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I want to make a simple setum, where I can measure the opacity of air. I want to use a simple laserbeam, which I shoot across the room, and on the other side I want to measure how much is blocked/travelled through.

Is it a good idea to use an LDR or should I use something else? I saw these photodiodes, but they cant determine how much it is, just if or if not.

Ideas?

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    \$\begingroup\$ Are there other sources of light in the room? Will the room be filled with smoke...exactly what are you trying to measure? How precisely do you want to measure opacity and/or scattering? \$\endgroup\$
    – Elliot Alderson
    Commented Nov 20, 2018 at 20:26
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    \$\begingroup\$ Why do you think a photodiode can't measure the optical power applied to it? \$\endgroup\$
    – The Photon
    Commented Nov 20, 2018 at 20:48
  • \$\begingroup\$ For proper power measurement, use a "laser power meter" \$\endgroup\$
    – tuskiomi
    Commented Nov 20, 2018 at 21:52
  • \$\begingroup\$ A word of caution - air is pretty damn transparent, especially over the length of a room. Unless you're looking to measure propagation through smoke or something similar, you're not likely to get much usable results. Plus, you'll need a reference path to deal with drift in your measurement system, unless you learn a good deal more than you seem to know about making high-stability amplifiers. \$\endgroup\$
    – WhatRoughBeast
    Commented Nov 21, 2018 at 1:08

2 Answers 2

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Is it a good idea to use an LDR or should I use something else?

I'd go with something more sensitive, a photodiode or phototransistor.

I saw these photodiodes, but they can't determine how much it is, just if or if not.

That's simply wrong. Just because you haven't calibrated them doesn't mean they wouldn't be the better measurement device!

In fact, CCD camera sensors work based on the principle that you bias a diode reversely, employing the depletion zone as capacitor. Incident photons lead to charge migration (i.e. current), which leads to a readable voltage change across said capacitor. So, as CCD sensors are typically these that you use on the more demanding light measurement settings: can't agree.

From a personal experience: red laser pointer, pointed at red LED with a Darlington array amplifying the photocurrent works pretty well as a demonstrator.

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In order to measure the optical absorption of something, you need to measure the power ratio between input and output.

You also want to use a detector that is linear with optical power. A photodiode is pretty good in this regard. Hamamatsu's application note says to nine orders of magnitude!
https://www.hamamatsu.com/resources/pdf/ssd/e02_handbook_si_photodiode.pdf

An optical filter in front of the photodiode to remove light that is in wavelengths other than your laser is required.

You might also need a lock-in amplifier to reduce the electrical bandwidth, remove sources of interference and to move the signal away from 1/f sources of noise.

Philip C. D. Hobbs' book (Building Electro-Optical Systems: Making It all Work) and papers are a good source in building these sorts of things.

Photodiode Front Ends: The REAL Story
https://electrooptical.net/static/oldsite/www/frontends/frontends.pdf

Ultrasensitive laser measurements without tears
https://electrooptical.net/static/oldsite/www/canceller/noisecan.pdf

Jerald Graeme's book is useful too (Photodiode Amplifiers: OP AMP Solutions)

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