Light fastness testing evaluates the ability of printed inks and colors to resist fading or discoloration when exposed to light, making it an essential quality control step for packaging, labels, publications, and outdoor graphics. This article explains the technical principles of printed matter light fastness testing, the applicable international standards such as ISO 12040, ASTM D3444, and GB/T 22771, along with sample requirements, test procedures, grading scales, and typical industrial applications.
Technical Overview
Light fastness, also known as color fastness to light, refers to the resistance of printed colors to fading, yellowing, or other visible changes caused by exposure to light, particularly the ultraviolet (UV) and visible portions of the solar spectrum. Printed matter is constantly exposed to light during storage, retail display, and end use, and the photochemical degradation of organic pigments and dyes can significantly reduce the visual quality and commercial value of a printed product.
Light fastness testing simulates and accelerates this degradation process under controlled laboratory conditions. Test specimens are exposed to a filtered xenon arc lamp or another standardized light source alongside blue wool reference standards, and the degree of color change is evaluated visually against grey scales or instrumentally using colorimetry. The principal international standards for this testing include ISO 12040 (graphic technology, assessment of light fastness of prints using filtered xenon arc lamp), ASTM D3424 (standard test methods for evaluating the lightfastness of printed images), GB/T 22771 (the Chinese national standard technically aligned with ISO 12040), and, for printed textiles, ISO 105-B02. Test results are normally expressed on the blue wool scale from grade 1 (very poor) to grade 8 (outstanding), supplemented by grey scale ratings from 1 to 5.
Accredited third-party laboratories routinely perform light fastness tests to support incoming material inspection, ink and substrate qualification, product specification compliance, and failure analysis for printed products destined for extended display or outdoor service.
Testing Samples
Paper and Paperboard Prints
Typical samples include books, magazines, brochures, maps, greeting cards, and folding cartons produced by offset lithography, gravure, flexography, screen printing, or digital printing processes. These are the most common sample categories submitted for light fastness evaluation.
Labels and Flexible Packaging
Self-adhesive labels, shrink sleeves, tags, and printed plastic films used for fast-moving consumer goods are frequently tested, since prolonged shelf display under retail lighting can cause noticeable fading of brand colors.
Plastic, Metal, and Synthetic Substrates
Printed PVC banners, synthetic paper, pressure-sensitive films, decorated metal containers, and plastic cards can be evaluated using the same exposure principles, sometimes in combination with weathering standards such as ISO 4892 or ASTM G155 where UV, heat, and moisture act together.
Textile Prints and Soft Signage
Printed fabrics, flags, banners, and home textile products are assessed following ISO 105-B02, which uses a xenon arc exposure comparable to ISO 12040 but with test conditions tailored to textile materials.
Sample Preparation Requirements
Specimens are cut from representative printed areas at dimensions suitable for the exposure device, typically not less than 30 mm x 100 mm. Each specimen is mounted on a sample holder with a portion covered by an opaque masking card so that an unexposed reference area remains available for comparison. Prior to testing, samples are normally conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity to ensure reproducible results.
Testing Items
The core testing items for printed matter light fastness evaluation include:
- Degree of color change (fading) after exposure, assessed against the grey scale for color change in accordance with ISO 105-A02
- Blue wool light fastness grade, determined by comparison with reference standards 1 to 8 as specified in ISO 105-B08
- Instrumental color difference (ΔE*) between exposed and unexposed areas, measured with a spectrophotometer and reported in CIE color coordinates
- Verification of compliance with contractual or product specification grades, such as a minimum blue wool grade 4, 6, or higher
- Comparative evaluation of different inks, pigments, coatings, varnishes, or substrates under identical exposure conditions
Testing Methods
Filtered Xenon Arc Lamp Method
The xenon arc method described in ISO 12040 and GB/T 22771 is the most widely used procedure for printed matter. A xenon arc lamp fitted with daylight filters simulates sunlight transmitted through window glass. Test specimens and blue wool references are exposed simultaneously in the chamber, with key parameters controlled and recorded:
- Irradiance level, commonly set around 1.10 W/m² at 420 nm or 0.51 W/m² at 340 nm depending on the filter system
- Black standard or black panel temperature, typically maintained in the range of 45 °C to 60 °C
- Chamber temperature and relative humidity, kept within the tolerances specified by the standard
- Exposure duration, determined by the endpoint criterion rather than a fixed time
Exposure proceeds in stages, with the specimen periodically compared against its masked, unexposed area. When the specimen reaches a color change corresponding to grey scale grade 4, or when the contractual endpoint is reached, the blue wool reference that displays an equivalent degree of fading defines the reported light fastness grade. Half-step grades, for example 5-6, are assigned when the specimen fades between two references.
