Museum Textile Light Fading & Cumulative Lux-Hour Budget

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 9 inputs are read from the form on every keystroke: Display Illuminance, Lit Hours per Day, Display Days per Year, UV Content, UV Reference Level, Blue Wool Rating, Dose to Fade Blue Wool 1, Dose Ratio per Blue Wool Step and Target Display Life.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Years to Just-Noticeable Fade together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Annual Dose, UV-Weighted Dose, Total Fading Budget, Illuminance for Target Life and Annual Budget Consumed — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

The tool loads with this case already solved — the Years to Just-Noticeable Fade shown above is its answer. Change one value and the difference from this baseline is the sensitivity of the result to that variable.

How to use it

1. Work through the input groups in order — Display Conditions and Object Sensitivity. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Years to Just-Noticeable Fade in the dark results panel — that is the headline figure, expressed in years.
4. Check the supporting rows underneath (Annual Dose, UV-Weighted Dose, Total Fading Budget, Illuminance for Target Life and Annual Budget Consumed) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Years to Just-Noticeable Fade before a trial is booked, so machine time and material in Conservation, Forensics & Legal Textiles are committed against a calculated figure rather than an estimate.
• Costing and quotation — Years to Just-Noticeable Fade is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Display Illuminance) shows how much of the gap in Years to Just-Noticeable Fade each variable explains.
• Teaching and study — the accepted ranges bracket normal Conservation, Forensics & Legal Textiles practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Assumptions and limits

• Reciprocity is assumed — that lux and hours trade freely and only their product matters — which holds well enough over normal display ranges but breaks down at very high illuminance, where damage runs ahead of the dose. The Blue Wool scale is also a coarse ordinal ranking rather than a measurement of a particular object, and a real historic textile carries several dyes of different sensitivity, so the most fugitive one governs and it is rarely the one the eye notices first. Damage from heat, humidity cycling and pollutants accumulates alongside light and is not in this budget. Display decisions for significant objects belong with a qualified conservator, informed by microfading testing where the object justifies it.
• Every input is bounded to the range normal practice occupies (Display Illuminance 5 to 500 lux, Lit Hours per Day 1 to 24 h and Display Days per Year 1 to 365 days, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Questions people ask