Googled "luminescence" and OpenGL -- don't seem to return relevant results. Could you point to where it's described?
replace/luminescence/luminance (thanks autocorrect).
Legacy OpenGL APIs used to assume sRGB, so you had to specify GL_LUMINANCE for non-color 'intensity' maps (which couldn't be blitted to FBOs, e.g.).
Modern OpenGL assumes linear color, so instead you have to specify sRGB on texture load to direct the driver to do colorspace conversion (e.g. GL_SRGB8 for typical RRGGBB byte triples).