Portable Spectrometer and questions about spectra collection....

[fog37]

TL;DR Summary

Understand the correct output of a portable Vis-NIR spectrum

Hello everyone,

I have a small portable VIS-NIR spectrometer with a fiber cable connected to it to collect light and see the light spectrum on the screen.

I have a question about the difference between collecting the so-called "unprocessed" intensity vs wavelength (counts vs lambda) spectrum and relative intensity vs wavelength (% vs lambda) spectrum... Before I get into the details of my question and waste your time and space, does anyone have experience with such tool?

Thank you as always.
Fog37

 

[pinball1970]

TL;DR Summary: Understand the correct output of a portable Vis-NIR spectrum

Hello everyone,

I have a small portable VIS-NIR spectrometer with a fiber cable connected to it to collect light and see the light spectrum on the screen.

I have a question about the difference between collecting the so-called "unprocessed" intensity vs wavelength (counts vs lambda) spectrum and relative intensity vs wavelength (% vs lambda) spectrum... Before I get into the details of my question and waste your time and space, does anyone have experience with such tool?

Thank you as always.
Fog37

Can you send details on the model and spec?
We use spectros for solid samples, paper, plastic and textiles. These give reflectance data.
We had a portable but the accuracy was much reduced.
From these we can get spectral curves between 360 and 750nm.

We access to a radiometer, so that is measuring light in a given environment.
From that we can get the Intensity and SPD. The machines are about £1800.

@Drakkith I think is skilled in this area.

 

[Drakkith]

@Drakkith I think is skilled in this area.

Unfortunately I am not. I had a few semesters of Optical Engineering education before I had to drop out due to health problems. I've never even used a spectrometer, just some hand held spectroscopes that I bought off of Ebay a number of years back.

 

[pinball1970]

TL;DR Summary: Understand the correct output of a portable Vis-NIR spectrum

Hello everyone,

I have a small portable VIS-NIR spectrometer with a fiber cable connected to it to collect light and see the light spectrum on the screen.

I have a question about the difference between collecting the so-called "unprocessed" intensity vs wavelength (counts vs lambda) spectrum and relative intensity vs wavelength (% vs lambda) spectrum... Before I get into the details of my question and waste your time and space, does anyone have experience with such tool?

Thank you as always.
Fog37

It depends what you want to measure, how accurate you need it and what sort of detail you require.
Photographers use versions that just measure Lux like the below (£50)

We have access to the below which covers a lot more ground, these about £1500-2000, CCT, Lux and SPD

The below is bench top used for measuring physical samples (paper, plastic textiles). You can get portable versions.

 

[pinball1970]

Unfortunately I am not. I had a few semesters of Optical Engineering education before I had to drop out due to health problems. I've never even used a spectrometer, just some hand held spectroscopes that I bought off of Ebay a number of years back.

Ok. I have posted a couple of images that will hopefully give him some insight.

 

[fog37]

Hello everyone. Sorry it took me some time to get back and thank you. So, I have the following USB spectrometer (see below) that collects light via a fiber cable and sends it to a spectral software:

I am confused on how to correctly use the software. There are 3 different types of spectra that can be collected (i.e. 3 different modes):

a) absolute irradiance: Y-axis is in Watt/m^2. A source of known spectral intensity must first be used for calibration. We then get the real Watt/m^2 in the spectrum that we measure at the wavelengths of interest. Without this calibration, we may be looking at lambda 1 having intensity I1 and lambda 2 having intensity I2<I2. But that may be incorrect and solely due to the lower response of the spectrometer detector at lambda 2...

b) relative irradiance: Y-axis between 0 and 1 (0% to 100%). The reference source spectrum, whose blackbody radiation curve is known, must first be collected and stored for calibration. A problem with illuminating fluorescent materials would be to get >100% value if the fluorescent wavelength is not present in the illuminating spectrum...

c) unprocessed spectrum: Y-axis measures Intensity in counts. This is where we simply turn the spectrometer on and take measurements.

I am not interested in absolute intensity but in relative intensity spectral measurements to compare transmission spectra in the same small wavelength ranges: I am illuminating two porous material samples and collecting the spectrum of the transmitted light behind the material samples. What is wrong with the using the unprocessed spectrum to perform the spectral comparison?

Why should I use the relative intensity mode? That would require first saving the spectrum of the illuminating source and then take the transmission spectra for each material sample...The goal is to determine which sample has the largest transmission at particular wavelengths of interest...

What is the difference, under the hood, between taking an unprocessed spectrum versus taking spectra in the relative mode (after collecting a reference spectrum and a dark spectrum)?

Thanks!

 

[fog37]

To be clear, reflectance is the ratio between the incident light and the reflected light at a specific wavelength: $$R= \frac {I_{reflected}(\lambda)} {I_{incident }(\lambda)} $$

And transmission is $$T= \frac {I_{transmitted}(\lambda)} {I_{incident }(\lambda)} $$

 

[pinball1970]

To be clear, reflectance is the ratio between the incident light and the reflected light at a specific wavelength: $$R= \frac {I_{reflected}(\lambda)} {I_{incident }(\lambda)} $$

And transmission is $$T= \frac {I_{transmitted}(\lambda)} {I_{incident }(\lambda)} $$

I have not used one of these. I measure reflectance data, for calibration of the environment I use the radiometer we have access to.
@sophiecentaur and @PeroK help?

 

[sophiecentaur]

I measure reflectance data, for calibration of the environment I use the radiometer we have access to.
@sophiecentaur and @PeroK help?

Sorry, I can't help. Not familiar enough with the topic.

 

[hutchphd]

@fog37

I have several old Ocean optics spectrometers (on ISA cards) on my win95 computer in the basement lab and have also used the USB version pictured. It seems to me that you understand what you are doing...if you use counts you must correct for the illuminant not being "flat" and for the dark current. If you are really at the same color then only the dark counts need be corrected. But you do need the dark counts subtraction before the ratio.
I would certainly compare the two kinds of results before commiting to a big data effort as a sanity check. And you understand that the dark counts should be measured (at least) at the beginning and at the end...they will drift with temperature. Depends upon your accuracy needs.

 

[Tom.G]

The spectrometer displays the power it receives at each wavelength. That received power at each wavelength is the product of the light source power, the sensor sensitivity, the specimen color, and the specimen reflectance (or transmittance).

Take a look at the spectra here when Google is asked about incadescent lamps:
https://www.google.com/search?tbm=i...scent+lamp&gbv=2&oq=spectrum+incadescent+lamp

You will see that there is a huge variety of available spectra. If you use a different lamp, an aged lamp, or even a dirty one, the spectrometer will give a different reading. (Generally speaking, light sources of all types get dimmer with usage.)

That is why the Relative Intensity mode is the most applicable for your usage.

Depending on how tight your requirements are, you may need to save the illuminant spectrum at the start of each run, and again check it at the end of each run to be sure the lamp has not drifted (or that the sensor hasn't gotten dirty <did someone sneeze on it? >).

Hope this helps!

Cheers,
Tom

 

[fog37]

No problem. Thank you all anyway!