Museum Lighting Design: Lux Levels, UV Limits & Conservation in Practice

Museum lighting design is judged on two numbers, and only one of them is a light level. The first is illuminance, in lux. The second is the total dose that illuminance delivers over a year, in lux-hours. A painting can survive 50 lx for a thousand hours, or 100 lx for five hundred — the object sees the same 50,000 lx·h either way. That product, not the lux figure, is what a conservation department actually protects.

This guide sets out how to run that budget, how to quantify the ultraviolet load that comes with it, and how to translate both into a specification you can hand to a contractor. All the geometry and arithmetic below is the same arithmetic used to draw the figures, and it is printed so you can check it. The exposure-budget and UV-limit conventions referenced here follow current museum practice as set out by the Illuminating Engineering Society and the International Commission on Illumination (CIE); for material-specific sensitivity guidance, the Canadian Conservation Institute publishes the most widely used agent-of-deterioration references.

Why museum lighting design is a dose problem, not a level problem

Photochemical damage is cumulative and irreversible. Bleaching, yellowing and embrittlement are the result of photons breaking chemical bonds, and the number of bonds broken is proportional to the number of photons absorbed — not to how bright the room looked on any given afternoon.

That is why the standard is written as an annual exposure budget. For highly sensitive material — works on paper, textiles, watercolours, illuminated manuscripts — a common curatorial budget is 50,000 lux-hours per year. A 90% safety factor is normal practice, which leaves 45,000 lx·h of usable budget.

The consequence is counter-intuitive for anyone used to specifying domestic lighting. Raising illuminance from 50 lx to 100 lx does not make the room slightly brighter — it halves the display time.

Turning the budget into a daily schedule

A budget of 45,000 usable lx·h at 50 lx works out to 900 hours a year. Spread across 365 days that is 2.47 hours per day, or a duty cycle of 10.3% of the year. That is the number a gallery actually operates against: not “the dimmer is at 50 lx”, but “this case is lit 2.5 hours a day”.

In practice most institutions run a shorter, brighter daily window. Three forty-minute guided tours at 50 lx is two hours — comfortably inside budget, with headroom for conservation photography and condition checking.

Lux levels by material sensitivity

The budget is what limits exposure; the illuminance target is set by the material’s sensitivity. The figures below are the working range used across conservation literature, paired with the hours each one buys inside a 50,000 lx·h budget.

Material class	Typical illuminance	UV limit	Hours at 50,000 lx·h
Highly sensitive — works on paper, textiles, manuscripts, dyed leather	50 lx	75 µW/lm	1,000 h
Sensitive — oil and tempera paintings, wood, ivory, horn	150–200 lx	75 µW/lm	333–250 h
Moderately sensitive — stone, ceramics, glass, metal with applied colour	300–500 lx	75 µW/lm	167–100 h
Insensitive — uncoloured stone, most metals, glass without coatings	No practical limit	UV still controlled	Not the constraint

Two things about that table catch people out. First, low illuminance does not mean the UV problem went away — UV sensitivity and visible-light sensitivity are different mechanisms, and a bronze can be lit hard and still be damaged by an unfiltered source. Second, the hours column is where the operative constraint lives. A 200 lx painting in a room open seven hours a day exhausts its budget in under six weeks of continuous display.

UV limits: how to quantify what the lamp delivers

Ultraviolet is not specified in lux or in watts. It is specified as microwatts of ultraviolet per lumen (µW/lm), which is a ratio: it tells you how much UV comes along for the ride with every lumen of visible light the lamp produces. The museum limit is 75 µW/lm.

To see whether a source passes, convert the ratio into an irradiance at the object. At 50 lx the object receives 50 lm/m², so:

UV load (mW/m²) = illuminance (lm/m²) × UV content (µW/lm) ÷ 1000

Run the numbers for the two sources that matter in a gallery and the picture is stark.

The arithmetic

• Quality LED — 0.2 µW/lm. At 50 lx that is 0.010 mW/m². The limit of 75 µW/lm is 375 times higher, so an LED installation passes with an enormous margin.
• Unfiltered daylight — around 700 µW/lm. At 50 lx that is 35.0 mW/m², roughly 3,500 times the LED load and 9.3 times the limit.
• Filtered daylight — a UV-absorbing glazing or film transmitting 2% still yields 0.700 mW/m², which is why filtering is mandatory wherever daylight reaches an object.

This is the single most useful reframing in museum lighting design practice: UV control is a daylight problem before it is a lamp problem. A conservation-grade LED on a track contributes nothing worth measuring, while an unfiltered rooflight fifteen metres away can be the dominant risk in the room.

Specify UV-absorbing filters on any aperture with a direct line of sight to an object, and require a UV content declaration — in µW/lm — from the luminaire manufacturer. Where an existing scheme uses incandescent-era halogen, note that it sits essentially at the limit: 75 µW/lm produces 3.75 mW/m² at 50 lx, which is to say at the ceiling of what is acceptable, and that is before the infrared contribution is considered.

Infrared and heat: the other half of the problem

Radiation below 700 nm does not break chemical bonds the way UV does, but it deposits heat. On a framed panel the failure mode is mechanical rather than photochemical: a canvas expands and contracts against its stretcher, gesso and paint develop differential movement, and the result is craquelure and lifting.

Two design consequences follow. Keep infrared-emitting sources off the object’s axis and out of the frame’s plane, and aim so that the beam passes across a surface rather than parking on it. Track lighting suits this because each head can be angled independently — which brings the geometry into play.

