Help me understand: 5mm viewing angles

Got a question for you:

what determines the viewing angle of a 5mm LED?

Some shine a very narrow beam, some a very wide one--anywhere from 15 degrees up to 60 degrees and more.

What puzzles me is this: my first thought is that the epoxy lens shapes the beam. But the physical shape of the epoxy lens is the same for all of these different angles.

(Almost all of them--there are also so-called "straw hat" LEDS which are very wide angle, and have an obviously different epoxy lens. Let's ignore those for now, since the answer is easier.)

So we have two 5mm LEDs in front of us, with what look like identical epoxy lenses, and one of them is a 15 degree beam and the other is a 60 degree beam.

What makes the difference?

Is it the tiny reflective cavity around the tiny die?
Is it the position of the die in relation to the epoxy lens? (e.g., the die is set further down for the narrower angles, and closer to the top for the wide angles--like a zoomy!)
Is it the composition of the epoxy lens? (different refractive indices make for different lensing properties?)
Some other factor?

Related question: if I hand you a pair of bare 5mm LEDs, can you look at them with your bare eye and tell me which is the 15-degree and which is the 60-degree emitter?

All information gratefully accepted--thanks!

Interesting question. I will have to study a couple of mine to check a few ideas I have based on the little bit of optics we covered in physics. Here's my attempt at applying what I remember:

- Setting the die deeper in the reflective cavity narrows the beams (as you suggested).
- A taller lens affects beam width (narrows it, I think, depends on refraction index)
- Setting the die higher or lower within the lens affects beam width (higher = wider, I think)
- A larger radius on the lens dome tends to broaden the beam (and flat tops tend to be very wide).
- Refraction tends to be the main factor defining beam pattern in these emitters, but internal reflection can also play a role, especially in straw hat lenses.
- Thicker phosphor coatings can scatter the beam more, for wider overall beam

Thanks, Lucky! Those are all good possibilities to keep in mind. Tell us if you notice any differences by comparing the 5mms that you have on hand.

Hey Lampedepeche, I didn't forget this, I've just been busy.

Comparison is between the Yuji 3200K, 45 degree beam, and the cool white, probably 15-20 degree beam emitter I swapped out of my Inova Microlight.

I could not see much difference to the naked eye, except that the radius at the end of the lens appears to be larger on Yuji, and the distance from the emitter to the dome seems about equal.

I set up my macro lens to get a close up photo. From there I could see the reflective cavity on the Yuji is larger, and possibly has a wider angle.

Most significantly the close up revealed that the Yuji has some sort of diffuser applied over cavity, while the stock Microlight LED appears to have a bare cavity. It seems safe to assume that this is the crucial difference. Let me know if the photo doesn't show up:

This makes sense as the Yuji has a smooth even colored beam with no discernible hotspot whereas the outgoing Nichia had all kinds of artifacts and a hint of an angry blue spot in the center of the hotspot up close.

MMmm...Yuji..so nice....

Lucky13--thanks so much for that!

So from your photo, it really does look as though the two epoxy domes have different curvatures. The one on the left (broader beam) is like a hemisphere on the end of a cylinder, whereas the one on the right is like a smaller hemisphere on the end of a truncated cone--the sides taper throughout the length of the epoxy molding.

I cannot see the diffusing coating in your photo, but I certainly have noticed it staring down into the LED itself. So perhaps the throwier 5mms have a reflective cavity which is more like a smooth reflector to begin with, whereas the broader-beamed 5mms have a reflective cavity that is more like an extreme orange-peel, or even that they do not use the reflective cavity to shape the beam at all.

Here's the test: can we take a handful of mixed anonymous LEDs, using these characteristics, and tell which will be narrow-beam and which will be broad-beam, before turning them on?

lampeDépêche said:

I cannot see the diffusing coating in your photo, but I certainly have noticed it staring down into the LED itself. So perhaps the throwier 5mms have a reflective cavity which is more like a smooth reflector to begin with, whereas the broader-beamed 5mms have a reflective cavity that is more like an extreme orange-peel, or even that they do not use the reflective cavity to shape the beam at all.

Click to expand...

Maybe it's not as easy to tell from the downsized image, but it's more like the cavity of the Yuji (on the left) is covered by a frosted lens, or perhaps filled up with a translucent epoxy.

Here's the test: can we take a handful of mixed anonymous LEDs, using these characteristics, and tell which will be narrow-beam and which will be broad-beam, before turning them on?

Click to expand...

I think we have enough info to stand a decent chance, but I'm not quite certain of it.

iamlucky13 said:

Maybe it's not as easy to tell from the downsized image, but it's more like the cavity of the Yuji (on the left) is covered by a frosted lens, or perhaps filled up with a translucent epoxy.

Click to expand...

Yeah, I can see it better now.

I'm sure you are right that this affects the beam-shape. But another difference may be relevant here, too:

The way that the Yuji gets its Hi CRI goodness is by having a huge amount and variety of phosphors. The reason why the other light has an angry blue tint is because it has fewer phosphors.

That is the white LED revolution: you start with an LED which is basically monochromatic, with a native wavelength way out in the blue-purple region, energetic enough to excite a variety of phosphors. Then you let the phosphors soak up a lot of the blue light and re-emit the photons in lower wave-lengths--green, yellow, orange, red, etc. The more phosphors you layer on top of the native LED die, the more you can enrich the spectral output--and the more you drop the total luminous flux. The fewer phosphors you layer on, the greater luminous flux you get, but it's predominantly that native blue-purple wavelength, the one that you see spiking off to the left in color-profiles.

So the extra white-stuff visible in the YUJI, and largely absent in the LED on the right, may have to do with beam shape (e.g. a diffuser) or it may have more to do with tint and CRI.