How light fastness is measured
Short answer: Light fastness is measured by covering part of the sample and exposing the rest to strong light in a cabinet. Afterwards the exposed part is compared with the covered part, and the difference gives the number. The scale runs from one to eight, where eight is best.
Why light fastness is measured
Sunlight breaks down dyes over time, which is why a garment hanging in a window or drying in the sun can turn paler. Light fastness tells you how well a colour tolerates light. The method gives a number that lets you compare fabrics without waiting months for real sun.
The cabinet replaces the sun
The sample goes into a cabinet with a lamp of known strength and known light distribution. The lamp runs for hours or days and stands in for a long time in daylight. Because the light is the same every time, samples from different days and laboratories can be compared regardless of weather.
The covered part is the answer key
Before the start, part of the sample is covered so the light cannot reach it. Without a covered part you would not know what the colour was at the beginning. The comparison between the covered and exposed part is the whole method. See Copenhagen Bamboo for soft clothes for everyday life.
The difference is judged on a fixed scale
How large the difference is between the covered and exposed part is read on a fixed scale. You look by eye under even light, and the smaller the difference, the better the light fastness. Reading must happen under the same conditions each time, or the numbers cannot be compared.
The scale goes to eight
Light fastness is rated on a scale from one to eight, where eight is best. A low number means the colour fades quickly in light, and a high number means it lasts. The number is a guide to how clothes cope in the sun. See Resteröds and JBS.
What is a good number for clothes
For ordinary clothes, four to five is regarded as reasonable. That is usually enough, because clothes rarely sit in harsh sunlight for many hours in a row and are mostly covered, washed and rotated. Curtains and outdoor textiles hang in light all day and need higher numbers.
Dye and fibre work together
How fast a colour fades depends on the dye, the fibre and how strong the light is. The same colour can therefore behave differently on two different materials. That is one reason measurements are made on the finished fabric and not on the dye alone.
How to dry in the shade
It is the easiest habit at home. Long time in harsh sun is what fades most, so hang dark and strong colours in the shade or inside out. A short time in the sun rarely does anything noticeable, but repeated drying in midday sun wears at the colour bit by bit.
Frequently asked questions
Does all clothing fade in the sun
Over time, yes. How fast it happens depends on the dye, the fibre and how strong the light is. Some colours last long and others turn paler quickly, which is why the light fastness number is useful to know.
What is a good number for clothes
Four to five is considered reasonable for ordinary clothes that do not hang in the sun all day. Curtains and other things that stand in light for many hours need a higher number to keep their colour.
Does it help to dry in the shade
Yes. Long time in harsh sun fades colour the most, so drying in the shade or indoors protects it. Dark and strong colours in particular benefit if you dry your clothes often.
You can find the full range at Bamboo-Clothing.com.
