Gloss measurement of printed matter quantifies the specular reflectance that determines the visual brilliance and perceived quality of printed products such as packaging, labels, magazines, and commercial prints. This article provides a comprehensive guide to printed matter gloss determination, covering measurement principles, internationally recognized standards such as ISO 8254, ISO 2813, ASTM D523, TAPPI T480, and GB/T 8941, instrument geometries, sample preparation, test procedures, and practical applications in quality control and product development.
Technical Overview
Gloss is the degree to which a surface reflects light in the mirror (specular) direction, and it is one of the most important optical quality attributes of printed matter. High-gloss printed surfaces produce sharp reflected highlights and vivid color saturation, while matte surfaces minimize glare and convey a different visual impression. Because gloss directly affects shelf appeal and brand perception, objective gloss determination is a routine requirement in the printing, packaging, and paper converting industries.
Specular gloss is measured with a photoelectric glossmeter. A calibrated light source projects a well-defined beam onto the specimen surface at a specified incidence angle, and a photodetector measures the intensity of light reflected in the specular direction. The result is expressed in gloss units (GU), defined so that a highly polished flat black glass standard with a refractive index of 1.567 reads 100 GU at every measurement geometry.
The gloss of a printed product depends on multiple interacting factors, including:
- Substrate smoothness, formation, and calendering level of the paper or paperboard
- Ink film thickness, pigment concentration, and degree of ink penetration into the substrate
- Type and application weight of coating, varnish, or lamination film
- Drying and curing conditions, such as UV curing completeness or oxidation drying time
- Printing process characteristics, including offset dampening balance, gravure and flexo impression settings, and digital fusing parameters
- Surface contamination, scratches, fingerprints, and blocking or rub damage after printing
Gloss measurement is standardized at the international and national levels. The principal standards include ISO 8254 (paper and board, 75° and 20° methods), ISO 2813 and ASTM D523 (20°, 60°, and 85° geometries), TAPPI T480 (75° specular gloss of paper), TAPPI T653 (20° specular gloss), JIS Z 8741 (specular glossiness), and GB/T 8941 (Chinese national standard for paper and board specular gloss at 20°, 45°, and 75°).
Testing Samples
Printed Paper and Paperboard
Typical specimens include coated and uncoated printed sheets, magazine and book covers, commercial brochures, cast-coated labels, and folding cartons. Specimens should represent the production run and include both solid printed areas and, where relevant, unprinted substrate areas for baseline comparison.
Packaging and Labels
Printed flexible packaging, self-adhesive labels, varnished cartons, film-laminated surfaces, and metallized or holographic foils are frequently tested. For these products, gloss consistency between print batches is often as important as the absolute gloss value.
Plastic Films and Printed Metal
Printed or plain plastic films such as BOPP and PET, decorated metal sheets, beverage cans, tubes, and caps can also be evaluated using plastic-film gloss procedures such as ASTM D2457, provided the specimen can be held flat and rigid during measurement.
Sample Preparation Requirements
Specimens must be flat, clean, and free of wrinkles, fingerprints, and abrasive damage. The test area must be significantly larger than the instrument aperture. Samples are normally conditioned in a standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 187 or GB/T 10739 before measurement, because moisture content affects paper surface properties and measured gloss.
Testing Items
Common gloss determination items for printed matter include:
- Specular gloss at 20° geometry, for high-gloss surfaces such as cast-coated paper, UV varnish, and laminated prints
- Specular gloss at 60° geometry, the universal geometry for medium-gloss printed and coated surfaces
- Specular gloss at 75° geometry, the primary method for low-gloss paper and printed paperboard surfaces
- Specular gloss at 85° geometry, for matte and near-matte printed surfaces
- Specular gloss at 45° geometry, as specified in GB/T 8941 for certain paper and board products
- Gloss uniformity and gloss mottle, evaluated by taking multiple readings across a printed sheet and reporting the standard deviation or range
- Gloss retention after rub, scratch, or abrasion testing, to assess surface durability
- Gloss difference (ΔGU) between specimens, batches, or between printed and unprinted areas
Testing Methods
60° Universal Method
The 60° geometry described in ISO 2813, ASTM D523, and JIS Z 8741 is the general-purpose method for printed and coated surfaces. As a screening rule, if the 60° value exceeds approximately 70 GU, the 20° geometry should be used for better resolution of high gloss; if the 60° value is below approximately 10 GU, the 85° geometry provides better differentiation of low gloss.
