UV filter radiometer

For instrumentation without a built-in sensor, calibration can be performed manually using a UV filter radiometer, luxmeter, thermopile or chemical actinometer (18-20). 

A UV filter radiometer provides a convenient measure of the total UV output of a photon source, if properly designed for a specific source. They are broadband meters designed to measure incident radiation in the entire selected, UV region. An optical filter is used to limit the spectral responsivity to a certain band (e.g., UVB, near-UVA, or far-UVA). Ideally, the spectral responsivity is constant within the band and zero outside, but this is not the case in practice. Also, the characteristics of the filters may change with time and the device should be calibrated at least once per year. 

It is important to differentiate between calibration of the UV filter radiometer or luxmeter as prescribed in the ICH guideline and calibration of these devices for a specific lamp. In the first case, the calibration is carried out by the manufacturer to establish the response curve of the device. If the meters are calibrated against a standard lamp and used as received, they are well suited for measuring any variation of irradiance across the sample area and changes in total output with time. But they cannot be used to give an absolute measurement of irradiance or to compare irradiance between sources unless they are calibrated specifically for each source (21). 

A procedure for the calibration of radiometers against a specific light source is suggested in Table 2. It is important to remember that neither the UV filter radiometer nor the luxmeter provides information on the spectral power distribution of sources. A detailed plot of the irradiation as a function of wavelength is only obtainable by use of a spectroradiometer. At present, such equipment is not used on a regular basis. 

UV filter radiometers may be calibrated to give an absolute measurement of irradiance for a particular type of source, e.g., xenon or fluorescent tube of a... 

The spectral sensitivity of filter radiometers is not constant over the sensitive wavelength region. Two typical spectral sensitivities of UV radiometers are shown in Figure 5.13. The response of such radiometers depends on the measured spectral power distribution. If the calibration distribution differs from the measured distribution, a spectral mismatch must be taken into account. If the measured spectral distribution is known, the mismatch can be calculated. 

Neither filter radiometers nor luxmeters can be used to obtain an absolute measurement of irradiance or to compare irradiance between sources unless they are calibrated specifically for each source (Tpnnesen and Karlsen, 1997). A spectro-radiometer is needed for a detailed estimate of the SPD but at present such equipment is not widely used on a regular basis because of cost and convenience. The total irradiance (i.e., actual number of photons) can be determined by chemical actinom-etry using a reaction of known photochemical efficiency (Chapter 3 and Chapter 6). The chemical actinometer listed in the ICH guideline (quinine hydrochloride) has its limitations and its suitability as actinometer has been questioned (Baertschi, 1997 Bovina et al., 1998 Drew, 1998). This actinometer is not suitable for calibration of option 1 radiation sources (Thatcher et al., 2001a, b). An alternative actinometer (2-nitrobenzaldehyde) has recently been discussed for calibration of option 1 (and other sources) for UV irradiance (Allen et al., 2000). 

Preparations of DNA and bacterial suspensions irradiated using a germicidal bulb (Osram, Germany) through an optical filter of 254 nm with a power light exposure of 6.7 W/m as determined by UV radiometer (TKA-PKM, Russia). 

The measuring device is Robertson-Berger broadband filter UV radiometer (RB-biometer). The time series of 10 min sums obtained during August 1, 1996 to June 25, 1997 was supplemented by (i) daily averages of total ozone concentration measured by Dobson spectrophotometer in HK [17, 18], (ii) hourly totals of global solar radiation measured by a Kipp-Zonen CP-11 in HK, and hourly fractions of sunshine... 

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