Light and exposure

Three calculators: lux and footcandles converted and placed against the published ceilings; a year of exposure on one wall compared with the published budget; and the months on display and years of rest a published table gives.

Every figure below is published for institutional collections on exhibition, because that is where the figures are. A house is not a gallery. What these boxes give you is what a wall costs in light per year, and what the published budgets say that buys; they do not say a picture is ruined, and they cannot see your piece. Where it can hang is the chapter they belong to.

1. Lux, footcandles and the published ceilings

Enter the reading your meter gave you. Measuring the light covers where to hold the meter and when to read it.

Assumptions

• The conversion is 10.76 lux to the footcandle, stated on the conservation institute's own page on light, ultraviolet and infrared, which also offers multiplying by 10 as the working rule. Two preservation centers publish approximately 11, which is about two percent high. Physical principle
• The ceilings are a convention copied forward rather than four separate findings: 50 lux for sensitive material, 150 for less sensitive and 300 for the least sensitive appear in near-identical wording in a conservation center's handout, a state agency's leaflet, a museum's duration guidelines and a state museum body's fact sheet. The same conservation institute is the only source in the research that explains where 50 lux comes from, as a visibility benchmark for a 25-year-old viewer that can be adjusted upward for dark surfaces, low contrast, fine detail and older eyes. Convention
• Where publishers differ: the North American sources give 150 lux for moderately sensitive material and the Australian convention gives 200. One state agency reprints the Australian text, so it is not independent corroboration. We show both. Convention
• One national library works in footcandles instead, at a maximum of 3 for very light-sensitive material and 5 to 10 for moderately sensitive.
• Ultraviolet is not something a household can measure. The published limit is 75 microwatts per lumen from five bodies, and 50 from the agency that alone shows its derivation, from an incandescent bulb's output of 40 to 70 in the late 1970s. Treat ultraviolet as a glazing choice rather than a measurement. Ask the authority

2. A year on this wall

Multiplies the level by the hours and the days, and compares the total with the published annual budget for the medium. The last two lines are the only honest answer to whether ultraviolet-filtering glazing is worth the money.

Assumptions

• Dose is level multiplied by time, in lux hours. A conservation center states the arithmetic and works it twice: 100,000 lux for one hour is the same dose as 50 lux for 2,000 hours. The conservation institute's own light damage calculator publishes the same formula, illuminance times hours a day times days a year times years. Physical principle
• The annual budgets are published: a preservation leaflet gives 50,000 lux hours a year for light-sensitive material, with its own worked example of ten hours a day at 50 lux for a hundred days, and says ideally that is the annual exposure regardless of whether the object is displayed every year. A professional group's exhibition guidelines give the same 5,000 footcandle hours for very light-sensitive material with three years of rest, and 10,000 footcandle hours for moderately light-sensitive with two years of rest. Convention
• The fade figures are the conservation institute's published table of the light dose that causes a just-noticeable fade of each ISO Blue Wool standard, in millions of lux hours: with ultraviolet present, 0.22 for Blue Wool 1 up to 120 for Blue Wool 8; with ultraviolet removed, 0.3 up to 1,000. A just-noticeable fade is defined there as grey scale step 4. Physical principle
• The same institute states that for colors sensitive to light, ultraviolet usually contributes less than half of the fading and often only about one tenth. That is why removing it buys a great deal on stable material and almost nothing on fugitive material. Physical principle
• Not modeled: which Blue Wool number your piece actually is, which nobody can tell you without testing it; light reaching the piece from more than one direction; or the difference between a reading at the brightest hour and an average over the day. Read at the brightest hour and enter the hours the sun is actually on the wall. Test it

3. Months on display, years of rest

A museum publishes a duration table by material category. Enter the category, your measured level and the date the piece went up, and the box gives the date to take it down. Rotation explains what to do then.

Assumptions

• The categories and the months are a museum's published light duration guidelines for works of art and for photographs: three months over five years at 50 lux at the sensitive end, through six, twelve and twenty-four months, to unlimited display at 300 lux. Its own rules apply: an object's category is set by its most sensitive component, and anything hand-colored, tinted or on colored mount moves to the top category. Convention
• Scaling the published months to a level above the table's own is our arithmetic on their figures, using reciprocity: a conservation center states in print that dim light for a long time does the same damage as bright light for a short time. The hours-a-day line is ours too, against the eight to ten hours a gallery runs. Physical principle
• The rest periods come from a national library and a professional group: a minimum of three years of dark storage after a display cycle for very light-sensitive material, two for moderately sensitive.
• Not modeled: how long the piece has already been on a wall, which is usually unknown and usually longer than anyone thinks; light from a second source; or any of the other agents, since a dark damp wall is not safe either.