Does 96% area coverage mean 96% of the UV is blocked?

No, and the assumption is the reason guards get signed off that do not protect anyone. The covered part blocks whatever its material blocks; the open part blocks nothing. What escapes through a 4% aperture is full-strength UV travelling in a specific direction, and whether it matters depends on whether that direction reaches a person, not on how small the aperture is as a fraction of the enclosure.

What material should a UV guard be?

One specified for attenuation at the wavelengths present, which is not the same as one you cannot see through. Visible opacity says little about UV transmission: some plastics that look solid pass appreciable UV, while certain clear glasses and acrylics block UVC entirely. Ask for transmission data over the range your lamp emits, and prefer materials that also handle the heat.

How do I deal with conveyor slots that cannot be closed?

Make the light take turns it cannot take. Baffles, labyrinth entries, brush seals and extended tunnels attenuate by forcing multiple reflections, each of which absorbs some energy, and they let parts through while UV largely does not. Then measure. A well-designed labyrinth can reduce escaping irradiance by orders of magnitude, and the measurement is what proves it.

Is the coverage efficiency an attenuation figure?

No. It is a material take-off allowance covering seam overlaps, fixings and trim waste, so it tells you how much to buy rather than how well the result works. It sits on this page because ordering material is a real task; it should never be quoted in a safety argument.

Do walls and machine surfaces matter?

More than most people expect. Bare aluminium and stainless steel reflect UV efficiently and light-coloured paint reflects a useful fraction, so a cell whose interior is bright will push UV out of apertures that have no direct view of the lamp. Darkening interior surfaces where practical is a cheap and underused control, and it is one of the reasons a measurement can come out higher than any line-of-sight sketch predicts.

When is the guard finished?

When a spectrally weighted stray-irradiance measurement at every position a person occupies gives an exposure budget the work can actually live within, and that budget comes from the exposure page, not this one. Area is an input to the purchase order. Attenuation, measured with the guard in place and the lamp at running power, is the acceptance criterion.

UV Curing calculator

UV Shielding Calculator: Area to Buy, and the Gaps That Decide It

Size the guarding material a UV cell needs and, separately, state the aperture that stays open. Enter the open face area, the number of operator-exposed sides, the take-off efficiency after overlaps and cut-outs, and the area of slots and windows the process cannot do without. The calculator returns the guard area to buy, the theoretical coverage behind it, the overlap allowance, and the fraction of the face that remains an open optical path. That last row is the one that decides safety: a guard is judged on the stray irradiance measured afterwards, not on the square feet ordered.

What this calculator does

Size the guard material a UV cell needs, and state separately the aperture left open, because a guard is judged on the stray irradiance that escapes, not on the area it covers.
Use it for estimating guard material for a UV cell enclosure, quantifying how much aperture a process design leaves open, comparing enclosure options at quotation stage, setting up the measurement that decides whether a guard is adequate, justifying baffles or light traps on conveyor apertures.
Size the guard material a UV cell needs, and state separately the aperture left open, because a guard is judged on the stray irradiance that escapes, not on the area it covers.

Formula used

Theoretical shield area = open face area × exposed sides
Total shielding area required = theoretical area ÷ coverage efficiency
Overlap and cut-out allowance = required area − theoretical area
Aperture left open (%) = openings ÷ theoretical area × 100
Face area actually covered = theoretical area − openings

Inputs explained

Open UV emission face area to enclose: Area of one face through which UV can escape towards a person.
Number of operator-exposed sides: Faces needing guarding because a person can be there. Count the ones reached during maintenance as well as during running.
Guard coverage efficiency after overlap and cut-outs: Share of purchased material that ends up as effective coverage, after seam overlaps, fixings and trim waste. A take-off allowance, not an attenuation figure.
Openings the process needs left open: Conveyor slots, viewing windows, loading apertures and access gaps that cannot be closed. These are optical paths at full strength, not a percentage of the hazard.

How to use the result

Best suited to estimating guard material for a UV cell enclosure, quantifying how much aperture a process design leaves open, comparing enclosure options at quotation stage, setting up the measurement that decides whether a guard is adequate, justifying baffles or light traps on conveyor apertures.
This is a material take-off, not a safety assessment. Area is what you buy; attenuation measured at the operator position is what protects anyone. Says nothing about the guard material's UV transmission. Many plastics that look opaque pass appreciable UV, and some clear materials block it entirely. Ignores reflected paths, which routinely dominate: bare aluminium, stainless steel and light paint redirect UV around corners a line-of-sight sketch says are safe. Treats openings as a single area, when what matters is where each one points and who stands there. Covers no other function of a guard. Mechanical trapping, thermal protection, ozone containment or interlocking, all of which usually govern the design too.

Common questions

Does 96% area coverage mean 96% of the UV is blocked? No, and the assumption is the reason guards get signed off that do not protect anyone. The covered part blocks whatever its material blocks; the open part blocks nothing. What escapes through a 4% aperture is full-strength UV travelling in a specific direction, and whether it matters depends on whether that direction reaches a person, not on how small the aperture is as a fraction of the enclosure.
What material should a UV guard be? One specified for attenuation at the wavelengths present, which is not the same as one you cannot see through. Visible opacity says little about UV transmission: some plastics that look solid pass appreciable UV, while certain clear glasses and acrylics block UVC entirely. Ask for transmission data over the range your lamp emits, and prefer materials that also handle the heat.
How do I deal with conveyor slots that cannot be closed? Make the light take turns it cannot take. Baffles, labyrinth entries, brush seals and extended tunnels attenuate by forcing multiple reflections, each of which absorbs some energy, and they let parts through while UV largely does not. Then measure. A well-designed labyrinth can reduce escaping irradiance by orders of magnitude, and the measurement is what proves it.
Is the coverage efficiency an attenuation figure? No. It is a material take-off allowance covering seam overlaps, fixings and trim waste, so it tells you how much to buy rather than how well the result works. It sits on this page because ordering material is a real task; it should never be quoted in a safety argument.
Do walls and machine surfaces matter? More than most people expect. Bare aluminium and stainless steel reflect UV efficiently and light-coloured paint reflects a useful fraction, so a cell whose interior is bright will push UV out of apertures that have no direct view of the lamp. Darkening interior surfaces where practical is a cheap and underused control, and it is one of the reasons a measurement can come out higher than any line-of-sight sketch predicts.

Last reviewed 2026-08-25.