Light fastness, or colour fastness to light, is the resistance of printed or pigmented materials to fading or colour change due to exposure to sunlight or an artificial light source.
The light fastness of print is predominantly governed by the pigment type used. Print exposed to high levels of sunlight such as on posters and compost sacks require a high degree of lightfastness.
Smithers Pira tests for print fade and lightfastness of paper, print and pigmented plastics using a James Heal Apollo weather fastness tester fitted with a xenon arc lamp source and irradiance measurement.
Lightfastness testing is made using filtered xenon arc lamps which simulate UV and visible solar radiation more closely than any other artificial light source. They provide the best method for assessing the accelerated light ageing of products such as pigments, dyes and inks, which are sensitive to the long-wave and visible wavelengths of sunlight.
With appropriate filters fitted, direct sunlight, sunlight through window glass and in-store lighting conditions can be reproduced. A series of Blue Wool standards are included with every set of test samples to ensure that results are reliable and reproducible.
Accelerated weathering includes a simulated rain cycle which can introduce an element of thermal shock to the test sample
Standards and Accreditation
There are many international standards applicable to lightfastness testing, including ISO 105-B02, BS EN 20105-B02, BS EN 4892-2 and ISO 12040.
The Blue Wool standards included with each set of test samples conform to ISO 105-B01, BS EN 20105-B02 and BS 1006.