## A first calibration-study for the Delta_T

UV_A and UV_B sensors

*Francis Massen & Nico Harpes*

**This report is now out of date ! **

Please read
" A second calibration-study for the Delta_T UV_A and UV_B sensors"
dated February 1997.

The following lines are left as an archive.

A first tentative approach to yield a "calibration-factor" for the 2 Delta_T UV sensors was done with the help of Mr. Markus Wallasch, Umweltbundesamt, Offenbach (Germany).
Mr. Wallasch makes continuous precise solar spectrum measurements with a spectrophotometer; his values are assumed to reflect the true situation at a given day.
Mr. Wallasch used his known {S(lambda)} solar spectral density values and applied to them the relative spectral responses {E(lambda)} of our Delta_T sensors.

A calibration factor can be found by comparing the integral of the {S(lambda)*E(lambda)} values computed by Mr.Wallasch to the raw readings of the Delta_T sensors. This method assumes that the same weather condition exist at the same time in Diekirch and in Offenbach (same latitude; difference of altitudes between Diekirch (218m) and Offenbach (100m) considered negligeable). From the available data (1. July to 14 September 1994) we retained only those corresponding to cloudless days; cloudless means that the meteorological station of Luxembourgs airport Findel reported no clouds during a whole day; this left 767 remaining cases.

**UV_A sensor: **

The plot of the computed UV_A data versus the "raw" readings gives a linear fit with a slope of 2.69;
hence the UV_A readings stored in the data files should be multiplied by 2.69 to give the "true" irradiance
in [W/m2]. As the UV_A sensor does not extend markedly into the UV_B region, no windowing will be applied for a first approximation, so the definitive calibration multiplier is** 2.69**.

**UV_B sensor: **

The plot gives a linear fit of 0.94, close to 1. This means that the readings of the UV_B sensor correspond
approximately to the "true" power received; as this sensor extends largely into the UV_A domain, a
windowing function must be applied, which gives a multiplier of 0.27.

If we neglect for the moment the difference between 0.94 and 1, the definitive multiplier **0.27** will yield the best approximation to convert the stored UV_B readings into "true" UV_B irradiance in [W/m2].

*It should be noted that the UV_B sensor has a slight zero offset of ca. +0.13 ; this value should be
substracted from the readings before multiplying by 0.27!*