UV Dose & Fluence
The science of UV energy delivery — how much light reaches your target
UV Dose & Fluence in 30 Seconds
- Dose (Fluence) = Irradiance × Time (mJ/cm²)
- Irradiance = power per unit area (mW/cm²)
- Higher dose = more pathogen inactivation (log reduction)
- Drinking water standard: 40 mJ/cm² (DVGW / EPA)
Example: 10 mW/cm² for 4 seconds = 40 mJ/cm² (drinking water standard)
Practical Guide
The Core Formula
Dose (mJ/cm²) = Irradiance (mW/cm²) × Time (s). Double the irradiance → halve the time for the same dose. But for UV curing, reciprocity does NOT always hold.
Irradiance vs. Dose
Irradiance (W/cm² or mW/cm²) is instantaneous power density. Dose (J/cm² or mJ/cm²) is energy delivered over time. Both matter — a high-power flash and a weak continuous lamp can deliver the same dose.
How Much Dose Do I Need?
Drinking water: 40 mJ/cm² (DVGW W 294). Surface bacteria (4-log): 8–27 mJ/cm². SARS-CoV-2 (3-log): 3.7 mJ/cm². Adenovirus (4-log): 186 mJ/cm². Always design for your worst-case target.
Measuring Correctly
Measure irradiance at the target surface, not at the lamp. Account for distance (inverse square law), reflections, and medium absorption (UVT for water). Radiometer spectral response must match your lamp.
Common Mistakes
Measuring too close to the lamp. Ignoring lamp aging (20–40% output loss over lifetime). Confusing radiometer readings across wavelengths. Not accounting for UVT in water applications.
Next Step
Our UV Simulator calculates dose distribution for your specific reactor geometry — including reflections, flow dynamics, and lamp aging factors.
The global benchmark for UV water disinfection, established by DVGW W 294 (Germany) and EPA UVDGM (USA). Validated biodosimetrically — not just by lamp output.[5]
Dose Requirements by Application
| Application | Required Dose | Standard | Target |
|---|---|---|---|
| Drinking water (DE) | 40 mJ/cm² | DVGW W 294 | 4-log bacteria |
| Drinking water (USA) | 40 mJ/cm² | EPA UVDGM | 4-log viruses |
| Cryptosporidium (3-log) | 12 mJ/cm² | EPA 40 CFR | 3-log |
| Giardia (3-log) | 11 mJ/cm² | EPA 40 CFR | 3-log |
| Surface bacteria (general) | 8–27 mJ/cm² | Various | 4-log (99.99%) |
| SARS-CoV-2 | 3.7 mJ/cm² | Nature (2021) | 3-log (99.9%) |
| Adenovirus | 186 mJ/cm² | EPA | 4-log |
| C. difficile spores | ~2,200 mJ/cm² | Various | 4-log |
| Upper-room UVGI (air) | 30–50 µW/cm² fluence rate | CDC/NIOSH | Continuous |
Unlike disinfection, UV curing does not follow simple reciprocity. A fast, high-intensity exposure gives different results than a slow, low-intensity exposure — even at the same total dose. Always specify both irradiance (W/cm²) and dose (J/cm²).[10]
UV Unit Quick Reference
| Criteria | Common (CGS) | SI |
|---|---|---|
| Dose / Fluence | mJ/cm² | J/m² (= mJ/cm² × 10) |
| Irradiance | mW/cm² | W/m² (= mW/cm² × 10) |
| Power | Watt (W) | mW (= W × 1000) |
| Time | Seconds (s) | Minutes (min) |
| Wavelength | nm (nanometers) | µm (= nm / 1000) |
Irradiance drops with the square of the distance from a point source. At 2× the distance, you get 1/4 the irradiance. This is why lamp-to-target distance is critical for system design.[6]
Deep Dive
What's the difference between Dose, Fluence, and Fluence Rate?
Fluence (H₀) is the total radiant energy incident on a surface per unit area, from all directions. Unit: J/m² or mJ/cm². In UV disinfection, "fluence" and "dose" are used interchangeably, though fluence is technically more precise.
Fluence rate (E₀) is the instantaneous fluence per unit time — essentially irradiance from all directions. Unit: W/m² or mW/cm².
The standardized method by Bolton & Linden (2003) uses collimated beam apparatus with petri factor, reflection factor, divergence factor, and water factor corrections to calculate the true average fluence delivered to a microbial suspension.[6]
Why doesn't reciprocity work for UV curing?
In UV disinfection, 10 mW/cm² for 4 seconds delivers the same biological effect as 40 mW/cm² for 1 second (same 40 mJ/cm² dose). This is because DNA damage is cumulative.
In UV curing, the photochemistry is more complex. Free-radical polymerization competes with oxygen inhibition: at low irradiance, oxygen diffuses into the surface faster than radicals can consume it, leaving a tacky surface. At high irradiance, the radical generation overwhelms oxygen — giving a proper cure.
This means a fast, high-power exposure produces a different result than a slow, low-power exposure at the same total dose. For UV curing, always specify both peak irradiance (W/cm²) and dose (J/cm²).[10]
How do I convert between common UV units?
The UV industry uses two parallel unit systems, which causes constant confusion:
CGS system (common in practice): mW/cm² for irradiance, mJ/cm² for dose
SI system (scientific): W/m² for irradiance, J/m² for dose
Conversion: 1 mW/cm² = 10 W/m² and 1 mJ/cm² = 10 J/m².
The DVGW W 294 standard uses 400 J/m², which equals 40 mJ/cm² — the same number the EPA uses. This is intentional harmonization.
For UV curing, you'll also see J/gsm (energy per gram per square meter) and W/cm (power per unit length of lamp). These are application-specific and not directly comparable to mJ/cm².
Need Expert Guidance?
Our team helps you select the right UV technology for your application — vendor-neutral, data-driven.
- [5] EPA UV Disinfection Guidance Manual (EPA 815-B-21-007, 2022)
- [6] Bolton & Linden (2003): Standardization of Methods for Fluence UV Dose
- [8] DVGW W 294 (2006): UV-Geräte zur Desinfektion in der Wasserversorgung
- [10] Kosar (2022): UV Curing: Science and Technology — Photoinitiator Chemistry and Formulation