# How to measure small change in the intensity of laser (red - near IR) light after absorption of gas?

#### viswapravin

Joined Mar 16, 2016
21
Using photodiodes/ phototransistors are effective for this measurement, as the absorption spectrum traces are in the range of 10^-24.
Also how to choose the photodiodes/phototransistors

#### jpanhalt

Joined Jan 18, 2008
10,080
Using photodiodes/ phototransistors are effective for this measurement, as the absorption spectrum traces are in the range of 10^-24.
10^-24 of what? Please add units if they exist, or if not, then what is it a count or ratio of? If you already know what devices will work, please give an example of that device.

John

#### Alec_t

Joined Sep 17, 2013
11,517
How small is a "small change in the intensity"?

#### viswapravin

Joined Mar 16, 2016
21
10^-24 of what? Please add units if they exist, or if not, then what is it a count or ratio of? If you already know what devices will work, please give an example of that device.

John
I read the absorption line intensity of the order of 10^-24 cm/molecule after absorption. The laser power is 10mW. Can I use photo diode/transistor to measure the change in intensity. If so, then what will be the range of voltage difference.

#### viswapravin

Joined Mar 16, 2016
21
How small is a "small change in the intensity"?
I read the absorption line intensity of the order of 10^-24 cm/molecule after absorption. The input power of laser is 10mW. Can I use photo diode/transistor to measure the change in intensity. If so, then what will be the range of voltage difference.

#### Sensacell

Joined Jun 19, 2012
2,587
Convert this "small change" to an AC signal.

Then you can process this signal with simple electronics.

I once worked for a company that made IR gas analyzers using rotating IR filters on a wheel...

#### viswapravin

Joined Mar 16, 2016
21
Convert this "small change" to an AC signal.

Then you can process this signal with simple electronics.

I once worked for a company that made IR gas analyzers using rotating IR filters on a wheel...

#### Sensacell

Joined Jun 19, 2012
2,587
It's was all a bit complex...

They used a black body IR source (broad spectral output) with a rotating filter wheel in front of it.
The filter wheel had IR band pass filters that would select a specific wavelength of IR light to pass through the sample chamber.

One wavelength was chosen to be right on the absorption band of the gas in question, the other was a reference wavelength that was not absorbed. The output of the detector was an AC signal that was amplified and fed into an ADC, along with a sync pulse from the rotating wheel.

By comparing the "reference" and the "absorption" signals, you could determine the concentration of gas, even when the sample chamber windows got dirty. The wheel had filters for lots of different gasses.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,153

#### wayneh

Joined Sep 9, 2010
16,399
It's also a common technique in to split the beam, and pass only part of it thru the sample. This allows precise zeroing when the sample is not present, and corrects for any fluctuation in the sampled intensity due to changes in the beam intensity.