Daylight Exposure Behind Glass
Natural daylight exposure behind window glass, covered by ASTM D3424 and ISO 105-B01 methodology, exposes specimens and blue wool references to real sunlight under glass in a southerly orientation. Although slower and season-dependent, this method provides a realistic reference correlation for indoor display conditions.
Fluorescent Lamp Exposure
ASTM D3424 also permits exposure to fluorescent lamps to simulate indoor illumination environments such as offices, supermarkets, and exhibition spaces. This method is useful for evaluating prints that will spend their service life under artificial retail lighting rather than daylight.
Evaluation and Grading
After exposure, grading is performed by trained assessors under standardized D65 illumination. The grey scale for color change (ISO 105-A02) assigns a rating from 5 (no change) to 1 (severe change), while the blue wool scale expresses durability from grade 1 to grade 8. Instrumental measurement of ΔE* between exposed and shielded areas provides objective supporting data. For high-durability applications, grades of 6 and above are generally expected, whereas short-life indoor prints may only require grades 3 to 4.
Applications
Light fastness testing is applied across the printing, packaging, and graphics industries for several key purposes:
- Packaging and labeling: verifying that brand colors on cartons, labels, and flexible packaging remain stable throughout the shelf-life and retail display period
- Outdoor advertising: qualifying inks and substrates for billboards, banners, vehicle graphics, and point-of-sale displays exposed to direct sunlight
- Publishing and archival applications: confirming the longevity of books, maps, posters, and photographic prints intended for long-term storage or repeated display
- Textile and interior decoration: evaluating printed fabrics, wallpaper, floor graphics, and furnishings subjected to window-filtered daylight
- Ink and material development: comparing pigment formulations, UV-absorbing varnishes, laminating films, and substrate coatings during research and quality improvement programs
- Supplier qualification and dispute resolution: providing objective, standards-based evidence of conformity when buyers and suppliers define minimum light fastness grades in specifications
FAQ
What does a blue wool grade of 6 mean?
A grade of 6 indicates very good light fastness, roughly corresponding to resistance over several months of direct daylight exposure before a just-noticeable fade occurs. Grade 8 represents outstanding resistance suitable for permanent outdoor or archival use, while grade 3 or below indicates poor fastness appropriate only for short-lived indoor products.
How is light fastness different from weather resistance?
Light fastness testing evaluates color stability under light exposure alone. Weather resistance testing, such as that performed under ISO 4892 or ASTM G155, adds cycles of water spray, humidity, and temperature fluctuation to simulate rain and condensation, making it more severe and relevant for outdoor products.
Why do different colors on the same print fade at different rates?
Fading depends on the photochemical stability of each pigment or dye. Organic pigments such as yellows and magentas often fade faster than inorganic or high-performance pigments such as carbon black and certain blues. The binder, substrate, and any protective coating also influence the result.
How long does a typical test take?
Because exposure is terminated at a defined endpoint rather than a fixed time, duration depends on the fastness of the sample. Low-fastness prints may reach the endpoint within a few dozen hours of xenon exposure, while grade 7 to 8 materials may require several hundred hours of continuous operation.
Instrumental measurement and visual assessment are both accepted. Visual grading against blue wool references is the basis of the standardized rating, while spectrophotometer data on ΔE* provides objective documentation that is valuable for quality records and customer communication.