UV disinfection questions combine photometry, microbiology and safety in a way that general lighting questions do not, and the failure mode of a badly specified system is silent: the fixture runs, the indicator light is on, and the target organism survives. These are the questions we handle most often when a customer specifies a 275 nm LED disinfection system, with the numbers needed to check a design.
1. Dose and irradiance
Q1. What is the difference between dose and irradiance?
Irradiance is the power per unit area arriving at the target, in milliwatts per square centimetre. Dose is irradiance multiplied by exposure time, in millijoules per square centimetre. Inactivation depends on dose, so a specification that gives only a time is incomplete unless the irradiance is also fixed.
Q2. How do I calculate the exposure time?
Divide the required dose by the irradiance you can achieve at the target surface. If the target dose is 40 millijoules per square centimetre and the delivered irradiance is 1.0 milliwatts per square centimetre, the exposure time is 40 seconds. The chart below shows how the required time falls as irradiance rises.
Q3. What dose should I specify?
It depends on the organism and the required log reduction, and published values vary widely across species. For surfaces and air, most published work sits between 10 and 100 millijoules per square centimetre for a three-log reduction. Treat any single number as a starting point for validation rather than as a guarantee.
Q4. Why does my measured dose differ from the design value?
Three causes account for most of the gap. First, the irradiance falls with distance and with the cosine of the incidence angle, and both change across a real target surface. Second, LED output falls with junction temperature, so a fixture measured cold delivers less after it warms. Third, absorption and reflection losses between the LED and the target are rarely included in the design figure.
2. Wavelength and source selection
The germicidal effectiveness curve peaks in the UVC band near 265 nm, so a 275 nm LED sits close to the maximum. That proximity, not the exact peak, is what makes a 275 nm LED a practical germicidal source.
Q5. Why is 275 nm chosen for LED disinfection?
Because it sits close to the peak of the germicidal effectiveness curve while remaining producible at reasonable efficiency in an aluminium gallium nitride LED. Sources further from the peak require proportionally more electrical power for the same dose.
Q6. Are UV LED emitters interchangeable with mercury lamps?
No. The spectral output differs, the optical source size differs, and the material requirements differ. A mercury lamp at 254 nm and an LED at 275 nm require different dose derivations and different housing materials. Treat a substitution as a new design.
Q7. What is the difference between UVA, UVB and UVC?
The bands are conventionally defined as UVA from 315 to 400 nm, UVB from 280 to 315 nm and UVC from 200 to 280 nm. Germicidal action is strongest in UVC, which is also the band with the strictest human exposure limits, so a UVC installation carries a higher safety burden than a UVA application at the same optical power.
Q8. Do I need optical filtering for a 275 nm LED?
A well-made 275 nm LED package has a narrow emission band and does not need filtering for the germicidal function itself. Where the application also requires visible light to be excluded, a filter may be used, but it reduces the delivered dose and must be accounted for in the design.
3. LED performance and lifetime
| Question | Short answer | Detail |
|---|---|---|
| What wall-plug efficiency should I expect? | 2 to 6 percent for 275 nm | UVC LEDs convert a small fraction of electrical input into germicidal photons. Plan for the heat that the remaining input power produces. |
| How does temperature affect output? | Strongly, more than visible LEDs | UVC LED output falls faster with junction temperature than visible emitters do. Thermal design is not optional in a disinfection fixture; it directly sets the delivered dose. |
| What lifetime should I plan for? | Use a UV-specific maintenance figure | Standard LM-80 projections are made in the visible range and do not transfer directly. Ask for UV-band maintenance data at the application junction temperature. |
| Does the output degrade gradually? | Yes, and the degradation is faster | Optical degradation in the UVC band typically outpaces visible-band lumen depreciation, so replace or re-calibrate on a schedule derived from measured data. |
| How do I monitor degradation? | With a UV sensor | Fit a calibrated UVC sensor to the chamber and log irradiance, then drive the exposure time from the measured value rather than from a nominal figure. |
| Can I overdrive for more output? | Only with thermal headroom | Pulsed overdrive raises instantaneous output but also junction temperature, which accelerates both optical degradation and the failure of the encapsulant. |
4. Materials and housing
| Component | Recommended | Avoid |
|---|---|---|
| Housing | Anodised aluminium, 304 or 316 stainless | Untreated polycarbonate, painted mild steel |
| Optical window | Fused quartz, UVC-transmitting glass | Soda-lime glass, acrylic, untreated polycarbonate |
| Seals and gaskets | Silicone or EPDM rated for UV | Natural rubber, standard nitrile |
| Reflector | Polished aluminium with UVC coating | Standard anodised finishes that degrade in the UV band |
| Fasteners | Stainless steel | Zinc-plated steel, which corrodes and can shadow the beam |
| Driver | Constant current with open-circuit protection | Drivers without a defined no-load behaviour |
Q9. What housing materials resist UVC?
