Glossmeter 75-degree angle measurement for paper gloss

This article introduces the principle and method of measuring paper gloss using a 75-degree mirror gloss meter. Gloss is the ratio of the light reflected from the paper surface to that reflected from a standard plate, with the 75-degree angle commonly used for measuring medium to high gloss papers. The article explains the instrument's structure and workflow, including calibration, sample placement, and reading steps. It also mentions that measurements are influenced by factors such as paper smoothness, color, and ambient temperature and humidity, emphasizing the importance of instrument maintenance and calibration. This method is widely applied in quality control and product development within the papermaking industry.

Measurement principle

Specular gloss is one of the key parameters for evaluating the optical properties of paper surfaces, defined as the ratio of the luminous flux reflected by the sample surface in the specular direction to the light flux reflected by the standard plate under the same conditions, usually expressed as a percentage. Its calculation formula is:

G_u = (φ_s / φ_m) × 100%

Among them, G_uIndicates the gloss value of the sample, φ_sRepresents the reflected light flux of the sample at a specified angle, φ_mRepresents the reflected light flux of the standard plate under the same conditions. For paper, 75-degree angle measurement is a common method, especially for medium to high gloss paper surface evaluation.

Standard basis

In paper gloss measurements, the angle of incidence is chosen depending on the gloss of the sample surface. The 75-degree angle is mainly suitable for papers in the medium gloss range, such as coated fine art paper, some packaging papers, etc. This angle is clearly defined in several international and national standards and can effectively distinguish between subtle gloss differences between samples and has good measurement repeatability.

Standards often specify light source characteristics, receiver spectral response, standard plate calibration, and instrument geometry to ensure data comparability across laboratories. Following standard procedures is the basis for reliable measurement results.

Workflow

A specular gloss meter for 75-degree angle measurement typically consists of a light source system, a collimation system, a sample stage, a receiver, and a data processing unit. The workflow follows: First, the instrument is calibrated to zero using a calibrated high-gloss standard plate; Subsequently, the paper sample is placed flat in the measuring hole; The light source is incident at an angle of 75 degrees, and the receiver receives the reflected light at the corresponding mirror reflection angle. Finally, the gloss value is calculated and displayed internally by the instrument.

Ensure that the sample surface is clean, flat, free of creases or contamination, and that the measurement location is representative. Environmental conditions such as temperature and humidity also need to be recorded, as they can affect the surface condition of the paper.

Influencing factors

Paper gloss measurement results are affected by a variety of factors. Understanding these factors can help with proper operation and interpretation of results.

Flatness and uniformity of the paper surface	Uneven surface texture or coating can lead to variations in the distribution of reflected light.
Paper color and absorption properties	Dark paper may absorb more incident light, affecting the reflected light flux.
Instrument calibration status	Regular calibration with certified standard plates is essential.
Ambient temperature and humidity	May cause changes in paper size or the physical state of the coating.
Measure pressure and contact	The contact pressure between the probe and the sample should be consistent and moderate.

Applications:

75-degree angular gloss measurement is widely used in quality control, research and development, and product classification in the paper industry. For example, in the evaluation of printing paper, gloss is correlated with the color vividness and contrast of printed products. In packaging paper, gloss affects the appearance texture and brand image.

When interpreting the data, it should be combined with the specific type and process of the paper. A single measurement is of limited significance and usually requires averaging over multiple measurements and observing how it tends to deviate from standard values or batch historical data. The measurement data can be used for process adjustments, such as adjusting the amount of coating or calendering process, to achieve the desired surface gloss effect.

Maintenance and calibration

To ensure long-term reliability of measurements, instrument maintenance and calibration are essential. Standard plates should be properly stored to avoid scratches or contamination, and the measurement value should be traced according to the standard cycle. The optical window of the instrument should be kept clean, and the power should be turned off and stored in a suitable environment after use. Operators should be trained to understand the principles and operational details of the standard method to reduce human error.

References

ISO 8254-1: Determination of specular gloss on paper and paperboard - Part 1: Determination of 75-degree angle by concentrated beam, TAPPI T480 om-15: Specular gloss of paper at 75 degrees, GB/T 8941: Determination of specular gloss on paper and paperboard.