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And how different is 18% gray from 18% red?

I was thinking about the properties of light and spectrum to better understand the camera metering mode. 18% means approx 1 out of 5 photons will be reflected. If 5 photons of Red Green and Blue colours are incident on 18% gray and 18% red than gray will reflect 1 photon of each colour, so total 3 photons. At the same time red will reflect 1 red photon only or 3 red photons ? If it reflects only 1 photon than 18% gray might equals the 54% red properties. Please guide me so that I can better understand the metering. Metering modes , zones etc are clear so please dont bother about that. I tried my best to explain, hope you understand my question.

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  • \$\begingroup\$ Does this answer help you? photo.stackexchange.com/a/19431/4892 \$\endgroup\$
    – dpollitt
    Commented Nov 26, 2014 at 18:37
  • \$\begingroup\$ I had gone through that post already, that is a good explanation of why not white card. But in my case I really feel that gray shade is better choice but can't pin point why. According to me if someone make a camera which is calibrated using 18% red than the camera will overexpose the images with respect to normal 18%gray calibrated cameras. This can only be proven wrong if 18% for red means 18% of red incident light but not all. \$\endgroup\$
    – Alex
    Commented Nov 26, 2014 at 19:14

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Gray is used because it's indifferent to differences in color temperature. If you used an 18% red card in the shade, the cooler light would make the red card be a biased measurement standard. The same thing would happen if you used a blue card in tungsten lighting- the lack of blue light frequencies would make it appear darker. With a gray card, such situations are not an issue.

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    \$\begingroup\$ Yes indeed. See this experiment I did for evidence how white balance can affect metering. \$\endgroup\$
    – mattdm
    Commented Nov 26, 2014 at 19:23
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    \$\begingroup\$ Thanks!!! your explanation cleared why not red, exactly to the point. Gray reflects all colour light so its better to use in all types of lightning. Still one doubt is not clear that how 18 % is calculated for gray, red, blue or any other colour tone ? \$\endgroup\$
    – Alex
    Commented Nov 26, 2014 at 19:30
  • \$\begingroup\$ @mattdm great experiment. You saved me couple of hours of experimentation. \$\endgroup\$
    – Alex
    Commented Nov 26, 2014 at 19:58
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    \$\begingroup\$ @user34928 - "Real" grey cards (like the old Kodaks, or similar grey cards/calibration targets made by Xrite/Macbeth or Datacolor) reflect a very even amount of light across the visible spectrum and into the near-visible IR and UV. Whatever hits it will come back (at 18% or 12% or whatever the calibrated value is for the target). They're not just paper or plastic that happens to look grey, and they will carry an expiration time after opening the package and exposing the card/target to light since fading is not guaranteed to be equal across the spectrum. \$\endgroup\$
    – user34820
    Commented Nov 26, 2014 at 20:08
  • \$\begingroup\$ Yes expiry date comes because gray card will not reflect 18% of light after some time due to many reasons like dust, scratches etc \$\endgroup\$
    – Alex
    Commented Nov 27, 2014 at 6:10

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