Aluminium and stainless steel housings with UV-stabilised seals hold up well. Untreated polycarbonate and many elastomers yellow and crack under prolonged UVC exposure, which is why a housing that performs well in a general lighting fixture may fail within months in a disinfection product.
Q10. Do I need a quartz window?
Where an optical window is required, use quartz or a UVC-transmitting glass. Standard soda-lime glass absorbs strongly in the UVC band and would remove most of the output before it reaches the target.
Q11. How should the reflector be specified?
Use a polished aluminium reflector with a UVC-compatible coating. Standard anodised finishes degrade and lose reflectance over time, and a reflector that loses 30 percent of its reflectance quietly removes 30 percent of the dose.
Q12. Can I use a standard LED driver?
Usually yes, with two conditions. The driver must be rated for the forward voltage range of the UVC LED string, which is typically higher than a visible LED string of the same length, and it should include protection against operation with no load, because an open circuit will apply the full compliance voltage to the fixture.
5. Safety and compliance
| Parameter | Design requirement | Reason |
|---|---|---|
| Target dose | State in millijoules per square centimetre | Time alone is not a specification; dose is irradiance multiplied by time. |
| Irradiance at the surface | Measured with a calibrated UVC sensor | A nominal LED output figure does not account for distance, angle or fouling. |
| Dwell or flow rate | Derived from the dose target | For water and air systems the residence time sets the effective dose. |
| Sensor logging | Continuous, with a maintenance trigger | Catches the optical degradation that otherwise silently reduces the dose. |
| Exposure control | Interlock or shielded enclosure | Signage is secondary; the housing is the primary protective measure. |
| Validation record | Sensor identification, calibration date, temperature | All three affect the reading and are needed to defend the result. |
| Question | Short answer | Detail |
|---|---|---|
| Which standard covers photobiological safety? | IEC 62471 | It assigns a risk group for the source, and the assignment depends on the measurement distance as well as on the output. |
| What risk group will my fixture be? | Almost certainly the highest | A UVC source at usable dose levels normally falls in the highest risk group, which places the burden on the product to prevent exposure rather than on the user to avoid it. |
| Do I need interlocks? | Yes, wherever a person can be present | An enclosure switch or a door interlock that removes power on opening is the standard protective measure for any accessible UVC fixture. |
| What about eye and skin protection? | Full shielding, not just signage | Signage is a secondary measure. The primary measure is a housing that does not permit direct or reflected exposure. |
| Can a UVC fixture be used in an occupied space? | Only with verified upper-air or shielded configurations | Unshielded UVC in an occupied room is not acceptable. Configured upper-air systems and in-duct installations are the usual approaches. |
| What documentation is expected? | A risk assessment and a dose validation report | Regulators and safety officers increasingly ask for both, and a dose validation report with a calibrated sensor is the evidence that the system performs as designed. |
6. Validation and field questions
Q13. How do I validate that the system works?
Measure irradiance with a calibrated UVC sensor at representative points on the target surface, multiply by the exposure time to get the delivered dose, and compare with the target. Record the sensor identification, the calibration date and the ambient temperature, because all three affect the result.
