Ink gloss testing quantifies the specular reflectance of printed and proofed ink films, providing an objective measure of surface brilliance that directly affects visual quality and brand perception. Using standardized geometries defined by ISO 2813, ASTM D523, and GB/T 9754, accredited third-party laboratories can evaluate gloss performance for inks, coatings, and printed substrates. This article explains the technical principles, sample types, testing procedures, and typical applications of ink gloss measurement.
Technical Overview
Gloss is the optical attribute of a surface that causes it to exhibit a shiny or lustrous metallic-like appearance when illuminated and observed under defined geometric conditions. In the printing and ink industry, gloss is one of the most important visual quality characteristics, because it influences color saturation, contrast, perceived depth of image, and overall consumer appeal of printed products such as packaging, magazines, labels, and decorative materials.
Ink gloss is generated by the interaction of light with the ink film surface. A smooth, well-leveled ink film with a high degree of surface uniformity reflects incident light predominantly in the specular direction (mirror-like reflection), producing high gloss. Conversely, a rough, poorly leveled, or absorbent surface scatters light diffusely, resulting in low gloss or a matte appearance. Key factors affecting ink gloss include:
- Pigment particle size and dispersion quality within the ink vehicle
- Vehicle resin type, film-forming ability, and solids content
- Ink film thickness and leveling behavior after printing
- Substrate smoothness, absorbency, and coating structure
- Drying and curing conditions (oxidative, penetration, thermal, or UV curing)
- Post-printing processes such as varnishing, lacquering, or lamination
Instrumental gloss measurement overcomes the subjectivity of visual assessment by quantifying specular reflection. A gloss meter projects a beam of light onto the surface at a defined incidence angle and measures the reflected light intensity relative to a polished black glass reference standard, which is assigned a value of 100 gloss units (GU) at each geometry. The most widely used geometries are 20°, 60°, and 85°:
- 60° geometry – the universal reference angle suitable for most semi-gloss surfaces, including the majority of printed ink films
- 20° geometry – recommended for high-gloss surfaces above approximately 70 GU at 60°, offering improved resolution and sensitivity
- 85° geometry – designed for low-gloss, matte surfaces below approximately 10 GU at 60°, where grazing illumination enhances measurement discrimination
Relevant national and international standards governing gloss measurement include ISO 2813 and ASTM D523 for paints, coatings, and ink films on non-metallic substrates, ASTM D2457 for plastic films and solid plastics, GB/T 9754 (the Chinese national standard aligned with ISO 2813), JIS Z 8741 for specular gloss in general, ISO 8254 and TAPPI T 480 for paper and board gloss (although TAPPI is an industry association standard rather than an official standards organization, it is widely recognized), and DIN 67530 as the corresponding German method.
Testing Samples
Ink Drawdowns and Proofed Films
Liquid inks of all major classes can be evaluated by preparing standardized drawdowns or proofs on controlled substrates. Typical sample categories include:
- Offset (lithographic) inks, both sheet-fed and web-fed, conventional and UV-curable
- Gravure and flexographic solvent-based and water-based inks
- Screen printing inks for graphic and industrial applications
- Digital and inkjet ink formulations
- Overprint varnishes, lacquers, and coating vehicles
Drawdowns are typically prepared with an applicator bar or proofing device at specified wet film thicknesses, then dried or cured under defined conditions before measurement.
Printed Production Samples
Finished printed materials can be submitted directly for gloss measurement, provided the test area is flat, uniform, and sufficiently large for the instrument aperture. Common sample forms include printed paper and paperboard, folding cartons, flexible packaging films, labels, metal decoration panels, and plastic printed parts.
Comparative and Reference Samples
To support gloss consistency evaluations, laboratories frequently test matched sets of samples, such as retained reference prints versus current production prints, competitor benchmark samples, or specimens before and after process changes. Sample size requirements generally call for pieces of at least 50 mm × 50 mm, flat and free of wrinkles, embossing, or surface contamination in the measurement area.
Testing Items
Core measurable parameters in ink gloss evaluation include:
- 60° specular gloss value (GU) – the primary reported result for most printed and proofed ink films
- 20° specular gloss value (GU) – high-gloss differentiation for glossy coatings, UV-cured films, and varnished surfaces
- 85° specular gloss value (GU) – matte and low-gloss surface characterization
- Gloss uniformity – multi-point or multi-location measurements across a printed sheet to assess mottling, streaking, or drying inconsistency
- Gloss difference (ΔGU) – comparison between test samples and agreed references, or between ink systems, substrates, or process conditions
- Gloss retention – evaluation of gloss stability after aging, abrasion, rub testing, or environmental exposure
Results are expressed in gloss units (GU), defined as 100 times the ratio of the light reflected by the specimen to the light reflected by the black glass reference standard under identical geometric conditions. Measurement uncertainty typically ranges from approximately 1 GU for instrument repeatability to several GU when sample non-uniformity is included.
