The calibration should be made in a temperature-controlled room if non-temperature-controlled
detectors are used. The recommended temperature is 23 °C. Humidity control may be
necessary if humidity-sensitive optical detectors are used, or if there is the possibility of
condensation on the components. A change of the laboratory's humidity may change the
absorption of air and thereby change the power. This effect is relatively strong between 1 360 nm
and 1 410 nm, especially when a sequential-type, open-beam calibration is used and the humidity
changes between the steps. In parallel-type calibrations with open-beam paths of approximately
the same lengths, the measurement results of both the reference meter and the test meter will
change at approximately the same time, with negligible effect on the calibration result.