Programs will expect that SIGNAL represents the integration of the photon flux for the exposure time given in the EXPTIME column (see section I.6.1). That is, it is the ratio of SIGNAL to EXPTIME that represents an actual photon count rate, or intensity, if the exposure times vary.
This assumes the data are in arbitrary intensity units (times time). Sometimes one must deal with data in the form of magnitudes, as in re-reducing old data, or data measured from strip-charts with a magnitude ruler. In that case, the column label should be RAWMAG; see Table I.18. Programs will expect RAWMAG to be the usual negative logarithms of intensities; that is, RAWMAG values only use EXPTIME in determining weights.
Except for pulse counting with photomultipliers, where reasonably accurate models exist for the nonlinearity, the SIGNAL values should have been corrected for nonlinearity. In any case, the EXPTIME values should have been corrected for differences between nominal and actual exposure times. Such corrections are especially important for CCD data, where they can vary across the frame (see Stetson ).