From MSDN:
GL_LUMINANCE: Each element is a single luminance value. It is converted to floating point, and then assembled into an RGBA element by replicating the luminance value three times for red, green, and blue, and attaching 1.0 for alpha. Each component is then multiplied by the signed scale factor GL_c_SCALE, added to the signed bias GL_c_BIAS, and clamped to the range [0,1] (see glPixelTransfer).
[This message has been edited by gvm (edited 02-18-2004).]