How is it possible to measure an IR LED source in mW/sr?

[Valentin]

TL;DR

How is it possible to measure an IR LED source in mW/sr?

Hello everyone,
I need to make measurements of a collimated infrared light source at different angles, practically to create an intensity map, all this in mW/sr.
I can't find any equipment able to measure in mW/sr, while there are plenty of instruments capable to measure in mW/cm^2.
As I understand, there is no conversion from mW/cm^2 to mW/sr.
Any solutions or advice?

 

[DaveE]

To convert from mW/cm² to mW/sr, you would have to know the distance from the source to the detector, your instrument probably doesn't know that. OTOH, I don't see why you can't do the conversion yourself.

 

[Valentin]

To convert from mW/cm² to mW/sr, you would have to know the distance from the source to the detector, your instrument probably doesn't know that. OTOH, I don't see why you can't do the conversion yourself.

This is not a problem, I can place the sensor at a known distance, say 1m.
I assume that also the sensor's diameter/surface is a factor?

 

[hutchphd]

This conversion method assumes the source is small compared to 1m and fills the detector. Try a few distances if you need to be sure.

 

[Valentin]

This conversion method assumes the source is small compared to 1m and fills the detector. Try a few distances if you need to be sure.

The source is an IR lambertian LED, collimated to an oval beam of 20/40°. I need to measure the radiant intensity at several points/coordinates. Many sensors are 1cm diameter so I believe it fills the sensor's surface.
The question is if there is any instrument capable of measuring directly in mW/sr.

 

[DaveE]

Have you tried google search yet?
https://en.wikipedia.org/wiki/Steradian

 

[Valentin]

Have you tried google search yet?
https://en.wikipedia.org/wiki/Steradian

Never thought about it, thanks for the "illumination".
I asked a simple question, if anyone knows if there is any instrument capable to measure W/sr, I don't need a lecture about what a steradian is or ways to go around with conversions.

To explain my point, we have at work a goniophotometer we use to test automotive lamps.
It has sensors at 3.16, 10 and 25m capable to measure lux or cd.
I install the lamp in the machine and it runs the tests by moving to the correct coordinates and measuring the intensity at each point.
I am looking for something similar but in mW/sr, the automatic part is not necessary, only a meter capable to measure in these units. I simply find it hard to believe that such thing doesn't exists.

 

[hutchphd]

For each distance you can derive a single conversion number. This will contain conversion from an area to solid angle. It will, for your measurement, also need to convert from photometric to radiometric units (this is wavelength dependent).

 

[DaveE]

I don't need a lecture about what a steradian is or ways to go around with conversions.

Sorry, I didn't mean to be demeaning. I was just trying to help. This was in response to your question about whether the size of the detector mattered (which BTW, yes).

I assume that also the sensor's diameter/surface is a factor?

So I assumed that you might not be familiar with how steradians are defined.

Good luck, I hope you find the instrument you seek. I don't know of anything like that. Most instruments give you the data they actually measure in what ever units make the most sense for the most users; that way they can sell more of them.

 

[Valentin]

Sorry, I didn't mean to be demeaning. I was just trying to help. This was in response to your question about whether the size of the detector mattered (which BTW, yes).So I assumed that you might not be familiar with how steradians are defined.

Good luck, I hope you find the instrument you seek. I don't know of anything like that. Most instruments give you the data they actually measure in what ever units make the most sense for the most users; that way they can sell more of them.

Everything is OK Dave, simply the discussion was going in another direction.
Meanwhile I found a Chinese device that according to what they claim is measuring in mW/sr and has a setting of the distance. God knows if it is reliable or not, too good to be true.

 

[Valentin]

For each distance you can derive a single conversion number. This will contain conversion from an area to solid angle. It will, for your measurement, also need to convert from photometric to radiometric units (this is wavelength dependent).

So if I take a sensor that measures in W/cm^2 with a surface of 1cm^2 placed it at 1m, I just multiply the measured value by 10,000 and that's it?

 

[hutchphd]

You said your instrument measured lux as I recall. This is not a radiometric unit so I cannot be sure you understand photometry vs radiometry.

 

[Valentin]

You said your instrument measured lux as I recall. This is not a radiometric unit so I cannot be sure you understand photometry vs radiometry.

I was talking about a goniophotometer we use for other things and mentioned it as an example of the measurement PROCEDURE I am looking for. No, I have no intention to measure radiant intensity with a lux meter.
This is what I wrote in the only reply where I mentioned the word "lux"

I am looking for something similar but in mW/sr, the automatic part is not necessary, only a meter capable to measure in these units.

Maybe my English is wrong or my explanations or.....

 

[Valentin]

Most instruments give you the data they actually measure in what ever units make the most sense for the most users; that way they can sell more of them.

In the LED industry, the radiant intensity of IR LEDs is always specified in mW/sr. So I imagine that such equipment exists.

 

[DaveE]

In the LED industry, the radiant intensity of IR LEDs is always specified in mW/sr. So I imagine that such equipment exists.

I'm glad you found a suitable instrument.

How experimental data is acquired and how it is reported are often different things. Those data sheets could have been made with a conventional power meter; in fact I would guess that they were since, as you've seen, the unconventional meters are... well..., unconventional.

 

[Vanadium 50]

This conversion method assumes the source is small

The units assume it as well. What is a steradian if you have a big, perhaps oddly shaped, source?

 

[Valentin]

The units assume it as well. What is a steradian if you have a big, perhaps oddly shaped, source?

This is a good question.
I am working on 2 projects, one is an IR illuminator with an oval beam 20/40° with radiant intensity up to 500mW/sr. The other project is an IR indicator with diffused beam and very low intensity, in the range of practically uW/sr.
I need to create for the first one an intensity map with a resolution of 1° vertically. I assume that I need to place the sensor at a distance where the sensor's diameter creates an angle of 1° from the source.
But what about the diffused one? This is supposed to be measured in an integrating sphere.