There are several equations to combine the R,G,B, of each pixel into a simple luminance value. By example, grayvalue= (30*r+59*g+11*b)/100. Also, we may use the most simple way which means the average grayvalue=(r+g+b)/3.

Like Rhadamés said, there are several ways to do it.

But it might well be that the signal you are interested in, is the strongest in one of the RGB channels, and by converting to monochrome, you will loose some of that signal. It could also be worthwhile to investigate whether your signal is stronger in other color representations, like CMYK, HSL and HSV.

Thanks for your answers