Since the appearance of the ISO publication "Guide to the expression of uncertainties in measurement" (GUM), in 1993, many practitioners in metrology have realised its limitations already indicated in the guide. The guide was intended for general use, special requirements like that of radiometry, propagation of distributions, multiple output, etc. are not met by it. Therefore, CIE dedicated its 2001 Symposium to this hot topic and held a symposium at the CIE Central Bureau, Vienna, Austria, on the 23 and 24 January 2001.

Two sections were organized:

•Advances in Uncertainty Evaluation
•Key Comparisons and the Mutual Recognition Arrangement.

Papers in the first section used mainly theoretical approach to uncertainty and among others dealt with uncertainties of quantities derived from spectral sums, with numerical methods in colorimetry, with uncertainties in models having more than one output quantity, with correlations in photometric measurements and with GUM supplements. The second section was based on the practical implementation of photometric and radiometric measurements mainly in the frame of key comparisons. Papers were read on the BIPM key comparison database, on the objectivity of key comparison reference values, on heuristic and optimal estimators in photon counting and on the evaluation of photometric measurements in different national measurement laboratories.

The Proceedings contain on 83 pages the text of the nine papers read as well as reports on the discussions.