Dose & Fluence

You may hear these terms used interchangeably. Regardless the term, mJ/cm2 is the important factor when evaluating efficacy of a UVC disinfection system with respect to the specific pathogen(s) being treated.

UV Dose: The amount of UV irradiation absorbed by an exposed population of pathogens, typically in units of mJ/cm² (1.0mJ/cm² = 1,000 µW/cm² per second). mJ is milliJoules.

UV Fluence: The total radiant energy incident on an infinitesimal sphere. Technically, this term is more accurate than the term “UV Dose.”

1: Newton’s Inverse Square Law

The delivery of UV-C energy is based on the simple laws of physics, specifically the Inverse Square Law. This law states that every time the distance is doubled, the intensity of light energy is reduced by 2 squared or ¼ of the power, not simply by ½. For example, if the distance to a surface increases from 1ft to 2ft from the UVC lamp, we are left with only ¼ of the energy. If the distance to a surface increases from 1ft to 10ft. from the lamp, we are left with only 1/100 of the energy.

2: UVC Energy

This is simply the power output of the lamp(s) in the UVC system. Not all lamps put out the same power, which is typically measured in uW/cm2 or mW/cm2. 1000uW = 1mW.
IMPORTANT: Be sure to know how far from the lamp(s) this energy is being measured. Remember the Inverse Square Law above. One product may appear to have higher energy, but the manufacturer may be measuring it closer to the lamp source.

  • HiTech UVC systems use multiple UVC lamps for higher power.

3: Exposure Time

Each pathogen, whether a virus or a bacteria, require a specific amount of fluence/dosing to kill the bacteria or deactivate the virus. As stated above, the dose is a function of energy level and duration of exposure. Double the time and you have doubled the dose.

4: Angle of Incidence

With a UVC tube, the angle that UVC radiation emits from it is uniform around the tube. Thus, angle of incidence relative to which part of the tube is directed at the surface is not an issue. For UVC LEDs, this is not true. As the treatment surface get further from the center beam of the LED, the energy level can drop significantly. For UVC lamps the angle of incidence is more a function of shadows or surface pores that may not irradiated or only receives partial irradiation.

5: Type of Surface

As mentioned in the Angle of Incidence discussion above, surfaces are rarely absolutely smooth. They may have tiny pores, cracks/crevices, holes, etc. Fabric and similarly porous and uneven materials are particularly difficult to disinfect and require higher dosing. This is a general problem for all UVC products.

6: Specific Pathogen(s)

As we are all aware, there is no shortage of bacteria and viruses that can make us sick. Some are similar in nature and others are completely different. Thus, the amount of fluence required to kill or deactivate pathogens is not a fixed amount of mJ/cm2 . It will vary from one pathogen to another. With that said, many of the most troublesome pathogens can be destroyed at specific thresholds.

  • Building owners and managers should consult with local experts or do research to determine if a threshold is valid for the types of pathogens they wish to destroy.

  • Airborne pathogens are more easily destroyed by UVC than those on surfaces.