Saint Helin Cathedral

Limestone that has weathered for a hundred and forty years is not one colour. The lower six metres of the west front are darkened by traffic film and the upper courses are almost white, so a single flat wash made the building look like two buildings stacked on top of each other. We resolved it by splitting the exterior into three vertical bands, each with its own illuminance target, so the wash falls from roughly 25 lux at the parapet to 12 lux at the plinth and the eye reads one continuous surface.

Winter changed the arithmetic. From December to March the forecourt is under snow, ground reflectance rises from about 0.12 to over 0.7, and the reflected component alone was lifting the lower facade by half a stop. Rather than accept a scheme that only worked in summer we built a second control scene: the ground-recessed uplights drop by thirty percent and the tower floods hold, which keeps the composition stable across the year. The seasonal switch is on a calendar trigger, not an operator decision.

Inside, the constraint was the ceiling. Nothing could be fixed to the vault ribs, so all chancel light comes from twelve framing projectors on a discreet gantry above the triforium, individually shuttered to the stonework so that not one photon crosses onto the plaster. We ran a full-scale mock-up over two nights with the Fabric committee present, which is the only reliable way to settle an argument about whether 2700K or 3000K flatters a limestone reredos. They chose 2700K, and they were right.