UV exposure and plant light are both “how much light” questions, but they measure different light for different purposes. The UV exposure calculator is about people: it turns the daily UV index, minutes outdoors, and skin type into a relative sun-exposure level. The plant light calculator is about plants: it turns measured photosynthetic photon flux density (PPFD) and photoperiod into daily light integral (DLI). They share the same weather-and-environment category, yet one measures radiation that damages skin while the other measures the light plants use — and neither number can be converted into the other.
What each calculator does
The UV exposure calculator takes three inputs: the day’s UV index (0 at night up to 11+ in extreme conditions), the minutes you plan to spend outdoors, and your Fitzpatrick skin type (I–VI, weighted from 2.0 for type I down to 0.5 for type VI). It computes a relative score as UV index × minutes × skin-type factor. A score below 50 is labeled Low, 50–149 Moderate, 150–299 High, and 300 and above Very high. The result also shows the official WHO/EPA band for the UV index you entered — 0–2 low, 3–5 moderate, 6–7 high, 8–10 very high, 11+ extreme — with the protection actions that band carries. The page stresses that the score is a heuristic for comparing scenarios, not a measurement of UV dose and not a predictor of sunburn.
The plant light calculator takes measured PPFD in µmol/m²/s and photoperiod in hours per day (0–24), plus documentation fields for the sensor and its uncertainty. It applies DLI = PPFD × photoperiod × 3,600 ÷ 1,000,000, displaying the result in mol/m²/day to two decimal places — for example, 300 µmol/m²/s over 12 hours gives 12.96 mol/m²/day. The single-value method treats the entered PPFD as constant across the entered photoperiod, and the page deliberately includes no lux-to-PPFD conversion, no species requirement, no sufficiency category, and no lighting recommendation.
Side-by-side
| UV exposure calculator | Plant light calculator | |
|---|---|---|
| Question answered | How much sun does my skin get in a planned outing? | How much light does a measured grow-light scenario deliver to plants? |
| Inputs | UV index, minutes outdoors, Fitzpatrick skin type | PPFD (µmol/m²/s), photoperiod (hours/day), sensor documentation |
| Core math | Score = UV index × minutes × skin-type factor | DLI = PPFD × photoperiod × 3,600 ÷ 1,000,000 |
| Result | Relative score plus Low–Very high level and UV index band | DLI in mol/m²/day, two decimals |
| Guidance | Mirrors WHO/EPA protection actions for the UV band | No species or sufficiency recommendation |
| Reference scale | WHO/EPA UV index bands (0–2 low to 11+ extreme) | None — input documented, not prescribed |
When to use which
Use the UV exposure calculator when the question is about people and the sun: how the day’s UV index, planned time outdoors, and skin sensitivity compare across scenarios — 30 minutes at UV 6 versus 60 minutes at UV 3, for example. Its exposure level and band guidance come from the WHO/EPA scale, and the page points to official sun safety guidance for real protection decisions.
Use the plant light calculator when the question is about plants and grow lights: translating a measured PPFD and photoperiod into daily light integral so a light scenario can be documented and compared — for example, 100 µmol/m²/s for 8 hours gives 2.88 mol/m²/day, while 500 µmol/m²/s for 16 hours gives 28.80. Keep each reading’s sensor provenance and uncertainty with its DLI so changes can be traced to measurement, duration, or both.
Limits and disclaimer
Both pages are educational tools rather than prescriptions. The UV exposure score does not calculate UV dose in joules per square meter, does not account for time of day, latitude, altitude, cloud cover, reflection off snow or water, or clothing coverage, and does not predict individual burn times — two people with the same score can respond very differently. The plant light result summarizes only the entered photon flux over the entered duration: sensor placement, calibration, spectrum, and daily variation can limit how representative one PPFD value is, and the page gives no plant target bands or lighting advice. Neither page is medical or horticultural advice for a specific person, plant, or situation.