for 95% of readings must be less than 0.5 seconds; while ‘spectrally flat’ can apply to sensors with a spectral selectivity of less than 3% in the 0,35 to 1,5 μm spectral range. While most Class A pyranometers are ‘spectrally flat’, sensors in the ‘fast response’ sub-classification are much rarer. Most Class A pyranometers have a response time of 5 seconds or more. The calibration is typically done having the World Radiometric Reference (WRR) as an absolute reference. It is maintained by PMOD in Davos, Switzerland. In addition to the World Radiometric Reference, there are private laboratories such as ISO-Cal North America who have acquired accreditation for these unique calibrations. For the Class A pyranometer, calibration is done following ASTM G167, ISO 9847 or ISO 9846. Class B and class C pyranometers are usually calibrated according to ASTM E824 and ISO 9847. Photovoltaic pyranometer Photovoltaic pyranometers are standardized and calibrated under IEC 60904-4 for primary reference samples and under IEC 60904-2 for secondary reference samples and the instruments intended for sale. In both standards, their respective traceability chain starts with the primary standard known as the group of cavity radiometer by the World Radiometric Reference (WRR). Signal conditioning The natural output value of these pyranometers does not usually exceed tens of millivolt (mV). It is considered a "weak" signal, and as such, rather vulnerable to electromagnetic interferences, especially where the cable runs across decametrical distances or lies in photovoltaic systems. Thus, these sensors are frequently equipped with signal conditioning electronics, giving an output of 4-20 mA or 0-1 V. Another solution implies greater immunities to noises, like Modbus over RS-485, suitable for ambiances with electromagnetic interferences typical of medium-large scale photovoltaic power stations, or SDI-12 output, where sensors are part of a low power weather station. The equipped electronics often concur to easy integration in the system's SCADA. Additional information can also be stored in the electronics of the sensor, like calibration history, serial number. See also Actinometer Photodiode Heat flux sensor Net radiometer Pyrgeometer Pyrheliometer Radiometer Sunlight Solar constant Sun path References External links Meteo-Technology instrumentation website Measuring instruments Meteorological instrumentation and equipment