Light Fastness and the Blue Wool Scale: A Complete Guide for Interior Designers and Specifiers
Light fastness is a fabric’s resistance to fading when exposed to light. For interior designers, it is one of the most practically consequential specifications you will make. A fabric that fades within two years in a south-facing room represents a specification failure regardless of how well it performs on every other measure. This guide explains how light fastness is tested, what the Blue Wool Scale grades mean, which grades to specify for different applications, and how light fastness interacts with fibre type, dye method, and room orientation.
This is the third in a series of technical specification guides from Kothea. The first covers the Martindale rub test and the second covers BS 5852 Crib 5 fire certification.
For why velvet in pale colourways in south-facing rooms is a specific light fastness risk, see our when not to use velvet guide. For colour naming, systems, and metamerism, see our colour naming and specification guide. For the companion test covering dye transfer and crocking, see our colour fastness and crocking guide.
What Light Fastness Means
Light fastness measures how much a fabric’s colour changes when exposed to light. It is not the same as colour fastness generally, which covers a broader range of stressors including washing, rubbing, and perspiration. Light fastness specifically measures the effect of ultraviolet and visible light on the dye or pigment within a fabric.

Fading occurs when light energy breaks down the chemical bonds in a dye molecule, altering its ability to absorb and reflect specific wavelengths of light. The result is a shift in the perceived colour of the fabric, which may manifest as bleaching, yellowing, or a change in hue depending on the dye type and fibre.
The speed and extent of fading depends on the fibre type, the class of dye used, the dyeing method, the intensity and spectrum of light the fabric is exposed to, and the presence of UV filtering in the glazing of the windows in the room.
Light fastness should not be confused with shade change in velvet, which is the apparent change in colour caused by pile being pushed in different directions through use. Shade change is a mechanical phenomenon and is not related to dye degradation or light exposure.
How the Test Works
The standard test for light fastness in the United Kingdom and Europe is ISO 105-B02: Colour Fastness to Artificial Light: Xenon Arc Fading Lamp Test. The fabric specimen is placed in a controlled chamber and exposed to a xenon arc lamp, which produces a spectrum of light representative of natural daylight at the D65 standard illuminant. This simulates the conditions of a south-facing interior window.
The specimen is assessed at intervals by comparing the degree of colour change against a set of eight reference fabrics known as blue wool references, numbered 1 to 8. These references are produced and calibrated by specialist manufacturers such as James Heal, who supply accredited test houses worldwide. Each blue wool reference is dyed with a different dye to produce a known and calibrated resistance to fading. Blue wool 1 is the most fugitive and blue wool 8 is the most resistant. Each successive reference is approximately twice as resistant to fading as the previous one, giving the scale a geometric rather than linear progression. The difference between grade 5 and grade 6 represents twice the resistance of grade 4 to grade 5, not an equal step.
The result awarded to the fabric is the number of the blue wool reference that most closely matches the degree of fading shown by the test specimen. A fabric rated at grade 5 has faded to a degree equivalent to blue wool reference 5 under the same exposure conditions.
The Blue Wool Scale: What Each Grade Means
Grade 1 indicates very poor light fastness. The fabric will fade rapidly under even moderate light exposure. No upholstery or curtain fabric should be specified at this grade.
Grade 2 indicates poor light fastness. Significant fading is expected within a short period. Not suitable for any interior application where appearance durability matters.
Grade 3 indicates moderate light fastness. Acceptable only for very low-light environments with no direct sunlight exposure. Not recommended for curtains or upholstery in standard residential or contract use.
Grade 4 indicates good light fastness and is the recognised minimum for interior furnishing fabrics. Suitable for residential upholstery and curtains in rooms with indirect or limited natural light. Not recommended for south-facing rooms with large glazed areas or for high-light contract environments.
Grade 5 indicates very good light fastness and is the recommended minimum for most residential upholstery and curtain specifications. Suitable for rooms with moderate natural light including east and west-facing rooms.
Grade 6 indicates excellent light fastness and is recommended for south-facing rooms, high-light residential environments, and standard contract interiors including hotels and restaurants.
Grade 7 indicates very high light fastness. Recommended for environments with prolonged or intense light exposure including glazed atriums, conservatories, and south-facing hospitality spaces.
Grade 8 indicates the maximum achievable light fastness and is reserved for the most demanding light environments including marine, semi-outdoor, and direct sunlight applications.
