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Greyscale is another way of saying Brightness
Kind of . . .
G
reyscale is another way of saying monochrome, or one-coloured, or more precisely one-hued. Monochrome is maybe a better word as greyscale does not really have to be grey, but we’ll stick with the word greyscale, to mean an image in different levels of brightness of the same hue.
Different brightnesses of the one hue: therefore by converting an image to greyscale we get a picture of how bright or dark each section – each pixel – of the picture is. Or should do in theory.
Perceptually, however, we are able to distinguish differences in brighter shades more closely that we are darker ones, so it can be more practical to convert colour to greyscale non-linearly (see Frequently Asked Questions about Gamma by Charles Poynton). This is especially demonstrable when looking a text legibility over a background, see Text Readability in Colour.
There are different formulas for converting a colour image to greyscale, and they do not all give the same results, yet on a monochrome conversion all of them look fine, it is when the conversion needs to be for comparative purposes that the formula used becomes significant.
The following two greyscale images use different formulas for converting the colour image and it can be seen that the results are quite different, though still both greyscale. The leftmost uses an non-linear formula and the rightmost a linear one; probably the leftmost one is the more percetually convincing. The formula differences are explained more fully on Greyscale Formulas and an example showing how different results can be depending on which formula is used, on Greyscale Comparisons.
colour pic missing inverse gamma pic missing lightness pic missing

 

 
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