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I have a Sekonic C700 spectrometer. I notice that when I makes a measurement even of a colored test card illuminated by an incandescent light, it still looks like the normal tungsten curve:

enter image description here

It looks like the upper histogram in the picture above. If I test a blue or a red card, it looks virtually the same. There are only slight differences. I assume this is because the spectrometer is not "focusing" on the card, so it sees the ambient light, not the light coming from the card.

But I don't really understand this because I am holding the meter right up to the card, so the only light it should be getting should be light coming from the card. Therefore, I would have expected the red or blue color to be more clearly shown, but this is not happening. Why not?

  • How is this about photography? -1 – user29608 Aug 29 '17 at 10:17
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    @fkraiem how is this not about photography? This question is asking about using a color meter, specifically intended for photographers and videographers. – scottbb Aug 29 '17 at 13:09
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    "There are only slight differences." Those 'slight' differences are what our eyes interpret as different 'colors'. Hint: colors and wavelengths of light are not the exact same thing. Our eyes perceive many colors that do not correspond to a single wavelength of light. There is nothing intrinsic about a wavelength of light with regard to a certain color. It is the sensitivity of the various parts of a vision system to that wavelength that determine how that wavelength is perceived by a vision system as a certain color. There is no such thing as color in nature - it is only in our brains. – Michael C Aug 30 '17 at 7:31
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    I think it would be significantly more likely for you to get good answers here if you posted the actual two histograms you are comparing, and perhaps also a photograph of the test card using preset WB. – mattdm Sep 10 '17 at 12:26
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    I'm a little confused, but what you have now seems to be an example set of histograms from some documation, the second of which we are supposed to ignore. (Is that right?) – mattdm Sep 10 '17 at 12:27
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+50

I can only guesstimate as to what is happening. This meter sports and integrating sphere entry thus its angle of view is 180°. You might try: construct at tube, blackened on the inside, and hand-hold it over the entryway. This likely will shield the instrument from the ambient light. Maybe you can use a tube from a paper towel roll and spray paint its interior flat black.

  • Excellent suggestion. Restricting the aperture, which is what the black coated tube does, is the only way to restrict the spectral Sekonic response to a non Lambertian surface. en.wikipedia.org/wiki/Lambertian_reflectance – doug Sep 7 '17 at 14:57
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"There are only slight differences."

Those 'slight' differences are what our eyes interpret as different 'colors'.

Colors and wavelengths of light are not the exact same thing. Our eyes perceive many colors that do not correspond to a single wavelength of light. There is nothing intrinsic about a wavelength of light with regard to a certain color. It is the sensitivity of the various parts of a vision system to that wavelength that determine how that wavelength is perceived by a vision system as a certain color. There is no such thing as color in nature - it is only in our brains.

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Theory

Each material (such as an actual blue card or actual red card) has a reflectance spectrum. The reflectance spectrum of a material:

  1. Varies in accord with the wavelengths in the illuminating source.

  2. Varies in accord with the angle of incidence of the illuminating source.

The actual spectrum of the reflected light from an actual object with an actual reflectance spectrum will be determined by the actual spectrum of the light source and the angle of incidence of the actual light source. The spectrum is determined by the integration the spectrum of the light source and the reflectance spectrum of the material at every wavelength.

Discussion

The incandescent light source in the experiment produces visible light across a broad spectrum. This is typical of incandescent light sources and indicated by the measured reflectance spectrum of the blue and red cards. In theory a card that reflects a single frequency or very narrow spectrum is possible. In ordinary practice having such a card at hand is very unlikely at best. And it is worth keeping in mind that spectral reflectance also varies with angle of incidence so either such a card becomes even more theoretical or carefully controlled illumination and/or observation/measurement becomes critical. For serious work they are critical anyway.

Remarks

Part of the conundrum may be due to not maintaining continuous awareness of the differences between radiometry and photometry. Photometry is normalized to the tristimulus response of the human eye rather than the equations of Newtonian physics. This plays out as a difference in kind between ordinary objects such as red and blue cards (photometry) and the frequencies of the electro-magnetic spectrum (radiometry). The human eye is very sensitive to slight changes in reflectance spectra and photometry includes the detailed study of those sensitivities.

Reference

SIGGRAPH has a very good introductory article on reflectance spectra.

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