Introduction
Light damage is cumulative and often irreversible. This calculator helps museum staff plan exhibit lighting so that a light-sensitive object stays within a defined exposure budget measured in lux·hours (illuminance in lux multiplied by time in hours). Many conservation policies express limits as a maximum lux·hour dose over a period (for example, per year) or as a remaining lifetime allowance based on material sensitivity and condition.
Use this page to estimate daily exposure, planned total exposure, and the maximum allowable exhibit days at your proposed lighting level. The results table also shows two quick alternatives (−20% illuminance and −20% daily hours) to support practical trade-off discussions.
How to use the planner
- Enter the artifact sensitivity limit in lux·hours. This should represent the remaining allowable dose for the planning period you care about (e.g., the next year or the object’s remaining lifetime), not necessarily a generic category value.
- Enter the display illuminance (lux) measured at the object surface under typical operating conditions.
- Enter daily display hours (hours/day) for the time the lights are on at that illuminance. If lights are off when the gallery is closed, use open hours.
- Enter planned exhibit days (days). Use calendar days if lighting is consistent every day, or open days if the object is dark when closed.
- Select Calculate to generate a scenario table. Use Download CSV to save the table for documentation, loan files, or internal review.
Formula and units
The calculator uses a dose model based on constant daily conditions:
- Daily exposure =
lux × daily hours(lux·h/day) - Planned total exposure =
daily exposure × planned days(lux·h) - Allowable days =
sensitivity limit ÷ daily exposure(days) - Remaining (days) =
allowable days − planned days
If Remaining (days) is negative, the plan exceeds the budget at the entered settings.
Worked example (single example)
Suppose a light-sensitive poster has a remaining limit of 50,000 lux·h. You plan to display it at 200 lux for 8 hours/day. The daily exposure is 200 × 8 = 1,600 lux·h/day. The maximum allowable display time at those conditions is 50,000 ÷ 1,600 ≈ 31.3 days. If you schedule 90 days, the planned total is 1,600 × 90 = 144,000 lux·h, which exceeds the limit. To comply, you could reduce illuminance, reduce hours, rotate the object, or use motion-triggered lighting.
Assumptions, limitations, and conservation notes
- Constant conditions: Illuminance and hours are treated as constant each day. The model does not simulate dimming schedules, daylight variation, or occupancy-based controls.
- Spectrum not modeled: Lux·hours are a practical proxy, but fading depends on spectral power distribution (UV and blue content), filtering, and material chemistry.
- Non-linear response: Some materials fade non-linearly (e.g., faster early fading). A conservative limit is still useful for planning, but condition checks remain essential.
- Prior exposure: If the object has display history, subtract past lux·hours from the limit before planning. This tool assumes the limit you enter is the remaining budget.
- Measurement uncertainty: Lux readings vary with meter calibration, position, and angle. Measure at the object plane and re-check after lamp changes or re-aiming.
- Not a substitute for policy: Always follow your institution’s conservation policy and lender requirements. This calculator supports documentation and scenario testing.
Quick glossary (plain language)
Conservation lighting discussions can be jargon-heavy. This short glossary is included so that curators, designers, and registrars can read the same output table and understand what it means without needing a separate reference.
- Lux
- A measure of illuminance: how much light falls on a surface. Lux is measured at the object plane (where the artifact is), not at the lamp. A spotlight can read high lux at the center and much lower at the edges, so measurement position matters.
- Lux·hours (lux·h)
- A cumulative dose: lux multiplied by hours. If you keep an object at 100 lux for 10 hours, that is 1,000 lux·h. Lux·hours are useful because they add up over time, making it easier to track exposure across multiple exhibits.
- Exposure limit / sensitivity limit
- The maximum dose you are willing to allocate for a planning period (annual) or for the remaining lifetime of the object. The right number depends on institutional policy, lender requirements, and the object’s condition and materials.
- Allowable days
- The number of days you can display the object at the entered lux and hours before reaching the limit. This is a planning estimate; condition checks and policy may require additional safety margins.
Why light budgets matter in museums
Museum professionals balance access and preservation. Light—especially ultraviolet and high-energy visible wavelengths—can fade dyes, yellow paper, and embrittle textiles. Because this damage accumulates, conservators often manage exposure using a budget expressed as cumulative lux·hours. Once an object’s budget is consumed, additional exposure increases the risk of noticeable and irreversible change.
A budget approach turns daily lighting decisions into a clear planning constraint. If you know the illuminance at the object and how long the lights are on, you can estimate the daily dose and translate it into allowable exhibit days. This supports rotation schedules, loan negotiations, and documentation for curatorial sign-off.
Interpreting the scenario table
After you calculate, the table shows three scenarios:
- Baseline: your entered lux and hours.
