Vernier PAR sensors are calibrated before shipping and should not need calibration. You can verify the calibration using the Clear Sky Calculator at:

The Clear Sky Calculator for Quantum Sensors reports the theoretical PPFD at any time of day at any location in the world on a cloudless day. The application is most accurate at solar noon in the spring and summer.

Please note that for best accuracy the sky must be completely clear. To determine the theoretical PPFD at your location, you will need to know your latitude, longitude, altitude, time of day, air temperature and humidity. Complete the required fields in the web application and compare the Model Estimate of PPFD to the PPFD that you measured with your Vernier PAR Sensor.

The measured PPFD should be within (+/-) 5% of the Model Estimate for full sunlight. Repeat several times near solar noon over several days. If the reading from a Vernier PAR sensor is consistently different from the Clear Sky Calculator, you may need to clean the sensor. If cleaning the sensor does not correct the problem, please contact Vernier for technical assistance.