"Light fastness 5 after 40 hours xenon." This line appears on many supplier reports, and under ISO 105-B02 it is close to meaningless.
The standard does not grade a fabric by how long it was exposed. It grades the fabric by comparing its fading with a set of reference materials that faded alongside it. The hours on the counter are a by-product.
Two sets of blue wool, not interchangeable
ISO 105-B02:2014 allows two sets of references:
| Blue wool 1 to 8 | Blue wool L2 to L9 | |
|---|---|---|
| Origin | Developed in Europe | Developed in America |
| Construction | Wool dyed with eight different dyes | Blends of two dyed wools in varying proportions |
| Relation between steps | Each reference roughly twice as fast as the one before | Each reference roughly twice as fast as the one before |
| Used with | Exposure cycles A1, A2, A3 | Exposure cycle B |
The standard is blunt: the results from the two sets are not interchangeable. ISO 105-B01 goes further and says the correlation chart between the sets must not be used to convert a rating from one to the other. A "grade 5 on L-series" and "grade 5 on 1–8" are two different statements.
References 1 to 8 must also meet the quality requirements of ISO 105-B08. Old cards from an unknown batch, stored in daylight, are not references any more.
The five methods
The standard gives five exposure methods. They answer different questions:
- Method 1 — one set of references per specimen, exposure controlled by inspecting the specimen itself. The most informative method and the one to use in a dispute over the numerical rating.
- Method 2 — several specimens exposed with one set of references.
- Method 3 — a pass/fail check against a single blue wool reference.
- Method 4 — a pass/fail check against an agreed reference sample of known performance.
- Method 5 — exposure to a specified radiant dosage; the end point is the dose, and intermediate assessment may not be needed.
The standard is explicit that for Methods 1 to 4, it is not sufficient to rely on exposure time to decide when each stage ends. You stop when the specimen or the reference has faded to the specified contrast on the grey scale. Only Method 5 runs to a dose.
So "40 hours" can be a valid Method 5 statement if the dose and conditions are reported. It is not a Method 1, 2 or 3 result.
Effective humidity: the step most labs skip
Moisture changes how fast many dyes fade. ISO 105-B02 therefore controls effective humidity — the combined effect of air and surface temperature and humidity at the specimen surface — and it does not trust the machine's humidity reading to do so.
Instead, a red azoic-dyed cotton humidity-test control fabric is exposed with the blue wool references until it shows a contrast equal to grey scale grade 4. Then the blue wool is checked. Under normal conditions (cycle A1), the target of roughly 40 % effective humidity is typically achieved when blue wool 5 shows a contrast equal to grade 4. If it does not, the machine settings are adjusted and the check is repeated with fresh materials.
The four exposure cycles, as tabulated in the 2014 edition:
| Cycle | Climate replicated | Black standard temp. | Black panel temp. | Humidity |
|---|---|---|---|---|
| A1 | Temperate (normal) | (47 ± 3) °C | (45 ± 3) °C | ≈ 40 % effective |
| A2 | Dry (extreme low humidity) | (62 ± 3) °C | (60 ± 3) °C | < 15 % effective |
| A3 | Semi-tropical (extreme high humidity) | (42 ± 3) °C | (40 ± 3) °C | ≈ 85 % effective |
| B | — | (65 ± 3) °C | (63 ± 3) °C | (30 ± 5) % relative |
Where irradiance control is available, it is set at (42 ± 2) W/m² over 300–400 nm or (1.10 ± 0.02) W/(m²·nm) at 420 nm. The standard also notes that chamber air temperature control should not be used in place of black standard or black panel temperature — they are different values.
A lab that sets 40 % RH on the display and never runs the humidity control fabric is not running ISO 105-B02 cycle A1. It is running something similar.
Specimen preparation errors
Several clauses exist because the eye is easily fooled:
- Same size and shape. Specimens and blue wool strips must be equal in size and shape. A wide specimen next to a narrow reference looks more faded than it is.
- Sharp masking. Covers must touch the unexposed area closely to give a clean line, but must not crush the pile.
- Thick fabrics. References are raised to the same distance from the lamp as the specimen's upper surface. For thick or shifting textures, the exposed area should be at least 50 mm × 40 mm.
- Empty holders. Fill unused positions with non-reflective material such as white card, so the light field does not change with the load.
Grading
Assessment uses the grey scale for change in colour (ISO 105-A02), in an assessment cabinet, with the specimen masked in the same colour as the grey scale's sleeve. The rating is the number of the blue wool reference that shows a similar change — not a grey scale grade. Mixing the two vocabularies on one report is a frequent source of confusion.
Before you report
- Which reference set (1–8 or L2–L9) and which exposure cycle?
- Which method (1–5)?
- Was effective humidity verified with the control fabric, not read from the display?
- Was the end point a visual contrast, or a dose (Method 5 only)?
- Were specimens and references the same size, correctly masked?
TLI-TexLabInstruments manufactures xenon light fastness testers in its own workshop in Istanbul.