I am reading a book for image processing algorithms (Andreas Koschan, Mongi Abidi: Digital Color Image Processing) and for the contrast algorithm it says that I can either go
RGB->HSL or RGB->HSI first and than apply a contrast technique for grayscale images, on the lightness component.
Then it gives this formula only, not other formulas for color conversion:
$$L(x,y) = 0.299*R(x,y) + 0.587*G(x,y) + 0.114*B(x,y)$$
This formula is neither for the L in HSL, neither for the I in HSI and that is what confuses me.
Can you explain what this formula stands for, and maybe help with the appropriate conversion formulas?