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Converting to black and white using luminance and luma

Posted September 7, 2015.

Comparing luminance and luma when converting a photograph of red, green, and blue glass to black and white.

Compared to the luminance conversion, the luma conversion flattens the tonality of the red glass and renders the deep blue glass as a dark undifferentiated mass.

Comparing luminance and luma when converting a color portrait to black and white. The original color image "Portrait" was taken by photographer Jérôme Decq and released under a Creative Commons Attribution 2.0 Generic (CC BY 2.0) license.

When converting images that don't have saturated colors to black and white, the difference between luminance and luma is subtle, but still important: compared to the luma conversion, the luminance conversion has more subtle (and to my eye more pleasant and believable) tonal transitions, especially evident across the nose, cheeks, and forehead.