Testing materials for light fastness

Poor resistance to light fading can be very noticeable when it occurs, and it can lead to product returns.

Test method SATRA TM160:1992 – 'Colour fastness to light from a xenon arc' is our routine test for light fastness. This method is intended to determine the resistance of a coloured item to the action of an artificial light source that closely approximates to natural daylight through glass but is of a higher intensity. It is applicable to all types of material.

Light fastness

Light visible to the human eye forms only a very small part of a much larger spectrum of radiation, which includes X-rays, microwaves and radio waves. The shorter the wavelength, the more penetrating the radiation and, generally, the more damaging. The part of the spectrum visible to the human eye is bounded by ultraviolet light at shorter wavelengths and infrared at longer wavelengths.

While ultraviolet (UV) radiation often causes discolouration and degradation of materials, it is by no means the only cause. Visible light also plays a part, and can be more damaging than the shorter UV wavelengths.

The purpose of light fastness testing is principally to predict a material’s resistance to fading, yellowing or darkening. However, the test can also be employed to assess physical degradation such as cracking and shrinkage. Here it is common to carry out physical tests before and after exposure to determine the degree of degradation. This article covers only the subject of discolouration due to light.

Test principles

In our light fastness test, samples are exposed to an intense artificial light generated by a Xenon arc lamp. The light passes through a series of filters to ensure that its spectrum (wavelength make-up) closely matches that of natural daylight through glass. The humidity and temperature of the test atmosphere are controlled.

A small sample of material, together with a series of eight Blue Wool Standard fabrics (reference 1 to 8, see paragraph on these below), are simultaneously exposed to the high intensity light. Two exposure times are used to determine whether fading progresses steadily, or initially at a different rate from the longer-term exposure.

The test involves assessing samples once during the test and again at the end. The assessment is completed by comparing the degree of fading shown on the sample with the fading shown by the Blue Wool Standard fabrics.

Each sample is assigned a light fastness rating based on the number of the Blue Wool Standard which shows the equivalent degree of colour change.

Blue Wool Standards

The eight Blue Wool Standard fabrics (reference 1 to 8) have known, but varied, resistance to light fading. Each standard in the series is approximately twice as resistant to light fading as the preceding fabric. The Blue Wool Standards range from number 1, which fades rapidly, to number 8, which has an excellent resistance to fading.

We normally make an assessment halfway through the test when Blue Wool Standard 4 reaches Grey Scale 4 (see box 1). The test is continued and finally terminated when Blue Wool Standard 4 reaches Grey Scale 3. This takes approximately 8 to 10 days, depending on the age of the lamps (which have to be regularly changed as the light intensity degrades with usage).

Materials requiring testing

Suedes and nubucks for all applications should be tested, as strong sunlight can cause deep colours to fade and lighter colours to yellow.

Many resin-finished black, navy and brown leathers have a good light fastness. However, SATRA has recently examined a black finished leather which faded to grey, so it is impossible to predict that no problems will occur with these types or colours of leather. Those used particularly for upholstery should definitely be tested. White and pastel-coloured finished leathers should always be assessed since these do tend to discolour.

Textiles are frequently tested, as again deeper colours can fade and the lighter colours and white can yellow or discolour.

Coated fabrics rarely need to be tested since they normally have a good resistance to light, although we have seen materials of this type giving rise to discolouration problems. So again, it is unwise to assume that there will be no problems.

Materials which have a slightly low resistance to light may be generally adequate if they fade uniformly. However, discolouration often appears in patches, or materials fade to a slightly different colour, rather than to a slightly lighter shade of the same colour.

All of the above types of materials are used for footwear and clothing, leathergoods and accessories, furniture and carpets, automotive interiors and household textiles. Wood veneers for furniture, picture frames and automotive applications can also be tested.

Problems encountered

SATRA tests materials for many different applications for light fastness. Most of the materials are new and unused examples but often we see products that have already changed colour. These problems are often caused by poor light fastness.

Products such as furniture and carpets in any indoor situation which are placed near a window may be subjected to intense light. Since this light rarely covers the whole of the article, the fading or discolouration will be patchy and very noticeable.

Household textiles can give rise to problems. Dark coloured curtains have to have a very good light fastness to prevent fading. Even the thread used on these products needs to be tested, since a thread fading to a pale colour on dark coloured curtains will be very noticeable. We have seen navy curtains where the navy thread has changed to a pale pink colour over time. There have also been problems with a deep gold coloured tablecloth (see photograph at top of page) that was used to cover a table close to a window. The part of the cloth subjected to the most light changed to a lighter pink colour.

Problems of fading and discolouration have been seen on handbags, briefcases and wallets made with deep coloured leathers. The correct colour of these articles can generally be found underneath a flap or clasp (see figure 1).

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