How to calibrate Si1133? How is UV index calculated?
To achieve the best performance, calibration is essential for Si1133 because of sensor-to-sensor and unit-to-unit variation in sensor's placement with respect to the diffuser (or window opening), as well as the variation in the material of the overlay and the diffuser. We recommend the customer to perform calibration on the prototype after the optical design is completed.
There're 2 options for the light source in the calibration procedure, one is to use the sun and the other is to use a solar simulator. A commercial UV index meter is also required as the reference. Here're the basic steps for calibration. First of all, set up a test such that the Si1133 sensor and the UV meter can be placed at the same location under the light source. Secondly, log multiple readings of both the sensor and the UV meter across different UV index levels (0 ~ 10 UVI). Lastly, fit the result to a second-order polynomial equation:
UV_index_reading = k(m × Si1133_raw_data ^ 2 + Si1133_raw_data)
The coefficients k&m can be later used to calculate UV index based on Si1133's readings.
Silicon Labs provides default configuration and coefficients to calculate UV index as well as optical design recommendations in the UV application note AN968:
The software example code is also attached.