Nobody loans a guard, a light meter, and a prayer. You loan light — a finite allowance of lux-hours that a work may absorb before cumulative damage begins. The chancellor's question is never "is it bright?" but "how many photons, over how many days, may a Rembrandt safely drink?" This is the arithmetic that answers it.
I.The allowance table
International conservation practice (below, cited to the ICOM-CC / CIE guidance and the CIE 157 standard on museum lighting) groups works into four dose classes. The real variable is photochemical dose — the product of illuminance and exposure time:
Class A · Light-sensitive
≤ 50 lux
Watercolours, silk, most dyes, photographic prints, insect pigments.
Class B · Moderate
≤ 150 lux
Oil and tempera, most woods, lacquers, undyed leather.
Class C · Fairly durable
≤ 300 lux
Stone, ceramic, undyed glass, metal, unframed bone.
Class D · Durable
no limit
Unpainted metal, unfired mineral, vitreous enamel.
II.Field tool — the dose ledger
Enter the loan's terms. The tool computes the annual light dose (lux-hours) against the recommended budget for that class, and warns when a temporary loan is burning through a work's lifetime allowance too fast.
III.Why lux-hours, not lux
A common desk error: ruling a gallery "safe at 50 lux" and walking away. The damage function for most fugitive colourants is roughly linear in dose — the same yellowing happens at 50 lux for ten years and 100 lux for five. Standards bodies therefore set annual dose budgets, then let the registrar trade lux against hours as the run's scheduling requires.
| Class | Practical lux ceiling | Recommended annual dose budget | Note |
|---|---|---|---|
| A | 50 lx | 25,000 lx·hr | CIE 157: ≤500 display-hours/yr at 50 lux |
| B | 150 lx | 75,000 lx·hr | same 500-hr protocol scaled to lux |
| C | 300 lx | 150,000 lx·hr | restricted only by thermal comfort |
| D | — | no practical cap | vitreous enamel / unfired mineral |
Grounded in references to the CIE 157:2004 standard on control of damage to museum objects, and the ICOM-CC conservation community's dose-based guidance as abstracted for desk use. Figures are planning numbers for a registrar's ledger, not a conservator's specification.
IV.A borrowed thought on restraint
My first small film, Lux and Time, argues the same case in ninety seconds — that conservation is the discipline of saying "not yet" and "not so bright." This sheet is its ledger twin. The pair belong together: the film gives the why, this tool gives the how. The monitoring guide carries the environmental paperwork.
Machines can read the model too — the data twin lives at light-budget.json.