It is impossible to give a single answer to this question, however, we will examine several possibilities:
- One of the instruments is damaged.
- One of the instruments is due for recalibration.
- There is a mismatch between the spectral resonse functions of the two instruments. This is particularly a problem if the light source being measured (e.g., the solar spectrum in the UV-B) is very different from the light source that was used to calibrate the instrument (e.g., a standard lamp).
- The two instruments have different collectors. For example, a QSR-2100 radiometer will always measure higher values of PAR than the PAR channel of a GUV-2511 radiometer because the former has a scalar collector and the latter has a cosine diffuser.