- Reduced Illuminance (−20%): a quick check of what happens if you dim the lights.
- Shorter Hours (−20%): a quick check of what happens if you shorten daily lighting time.
These are not prescriptions—just fast comparisons. If you need a specific target (for example, “How many lux can we use for 60 days?”), you can iterate by adjusting inputs. A practical workflow is to start with the desired exhibit duration, then adjust lux and hours until the Remaining (days) value is comfortably positive.
Planning workflow: from policy to a schedule
In many institutions, lighting decisions involve multiple stakeholders: conservation, curatorial, design, facilities, and sometimes lenders. A simple workflow helps keep the conversation focused and reduces last-minute changes.
- Confirm the planning limit: Decide whether you are budgeting for a year, a loan period, or a lifetime remaining allowance. Enter that number as the sensitivity limit.
- Measure the real lux: Take readings at the object plane after aiming and focusing. If the object has uneven illumination, consider budgeting against the highest sustained reading.
- Define operating hours: Use the hours the lights are actually on. If lights are reduced during cleaning, events, or closed days, reflect that in the hours or days.
- Run scenarios: Use the baseline and the two alternatives as a starting point. If the plan exceeds the budget, decide whether to dim, shorten hours, rotate, or substitute a facsimile.
- Document and monitor: Save the CSV, attach it to the exhibit file, and re-check lux after lamp replacement, re-aiming, or gallery reconfiguration.
Practical considerations for real galleries
Light exposure is only one part of preventive conservation. UV filtering, fixture choice (LED spectrum), beam shaping, and motion-triggered controls can reduce risk without compromising visitor experience. Condition assessments remain important: if early fading is observed, reduce exposure immediately even if the calculated budget is not yet exhausted.
Documentation is also key. Many institutions maintain exposure logs so that future exhibits can account for prior display history. When budgets are low, objects may be displayed under very low illuminance for short periods, rotated with similar works, or replaced with facsimiles. For rotating displays, it can be helpful to treat the limit as a “bank account”: each exhibit withdraws lux·hours, and periods in dark storage do not replenish the account.
Consider the difference between target lux and measured lux. Designers may specify “150 lux,” but the actual reading at the object can be higher due to reflections, nearby light sources, or a tighter beam than expected. Conversely, a dimmer setting may still produce hotspots. When in doubt, budget conservatively and verify with a meter.
Common questions (FAQ)
Is lux·hours the same as “damage”?
Lux·hours are a practical proxy for cumulative exposure, not a direct measurement of chemical change. Two light sources with the same lux can have different spectra, and some materials are more sensitive to certain wavelengths. Still, lux·hours are widely used because they are measurable, additive, and easy to track across time.
Should I use open days or calendar days?
Use whichever matches how the lights operate. If lights are off when the museum is closed, using open days (or reducing daily hours) better reflects reality. If the gallery remains lit for security or cleaning, include those hours. The goal is to approximate the hours the object actually receives the stated illuminance.
What if the object is lit at different levels during the day?
This calculator assumes a constant lux value. If you have a dimming schedule, you can approximate it by using an average lux weighted by time. For example, if an object is at 200 lux for 4 hours and 100 lux for 4 hours, the average is (200×4 + 100×4) ÷ 8 = 150 lux, and the daily exposure is 150×8 = 1,200 lux·h/day. For more complex schedules, break the day into segments and sum lux×hours manually, then enter the equivalent daily exposure by adjusting lux and hours.
How should we handle prior exposure history?
Enter the remaining budget, not the original category limit. If your records show the object has already accumulated 20,000 lux·h and the policy limit is 50,000 lux·h for the period you are managing, enter 30,000 lux·h. If you do not have reliable history, consider using a more conservative limit and increase monitoring.
Does the calculator account for UV filtering or LED spectrum?
No. UV filtering and spectral tuning can reduce risk, but lux is weighted to human vision and does not fully represent photochemical hazard. Use this tool for schedule planning and documentation, and rely on your conservation policy for fixture selection, UV limits, and acceptable spectra.
Related tools
If your exhibition work overlaps with other planning needs, you may also find these pages useful: Portable Darkroom Waste Neutralization Planner, Vitamin D Sunlight Exposure Calculator, and Jet Lag Recovery Time Calculator.
Summary: what this planner is best for
This page is designed for quick, defensible planning: it converts a proposed lighting level and schedule into lux·hours and compares that dose to a limit you provide. It is most useful when you need to answer practical questions such as: “Can we keep this manuscript on view for the full run?”, “How much do we gain by dimming 20%?”, or “What schedule can we propose to a lender that stays within their requirements?”
For best results, pair the calculation with good measurement practice (calibrated meter, readings at the object plane, checks after changes) and with ongoing condition assessment. A calculator can support decisions, but the object’s observed response and your institution’s policy should always take priority.