LED lighting in collections

Replacing halogen with LED in a gallery brings two genuine conservation benefits and a great many display decisions. It is worth being clear about which is which, because the conservation case is often overstated in ways that lead people to relax controls they should not relax.

The real gains

No ultraviolet. A white LED produces light by exciting a phosphor with a blue emitter. There is essentially no UV in the output. That removes an entire category of damage and, in a purely LED installation, makes lamp filtering unnecessary.

Much less infrared. A halogen spot puts a substantial amount of energy into the infrared, and at close range it warms the surface it illuminates. A warm surface sits at a lower local relative humidity than the surrounding room, which for a painted panel or a varnished surface is a genuine microclimate problem. LEDs put their waste heat into the heatsink at the back of the fitting rather than into the beam.

Controllability. LEDs dim smoothly over a wide range without the colour shift that dimmed halogen suffers, which makes running a gallery at a low illuminance far more practical.

What does not change

Visible light fades things. An LED at 200 lux and a halogen at 200 lux both deliver 200 lux of visible light to the object, and the fading difference between them is a second-order spectral effect, not an escape.

This matters because relamping is often presented internally as a conservation improvement that justifies brighter or longer display. It is not. The exposure budget is unchanged by the light source, and if the relamping is used as an excuse to raise illuminance the net effect on the collection is negative.

Specifications that decide how it looks

Colour rendering. The familiar CRI or Ra figure averages performance across eight moderately saturated test colours and can be high while deep red rendering is poor. Since deep reds are precisely where paintings, textiles and polychrome objects live, this is a real weakness. Ask for R9 specifically, or for a TM-30 report giving fidelity and gamut, which is considerably more informative.

Colour temperature. Lower temperatures around 2700 to 3000 K read as warm and are usually preferred for historic material and for spaces previously lit by halogen. Higher temperatures render blues and whites better. What matters at least as much is consistency: mixed colour temperatures across a gallery are immediately visible and look like a fault.

Binning consistency. LEDs vary in colour between production batches, and the tolerance is expressed in MacAdam ellipses or SDCM. A tight bin means two fittings side by side match. A loose one means a visible patchwork, and it means a replacement fitting in three years will not match its neighbours.

Dimming method and flicker. Ask how the fitting dims and what the flicker performance is at low output. Cheap drivers frequently perform worst exactly where a gallery will run them.

Beam control. The practical conservation advantage of good optics is that light lands on the object rather than the wall, so the same visual effect is achieved at lower total exposure. Well-controlled beams and accessories that limit spill are worth more to a collection than any spectral consideration.

Doing a relamping properly

Measure before and after, at the object, with the same meter. Illuminance frequently rises after a relamping simply because the new fittings are more efficient and nobody adjusted the dimming, which turns a neutral change into a damaging one.

Mock up one case or one bay before committing to the whole gallery, and look at it with the objects in place rather than at a sample board. Colour rendering problems are obvious on a polychrome object and invisible on a grey wall.

Keep spares from the same batch. It is the cheapest insurance against a mismatched fitting three years from now.