On Saturday 13 June, an old theatre in Lisbon reopened with our light in it. Two hundred and forty fittings, made on our benches through the winter and the spring, were installed over eleven weeks and switched on for the first time in front of a full house. We stood at the back of the stalls with our hands in our pockets, which is how we always stand on a night like this.

A house that remembers every lamp it once held

The building dates from the middle of the nineteenth century, and the plaster still carries the traces of earlier fittings: round ghosts where gas mantles sat, a shallow channel cut for a cable run in the 1950s, patches of paint one shade off from the ceiling rose. Nothing we sent could pretend the room was new, so we worked from the room instead. The wall brackets were drawn to the proportion of the boxes rather than of our catalogue. The brass was left warm to meet the gilding on the proscenium. The glass diffusers were made faintly cloudy, so that the source never announces itself.

What the night asked for

Three demands had to hold at once, and they pull against each other.

• Enough light on the stairs and in the boxes for a crowded interval, without spilling onto the ceiling plaster.
• A lower, warmer level in the foyer, where the audience waits forty minutes and the light should behave like a held breath before the curtain.
• Full dimming in the house, down to two per cent, with no flicker on camera and no buzz in the silence before the first line.

The last of those took the longest. Dimming a lamp is easy; dimming it honestly, all the way down, without the filament trembling, is a matter of driver, glass and patience. We rebuilt the smallest wall fitting twice and carried the second version to Lisbon in a hand-luggage case. Beforehand we tested the balance for three evenings, lifting and lowering the front-of-house rows by hand while one of our team sat up in the gallery and called out what she could see. At the interval we stood under the box we had redrawn three times, and nobody in our group said a word about lighting. That is the review we wanted. The house has played to full rooms since, and our part in it stays small, which is correct.

A good lamp behaves like a good stagehand: never in the story, always in the light.