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How to measure the Intensity of Light?

by HungryChemist
Tags: intensity, light, measure
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nicp
#19
Mar8-06, 12:15 PM
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Once upon a time, the intensity of a luminous source was measured by comparing it with a reference source using measuring sistems called photometers (Bunsen's one is the most famous).
The primary reference source was 1 cm^2 of platinum brought at its melting temperature, which was defined to have a luminous intensity, in direction normal to its surface, of 20 cd.
chwn
#20
Jan16-09, 02:16 PM
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A laser light going through a optic fiber, then with a focal lens, it is focused on a target. The focused light spot is about 0.2 mm. Can I achieve the best focus by measuring the focused light spot intensity with a photodiodes and judge the best focus based on its output current or voltage reading?
Topher925
#21
Jan16-09, 02:47 PM
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Quote Quote by chwn View Post
A laser light going through a optic fiber, then with a focal lens, it is focused on a target. The focused light spot is about 0.2 mm. Can I achieve the best focus by measuring the focused light spot intensity with a photodiodes and judge the best focus based on its output current or voltage reading?
I don't see why not. Just make sure you account for the size of the active area of the photodiode and that active areas distance behind the lens of the photodiode.
ZapperZ
#22
Jan16-09, 04:19 PM
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Quote Quote by chwn View Post
A laser light going through a optic fiber, then with a focal lens, it is focused on a target. The focused light spot is about 0.2 mm. Can I achieve the best focus by measuring the focused light spot intensity with a photodiodes and judge the best focus based on its output current or voltage reading?
But what if you are already collecting all of the light already and yet, you are not as a focus, as in not the smallest spot size you can get? The photodiode is already giving you the same current since all the photons are already hitting it. The focused beam is only characterized by having the highest photon density per unit time per unit area. A photodiode can't determine that if you are already collecting all of the light, and all you're changing is the beam spot size hitting the photodiode.

Zz.
Cthugha
#23
Jan16-09, 05:19 PM
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Maybe the easiest way to check your focus is to put a pinhole of sensible size at the position, where you want your spot to be smallest. Now you can just measure and maximize the amount of light, which goes through the pinhole.
chwn
#24
Jan19-09, 09:53 AM
P: 2
Originally Posted by chwn
A laser light going through an optic fiber, then with a focal lens, it is focused on a target. The focused light spot is about 0.2 mm. Can I achieve the best focus by measuring the focused light spot intensity with a photodiodes and judge the best focus based on its output current or voltage reading?[/I][/I]

[QUOTE=ZapperZ;2037501]But what if you are already collecting all of the light already and yet, you are not as a focus, as in not the smallest spot size you can get? The photodiode is already giving you the same current since all the photons are already hitting it. The focused beam is only characterized by having the highest photon density per unit time per unit area. A photodiode can't determine that if you are already collecting all of the light, and all you're changing is the beam spot size hitting the photodiode.

Chwn: I have the same puzzle when I posted the question as to whether I can "achieve the best focus by measuring the focused light spot intensity with a photodiodes and judge the best focus based on its output current or voltage reading". I thought since the effective sensing area of photodiode would collect the same amount of the beam, so it should give the same current/voltage output reading regardless if the beam is focused or not. Actually, my question should be: If the effective sensing area of photodiode is smaller than the focused beam spot size, will the focus change cause the light intensity change and generate defferent current/voltage output? If it is, is there such a photodide available for a 0.2 mm spot size focus adjustment? and where I can get it?
Skooage
#25
Feb8-09, 06:40 AM
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Quote Quote by rbj View Post
how is it, then, that they tell us that there is about 1360 watts/m^2 of solar radiation at our distance of 93,000,000 miles from the sun (and from that they calculate that the output of the sun is about 3.8 x 10^26 watts)? how do they measure that? i think there is some way to calibrate photosensors, but i dunno what it is.

r b-j
Light follows the inverse squared law. This is a trait discovered by Newton back in the day. so Light would spread 1/d^2 for every value of D as a distance, effectively quartering it's intensity at every distance. so if you know the distance between the sun and earth and the intensity on earth, you can work out the intensity on the sun.
Andy Resnick
#26
Feb9-09, 07:25 AM
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Quote Quote by rbj View Post
here's my curiousity (being an electrical engineer): how do you calibrate those detectors so you know what indication corresponds to some standard intensity of some given watts/m^2? how do you measure absolute intensity of light? (i can think of using some large black tank with water and measuring the rate of rise in temperature.)

r b-j
You have a calibrated blackbody source. They are commerically available.
Iforgot
#27
Oct18-10, 08:03 AM
P: 101
I realize this thread is 5 years old, but it's the first thing that came up on Google, so I figured I'd put my two cents in.

