Light fastness testing
Light fastness measures how much a dyed textile fades when it is exposed to light. Unlike washing or rubbing, which are short and mechanical, this is a slow chemical effect: light energy breaks down the dye molecule, and the colour changes as the dye is destroyed. Fading is progressive and cumulative, so the question the test answers is not whether a shade fades but how much exposure it survives before the change becomes visible. Reactive dyes on cellulose generally hold light well, but not uniformly, and pale shades and certain colour chemistries are the ones that go first.
What light fastness measures
The result is reported on the blue wool scale, an eight-point reference scale rather than a grey contrast, because fading is gradual and has to be measured against known reference samples rather than judged in one glance. A rating of 6 describes a fabric that withstands substantially more exposure than one rated 3. Buyers in furnishing, automotive, outdoor textiles and apparel that will hang in daylight specify it, because a faded display sample or a curtain that changes shade in a season is a return rather than a complaint.
The reference Nuchem follows
Nuchem runs the light fastness test to ISO 105-B02:2014, using a xenon arc instrument on dyed cotton. The instrument is calibrated and controlled to a fixed exposure cycle, so the fading conditions are reproducible rather than dependent on the weather. Cycle A1 settings are shown below.
| Setting | Value |
|---|---|
| Black standard temperature | 47 ± 3 °C |
| Irradiance at 420 nm | 1.10 ± 0.02 W/(m²·nm) |
| Effective humidity | Approximately 40 % |
| Instrument | Calibrated ISO 105-B02 xenon tester with daylight and heat filters |
| References | Blue wool 1-8 to ISO 105-B08 |
| Contrast assessment | ISO 105-A02 grey scale |
| Assessment lighting | D65 cabinet |
How the test is run
The specimen is mounted alongside its own set of blue wool references of equal size, with the middle third of the assembly masked so that an unexposed area remains for comparison. Exposure is then interrupted at defined points so that fading can be read progressively rather than only at the end.
- The cotton specimen is mounted with its own blue wool set, cut to equal size and shape, with exposed and unexposed areas of at least 10 mm × 8 mm, and the middle third masked.
- Humidity is verified first: at a control-fabric contrast of 4, blue wool 5 must reach contrast 4. Chamber relative humidity alone is not sufficient evidence.
- Exposure begins and the specimen is inspected periodically.
- When blue wool 2 reaches contrast 3, a preliminary rating is recorded against references 1 to 3.
- When the sample reaches contrast 4, the mask is replaced so that only the right-hand third remains exposed, and exposure continues.
- Exposure stops when the sample reaches contrast 3, or when blue wool 7 reaches contrast 4, whichever happens first. Contrasts are read on the ISO 105-A02 grey scale.
How the result is read
After exposure the tested cards rest in darkness at room conditions for at least 24 hours before grading, because some dyes recover partially once the light is removed. The covers are then removed under D65 lighting and the contrasts of the corresponding panels are matched against the blue wool fading, using whole and half grades from 1 to 8, where 8 is best and anything worse than 1 is reported as below 1. Available panel grades are averaged, and a figure falling between whole and half grades is rounded up.
Where a preliminary grade of 3 is followed by a final grade of 4 or better, the preliminary figure is appended in brackets on the report, and photochromism is checked separately. The agreed values for each published COTFIX grade appear on the fastness and solubility data page.
Frequently asked questions
A set of eight dyed wool references that fade at known, graduated rates. A specimen's light fastness is the rating of the reference it fades in step with, so the result is 1 to 8 rather than a judgement of how faded the fabric looks.
Because fading is gradual and cumulative rather than a single visible change. The grey scale would not show how much exposure the fabric survived, which is what the buyer needs to know.
Some dyes partially recover their colour once the light source is removed. Grading immediately would overstate the fading; resting in darkness lets the colour settle so the result reflects permanent change.
It means the shade withstands substantially more exposure than a lower-rated one before a perceptible change. All dyes fade eventually under enough light.
Light fastness values are published for the range grade by grade, and a specific grade can be tested for an order where a figure is required against a buyer's own specification.
Need a light fastness figure for a furnishing or outdoor shade?
Give the COTFIX grade and the exposure requirement the buyer specifies, and the laboratory will run the xenon arc test and report the blue wool rating.