The fading effect of light on the delicate tints of a watercolour might seem unsurprising, but light can fade even a piece of ebony.

Conversely, it can turn almost white wood, like holly, an unrecognisably dark brown. It can make the most brightly patterned textiles look uniformly drab, and ultimately cause them to fall apart.

Damage is caused by artificial as well as natural light and, in both cases, not only by the light which can be seen.

Beyond the ends of the visible spectrum lie ultraviolet and infrared radiation. Ultraviolet radiation causes even more fading than the visible wavelengths.

Filters made from a colourless plastic film are attached to the inside of window glass to absorb the ultraviolet radiation. At the other end of the spectrum is infrared radiation, which we feel as heat. It softens some materials and reduces relative humidity.

Light intensity

The intensity of light is measured in ‘lux’ and, taking the duration of the exposure into account, the total light dose is measured in ‘lux hours’ (intensity multiplied by time).

Since the harm done is irreversible and cumulative, all light-sensitive materials will progressively deteriorate, but the rate at which this deterioration occurs can be minimised by reducing the lux hours of exposure per year.

Museums normally display light-sensitive objects at 50 lux, and are open for approximately 3,000 hours a year. National Trust houses are, on average, open for 1,000 hours a year.

The Trust aims to light rooms to between 50 and 150 lux depending on the position of objects in the room and their distance from a window. Exposure of 1,000 hours at 150 lux is neither more nor less harmful than 3,000 hours of exposure at 50 lux (both equal150,000 lux hours).

Black-outs

Outside opening hours, and after the houses are closed for the winter, a total black-out is maintained. Daylight is controlled by sun-blinds and sun-curtains. On very bright days these must be used to keep direct sunlight out altogether, but when skies are duller they need be only part drawn.

Changes in the light level during the day are measured as they occur and adjustments made to the blinds or curtains ensure the right balance between visibility and light control. Filters are applied to the inside of all windows to exclude ultraviolet light.