Colour Performance / Light / Xenon Arc
How Is Colour Fastness to Light Tested in Knitwear?
Light fastness compares colour change under a defined radiant exposure. It is a controlled method-specific result, not a promise that a shade will remain unchanged for a stated number of years.
Colour fastness to light is tested by mounting a representative textile specimen beside the reference materials required by the selected method, covering portions to preserve unexposed comparisons, and exposing the assembly to a controlled light source. ISO 105-B02 uses xenon-arc light intended to represent D65 daylight. Exposure conditions include the specified filters, irradiance control, chamber atmosphere, black-standard or black-panel temperature, humidity condition, and endpoint procedure. The specimen's exposed and covered areas are then compared, with blue wool references used to characterize the exposure and resistance to colour change. The report must name the exact method, option, reference system, exposure stage, specimen state, and result.
A xenon-arc test controls the light as well as the specimen
ISO 105-B02 applies to textiles of all kinds and forms and determines colour fastness to an artificial source representative of natural daylight. AATCC TM16.3 also uses a xenon-arc apparatus but provides its own procedures and exposure options. The methods are related in purpose, not interchangeable in notation or operating detail. A specification should identify the standard and edition rather than request a generic “light test.”
Xenon lamps produce a broad spectrum, while optical filters shape the radiation reaching the specimen. The apparatus controls irradiance and monitors chamber conditions because lamp output, filter state, temperature, and moisture can alter photochemical response. Black-standard or black-panel temperature indicates the thermal condition at a dark reference surface; it is not simply room temperature. Humidity control matters because moisture can change the response of fibres, dyes, pigments, finishes, and reference materials during exposure.
Part of each specimen remains masked. As exposure proceeds, masks may be repositioned according to the selected procedure so that several exposure stages remain visible. This creates direct comparisons between unexposed material and areas receiving known stages of radiant exposure. Assessment is made under the viewing conditions and reference system required by the method. A casual comparison beside a window cannot reproduce the controlled spectral distribution, irradiance, temperature, humidity, and endpoint logic.
Specimen mounting should present the surface that will face light in use without stretching, glazing, or changing pile direction. Open knits need an agreed backing where the method permits because colour seen through the apertures can influence visual assessment. Brushed, pile, chenille, metallic, lustrous, fluorescent, and high-relief surfaces require consistent orientation. A seam, fold, rib edge, or jacquard float should not enter the test area accidentally.
Do not convert the grade into years
A light-fastness result describes performance under the stated apparatus, method, reference set, and endpoint. It does not provide a universal conversion to hours in a shop window or years of wear.
Blue wool references make the exposure interpretable
Blue wool references are dyed wool standards with graduated responses to light. They are exposed with the specimen so that the degree of change can be judged against a known sequence. ISO 105-B02 recognizes two sets of blue wool references, and the standard notes that results from the two sets may not be identical. The report therefore needs to identify the reference system rather than presenting an isolated grade.
The blue wool sequence should not be confused with the grey scales used to assess colour change or staining in other colour-fastness tests. Blue wool references characterize resistance under light exposure; grey scales quantify visual differences for specified assessments. Some procedures use both kinds of comparison at defined stages, but their numbers do not mean the same thing.
The exposure may be controlled by change in a specified reference, by a prescribed radiant exposure, or by another endpoint provided by the selected method and client specification. A result such as a blue-wool grade has meaning only with that method and endpoint. Likewise, a statement of radiant exposure without the spectral, thermal, humidity, and specimen conditions is incomplete.
AATCC TM16.3 includes continuous-light and alternate light-and-dark approaches under defined apparatus conditions. Those options are not a general promise of a fixed acceleration factor. The official method expressly cautions against assuming that a particular exposure predicts a specific period of actual use. Real daylight changes with latitude, season, glazing, orientation, weather, indoor distance, and time of day.
Fibre, colourant, shade depth, and finish determine the response together
Photochemical change begins with the colourant and fibre system. Acid dyes on wool or polyamide, reactive dyes on cellulosic fibres, disperse dyes on polyester, pigments in a binder, natural dyes, fluorescent pigments, and optical brighteners respond through different mechanisms. A blended yarn can contain components with unequal resistance, and a marled or mélange shade can shift in balance even when the total colour difference appears moderate.
Shade depth affects the visible result but does not produce a universal rule. Pale shades can show a small absolute loss clearly, while deep shades may contain more colourant and complex mixtures. Bright reds, blues, violets, fluorescent colours, and delicate neutrals can pose different risks depending on chemistry. Each colourway should be represented; passing one dark shade does not validate a pale related shade, and visual similarity does not prove an identical dye recipe.
Yarn preparation and wet processing influence the available colour system. Dye selection, concentration, fixation, reduction clearing or wash-off, pH, metal ions, water quality, residual auxiliaries, antioxidants, ultraviolet absorbers, and aftertreatments can alter light response. A finish intended to improve one property may shift shade or affect another fastness route. Testing should therefore use the intended bulk fibre, yarn, dyehouse process, and final finishing sequence.
