Annex 8. Choosing a radiometer for measurement of ultraviolet C irradiation

The manufacturer’s specifications should be checked to determine whether the radiometer has the appropriate characteristics for wavelength, irradiance measurement and accuracy, based on the ultraviolet C (UVC) source being used (1)

Wavelength range

The radiometer chosen should be able to measure wavelengths of 220–280 nm with a peak response at 254 nm for standard UVC254 low-pressure mercury lamps.

  • If measuring sources other than UVC254 low-pressure mercury lamps, look for a radiometer calibrated to the peak output of the concerned source.
  • If using more than one type of UVC fixture with different wavelengths, consider purchasing a radiometer that can be programmed to measure multiple wavelengths (rather than using radiometers specific for individual wavelengths).

Irradiance measurement range

The radiometer chosen should be able to measure effective25 irradiance within a recommended range of at least 0.1–2000 µW/cm2 for standard UVC254 low-pressure mercury lamps.

  • The upper end of the range may need to be increased if high-output, unbaffled UVC fixtures are used.
  • For wavelengths other than 254 nm, the range may need to be shifted up or down based on the peak output of the lamp (depending on the manufacturer’s specifications).


Accuracy may be referred to as “measurement uncertainty” under specifications. The radiometer should have an accuracy (measurement uncertainty) for both of the following criteria:

  • Accuracy for measurements of UV irradiance of more than 1 to 2000 µW/cm2 should be ±10% of the reading (not ±10% of the upper end of the radiometer range), to measure irradiance and confirm performance of the source or lamp.
  • Accuracy for measurements of UV irradiance of 0.05–1 µW/cm2 should be ±0.05 µW/cm2 , to measure safety levels for occupants.

Some radiometers meet both of the accuracy criteria required; however, if a radiometer meets only one of the two criteria, a second radiometer that meets the other criterion will be needed. Reputable companies will disclose this information; if the information is not given on the company’s website, then it is best to speak with a representative of the manufacturer.

  • Calibration instructions: The radiometer should be calibrated according to the manufacturer’s recommendations. If no recommendation is provided, then annual calibration is recommended.
  • Field of view (FOV) cone: An FOV cone is a separate accessory for the radiometer that should be used for all safety measurements. It should be ±40 degrees (80 degrees total) and must be compatible with the radiometer model.

Reference for Annex 8

  1. Tuberculosis infection control: a practical manual for preventing TB. San Francisco, CA: Curry International Tuberculosis Center; 2022 (

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