Aiming and layout: from lux target to candela

Once you have an illuminance target, the luminaire specification is a calculation, not a guess. Illuminance falls with the square of the distance, so:

E = I / h², therefore I = E × h²

Take a gallery wall with the rail at 2.72 m and the artwork centreline at 1.60 m. The vertical drop from head to the point being lit is 1.92 m. For a 50 lx target at the wall, that requires 184 cd from each head — a modest output, which is exactly why conservation spaces are lit with low-wattage, tightly-controlled fittings rather than bright ones.

Beam angle sets the pool, and the pool sets the pitch

Pool width follows from beam angle and drop-off distance:

D = 2 × h × tan(θ/2)

A 24° head at 1.92 m produces a 0.82 m pool. At a spacing criterion of 1.0 — adjacent pools just touching — the centre-to-centre pitch equals the drop-off distance, so 1.92 m. Over an 8 m rail that yields four heads.

Where the work is longer than it is tall, the geometry changes. A 2.9 m scroll run lit by 24° heads tilted 30° off-axis needs 0.465 m pitch to hold 25% overlap, giving six heads. Tilting costs light: the off-axis illuminance falls by the cosine of the tilt, so 50 lx on-axis becomes 43.3 lx at 30° — 87% of target. Meet the target on-axis first, then angle the head.

Aim before you space

The most common layout error in conservation spaces is spacing heads before deciding the aiming point. Because illuminance falls with the square of the distance, a head aimed at a point further away needs disproportionately more output. Fix the aiming point on the object’s centreline, calculate the drop-off, derive the pool and the pitch, then place the heads. Changing the aim afterwards invalidates the whole chain.

Track, filters and the specification checklist

Track systems are the usual choice for conservation galleries for three reasons: the aim of each head is independently adjustable, head positions can be changed without touching the fabric of the building, and accessories — UV filters, glare control, snoots — clip on at the head rather than being designed into the ceiling.

A specification that survives contact with a conservation department covers these points:

Item	What to specify	Why
UV content	≤ 75 µW/lm, declared by the manufacturer	Converts a ratio into a checkable figure
Infrared	No IR-emitting source on axis with the object	Removes the differential-heating failure mode
Colour rendering	Ra ≥ 90, ideally ≥ 95 for pigment matching	Wrong colour temperature changes perceived hue and can force illuminance up
Beam angle	Matched to the pool size, not to a room dimension	Pool is the controlled variable; spill is wasted dose
Aiming	Independent per head, with a locking mechanism	Stops drift over the life of the exhibition
Glare	Deep-set or accessory-controlled heads	Visitor comfort, and it keeps the light off the glass
Dimming	Dimmable to at least 10%	Lets the daily budget be managed rather than fixed

On the luminaire side, the fittings that suit this work are the ones with narrow, well-controlled distributions and genuine aim adjustability. Our AIMER track light is built for exactly this pattern — a tight beam at modest output, with the head aimed per position rather than the whole run set to one angle. For wall-mounted cases and panel edges, the OVEGA zoomable wall-mounted fitting lets the beam be narrowed on site as the pool requirement firms up. Browse the wider lighting guides for the optics behind these numbers, or see the full track light range.

Frequently Asked Questions

What lux level is appropriate for museum lighting?

It depends on the material. Highly sensitive objects — works on paper, textiles, manuscripts — are held to about 50 lx. Oil paintings and other moderately sensitive works sit at 150–200 lx, and robust materials such as stone and metal can take 300–500 lx. The illuminance is set by sensitivity, but the display schedule is set by the annual exposure budget.

How do I calculate an annual exposure budget?

Multiply illuminance by the hours the object is lit, and keep the product under the budget. Highly sensitive material is commonly budgeted at 50,000 lux-hours per year. At 50 lx that allows 1,000 hours; applying a 90% safety factor gives 900 hours, or 2.47 hours a day.

What is the UV limit for museum lighting, and how is it measured?

The limit is 75 microwatts of ultraviolet per lumen (µW/lm). To check a source, multiply the illuminance in lm/m² by its UV content in µW/lm and divide by 1,000 to get milliwatts per square metre. A quality LED at 0.2 µW/lm yields 0.010 mW/m² at 50 lx; unfiltered daylight at 700 µW/lm yields 35 mW/m², which is 9.3 times the limit.

Does LED lighting emit UV?

Quality LEDs emit almost none — typically around 0.2 µW/lm, roughly 375 times inside the 75 µW/lm limit. The UV risk in a gallery almost always arrives as daylight through unfiltered glazing or rooflights, not from the artificial lighting.

How far apart should museum track lights be?

Set the pitch from the pool, not the rail. Calculate the drop-off from head to object, derive the pool width from the beam angle, then space heads at the drop-off distance for pools that just touch. In the worked example here — 2.72 m rail, 1.60 m artwork centreline, 24° heads — that gives a 0.82 m pool and 1.92 m centre-to-centre, so four heads over an 8 m rail.

Can I fit conservation filters to an existing track system?

Usually yes. UV-absorbing filters clip onto the head or the track accessory slot, but confirm two things first: that the filter does not overheat at the lamp’s output, and that the accessory is compatible with the head’s beam angle. Filters that sit in the beam can also alter the distribution, so re-check the pool width after fitting.

Putting it together

Museum lighting design comes down to running three numbers in the right order. Start with the material’s sensitivity to fix the illuminance, convert that into an annual budget in lux-hours to fix the display schedule, then derive beam angle, pitch and candela from the drop-off distance so the photometric specification is a calculation rather than a preference. Check the UV content against 75 µW/lm, and remember that in most galleries the daylight is the larger risk than any luminaire on the track.

If you are specifying a conservation or gallery project and want the optical side worked through against your own drop-off distances and object sizes, send us the drawings and we will run the numbers.