where is optical center of an LED with a typical lens - Page 1
Forum section: electronics / general

author: ballen
author_rank: Regular Contributor
timestamp: January 01, 2025, 11:22:50 pm
content: I've got some 1W green LEDs, mounted on standard 20mm diameter "star" aluminum PCB carriers. These LEDs have an 8mm diameter white package with a transparent plastic dome-shaped lens on top, about 5mm in diameter.I am using some of these as point sources for an autocollimator, and would like to know, where is the optical center of that point source located? Since the lens is collimating the light, I believe that this optical center will lie on the axis of the LED, but slightlyBEHINDthe physical chip. Can someone confirm or deny? Is there any documentation (from any manufacturer) which indicates where this optical center is located?[Mods: if this is the wrong forum, please move to the correct location.]

author: Someone
author_rank: Super Contributor
timestamp: January 01, 2025, 11:58:09 pm
content: Most power LEDs have dies so large they are a poor approximation of a point source.

author: Simmed
author_rank: Frequent Contributor
timestamp: January 02, 2025, 02:56:13 am
content: Quote from: ballen on January 01, 2025, 11:22:50 pmare you talking about documentation like this ?indication of the center ?

author: ballen
author_rank: Regular Contributor
timestamp: January 02, 2025, 09:25:46 am
content: Quote from: Someone on January 01, 2025, 11:58:09 pmIn my case, the LED is replacing a lamp filament whose size is comparable to the LED die. So my question stands: if I trace back the light rays to the source, where (in relation to the LED mount plane) do those converge?

author: ballen
author_rank: Regular Contributor
timestamp: January 02, 2025, 09:28:38 am
content: Quote from: Simmed on January 02, 2025, 02:56:13 amYes, that's the idea. Unfortunately the document which you display only shows the plane which is the origin of the optical model. I'm fairly sure that this does not correspond to the location which the rays diverge from. Note that the answer depends upon the specific form of the lens which is mounted over the LED.

author: tooki
author_rank: Super Contributor
timestamp: January 02, 2025, 09:32:22 am
content: Quote from: ballen on January 02, 2025, 09:28:38 amWhy do you make that assumption? It sure looks to me like it’s that point. And using my fingers as a compass, it sure looks to me like the origin shown corresponds to the center of the lens, too.Regardless, the answer will depend on your exact LED, so check its datasheet. If you have LEDs without a datasheet, then replace them with ones that do, if the application is so critical.

author: magic
author_rank: Super Contributor
timestamp: January 02, 2025, 09:32:53 am
content: Not sure what you mean. If the light is collimated then it appears to come from infinity behind the lens.

author: ballen
author_rank: Regular Contributor
timestamp: January 02, 2025, 05:27:02 pm
content: Quote from: magic on January 02, 2025, 09:32:53 amYou are correct that if the light is collimated into a parallel beam, then it appears to come from a point at infinity behind the lens. However, my understanding is that the lens in front of the LED is there to provide an impedance match between the index of refraction of the silicon die and the index of refraction of air. This lens causes the rays to diverge in a cone which is narrower than 2 pi steradians, but it does not collimate the rays into a parallel beam. Hence the rays to intersect, presumably some distance behind the LED. I'd like to know where that point lies.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 02, 2025, 05:31:34 pm
content: The position depends on the LED type. It can be still insider the case, but also well behind. The datasheet may tell if the LEDs are made to be used with an external lense.

author: ballen
author_rank: Regular Contributor
timestamp: January 02, 2025, 05:33:25 pm
content: Quote from: tooki on January 02, 2025, 09:32:22 amBecause the point identified as the "origin of the optical model" lies at the location of the chip. The lens is convex and converges the rays, so the virtual source must lie behind the chip. I believe that the point identified as the "origin of the optical model" is simply an arbitrarily defined origin of coordinates, nothing more.Yes, the answer will depend upon my exact LED. I have looked at its datasheet and not found what I was looking for. Hence, my question to the group.

author: ballen
author_rank: Regular Contributor
timestamp: January 02, 2025, 05:36:41 pm
content: Quote from: Kleinstein on January 02, 2025, 05:31:34 pmYes, I agree with this.The LED I am talking about is supplied from the manufacturer with a pre-installed lens. The datasheet does not indicate the location of the effective point source. I was hoping that someone here could tell me where to find that information.

author: Someone
author_rank: Super Contributor
timestamp: January 02, 2025, 08:46:30 pm
content: Quote from: ballen on January 02, 2025, 05:36:41 pmBecause there is no "effective" point source. There may be an origin for the optical model but that is typically a complex model and not your wished for point source.

author: thm_w
author_rank: Super Contributor
timestamp: January 03, 2025, 12:53:59 am
content: Quote from: ballen on January 02, 2025, 05:36:41 pmYou haven't provided a manufacturer part number, so how do we know anything about it? You could always ask the company.Or are you referring to a generic LED from aliexpress, in that case it might be better to try to find some way to test it yourself.