Testing Methods
Gloss Meter Method (Primary Method)
The specular gloss of ink films is determined with a calibrated gloss meter in accordance with ISO 2813, ASTM D523, or GB/T 9754. The instrument directs a parallel light beam onto the specimen surface at the selected incidence angle (20°, 60°, or 85°) through an entrance aperture, and a detector positioned at the specularly corresponding angle measures the reflected luminous flux. The optical source is typically a CIE standard illuminant C source, and the detector response is photopic.
Sample Preparation and Conditioning
Before testing, samples should be conditioned at standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity, for a defined equilibrium period. Surfaces must be clean, dry, and free of fingerprints, dust, and anti-set-off powder residues. For liquid ink evaluation, drawdowns are prepared at controlled film thickness and fully dried or cured using the process conditions specified by the client or the relevant product standard.
Measurement Procedure
The general procedure is as follows:
- Verify and calibrate the gloss meter using the working standard tile and the built-in or certified black glass reference traceable to a primary standard
- Place the specimen flat on a rigid, planar support so that the measurement aperture is fully covered by the test surface
- Take readings at the selected geometry, avoiding ink-free margins, drying-edge zones, and visibly defective areas unless these are specifically under investigation
- Record a minimum of three to five readings at different positions and directions on the specimen, and report the arithmetic mean and range
- Rotate the specimen 90° where directional surface effects may exist and note any directional dependence
- Clean or re-zero the instrument as required by the standard, and re-verify calibration at the end of the measurement series
Geometry Selection Logic
A preliminary measurement at 60° is used to classify the surface. If the 60° value exceeds approximately 70 GU, the 20° geometry should additionally be used for better resolution; if the 60° value falls below approximately 10 GU, the 85° geometry is preferred. For full characterization of a product across matte-to-gloss ranges, all three geometries may be reported.
Quality Control Considerations
To ensure reliable interlaboratory and inter-batch comparability, laboratories control substrate consistency, film thickness, drying state, instrument calibration frequency, and ambient conditions. Because gloss is highly sensitive to surface topology, even minor differences in substrate smoothness or ink film thickness can shift results by several gloss units, so comparative evaluations must hold all variables constant except the one under study.
Applications
Ink gloss testing serves a broad range of industrial and commercial purposes:
- Ink formulation and R&D – comparing vehicle resins, pigment dispersions, wax additives, and matting agents; optimizing pigment loading and grinding fineness for target gloss levels
- Printing process control – monitoring the influence of ink film thickness, dampening balance, printing speed, drying temperature, and UV lamp output on gloss development
- Packaging quality assurance – verifying gloss conformity for folding cartons, flexible films, labels, and laminates against brand owner specifications and color management targets
- Publication and commercial printing – ensuring consistent gloss of coated paper printing for magazines, catalogs, and promotional materials
- Substrate and coating evaluation – assessing how paperboard coatings, primers, and overprint varnishes contribute to final printed gloss
- Product certification and compliance – providing objective test reports for supplier qualification, incoming inspection, dispute resolution, and export documentation through accredited third-party laboratories
- Durability studies – tracking gloss retention after abrasion, blocking, heat aging, humidity exposure, or lightfastness testing to predict long-term appearance performance
FAQ
Q: What gloss unit (GU) values are typical for printed ink films?
A: Values vary widely with ink system and substrate. High-gloss UV-cured inks or varnished surfaces on coated board often measure 80 GU or higher at 60°, conventional offset inks on coated paper commonly fall between 40 and 75 GU, and matte inks typically measure below 10 GU, for which the 85° geometry is recommended.
Q: Which measurement angle should be selected?
A: The 60° geometry serves as the universal starting point. If the 60° result exceeds about 70 GU, add or switch to 20° for improved resolution of high-gloss surfaces; if it is below about 10 GU, use the 85° geometry for matte surfaces.
Q: How is the instrument calibrated and traceability ensured?
A: Gloss meters are calibrated against highly polished flat black glass reference standards with a refractive index of 1.567, which by definition reads 100 GU at every geometry. Working tiles are periodically verified against certified reference standards traceable to national metrology institutes in accordance with ISO 2813 or ASTM D523.
Q: Why do two visually identical prints show different measured gloss?
A: Gloss depends strongly on micro-surface topology. Small differences in ink film thickness, substrate smoothness, drying completeness, or anti-set-off powder can alter specular reflection measurably even when the human eye perceives the samples as equivalent. Multi-point measurement and consistent sample preparation reduce such discrepancies.
Q: Can gloss be tested on curved or textured printed products?
A: Standard gloss geometries require flat test areas. Curved, embossed, or heavily textured surfaces produce unreliable readings because the specular condition cannot be maintained; in such cases, laboratory preparation of flat printed specimens of the same ink and substrate is recommended.
Q: How many readings should be taken per sample?
A: Standards typically require at least three readings at different positions on the specimen, with the mean value reported. For heterogeneous surfaces or gloss uniformity studies, additional measurements on a defined grid are advisable, and both mean and range should be documented.
Q: What sample conditions are required for submission?
A: Samples should be flat, clean, dry, fully cured, and larger than the instrument measurement aperture, generally at least 50 mm × 50 mm per test area. Specimens should be protected from scratching, contamination, and pressure marking during transport, and relevant process details such as ink type, substrate, and film thickness should be provided for meaningful interpretation.
