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I really hope, someone makes a cheap spectrometer as a smartphone accessory. That would be the best way to quickly assess the properties of light sources. Beyon

by _ph_ 7y ago

I really hope, someone makes a cheap spectrometer as a smartphone accessory. That would be the best way to quickly assess the properties of light sources. Beyond the health impact, one wants to know the light spectrum when e.g. selecting lamps for fish tanks or plant lighting. As a very coarse but easily available poor-mans spectrometer, I have been taking pictures of a greycard with my camera in raw-mode. Looking at the histogram balanced for a fixed color temperature, e.g. sunlight, for the r,g,b-channels gives very different results depending on different light sources. While you don't see a true spectrum, just the strength of the blue vs. the green and red channel usually is very revealing.

scotty79 7y ago
It's cheap and works without the smartphone https://www.ebay.com/sch/i.html?_odkw=Diffraction+&_from=R40&_trksid=m4084.l1313&_nkw=Diffraction+grating+slides https://www.ebay.com/sch/i.html?_odkw=Diffraction+&_from=R40...
- _ph_ 7y ago
  Yes, that is a good start - I would like to have curve with the relative strengths of the spectrum. But maybe it is enough to carefully calibrate the smartphone camera on the diffraction sprectra to generate a course graph. Also, if no diffraction grating is at hand, a random audio CD usually is up to the task :)
gtrubetskoy 7y ago
Any compact disk works as a spectrometer. https://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html https://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html
- NIL8 7y ago
  I've never seen this. Thanks!
janekm 7y ago
That only shows differences in white balance and tone. It can't give you any information about the CRI since you only have 3 channels of colour sensors in your camera. If you use a colour swatch palette that would be different.
- _ph_ 7y ago
  Yes, of course. It doesn't give you a "spectrum" at all. Only a very rough guesstimate on how the colors are balanced. But good enough to see, whether there is a large blue peak (blue line much stronger than red and green) or not. Just using your camera and a rgb histogram. But using a color palette is a great idea. With some calibration work, this could give a rough spectral curve with a single picture. I need to investigate this :). One would require to get a palette, where the colors are generated not by rgb pigment mixing but by absorbing/reflecting distinct wavelengths. Of course, the MacBeth color targets would fufil that condition, now how to get a wavelength calibration from this? Have to read up on things...
  - janekm 7y ago
    Another option would be to use colour correction gels like from Rosco. They publish transmission curves for their gels: https://emea.rosco.com/en/products/filters/r3220-double-blue-ctb https://emea.rosco.com/en/products/filters/r3220-double-blue... (click on the little i). You could take pictures of the light source through a set of different gels and reverse to an approximation of the spectrum of the light source. Interestingly they are also now offering gels to correct the colour casts of LED lighting: https://emea.rosco.com/en/product/opti-flecs https://emea.rosco.com/en/product/opti-flecs There are samples swatches of their gels, which can be had by either asking a theatrical lighting supply company or on eBay.

by _ph_ 7y ago

scotty79 7y ago

It's cheap and works without the smartphone https://www.ebay.com/sch/i.html?_odkw=Diffraction+&_from=R40&_trksid=m4084.l1313&_nkw=Diffraction+grating+slides https://www.ebay.com/sch/i.html?_odkw=Diffraction+&_from=R40...

_ph_ 7y ago
Yes, that is a good start - I would like to have curve with the relative strengths of the spectrum. But maybe it is enough to carefully calibrate the smartphone camera on the diffraction sprectra to generate a course graph. Also, if no diffraction grating is at hand, a random audio CD usually is up to the task :)

gtrubetskoy 7y ago

Any compact disk works as a spectrometer. https://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html https://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html

NIL8 7y ago
I've never seen this. Thanks!

janekm 7y ago

That only shows differences in white balance and tone. It can't give you any information about the CRI since you only have 3 channels of colour sensors in your camera. If you use a colour swatch palette that would be different.

_ph_ 7y ago
Yes, of course. It doesn't give you a "spectrum" at all. Only a very rough guesstimate on how the colors are balanced. But good enough to see, whether there is a large blue peak (blue line much stronger than red and green) or not. Just using your camera and a rgb histogram. But using a color palette is a great idea. With some calibration work, this could give a rough spectral curve with a single picture. I need to investigate this :). One would require to get a palette, where the colors are generated not by rgb pigment mixing but by absorbing/reflecting distinct wavelengths. Of course, the MacBeth color targets would fufil that condition, now how to get a wavelength calibration from this? Have to read up on things...
- janekm 7y ago
  Another option would be to use colour correction gels like from Rosco. They publish transmission curves for their gels: https://emea.rosco.com/en/products/filters/r3220-double-blue-ctb https://emea.rosco.com/en/products/filters/r3220-double-blue... (click on the little i). You could take pictures of the light source through a set of different gels and reverse to an approximation of the spectrum of the light source. Interestingly they are also now offering gels to correct the colour casts of LED lighting: https://emea.rosco.com/en/product/opti-flecs https://emea.rosco.com/en/product/opti-flecs There are samples swatches of their gels, which can be had by either asking a theatrical lighting supply company or on eBay.