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For the case of spectrophotometers, we can easily calibrate the device(intensity wise) taking the light source inside the device as the reference, without any need of precise knowledge about its spectral properties. We can replace the light source with almost anything as long as it provides sufficient amount of light in the spectrum we want to work at and it's relatively stable, then recalibrate the spectrophotometer.

But when we build a spectrometer, a device that measures the spectrum of a light source, what we can use as an intensity reference other than a light source whose spectrum is already measured by an other calibrated spectrometer? How were spectrometers calibrated for intensity, lets say, when they were first made? Is there any light source whose intensity spectra is intrinsically known? Does such a light source need to be held in very specific conditions to be taken as an intensity reference(temperature, voltage, pressure, etc.)? Is there any simple reference that can be used to make a simple spectrometer which will be accurate to about +-5% in the visible spectrum?

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    $\begingroup$ 5% is a pretty lousy calibration. Various gas emission line sources are easy to get and use. $\endgroup$
    – Jon Custer
    Commented Oct 14, 2019 at 23:28
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    $\begingroup$ Early in my career, ~1980, I worked for a company that made color measurement instruments. Intensity calibration was a problem. We bought an incandescent light bulb from the National Bureau of Standards that they can calibrated. It was good to about 1%. It was expensive and we treated it very carefully. We kept it in a safe place, used it seldom (to calibrate secondary standards), and ramped the voltage up and down slowly when we did. I think the NBS had calibrated it with their own special light bulb. $\endgroup$
    – mmesser314
    Commented Oct 14, 2019 at 23:58
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    $\begingroup$ I have a recollection of doing a calibration along these lines for a Raman spectroscopy lab during my graduate-level experimental physics course, c. 2005. This involved (a) taking the spectrum of an incandescent bulb in the spectrometer, (b) using an optical pyrometer to measure the temperature of the filament, and (c) looking up data on the emissivity of tungsten. I honestly don't recall how accurate the results were, or indeed whether we did a very thorough error analysis on the calibration curve. $\endgroup$
    – Michael Seifert
    Commented Oct 15, 2019 at 1:04
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    $\begingroup$ Also: are you looking for a way to ensure that (say) 0.01 W/m<sup>2</sup> of intensity creates the same size peak at 400 nm as it does at 600 nm (i.e., calibrating relative intensity across wavelengths)? Or are you looking for a way to associate absolute intensity readings with a particular response in your spectrometer? The two are somewhat different questions. $\endgroup$
    – Michael Seifert
    Commented Oct 15, 2019 at 1:07
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    $\begingroup$ @S.McGrew The only photoelectric devices I know are photodiodes(which includes phototransistors, leds and solar cells). Yes, their output is proportional to the incident photon flux for a single wavelength, but the ratio differs among wavelengths. Their wavelength sensivity characteristics are given in their datasheet, but without any tolerance. They are generally used as the detector in a spectrometer. But the problem is, the other parts in the spectrometer heavily contribute to its spectral response and I can't easily predict it, so the spectrometer in whole need to be calibrated. $\endgroup$
    – Ömer Gezer
    Commented Oct 15, 2019 at 8:56

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