Luminance of a pixel (read more on Wikipeia) is a measure of perceived brightness and is calculated in SUB2r code using the following formula:
\[ luma = 0.2126R + 0.7152G + 0.0722B \]
This is in accordance with the HDTV's BT.709 recommendation (and not the “expected” CCIR 601) and the UVC descriptors for USB video transmission reflect that choice.
A.k.a. CCIR 601 If we were to chose the CCIR 601, which is used in most digital standard definitions, the coefficients would be:
\[ luma = 0.299R + 0.587G + 0.114B \]
According to Wikipedia the new standard's coefficients are yet different still:
\[ luma = 0.2627R + 0.678G + 0.0593B \]