Specification Thresholds by Application
For residential upholstery in rooms with limited or indirect natural light, grade 4 is the minimum acceptable threshold. For residential upholstery in rooms with moderate natural light, specify grade 5 or above. For south-facing rooms or rooms with large glazed areas, specify grade 6 or above. For contract upholstery in hotels, restaurants, and offices with standard glazing, specify grade 5 as a minimum with grade 6 preferred. For glazed atriums, hotel lobbies with skylights, or any environment with prolonged daylight exposure, specify grade 6 to 7.
For curtains, the same grading applies but the exposure is usually more direct and more sustained than for upholstery. A curtain fabric in a south-facing room should be specified at grade 6 or above regardless of whether the curtains are habitually closed or drawn.
For marine, yacht, or semi-outdoor applications, grade 7 to 8 is the appropriate range and specialist outdoor-rated fabrics should be specified rather than standard interior upholstery fabric.
The Effect of Room Orientation
Room orientation is one of the most underspecified variables in fabric selection. A north-facing room in the UK receives no direct sunlight at any time of year, and a grade 4 or 5 fabric is typically adequate. An east-facing room receives direct morning sun for a limited period. A west-facing room receives afternoon sun, which can be intense in summer. A south-facing room receives direct sunlight throughout the day from spring through autumn, with peak UV intensity between midday and 3pm.
The difference in light exposure between a north-facing and south-facing room in London over a twelve-month period is very significant. A grade 4 fabric that performs adequately in a north-facing study may show visible fading within eighteen months in a south-facing drawing room.
Always ask the client which direction the principal windows face and factor that into the light fastness requirement before specifying.
Fibre Type and Dye Method
Not all fibres accept dyes equally, and not all dyes are equally resistant to light degradation. The light fastness of a fabric is a product of both.
Silk is the most photosensitive natural fibre. Silk dyes are chemically susceptible to UV degradation, and silk fabrics typically achieve lower light fastness ratings than wool, cotton, or linen. Silk and silk velvet should be specified with caution in high-light environments, and the client should be advised of this limitation explicitly before specification is finalised.
Wool and mohair accept reactive and acid dyes that can achieve good light fastness ratings when correctly selected. Well-dyed wool upholstery fabrics typically achieve grade 5 to 6. Mohair, being a wool-derived fibre, has similar dye chemistry. Kothea’s Mohair Velvet Seven is tested independently to ISO 105-B02 and achieves grade 4 to 5 for light colourways and grade 5 to 6 for dark colourways. Darker colourways generally achieve higher light fastness grades because a greater proportion of colour loss is required before a visual change becomes perceptible.
Cotton and linen typically achieve moderate light fastness with standard reactive dyes. Pre-washed and solution-dyed cotton and linen can achieve higher grades depending on the dyestuff selection.
Polyester is inherently more resistant to UV degradation than natural fibres and typically achieves grades 6 to 7. Solution-dyed polyester, where colour is introduced into the fibre during extrusion rather than applied to the surface after weaving, achieves the highest light fastness ratings of any standard interior fabric and is appropriate for the most demanding high-light or semi-outdoor applications.
PVC and PU faux leathers are treated with UV-stabilising additives during manufacture and typically achieve high light fastness ratings. However, UV degradation of the substrate itself can cause surface cracking and loss of surface texture independent of colour change, which is a separate consideration for high-light environments.
What Light Fastness Does Not Measure
The ISO 105-B02 test measures colour change under controlled artificial light. It does not measure the effect of UV-filtering glass, which can significantly reduce UV exposure in modern double or triple-glazed windows. It does not measure the effect of cleaning on dye stability, which is covered by separate fastness tests. It does not predict how a specific fabric will behave in a specific room, because actual exposure varies by latitude, season, window orientation, glazing specification, and curtain or blind usage.
Low-e glazing and UV-blocking film can substantially reduce the UV component of light entering a room, extending the effective service life of a fabric beyond what the grade alone would suggest. If a client is refurbishing a property with high-specification glazing, this should be factored into the specification conversation.
Light Fastness and Crocking
Light fastness should not be confused with crocking, which is the transfer of dye from a fabric surface to another material through rubbing or friction. Crocking is measured by a separate test and graded on a different scale of 1 to 5. A fabric with good light fastness may still crock, particularly when wet.
For dark-coloured velvets in upholstery applications, crocking is a relevant concern particularly where light-coloured clothing is likely. Always check the crocking rating as well as the light fastness grade when specifying dark velvets for seating.
Kothea and Light Fastness
Mohair Velvet Seven from Kothea is tested independently to ISO 105-B02 and achieves grade 4 to 5 for light colourways and grade 5 to 6 for dark colourways, making it appropriate for moderate to high-light residential environments and standard contract interiors with adequate glazing.