Knit construction does not change colourant chemistry, but it changes what is exposed and what the observer sees. Pile, brushing, hairiness, loop orientation, plating, floats, and relief affect surface area, shadow, and directional appearance. In a plated fabric, technical face and body-facing yarn can be different colours or fibres. Intarsia and jacquard place distinct yarn systems together. Those components may need separate or composite specimens rather than one average grade.
White and near-white knitwear also require attention. Optical brightening agents absorb ultraviolet radiation and emit visible light; exposure can reduce that effect and create yellowing or loss of brightness without a conventional dye fade. Fluorescent fashion colours similarly need a method and assessment suitable for their response. Instrumental colour difference can supplement controlled visual assessment where allowed, but it does not excuse omission of the standard's reference and viewing conditions.
Light fastness is not the same as weathering or every daylight-related effect
ISO 105-B01 addresses colour fastness to daylight, while ISO 105-B02 uses artificial light representative of D65 daylight. Artificial exposure improves laboratory control and repeatability, but the result still belongs to that method. ISO 105-B04 addresses artificial weathering that includes wetting as well as light. Outdoor rain, condensation, pollution, salt, and thermal cycling are not supplied by a light-only result.
Photochromism is a temporary, often partly reversible colour change after brief light exposure and is detected through a separate method, ISO 105-B05. It should not be mistaken automatically for permanent fading. The observation interval after exposure matters when the material appears to recover. Perspiration combined with light is another distinct route, covered by purpose-specific procedures such as AATCC TM125; neither a perspiration-only nor light-only grade predicts the combined result.
Display and storage risk also depend on product geometry. A folded knit may develop a line between exposed and covered areas; a window display can expose one shoulder continuously; a showroom spotlight can create a local field; a garment bag can filter part of the spectrum. Light-fastness data support risk decisions, but they do not replace controlled display rotation, ultraviolet-filtering glazing where appropriate, or product-specific care and storage instructions.
Build the test around the actual colour system
During development, the sampling plan should identify fibre, yarn lot, dye or pigment recipe, shade, construction, face, finish, and intended exposure risk. High-contrast jacquards, engineered intarsia, mélange yarns, trims, embroidery, print, and body-facing plating may need separate specimens. The final production state matters: a laboratory dip, dyed yarn, knitted panel, finished garment, and laundered garment are not automatically equivalent.
When a result is weak, diagnosis should begin with the exact area and nature of change, not a blanket instruction to add protection. Review colourant selection, dye combination, fibre compatibility, fixation, clearing or wash-off, brighteners, auxiliaries, finish, and exposure face. A ultraviolet absorber or aftertreatment can affect handle, shade, yellowing, care performance, or other fastness properties and must be evaluated as part of the complete material system.
Retesting should use the same method, option, apparatus conditions, reference system, endpoint, and representative corrected route. A comparison across laboratories is meaningful only when those controls align. There is no single universal light-fastness threshold for all luxury knitwear; the requirement depends on product category, colour, fibre, expected display and use, market specification, and accepted risk.
Light exposure should be read beside other colour routes. Washing, rubbing, and perspiration answer different questions. Selected colour and construction outcomes appear in Work; Anne's technical remit is described in About. Project-specific enquiries can be made through the contact form.
Common questions about light-fastness testing
What does the blue wool scale mean?
Blue wool references are materials with graduated light responses exposed beside the specimen. The result compares the specimen with the stated reference set under a defined method. It is not a direct number of months or years.
Is xenon-arc testing the same as natural daylight exposure?
No. Xenon-arc systems use filters and controlled apparatus conditions to represent a defined daylight spectrum reproducibly. Natural daylight varies with location, season, weather, glazing, orientation, and time.
Does a high grade mean a colour will never fade?
No. It indicates stronger resistance under the reported test conditions. Sufficient exposure, different spectra, heat, moisture, chemicals, or combined wear can still produce change.
Is light fastness the same as weather fastness?
No. Weathering can combine light with wetting and other environmental stresses. A light-only result should not be used as a weathering result.
Do all colourways need to be tested?
Technically equivalent routes may be grouped through a justified risk plan, but visual similarity is not evidence of identical dye chemistry. New shades, fibres, suppliers, recipes, finishes, and fluorescent or brightened systems require suitable evidence.
What is photochromism?
Photochromism is a temporary or partly reversible colour change caused by light. ISO 105-B05 provides a specific method to detect and assess it, separately from ordinary permanent light fading.
The grade remains inseparable from the exposure
Light-fastness testing becomes useful technical evidence when the specimen identity, exposed face, apparatus, filters, irradiance, temperature, humidity, reference set, endpoint, and assessment remain attached to the result. That precision supports colour decisions without turning a laboratory comparison into an unsupported forecast of years in use.
Light fastness and metamerism answer different colour questions. The companion note on metamerism in knitwear colour matching explains why two unchanged materials can match under one illuminant and separate under another, and how spectral and visual controls diagnose the pair.
Colour change after exposure and ultraviolet transmission are different controls. The companion note on UPF measurement in knitted fabric explains erythemally weighted spectral transmission and the effects of cover, extension, wetting, care, and garment design.