission to publish his/her messages to me, which follows:

For Erin’s full reply, you can watch this video below:

Some Takeaways
​

No matter what specialties you’re interested in, these tips apply to anyone interested in an engineering career.

1. PRACTICAL EXPERIENCE — whether through internships or (even better!) a comprehensive co-op program — can hugely enhance your ability to get a great job after graduation.
2. ELECTIVES IN COMPLEMENTARY DISCIPLINES — take a look not just at the classes available in your specific curriculum, but also in the offerings of complementary engineering/science tracks. If you’re interested in machine vision from the optics side, it would behoove you to learn about it from the computer science, mechanical engineering, and electrical engineering sides, too! What good stuff is available outside your major and how many electives do you have to play with?
3. STUDY ABROAD – we’d highly recommend this one for both giving you an edge in your field and broadening your network. It also says something to an employer if you can handle being routinely thrown into strange, foreign situations. (Something tells us Jung-Mundi already has some international travel under his/her belt, though…)
4. IN-PERSON CONNECTIONS – if you can, reach out to prospective professors, both online and in person! In both academics and later professional life, personal connections are super important. When people meet you and like you, they’re more likely to help you out in extraordinary ways. You also learn a lot more interesting details when you show up and ask questions in person! A LOT more. A lot. So much. Infinitely more stuff, even. Can’t emphasize this enough.

Optics and Photonics-Specific Advice

Jung-Mundi had some specific questions about optics education. He/she noted that most of the curricula she came across was biology/eyeball focused, but these weren’t the types of classes she was interested in. She wanted to play with the equipment that had nothing to do with eye health.

1. Other Degrees Not “Optical Science” Might Be Right For You – Depends on Your Aims and the Specific Universities. Jung-Mundi wondered if she was barking up the wrong tree with looking at a purely optical science B.S.. Again, though, it depends on each university’s offerings and available electives. In one university, it might be best to get their Optical Science degree. In another with a strong optical science department, it might actually be better to get an Engineering Physics or Electrical Engineering or Computer Science degree and heavily fill up electives with optics coursework. Play around with ways you can fill up each curriculum and see what matches your interests best. HOWEVER: if you want to be the guy who builds the microscope, or builds the camera, instead of the person who uses or adapts those things in other applications, you probably want the Optical Science B.S..
2. Optical Simulation Software Instruction – unless you are taking very detailed courses on optic design, you might not come out of your education with any optical simulation software proficiencies to put on your CV. So if you want to be the guy who builds the telescope, you definitely want to ask universities about this part. The more you can put on your resume, the better. However, if you have to pick just one for imaging optics, we’d recommend Zemax. This is the software you’d statistically be most likely forced to use when jumping into a new corporate job.
3. What is the “Best” University for Optical Science – in Europe, we have no idea what holds the most clout. In the United States, the reigning view is that University of Rochester is considered top (it was the first university to offer an optical science program in the United States, but is also where our Director of Engineering was born, so there’s some bias there), with University of Arizona close behind (if not equal depending on your geographical proximity), then there’s a university in Florida we’ve heard rumors of where you can also get a degree in optics. If you’re not trying to, say, specifically make camera lenses for Canon, however, this makes little difference to your career. Outside of those types of jobs, the managers looking for someone specifically coming from University X tend to simply not understand the field enough in order to come to their own conclusions about an individual’s ability. We’ve seen plenty of grads from “top” institutions who in the real world can’t deliver – whether you’re talking business school or in finding fast, creative solutions to technical problems. At Spire Starter, we value more things like: past technical accomplishments and experience, ability to find creative solutions and look at problems from new angles, flexibility, and general ability to follow-thru. If you’re smart, tenacious when fixing technical problems, and have some degree of interpersonal aptitude, those are the things that put you leaps and bounds above other engineers and scientists in our view!

Share Your Tips!

We’ll post Jung-Mundi’s question on the optical engineering forum at “ELE Optics Community” found here: https://community.eleoptics.com/

If you have other tips and feedback for this student – or other students interested in optics and photonics in general – we’d love to hear from you! This is especially so if you have some Europe-focused insight. Feel free to comment here, on the ELE Optics Community forum, or on the LinkedIn post. 

And be sure to check out the other stuff offered at the ELE Optics Community page! There is the opportunity to ask your own questions of other optics professionals, and an excellent podcast featuring the experiences of other optics pros. That’s some niche stuff right there.

Got other questions that you’d like answered in laymen’s terms? Drop us a line on the Contact Form and we’ll be happy to help you out!

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.

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).

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.

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.

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.

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