--- ray/src/common/color.h	1990/12/14 16:28:20	1.8
+++ ray/src/common/color.h	1991/03/18 09:32:49	1.9
@@ -48,6 +48,8 @@ typedef float  COLOR[3];	/* red, green, blue */
 #define  normbright(c)		(int)((67L*(c)[RED]+168L*(c)[GRN]+21L*(c)[BLU])/256)
 #endif
 
+#define  luminance(col)		(683.0 * bright(col))
+
 #define  intens(col)		( (col)[0] > (col)[1] \
 				? (col)[0] > (col)[2] ? (col)[0] : (col)[2] \
 				: (col)[1] > (col)[2] ? (col)[1] : (col)[2] )