Calibration of LI-COR Radiation Sensors

LI-COR encourages our customers to return their LI-200 Pyranometer Sensors for recalibration (suggested every 2 years) between the months of March and September. More information can be found on our blog.

Recalibration of LI-COR radiation sensors is recommended every two years. Sensors must be returned to LI-COR for recalibration. Please contact LI-COR for a return materials authorization (RMA) number prior to returning any instruments.

Importance of Calibration

Calibration is an integral step in the manufacture or repair of all LI-COR optical radiation measuring instruments. All sensors are designed to provide data in standardized and accepted measurement units. Because of the normal variation in internal optical components, it is necessary to characterize each device by measuring its output from a standardized source. For fixed sensors, this calibration data is supplied as a "calibration constant" which indicates the amount of sensor output for a given amount of measurable energy.

There are two methods by which the readout device used with a sensor may be adjusted to the calibration constant to provide a direct readout. The constant is numerically entered into the instrument's internal electronic memory via the instrument's keypad. Adjustment of the internal "trimpots" (electromechanical memory) is done. Instruments without any provision for adjustment must have their data scaled by a factor determined from the calibration constant.

The characteristics of the optical components change with time and may also be affected by environmental conditions. Periodic recalibration is necessary to restore correct measurements. The time intervals suggested by LI-COR for recalibrations should be seriously considered. Data of a critical nature requires more frequent calibrations than those suggested, as well as calibrating under the conditions of actual use (e.g. same temperature). Ultimately, the only way to verify performance is to recalibrate.

Basis of Calibration

All LI-COR sensors (except pyranometers) are calibrated using working standard quartz halogen lamps, which have been calibrated against reference standard lamps traceable to the U.S. National Institute of Standards and Technology (NIST). Standard lamp current is metered to 0.035% accuracy. Microscope and laser alignment in the calibration setup reduce alignment errors to less than 0.1%. Stray light is reduced to less than 0.1% by black velvet background. The absolute calibration accuracy is limited to the uncertainty of the NIST-traceable standard lamp. The absolute calibration specification for LI-COR sensors is ± 5% (typically ± 3%) traceable to NIST.

Pyranometer sensors are calibrated against an Eppley Precision Spectral Pyranometer (PSP) under natural daylight conditions. Typical error under these conditions is ±5%.