Homogenization

Abstract

Photometry consists of quantitative observations. Besides measurement errors, which are discussed in another section, these observations are affected by the quality of the instrumentation. In most experiments, the measuring apparatus has to be calibrated in order to ensure conformity to the established standards. For instance, land-surveyors and geometers need to know whether they use an accurate unit of length. It is quite easy to apply a relative correction of e.g., 10^-4 to all distances, if it appears that one uses a tape- measure which is off by that amount. Such differences would be more difficult to correct if they were found to be temperature-dependent: one could write down the temperature during each measurement and take that into account. More complex cases arise, e.g., when the temperature varies along the length of the tape. This would occur when surveying a field with a long tape-measure. The ground temperature varies according to the nature of the surface (rock, sand, grass...), to the wind speed and direction, to insolation, etc. We are then confronted with a situation where the measurement depends in an intricate way on the measured object itself. And unfortunately the measure gives almost no clue on the nature of the subject of the analysis, so an iterative correction procedure cannot be applied.