ANALYSIS AND OPINION UV ILLUMINATION g
safe use requirements given in EN 62471:2008, ‘Photobiological safety of lamps and lamp systems’.
How much UVC light is required for effective sterilisation?
The action spectrum of the Covid-19 virus to UVC radiation is currently unknown. An action spectrum is the relative response as a function of wavelength for a given effect. However, a standardised germicidal action spectrum could be assumed to apply, and that published by the Illuminating Engineering Society of North America is based on the deactivation of the E. coli bacteria, which extends from 235 to 313nm with a peak response at 265nm. Anecdotally, a target
irradiance of approximately 2mW/cm2 m2
(equivalent to 20W/ ) at 254nm would require
that the UVC germicidal light source remain stationary over the surface to be disinfected for tens of seconds. For a more complete description of the UVC dose required to achieve a 90 per cent deactivation (the D90 value) with a variety of coronaviruses, I will refer to a paper published on
ResearchGate.net [1]
relationship, which means that if the source to sample separation is doubled, the irradiance will drop to 25 per cent of the original level. It is incumbent on manufacturers of UVC sterilisation products to specify the distance at which their lamp achieves its specified irradiance. If not, the user must purchase a suitable UVC radiometer or an appropriately configured UV spectrometer system to measure the irradiance in-situ.
. The range of
D90 values for the stated viruses ranges from 7 to 241J/m2, with an average of 67J/m2. This would be equivalent to exposing the virus to a mean irradiance of 1.116 W/m2 (or 0.1116 mW/cm2) for just over one minute. However, this throws up a major concern. As with any light source, the irradiance delivered onto a surface varies with the separation between lamp and the irradiated surface. The irradiance follows an inverse squared
8 Electro Optics June 2020
UVC light ‘wands’: caveat emptor The Covid-19 pandemic is being seen by a number of opportunistic business people as an excuse to market low-cost, UV light ‘wands’ as a kind of miracle cure for disinfecting domestic surfaces of the virus. Some of these devices are of very questionable efficacy and it is clear that uninformed consumers are being taken advantage of. An informal market survey conducted by the author revealed that most manufacturers offering low cost, consumer-orientated, UV light ‘wands’ failed to specify a working distance for their product, assuming even that they quoted an actual irradiance specification at all (most did not). This is worrying, as the general public are not light metrology experts and cannot be expected to know that the effectiveness of these devices varies enormously both with the distance they are held over a surface and with the time that the surface is left exposed to the UV radiation. Moreover, many of the UV
light ‘wands’ on offer seem to suffer from a number of fundamental flaws. Some seem to be equipped with violet LEDs
that do not emit in the required UVC band. Others seem to be fitted with plastic diffusers that would not transmit the UVC light anyway. Still more are being manufactured using UVC LEDs, but produce an intensity that is an order of magnitude lower than can currently be obtained from an equivalent discharge lamp, requiring that the wand be held over a surface for an excessively long time to achieve the required UVC dose. Also, few if any vendors take the trouble to point out that skin and eye exposure to the UVC radiation should be avoided, due to photobiological safety concerns.
Measuring UVC radiation There are two optical radiation measurement methods used to quantify the amount of light emitted from a germicidal UV lamp – radiometry and spectroradiometry. The simpler and lower-cost
method is to use a photodetector filtered to receive the 254nm peak emission from the low- pressure mercury lamp. This type of instrument is referred to as a radiometer and will be calibrated to record dose or irradiance in units of Joules per square metre or Watts per square
“The general public are not light metrology experts and cannot be expected to know that the effectiveness of these devices varies enormously both with the distance they are held over a surface and with the time that the surface is left exposed”
metre (or equivalent) at 254nm. In their favour, such radiometers are affordable, portable, and simple to operate. Their main drawback is that they only reveal how much light is present at 254nm; they yield no information on the amount of UVC radiation as a function of wavelength over the entire UV or UVC waveband. Pro-Lite supplies UVC radiometers made by Solar Light Corporation. The more complete
approach is to configure a spectroradiometer that measures
the complete UVC (or entire 200-400nm UV) waveband and reports the irradiance as a function of wavelength (spectral irradiance). Often equipped with a flexible optical fibre with cosine receiver as the collection optics, the spectroradiometer will generally require the use of a laptop computer, so will not be quite as simple to use or as portable as a handheld radiometer; however, the recorded spectrum of the light source allows for the computation of its effectiveness, taking into account the actual (or presumed) action spectrum of the germicidal effect. Pro-Lite supplies modular spectrometer systems made by Avantes (such as the AvaSpec-ULS2048) as well as the dedicated Specbos spectroradiometer made by Jeti (such as the Specbos 1211 UV).
In summary UVC germicidal lamps are a proven means of disinfecting surfaces infected by all manner of bacteria and viruses, and the Sars-Cov-9 virus responsible for the Covid-19 respiratory illness is expected to be no exception. Key to the effectiveness of the UVC lamp is the irradiance delivered at a surface, which in turn is a function of the separation between lamp and the object being irradiated. To ensure that a sufficient UVC dose is delivered, the use of a purpose- designed UVC radiometer or spectrometer configured for UVC measurements is highly recommended. Caution should be exercised when considering the use of low cost, consumer- orientated UV light ‘wands’ that are currently flooding the market, as these have a dubious efficacy and questionable photobiological safety. UVC germicidal lamps are emphatically unsafe for exposure to unprotected human eyes and skin, and must not be used for hand sanitisation under any circumstances. EO
References [1]
Kowalsky W., Walsh, T., Petraitis,
V. 2020 COVID-19 Coronavirus Ultraviolet Susceptibility, March 2020, https://doi:10.13140/ RG.2.2.22803.22566
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