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Could an LED be used as the probe for a LPM?

405nanoMatt

405nanoMatt

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I know that LED’s can pass a current when a light if shown into them so I was just wondering if a peltier cooler works by heat
Converted to current than wouldn’t using an led work just as well by converting light to current that could be measured?
 





405nanoMatt said:
I know that LED’s can pass a current when a light if shown into them so I was just wondering if a peltier cooler works by heat
Converted to current than wouldn’t using an led work just as well by converting light to current that could be measured?
Click to expand...
It can but im not sure how accurate they are. I recall a guy making them for green laser only with a green LED.
Problem there is its an optical sensor as opposed to a thermal sensor, so the readings will swing with wavelength.

Best to try and experiment if you have access to a known output laser or better yet, a power meter.
 
Many years ago at LPM, 100 mW Reds were becoming available.
All we had was a LED and viol meter. Not calibrated but gave some
indication of power.
You might try a small diode cooling module. Paint one side flat black.
HM
 
Matt --- I just happen to have a small Peltier cooler.
It is about 9 mm square and three 3mm thick.
I painted the hot side flat black and read 5 mV with a
100 mW green and 20 mV with a 400 mW.
If you are testing lower power, this might work. Just add a
heat sink on the back! I'll send it to you FREE if you are interested.
Email me or PM me here with an address.
HM
 
Sure, you could use a LED for this, but that's just a really weird and inefficient technique.

What you want is a photodiode, where a LED is good at emitting light and really shitty at picking light up a photodiode is basically the opposite: Doesn't emit light, but it's explicitly designed to pick light up.

And yes, photodiodes are used as LPMs up to a few mW. If you want to measure more, you either use an attenuator or reflect a tiny bit of the beam with a glass slide and measure that reflection.
 
Why_you said:
Sure, you could use a LED for this, but that's just a really weird and inefficient technique.

What you want is a photodiode, where a LED is good at emitting light and really shitty at picking light up a photodiode is basically the opposite: Doesn't emit light, but it's explicitly designed to pick light up.

And yes, photodiodes are used as LPMs up to a few mW. If you want to measure more, you either use an attenuator or reflect a tiny bit of the beam with a glass slide and measure that reflection.
Click to expand...
"inefficient" how so? you dont have to add any circuitry. No zeroing and can be used with any voltmeter within rating.
LED's can take a beating of optical power as well.

To each his own I guess.

"really weird" that tends to be where great things happen.
 
Last edited:
Inefficient because using a LED for that job is like using a set of fine china to hammer in a nail. Yes, it works in theory but why would you ever do it when there's a specialised tool available for that job. And that's a photodiode.

As a more technical explanation: LEDs have an extremely small emitter section/semiconductor, so you're a) focusing A LOT of light onto a very small and very heat sensitive part and b) you have to hit that small target in the first place. Plus, VIS LEDs use a direct semiconductor with a bandgap in the visible range (duh) -> let's say you have a theoretical LED that emits green light at 532 nm. In an ideal world, that LED could only register light between 531 and like 250 nm. Completely blind to green and red light, literally zero response.

A photodiode is the same size as a LED, costs the same, can be connected to a voltmeter just the same, even outputs a voltage in the same range but reacts to everything between 250ish and 1100 nm and has an orders of magnitude bigger "sensor area".

So why would you ever use a LED for this?
 
Why_you said:
Inefficient because using a LED for that job is like using a set of fine china to hammer in a nail. Yes, it works in theory but why would you ever do it when there's a specialised tool available for that job. And that's a photodiode.

As a more technical explanation: LEDs have an extremely small emitter section/semiconductor, so you're a) focusing A LOT of light onto a very small and very heat sensitive part and b) you have to hit that small target in the first place. Plus, VIS LEDs use a direct semiconductor with a bandgap in the visible range (duh) -> let's say you have a theoretical LED that emits green light at 532 nm. In an ideal world, that LED could only register light between 531 and like 250 nm. Completely blind to green and red light, literally zero response.

A photodiode is the same size as a LED, costs the same, can be connected to a voltmeter just the same, even outputs a voltage in the same range but reacts to everything between 250ish and 1100 nm and has an orders of magnitude bigger "sensor area".

So why would you ever use a LED for this?
Click to expand...

Why? In the spirit of the OP and my followup post suggesting "experimentation".

The question was not of which is better, have a look back it will make more sense. Not everybody has experience in these things
we all start off with many questions, some enjoy discovering the answers.🍺
 
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