The Measurement of Ultraviolet Radiation in Biomedical Field

The measurement of ultraviolet radiation in biomedical field may be categorized into three classes:

• Physical
• Biological
• Chemical

In general, physical devices measure power, whereas chemical and biological systems measure energy.

The common radiometric terminology is shown in the table below:

Table 1.0 Radiometric Terms

In clinical photobiology, the derived unit of milliwatt per square centimetre is commonly used and radiant exposure tends to be referred to as dose. Take note of the fact that, dose in this context differs from the term used in radiobiology where dose indicates energy absorbed per unit mass of the tissue.

To calculate for instance, the time for which a patient who is prescribed a certain dose (in J/cm²) should be exposed when a radiometer indicates the irradiance in mW/cm², we can use the relationship shown below between these quantities:

Physical UV Detectors

Physical UV detectors make use of either thermal or photon methods.

Thermal Detectors

In thermal detectors, the absorption of radiation increases the temperature in the detector element and this rise in temperature is measured by some mechanisms. Thermopile UV detectors are the simplest and commonest thermal device used to measure UV irradiance. A multi-junction thermopile is formed from a number of thermocouples in series, which generate a voltage that is proportional to incident energy in the form of heat. Thermopiles designed for use with UV radiation are fitted with quartz windows, which transmit well in the UV range. The merit of thermal detectors is that they have a relatively flat spectral response over a wide wavelength range.

Photon Detectors

Photon detectors operate by absorbing discrete quanta of photon energy, and therefore have a threshold wavelength above which no radiation is detected. The lower wavelength limit is related to optical properties of the detector or associated filters. The response of photon devices are therefore inherently wavelength dependent and thus have to be calibrated for each source of interest.

Photo emissive detectors have a photocathode from which electrons are ejected when photons are absorbed. These electrons are then collected by an anode and a current is produced. The simplest of this type of detector is the vacuum phototube consisting of an evacuated tube with a potential difference applied between the cathode and the anode. This device has a gain of unity and a low responsivity {approximately 0.05 A/W)}. A gas-filled phototube has a gain of   ̴10 due to secondary ionization of the gas in the tube that has the effect of producing a greater anode current. Photomultiplier tubes has a series of electrodes (termed to as dynodes) having successively greater potential differences applied between them. As electrons hit each dynode, they release further electrons which in turn release more electrons at the next dynode, etc., leading to a high overall gain (typically 10⁶). The responsivity of photomultiplier detectors tends to be high (approximately 5 x 10⁴ A/W).

Photodiodes or junction photodetectors have a depletion region formed by the junction of n and p doped semiconductor material. On absorption of a photon in this region, electron hole pairs are formed that are then swept out of the region and cause a current to flow in an external circuit. These devices can either be operated in a zero bias or reverse bias mode. For UV detection, GaAsP, GaP, or Si photodiodes are employed. These photodiodes are small, inexpensive, and rugged with good responsivity (approximately 0.1 A/W) and are perfect UV detectors.

Also read: The Measurement of Tissue Optical Properties

UV Radiometers

A radiometer is a complete ultraviolet (UV) radiation measurement device, consisting of a detector and a meter to amplify and display the detector output.

Narrow-band UV radiometers are used to measure the irradiance of a source in the different UV bands. The detector consists of a diffuser to collect UVR, a filter and the sensor e.g. a photodiode. Lambert’s law states that the irradiance falling on a surface varies with the cosine of the incident angle. A good diffuser should possess an angular response close to the ideal cosine response especially in phototherapy, where arrays of tubes are used leading to a large source area irradiation. Narrow-band radiometers have a response that varies with a wavelength, and therefore they must be calibrated for each type of source that they will be used to measure.

Broad-band radiometer possesses no filter, and therefore measurement of the total irradiance from a source is achieved. Typically these incorporate a thermocouple detector connected to an amplifier and display electronics.

Spectroradiometry

 Spectroradiometry is focused on the measurement of the spectrum of a source of optical radiation. Spectral measurements are used in the calculation of biologically weighted radiometric quantities.

We have three basic requirements of a Spectroradiometry system, namely:

1. Input optics, designed to conduct the radiation from the source into the Monochromator.
2. Monochromator, which usually incorporates one or two diffraction gratings as the wavelength dispersion elements.
3. An optical radiation detector, either a photomultiplier tube or a solid state photodiode.

The ultraviolet radiation is collected by a diffuser, which should have a good angular response, and transmitted to the Monochromator by a light guide. A quartz optical fiber is needed as quartz has a very high UV transmission, unlike ordinary glass. The Monochromator allows the separation of polychromatic radiation into very narrow bandwidths (typically 1 nm) thereby allowing the spectral irradiance of a source to be measured. This is achieved by using diffraction grating, which disperses the incident radiation. Scanning spectroradiometer systems operate by measuring at a given wavelength, then changing the angle of the diffraction grating, making another measurement, etc. For photobiology applications, it is recommended that a double diffraction grating system is employed. The second diffraction grating achieves a reduction in the radiation transmitted outside the waveband of measurement (stray radiation). Very small amounts of stray radiation may have large implications in the measurement of erythemally weighted irradiance if it occurs at biologically highly effective wavelengths.

The spectroradiometer system requires a high responsivity detector as narrow bandwidths are chosen.  Spectroradiometers need to be calibrated with reference to a standard lamp, which in turn has an output calibrated by a national standards laboratory. The shortcomings of double-diffraction grating scanning spectroradiometers systems are the high cost, large size, and relatively long time needed to obtain a complete spectrum. 

Chemical and Biological Methods

Chemical and Biological UV radiation measurement methods can be used in certain medical applications.

Polysulfone film changes its optical properties with the absorption of UV radiation. It is possible to measure this change and relate it to the exposure dose received by the film. These devices are useful for measuring human UV exposure as they are unobtrusive for individuals to wear.

Biological dosimeters utilize the inactivation of bacteria or viruses as a function of UV radiation dose. There are some applications, such as measuring UV doses in water flowing in a disinfection plant, where biological dosimeters are the only reliable way to measure UV dose.

Also read: Fiber-optic based blood gas sensors