A UV-B radiometer for making field measurements of UV-B (280 to 320 nanometers) irradiance from solar sunlight and a UV-B lamp source

A UV-B radiometer was developed for field use in measuring solar ultraviolet-B (UV-B) irradiance from 280 to 320 nm and UV-B irradiance from a UV-B lamp source. The steps taken in the analysis and design, implementation, and testing of this device are presented. The radiometer was designed for an operating temperature of -10 to 60°C and a supply voltage of 30 V dc.

The circuit was designed to be a three-wire field instrument, with the output being a 4- to 20-mA signal contained in the power supply loop used to bias the circuit. The circuit signal bandpass was designed to be 1.6 Hz, and this was measured and verified. The output noise of the circuit was calculated to be 88 µV and was measured to be 92 µV (less than 1 count of the analog-to-digital converter this signal interfaced with).

The output of this radiometer was compared with an off-the-shelf radiometer, and correction factors for both meters were calculated to correct their outputs for measuring only the UV-B irradiance of 280 to 320 nm. After taking into consideration the differences in the design of the sensors, difference errors in their readings were calculated to be~15%.

The worst-case circuit drift was calculated to be less than -0.25% of full scale over the temperature operating range of the instrument when the temperature-compensating resistor is used in the feedback path of the transimpedance amplifier. The output zero current error was measured to be approximately ±0.2% for a temperature span of 0 to 50°C.