Ultraviolet-C (UV) light is a component of the electromagnetic spectrum that falls in the region between visible light and X-Rays.
This invisible radiation includes the wavelength range of 100 nm to 400 nm. UV light can be further subdivided and categorized into separate regions:
Now we have established UV-C as a form of light, we must also accept that UV-C is governed by the exact same laws as visible light. Most importantly for disinfection purposes - shadow and intensity over distance.
100 nm to 200 nm
Far UV or vacuum UV
(these wavelengths only propagate in a vacuum)
200 nm to 280 nm
UV-C - useful for disinfection and sensing
280 nm to 315 nm
UV-B - useful for curing, tanning and medical applications
315 nm to 400 nm
UV-A (or ”near UV”) - useful for printing, curing, lithography, sensing and medical applications
Most people know that light travels in all directions in straight lines and objects in the path of light cast a shadow (see Fig. 1). That being said, it’s important to revisit the issue and understand the challenges shadowing creates when using light for disinfecting.
By far, the most powerful source of UV-C radiation in the solar system is the Sun. The amount of UV-C generated by the Sun every second is higher than all of the artificially generated UV-C in the history of UV-C disinfection combined (see Fig. 2).
Imagine being on holiday in Spain mid August and the dangers of spending too much time in the Sun. Sunburn is a well-known result of overexposure to UV light. Now, have you ever stopped to think why you cannot get a sunburn at night? The answer is simple! The part of The Earth facing the sun blocks the light from the part facing away from the Sun. Otherwise known as day and night (see Fig. 2).
If we now apply this fundamental law of physics to a patient room or operating theatre scenario, how can we expect a man-made device to accomplish what the Sun cannot? The only effective approach to avoid shadowing in a healthcare setting is to reposition the light source (UV-C device) as many times as necessary.
THE INVERSE SQUARE LAW
Just like shadow, another law of light that complicates the use of UV-C as a room disinfectant is intensity over distance. The loss of light intensity over distance can be easily calculated using the inverse square law.
We know that UV-C intensity at 1m is 100% therefore, the light intensity at 2m will fall to 25% (a quarter). At 3m the intensity drops further to 11% (a ninth) and at 4m, the intensity is only 6.25%.
The inverse square law dictates that when we want to reach the same level of UV-C intensity (or germicidal effect) achieved at 1m distance, it is necessary to radiate for 9 times longer from 3m distance and 16 times longer from 4m distance.
In 2017, an experiment to measure the intensity of light from a UV-C device in a local hospital was conducted by Blue Ocean Robotics. Light intensity was measured using a Spectrophotometer with the maximum intensity shown in red (see below). The experiment clearly proved how light intensity is significantly affected by distance even, in a small single patient room. Figs. 3 and 4 illustrate how light intensity drastically drops over distance when radiating from a single position. It was concluded that complete coverage of the single room with maximum UV-C illustrated in Fig. 5 was only achievable when the results from 6 separate positions were combined.
UV-C has a very short wavelength which means approximately 95% of the energy is ab- sorbed by many types of molecules present in modern plastics and paints. Only 5% of the UV-C energy can be utilized for disinfecting objects in shadow from direct UV-C rays.
In a realistic healthcare environment reflection is often combined with distance as the examples 1 and 2 illustrate. In order to radiate UV-C on to the blind side of a hospital bed rail, the light must first travel to the wall in order to be reflected. This means that the 95% loss of energy due to reflection is not the only loss of energy to consider. Addi- tionally, the loss of intensity as per the inverse square law must be calculated:
Calculation of UV-C radiated onto the blind side of hospital bed rail (marked in red).
Distance light travels is approx. 4 metres = UV-C light intensity is approx. 6.25% (a sixteenth). Energy reflected off the wall = 5% (95% absorbed).
Total UV-C radiated onto blind side of hospital bed rail = 5% of 6.25% = 0.3% Conclusion with example 1:
1 minute direct UV-C at 1 metre = 300 minutes of reflected UV-C