There are commercially available avalanche photodiodes (APD) and photomultipliers tubes (PMT) capable of registering a single photon. I believe a single APD or PMT generates a consistent electrical pulse every time it gets hit by a photon. At low light intensities, one can work backward from the pulse count and calculate the number of photons hitting the APD/PMT, and from that, calculate the energy and intensity.

After this is done, the APD/PMT is replaced with a regular photodiode, and the resulting current generated is scaled to some wattage value using the energy detected using the PMT.

I think this can only be done at very low intensities, because APD's will fry at higher ones.
ZapperZ
#28
Oct18-10, 08:45 AM
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Quote Quote by Iforgot View Post
I realize this thread is 5 years old, but it's the first thing that came up on Google, so I figured I'd put my two cents in.

There are commercially available avalanche photodiodes (APD) and photomultipliers tubes (PMT) capable of registering a single photon. I believe a single APD or PMT generates a consistent electrical pulse every time it gets hit by a photon. At low light intensities, one can work backward from the pulse count and calculate the number of photons hitting the APD/PMT, and from that, calculate the energy and intensity.

After this is done, the APD/PMT is replaced with a regular photodiode, and the resulting current generated is scaled to some wattage value using the energy detected using the PMT.

I think this can only be done at very low intensities, because APD's will fry at higher ones.
We need to be a bit careful here.

APD and PMT, while they do have single-photon responses (i.e. they can detect single photons), do NOT have 100% quantum efficiency. Hamamatsu PMT's, with proper coating, etc., may reach 50% or slightly higher, but this means that 1 out of every 2 photons, at best, triggers the detector.

So no, these devices do not generate an electrical signal "... every time it gets hit by a photon..."

Zz.
Iforgot
#29
Oct18-10, 09:03 AM
P: 101
As long as they know the QE, they can do a calibration. They question is how do they know ~50 QE?

I guess they could down convert one photon into two using a non-linear crystal? Then they would for sure know they have two? I'll admit, it's not a very straight forward method.


I was just throwing this idea out there. I don't know how they really measure light intensity either. That's what I'm trying to figure out. I'm still looking through equipment on the web. I'll post again when I find an intensity measurement whose calibration is straight forward.
Dr Lots-o'watts
#30
Oct18-10, 12:26 PM
P: 674
I just have to point out that an instrument such as the following will measure most of what one needs to know about a beam between PMT- and APD- range and 10 W:

http://www.gentec-eo.com/en/products...beamage.38.htm
ZapperZ
#31
Oct18-10, 02:09 PM
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Quote Quote by Iforgot View Post
As long as they know the QE, they can do a calibration. They question is how do they know ~50 QE?
What does this have anything to do with my post?

Note that you were claiming that ALL photons can be detected by such devices (i.e. PMT). This is what I was arguing against. Being able to detect single photon is NOT the same thing as being able to detect ALL photons.

Zz.
Andy Resnick
#32
Oct18-10, 04:10 PM
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Quote Quote by Iforgot View Post
As long as they know the QE, they can do a calibration. They question is how do they know ~50 QE?

I guess they could down convert one photon into two using a non-linear crystal? Then they would for sure know they have two? I'll admit, it's not a very straight forward method.


I was just throwing this idea out there. I don't know how they really measure light intensity either. That's what I'm trying to figure out. I'm still looking through equipment on the web. I'll post again when I find an intensity measurement whose calibration is straight forward.
Who is 'they'? NIST? Random scientists and engineers? Photographers? There's a large number of ways to measure intensity.


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