If I convert 100% green and blue to grayscale, the blue one becomes about twice as dark.
I can see that green looks brighter, but I also want to understand reason behind it.
Looking at colors in rgb model:
- pure blue = 0 red 0 green 255 blue
- pure green = 0 red 255 green 0 blue
So why is pure green brighter than pure blue?
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Human perception isn’t the same for all colors. Our eyes have different color pigments which absorb different frequencies of light.
There’s a bit about this over in Physics.SE: Why do green lasers appear brighter and stronger than red and blue lasers? From this question a chart is presented that shows the absorption of different frequencies of light. The three cones are represented by their colors and the rods are the dotted line.
Note here that that something that is in the blue range, while it will trigger the blue cones, but poorly trigger the rods or the green or red cones. Meanwhile, something that is deep red will trigger the red and a bit of the green cone, but not much of the rod or any of the blue cone. The sum of the absorbance of the three pigments, peaks in the green making green look the brightest.
There’s even more about this in the cone sensitivity curve found at Visible Light and the Eye’s Response and the spectral sensitivity wikipedia page. When looking at these graphs take care to note that many of them have been normalized into a 0 .. 100 scale.
There is also an aspect of distribution of the various color cells. More on this can be read about in “Blue” Cone Distinctions at HyperPhysics which notes that outside of the Forvea Centralis (most densely packed area of the retina for maximum sensitivity) they are only about 2% of the total count of cone cells (which then suggests that there is a ‘blue amplifier’ in our perception of the signal). The reasoning for this is likely because the blue light will get bent differently than red or green and we would have significant issues with chromatic aberration in our own eyes if we were to sample blue outside of the main focus significantly (just thinking about that gives me a headache). The root of the color vision tree at HyperPhysics is Color Vision Concepts which is a good read.
This predisposition for green being the important color can be seen in the various Bayer filters used in digital cameras which sample green more than red or blue.
Bryce Bayer’s patent (U.S. Patent No. 3,971,065) in 1976 called the green photosensors luminance-sensitive elements and the red and blue ones chrominance-sensitive elements. He used twice as many green elements as red or blue to mimic the physiology of the human eye. The luminance perception of the human retina uses M and L cone cells combined, during daylight vision, which are most sensitive to green light.
From Wikipedia Bayer Filter: Explanation
This can again be seen being utilized in the program ppmtopgm which converts a color image to grayscale. It uses the formula:
l = .299 r + .587 g + .114 b – note that green is more than twice as the other colors combined when creating the gray value, and 5x more than blue. Btw, be sure to read the quote in that man page… its quite amusing – especially when taken in context of its origins)
And thus, why green appears to be brighter than the other colors.
“Greyscale” is a print specification basically. Yes it removes color, but the “Greyscale” mode is only really needed for printing. Everything on screen is RGB even if it looks grey. In this respect, when you use the
Mode > Greyscale command, Photoshop ignores the RGB data and relies on CMYK color data and your color profile settings to convert.
If you hover your cursor over the colors and look at the
Info Panel you’ll see the wildly different CMYK colors:
You can see that one color is 69C and 100Y and the other is 90C and 72M (based on my profile settings). Mixing C and Y is always going to be lighter than mixing C and M.
(also note the ! next to the values indicating that the color is actually out of the CMYk gamut.)
This is a simplified explanation, as others have pointed out luminosity is a large factor as well in the conversion.
If you are strictly working with RGB documents, greyscale is a poor choice. You are better served with a
Hue and Saturation Adjustment layer which simply removes all saturation — this results in the same grey for both colors if that’s what you want.
You can also use
Image > Adjustments > Black & White to convert to a grey image (still in RGB color space though). This allows you to tweak the levels of the colors specifically to achieve the greys you want.
Essentially you’re comparing RGB and Greyscale when they really don’t have any correlation.
I find that’s it’s easier to evaluate RGB colors in the HSB model (Hue, Saturation, Brightness). In this case, it helps you in two ways.
1. It helps me remember that some colors have greater inherent luminance/brightness than others. Just take a look at the green and blue points on the Hue scale. Green is clearly brighter.
2. If you’re looking to control the value levels, you have a clear indication in the brightness scale (once you remove the saturation variable).
The best way to inspect the differences in lightness is to use the Lab colour space, rather than RGB or CMYK.
The Lab colour space was developed to be a better representation of human vision than the RGB colour space. The L channel in particular, which stands for “lightness” and is the relevant channel for this discussion, attempts to be a closer match of human perception of lightness.
If you compare a pure green vs pure blue image, you’ll see that the lightness channel has different values. While pure blue has a value of 30 (out of 100), green has a value of 88.
Its because of the function used internally when converting to grayscale. Its a weighted ratio addition of the three components, with more weight assigned to green compared to blue.