Question

I see that the algorithm for measuring luminance in LUX for a pixel is (obtained from here)

double luminance = r*0.299 + g*0.587 + b*0.114;

I’m interested in knowing the intensity of a given picture’s light in the “blue” spectrum, rather than pure luminance. How can this be done? Would simply taking the weighted “blue” value of each pixel in a camera image and somehow averaging/adding them do the trick?

Thank you for your input!

Answer 1

Luminance is intensity per area.

The formula you posted isn’t about converting red, green, and blue intensity to total luminance or vice-versa; it’s just about weighting red, green, and blue luminance to get total luminance. You’ve already got blue luminance, so the relevant formula for your purposes is:

b

Or, if you prefer:

b*0.114

For the other part, it depends on what you’re asking, but it’s probably somewhere along this path:

The intensity for each pixel is the luminance for that pixel times the area of the pixel.

The intensity for the whole array is the sum of all of those separate intensities.

The luminance for the whole array is the intensity for the whole array divided by the total area of the array, and it should be obvious that’s also trivial.

The luminance for the whole room is… well, presumably the whole point here is that you’re making the assumption that it’s roughly the same as the luminance for the camera array, so by assumption it’s just the same as the last value.

The intensity for any plane within the whole room is then the luminance times the area of that plane, and the intensity for the room as a whole is then the sum across all those planes, but those probably aren’t very useful to you.

The intensity for a picture is just the intensity across the camera array at the time you took the picture. That’s probably more useful than the intensity of the room.