Light Fastness

Light fastness is how quickly a fabric's colour fades under light. It is reported as a numbered grade, and — this is the part buyers get wrong most often — a higher number is better. The top grade barely fades; 1 means visibly pale within weeks.

Two scales are in use, and mixing them is a real source of RFQ disputes. ISO 105-B methods grade the faded specimen against a grey scale, 1–5. The blue wool scale is 1–8 and is anchored to dyed reference fabrics. Both appear on real specifications. A buyer asking for "grade 4" should say which one — see Color Fastness for the 1–5 grey-scale convention our wash and rub tests use. This page uses 1–8 where it names blue wool, and otherwise refers to the grade without implying a scale.

Every textile has two fastness properties: fastness to light and fastness to washing. They are different tests, different numbers, and a fabric can be excellent at one and poor at the other. A deep-dyed fabric that survives 40 industrial washes can still fade badly in a south-facing window.

What our print routes deliver

Fastness is a property of the route, not of the fibre — the same 100% polyester base behaves differently depending on how the colour got there. These are the grades we quote against the 1–5 grey scale:

Read that table as a sourcing decision, not a quality ladder. If the end use is a south-facing curtain or an upholstered piece in permanent daylight, specify digital printing. If it is a bedding program that will be washed hard and rarely sat in sun, disperse printing at 3–4 is the economical call.

How the number is produced

Testing uses exposure to controlled light for a set number of hours, then grading the change against a reference scale:

• ISO 105-B01 / B02 — Xenon lamp, the general method for textiles, graded on the 1–5 grey scale
• ISO 105-B04 — natural daylight exposure, the closest simulation of a real window
• Blue wool standards — dyed reference fabrics anchoring a 1–8 scale, widely used for light fastness and sometimes quoted where the grey scale is used. Ask which one a report uses.

Testing daylight rather than a lamp matters for the one application buyers care about most: a curtain in a real window. A lab xenon arc and a window are not the same spectrum, and grades can differ between them.

What to put on the RFQ

Three lines are worth having on the purchase order, and they are cheap to add before sampling and awkward to discover after bulk:

1. The grade, with the method and the scale — "Grade 4 to ISO 105-B04 on the 1–5 grey scale" is actionable; "light fast" is not.
2. The route — digital, disperse, pigment or piece-dye, since that is what sets the grade.
3. The end use and exposure — "residential curtain, west-facing, 6 hours direct sun" tells the lab what to test. Without it, a grade is just a number.

For a home-textile programme the honest summary is: our digital print route runs 4–5, our other print routes 2–4, and the route you pick should follow the light the finished product will actually see.

Where buyers over-ask

• "Does it fade?" — always, eventually, on any grade below 8. The useful question is how fast, and against which exposure.
• Comparing grades across methods — a grade from ISO 105-B04 and one from AATCC TM-16 are both "grade 4" and are not the same measurement. Ask which method produced the number.
• Expecting the fibre to protect the dye — in blends, the dye is on the fibre that takes it. A polyester-dominant fabric with a small cellulose content can fade on the cellulosic part and look blotchy rather than uniformly pale.

Related

• Color Fastness — the washing and rubbing properties, and the same scale applied to a different threat
• Martindale — abrasion cycles, the wear test rather than the fade test
• Pilling Resistance — surface wear, again a different axis