20° High-Gloss Method
The 20° geometry, specified in ISO 8254-2, TAPPI T653, and ISO 2813, is applied to high-gloss printed materials such as cast-coated stock, UV-cured varnishes, and glossy laminated packaging. The shallow angle increases sensitivity to surface smoothness and distinctness of the reflected image.
75° Paper Method
The 75° geometry is the standard method for most printed paper and paperboard. It is specified in ISO 8254-1, TAPPI T480, and GB/T 8941. The steep angle is well suited to the diffuse, lower-gloss reflection typical of coated and uncoated papers and yields good repeatability for printed magazine and catalog papers.
85° Matte Method
The 85° geometry in ISO 2813 and ASTM D523 is used for matte and textured printed surfaces, including matte-laminated book covers and uncoated board, where the 60° response is very low and insensitive.
Instrument Calibration and Measurement Procedure
A typical test procedure in an accredited third-party laboratory consists of the following steps:
- Condition the specimens in the standard atmosphere and allow them to reach equilibrium
- Verify and calibrate the glossmeter against its primary polished black glass standard (100 GU reference) and a low-gloss or mid-range working standard traceable to national metrology institutes
- Place the specimen on a rigid, flat backing to eliminate curvature effects, ensuring the printed test area is free of defects
- Take multiple readings, typically five to ten per specimen, distributed across the solid print area; where directional effects are expected, measure in both machine direction and cross direction orientations
- Record individual values, calculate the arithmetic mean, and report the standard deviation, maximum, and minimum values
- Report the measurement geometry, the standard followed, the instrument and calibration data, conditioning details, and the tested surface description
Applications
Gloss determination of printed matter supports quality control and product development throughout the graphic arts supply chain. Major applications include:
- Incoming inspection of paper, board, films, and coatings to verify that substrate gloss meets purchasing specifications before printing
- Printing process control, including monitoring ink-water balance in offset printing, coating application weight, varnish leveling, and UV or EB curing completeness
- Packaging quality assurance, ensuring uniform gloss across print runs, between print suppliers, and between production sites for consistent shelf appearance
- Research and development of inks, overprint varnishes, primers, laminating adhesives, and surface treatment processes
- Matte versus gloss finish qualification for cosmetic, pharmaceutical, and premium consumer packaging programs
- Verification of product conformity for contracts, tenders, and international trade, where accredited third-party laboratories issue test reports based on ISO, ASTM, TAPPI, JIS, or GB methods
- Failure analysis, such as investigating gloss loss caused by poor drying, back-trap mottle, rub damage, or incompatible coating and ink combinations
FAQ
What is a gloss unit (GU)?
A gloss unit is the conventional scale unit of specular gloss. The scale is anchored so that polished black glass with a refractive index of 1.567 equals 100 GU at all geometries. GU values are comparable only when the measurement geometry is identical; a 60° reading cannot be converted into a 75° reading.
Which measurement angle should be selected for printed matter?
For most printed papers and boards, 75° is the reference geometry. High-gloss printed surfaces such as cast-coated paper, UV varnish, and lamination are best measured at 20°. General printed and coated surfaces are commonly screened at 60°, and matte surfaces at 85°. The applicable product standard or customer specification normally defines the required geometry.
What gloss values are typical for printed products?
Indicative ranges are: uncoated printed paper, low gloss, typically below 15 GU at 75°; matte-coated printed board, roughly 10 to 30 GU at 60°; gloss-coated printed paper, commonly 40 to 75 GU at 60°; and UV-varnished or film-laminated surfaces, frequently above 85 GU at 60°. Exact values depend on the substrate, ink coverage, and coating system.
Why do gloss results sometimes differ between laboratories?
Differences can arise from instrument aperture and geometry tolerances, calibration standard traceability, specimen flatness, the choice of printed versus unprinted test areas, and conditioning history. Following the procedures of ISO 8254, ISO 2813, or ASTM D523, including proper calibration and adequate replicate readings, minimizes variability. Small differences of a few GU within laboratory reproducibility limits are generally not considered significant.
How do substrate and ink film affect printed gloss?
Smoother, well-calendered, and coated substrates produce higher printed gloss because the dried ink film mirrors the surface topography. Thicker ink films and full curing of varnishes increase gloss, while excess dampening solution, ink penetration into porous stock, and insufficient coating weight reduce gloss. Testing both printed and unprinted areas helps separate substrate gloss from the contribution of the printing process.
How many readings are needed for a reliable result?
Most standards require averaging multiple readings per specimen, typically at least five, with ten preferred for paper and board at 75°. Reporting the mean together with the standard deviation and range provides a measure of gloss uniformity, which is essential for detecting gloss mottle and batch-to-batch variation in printed matter.