Portable lamp irradiance calibration units of different size were used to transfer a relative calibration in a group of spectroradiometers operating at the same site. Every unit was used as normally in the field or at a home laboratory, i.e., including any errors related to the method itself. The calibration obtained for each instrument was used to reprocess their synchronous solar measurements. Solar measurements deviating by typically ¿10% got a standard deviation as low as 3--4% when corrections derived from a common lamp measurement were applied. Because instrumental features remained the same in the original and in the reprocessed solar data, the improvement of the agreement in the solar measurements was considered as an indication of a more homogeneous irradiance scale. The remaining disagreement must be a sum of the uncertainty related to the use of each calibration unit and any instrumental peculiarities. Each of the lamp units could bring the scales of the specific instrument group closer. This was especially true for distinct outliers in the group. Even the smallest units performed almost as well as more sophisticated equipment, and an agreement of about 5% in the sky measurements could be reached, which is better than that seen in most recent intercomparisons. ¿ 2000 American Geophysical Union