Is the permitted exposure per visit or per day?

Per day. The limit is a total for an eight-hour period, so every approach draws on the same budget. This is the single most important thing about the page and the thing the earlier version did not say: fifteen seconds is the whole shift's allowance, and twenty approaches share it between them.

What is an effective irradiance?

A spectrally weighted one. Each wavelength present is multiplied by its relative hazard weighting and the results are summed, which is what makes a single number comparable to a single limit. A broadband instrument reports total UV, which is a different and usually much larger quantity. Getting an effective irradiance needs an instrument matched to the weighting or a spectroradiometer, and someone who knows how to use it.

Can I manage this with a timed procedure?

Almost never, and the per-event row is there to make that obvious. A control that requires a person to limit themselves to a fraction of a second is not a control. Enclosure, interlocks, shielding and distance reduce the irradiance or remove the person, and those are the measures that survive a busy shift, a new starter, or a jam at the end of a run.

Why is the continuous-occupancy irradiance so small?

Because it spreads the same daily budget over 28,800 seconds instead of a few. The limit is a dose, so the irradiance that allows all-day presence is the dose divided by the whole day. A level far below what an industrial lamp leaks even through a modest gap. The ratio row expresses the same fact as a multiple, which is usually more persuasive than the decimal.

Does the safety factor make the process safer?

It makes the permitted time shorter. That is worth having as a margin against measurement uncertainty, posture variation and the events nobody counted, but it does not reduce anyone's exposure by a single millijoule. Only changing the irradiance or the presence does that.

What about reflected UV?

It is the usual explanation for a position that measures far higher than the geometry suggests. UV reflects efficiently off bare aluminium and stainless steel, and appreciably off light-coloured paint, so a well-placed guard can still deliver a strong indirect path. Measure at the position rather than reasoning from line of sight, and re-measure after any change to guards, walls or machine surfaces.

UV Curing calculator

UV Safety Exposure Calculator: A Daily Budget, Not a Per-Visit Allowance

Convert a UV exposure limit and a measured stray irradiance into permitted exposure time, and see it for what it is: a budget for a whole working day, shared among every time someone approaches the machine. Enter the applicable effective-irradiance limit, the spectrally weighted stray irradiance measured at the operator's position, your safety factor, and how many exposure events happen in a shift. The calculator returns the daily allowance, the seconds that leaves per event, the bare time to the limit, and the stray irradiance at which someone could stand there for a full shift. This page supports a risk assessment. It is not one.

What this calculator does

Convert an effective UV exposure limit and a measured stray irradiance into the permitted exposure for a whole working day, and divide that budget among the times an operator is actually exposed.
Use it for turning a measured stray irradiance into a daily exposure budget, showing why timed access is not a viable control around a UV cell, setting the target attenuation a guard has to achieve, supporting a risk assessment with the arithmetic behind its conclusion, comparing operator positions before and after a shielding change.
Convert an effective UV exposure limit and a measured stray irradiance into the permitted exposure for a whole working day, and divide that budget among the times an operator is actually exposed.

Formula used

Derated limit = exposure limit ÷ (1 + safety factor ÷ 100)
Permissible exposure per day = derated limit ÷ stray irradiance
Seconds per exposure event = permissible exposure per day ÷ events per day
Base time to the limit = exposure limit ÷ stray irradiance
Stray irradiance allowing 8-hour presence = derated limit ÷ 28800 s

Inputs explained

Effective UV exposure limit: The applicable eight-hour effective-irradiance limit for actinic UV. Take it from the standard that applies to your site rather than from memory.
Measured stray irradiance at the operator: SPECTRALLY WEIGHTED effective irradiance at the operator's position: eyes and face. A broadband reading is not this quantity and will not give a comparable answer.
Safety factor above bare limit: Margin held back below the limit. 300% divides the bare allowance by four. A margin is not a control: it does not stop the exposure, it only shortens the time before it matters.
Exposure events per day: How many times a shift the operator is actually in that field: jams, loading, inspection, adjustment. The budget is daily and is shared among all of them.

How to use the result

Best suited to turning a measured stray irradiance into a daily exposure budget, showing why timed access is not a viable control around a UV cell, setting the target attenuation a guard has to achieve, supporting a risk assessment with the arithmetic behind its conclusion, comparing operator positions before and after a shielding change.
This is not a risk assessment, an exposure assessment, or a compliance determination. It is arithmetic on numbers a competent person has to supply. Covers actinic UV against one limit. Separate limits exist for the near-UV region and specifically for the eye, and more than one can apply at once. Assumes a single irradiance for every event. Real positions vary with posture, distance and what the operator is doing. Ignores reflected UV, which is the usual reason a position measures higher than anyone expected. Bright metal, machine guards and even light-coloured walls redirect it. Says nothing about skin versus eye exposure, protective equipment, or the transmission of whatever eyewear is worn.

Common questions

Is the permitted exposure per visit or per day? Per day. The limit is a total for an eight-hour period, so every approach draws on the same budget. This is the single most important thing about the page and the thing the earlier version did not say: fifteen seconds is the whole shift's allowance, and twenty approaches share it between them.
What is an effective irradiance? A spectrally weighted one. Each wavelength present is multiplied by its relative hazard weighting and the results are summed, which is what makes a single number comparable to a single limit. A broadband instrument reports total UV, which is a different and usually much larger quantity. Getting an effective irradiance needs an instrument matched to the weighting or a spectroradiometer, and someone who knows how to use it.
Can I manage this with a timed procedure? Almost never, and the per-event row is there to make that obvious. A control that requires a person to limit themselves to a fraction of a second is not a control. Enclosure, interlocks, shielding and distance reduce the irradiance or remove the person, and those are the measures that survive a busy shift, a new starter, or a jam at the end of a run.
Why is the continuous-occupancy irradiance so small? Because it spreads the same daily budget over 28,800 seconds instead of a few. The limit is a dose, so the irradiance that allows all-day presence is the dose divided by the whole day. A level far below what an industrial lamp leaks even through a modest gap. The ratio row expresses the same fact as a multiple, which is usually more persuasive than the decimal.
Does the safety factor make the process safer? It makes the permitted time shorter. That is worth having as a margin against measurement uncertainty, posture variation and the events nobody counted, but it does not reduce anyone's exposure by a single millijoule. Only changing the irradiance or the presence does that.

Last reviewed 2026-08-25.