The 1931 2 degrees standard colorimetric observer can be used for photometry as well as for colorimetry. But there is confusion as ro whether this applies also to the 1964 10 degrees standard colorimetric observed. This system was derived by Judd, who did not fully report his procedure. Its validity in photometry and the way in which Judd derived the transformation equations are examined. (C) 2001 John Wiley & Sons, Inc.