Q14. My system passed at commissioning but not six months later. Why?
Three usual causes: LED optical degradation that was not compensated by a longer exposure time, fouling on the window or reflector, and a rise in ambient temperature that reduced output. A sensor-logged installation distinguishes between them quickly, and a maintenance interval that includes cleaning handles the second.
Q15. Can UVC reach shadowed areas?
No. UVC is line of sight, and a shadowed surface receives essentially nothing. Disinfection systems must either move the target, change the geometry, or use a reflective enclosure so that every surface sees the source. This is the most common reason a system underperforms in practice.
7. Part numbers in this FAQ
QUEENDOM UVC LED emitters at 275 nm sit in the UV emitter group, covering the standard package options used in air, surface and water disinfection products. The near-UV 365 nm to 405 nm emitters sit in the same group and serve curing, fluorescence and inspection applications rather than disinfection. Because these packages share thermal and reliability characteristics with the visible LED families, the derating and lifetime questions in the LED FAQ apply here as well, with the additional caution that UVC output falls more steeply with junction temperature.
8. Related resources and next steps
For the electrical questions behind these answers, see the LED diode and chip FAQ. For a review of a specific disinfection design, send the target organism, the required log reduction, the target surface geometry and the available electrical power, and our applications group will return an irradiance budget and a recommended exposure time with the assumptions stated.
Dose and Wavelength Questions
| Wavelength | Source | Relative effectiveness | Typical role |
|---|---|---|---|
| 254 nm | Low-pressure mercury lamp | 0.85 | Legacy upper-air and water units |
| 265 nm | AlGaN LED | 1.00 (peak) | Reference point for dose comparisons |
| 275 nm | UVC LED | 0.90 | Water, air and surface units – our standard |
| 365 nm | UVA LED | below 0.01 | Curing and fluorescence inspection |
| 405 nm | Violet LED | negligible | Surface sanitation aid in food areas |
Q16. Why choose 275 nm LEDs over 254 nm lamps?
Mercury-free construction, instant restart, compact optics and lower thermal load. Effectiveness at 275 nm is about 90 percent of the 265 nm peak and close to lamp output, while the package lasts far longer in humid service.
Q17. How is UV dose calculated?
Dose in mJ/cm2 equals irradiance in mW/cm2 multiplied by exposure seconds. Halve the irradiance and you double the time for the same log reduction; distance and dwell time are the two knobs.
Q18. UV or chemical disinfection?
They complement each other. UV leaves no residue and applies no selection pressure, but it is line-of-sight, so shadowed surfaces still need a wipe-down contact chemistry.
Application Design Questions
Typical 3-log reduction doses by target class:
| Target class | 3-log dose (typical) | Note |
|---|---|---|
| Vegetative bacteria | 5-15 mJ/cm2 | E. coli and S. aureus at 254-275 nm |
| Bacterial spores | 20-60 mJ/cm2 | Bacillus atrophaeus is the hard case |
| Yeasts and molds | 15-40 mJ/cm2 | Surface modules run the high end |
| Enveloped viruses | 5-20 mJ/cm2 | The most susceptible class |
| Non-enveloped viruses | 20-60 mJ/cm2 | Norovirus surrogates |
Q19. Which reflectors work under UVC?
Electropolished aluminium and PTFE hold their reflectance; standard chrome plating and most polymers yellow and fade within weeks. Optics design matters more than raw power at these doses.
Q20. What happens to plastics in the enclosure?
UVC embrittles ABS and polycarbonate over months. Specify UV-rated grades for anything with long exposure and put sensors and cable jackets behind quartz or PTFE windows.
Q21. How are safety interlocks designed?
Door and panel switches sit in series with the driver enable line, plus a labeled emergency stop. Our modules ship with an enable input that fails dark, so a broken interlock cannot light the array.
Related products and applications
- UV LED emitters (275 nm to 405 nm)
- High-power LED packages
- Glossary: LED package and reliability glossary
- More LED knowledge: LED knowledge resources