Budgeting light exposure

Light damage has one property that makes it uniquely tractable: it is cumulative and additive. The damage from an hour at 200 lux is very close to the damage from four hours at 50 lux. That fact turns an anxious guessing game into arithmetic.

The unit

Exposure is illuminance multiplied by time, expressed in lux-hours. An object displayed at 50 lux for six hours a day, five days a week, for a year receives roughly 78,000 lux-hours. The same object at 200 lux receives four times that.

Because the relationship is close to reciprocal over the range that matters in museums, a budget can be spent in whatever combination suits the display. That is the whole practical point: the curator gets to choose between a long dim display and a short brighter one, and the arithmetic says whether the choice is affordable.

There are limits to the reciprocity, and for some materials very high illuminance for a short time is worse than the arithmetic predicts. In practice this is rarely the binding constraint at museum light levels.

Setting an annual allowance

The allowance depends on how responsive the material is and on how much total change the institution considers acceptable over the object’s expected display life. Broad responsivity categories are the usual starting point: materials with essentially no response such as stone, metal, glass and ceramic; low responsivity such as oil and tempera paintings and undyed leather; medium responsivity such as most textiles, watercolours, prints, drawings and dyed leather; and high responsivity such as many early photographic processes and objects with known fugitive colourants.

The important part is the second half of the sentence: the allowance encodes a decision about acceptable change over a stated period. Two institutions can look at the same watercolour and legitimately set different budgets, because one intends to display it every year for a century and the other intends to display it twice in a generation. What is not legitimate is having no figure at all, because then nobody can say whether this year’s exhibition was affordable.

Write the allowance down with its reasoning, in lux-hours per year, per object or per category.

Spending it

Once there is an allowance, display planning becomes a subtraction.

Measure the actual illuminance at the object’s surface, in the plane of the surface, with a properly used meter. Multiply by the hours the object will be lit, which is not the same as opening hours if the lighting is on a sensor or a timer. Include daylight, which in a room with windows frequently dominates and which varies by a factor of many between seasons.

Subtract the result from the allowance and record the remainder. That remainder is the object’s remaining display capacity for the year, and it is the number that lets someone say no to a fourth loan request with a reason rather than a feeling.

Rotation is the lever

For medium and high responsivity material, no lighting technology solves the problem. Every LED, every filter, every clever optical arrangement still delivers visible light, and visible light fades. The only variable that changes the answer by an order of magnitude is time.

An object shown for three months and stored for nine spends a quarter of what a permanently displayed object spends. A facsimile on permanent display with the original brought out for a month a year spends a twelfth. This is why rotation, and not equipment, is the primary control for the most sensitive material.

Keeping the record

The budget only works if the spending is recorded, and the record has to outlive the person who set it up.

Keep it with the object record, not in a separate spreadsheet on someone’s desktop. For each display period record the dates, the measured illuminance, the estimated lit hours, the resulting lux-hours and the running total. Note the meter used and when it was last checked.

Twenty years of this record is worth a great deal: it is the difference between an institution that can say what an object has received and one that is guessing. It is also, in practice, the strongest argument available when a display proposal needs to be refused.