For high-light environments, colourway selection is material. A dark colourway at grade 5 to 6 will outperform a pale colourway at grade 4 to 5 in a south-facing room. If the client’s brief requires a pale colourway in a south-facing room, this should be discussed explicitly and the light fastness limitation acknowledged before specification is finalised.
For full specification data on light fastness across the Kothea range, see the mohair velvet upholstery page or contact Kothea directly.
How to Specify Light Fastness
Ask the supplier for the ISO 105-B02 grade and confirm whether the test was carried out by an independent third party laboratory or self-declared by the supplier. Self-declared ratings without an independent test certificate should not be relied upon for contract projects.
State the required minimum grade in your specification as a labelled field. For example: Light fastness minimum grade 5 to ISO 105-B02. This makes the requirement explicit and verifiable.
Where the project involves south-facing rooms, large glazed areas, or a light-sensitive colourway, note this in your specification and confirm with the supplier that the grade applies to the specific colourway being ordered. Light fastness can vary between colourways within the same range, particularly between light and dark shades.
Frequently Asked Questions
What light fastness grade do I need for a south-facing room?
For a south-facing room in the UK, specify a minimum of grade 6 to ISO 105-B02 for both upholstery and curtain fabrics. South-facing rooms receive direct sunlight throughout the day from spring through autumn, which represents the most demanding light exposure condition in standard residential interiors. Grade 4, the minimum for interior furnishing fabrics generally, is insufficient for sustained south-facing exposure and will show visible fading within one to two years in most cases. If the glazing incorporates UV-blocking film or low-e coating, this will extend fabric performance, but grade 6 remains the appropriate specification baseline regardless of glazing.
What does a Blue Wool Scale grade of 5 mean for upholstery fabric?
A Blue Wool Scale grade of 5, tested to ISO 105-B02, means the fabric’s colour has faded to a degree equivalent to blue wool reference 5 under controlled xenon arc light exposure. Grade 5 is the recommended minimum for most residential upholstery specifications and is appropriate for rooms with moderate natural light including east and west-facing rooms. It is not recommended for south-facing rooms with large windows, where grade 6 is the appropriate minimum. Each grade on the scale represents approximately twice the light resistance of the grade below it, so the difference between grade 5 and grade 6 is significant rather than marginal.
Does silk fabric fade faster than other upholstery fabrics?
Yes. Silk is the most photosensitive of the natural upholstery fibres and typically achieves lower ISO 105-B02 grades than wool, mohair, cotton, or linen under equivalent conditions. The dyes used on silk are chemically more susceptible to UV degradation. Silk velvet and silk upholstery fabrics should not be specified for rooms receiving significant natural light without an explicit conversation with the client about this limitation. For high-light environments, mohair velvet or solution-dyed synthetic fabrics are more appropriate choices.
What is the light fastness rating of Kothea mohair velvet?
Kothea’s Mohair Velvet Seven is tested independently to ISO 105-B02 and achieves grade 4 to 5 for light colourways and grade 5 to 6 for dark colourways. For south-facing rooms, dark colourways at grade 5 to 6 are the appropriate selection from this range. For rooms with indirect or moderate natural light, light colourways at grade 4 to 5 are suitable. Contact Kothea to confirm the grade applicable to a specific colourway before finalising your specification.
Can UV-blocking glazing improve the effective light fastness performance of a fabric?
Yes. Modern low-e glazing and dedicated UV-blocking film reduce the UV component of light entering a room, which is the primary driver of dye degradation in interior fabrics. A fabric at grade 5 installed behind UV-blocking glazing will typically outlast the same fabric at grade 5 behind standard single glazing by a considerable margin. However, UV-blocking glazing does not eliminate UV exposure entirely, and the ISO 105-B02 grade should still be specified at the appropriate level for the room orientation. Treat the glazing specification as a factor that extends fabric performance, not as a substitute for adequate light fastness in the fabric itself.
What is the difference between light fastness and colour fastness?
Light fastness is a specific type of colour fastness that measures resistance to fading caused by light exposure, tested to ISO 105-B02 and graded on the Blue Wool Scale from 1 to 8. Colour fastness is a broader term covering resistance to colour change or transfer from a range of stressors including washing, rubbing (crocking), perspiration, and dry cleaning, each tested to a separate standard within the ISO 105 series and graded on a scale of 1 to 5. A fabric can have excellent light fastness and poor crocking resistance, or vice versa. For contract upholstery, both light fastness to ISO 105-B02 and crocking resistance should be checked and specified independently.
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