Knowledge Base
Optical Testing Applications for Paper and Paperboard Materials
The optical properties of paper and board determine the appearance and appeal of a product, and they can be controlled and altered with rigorous paper testing.
Optical properties relate to all aspects of the product’s appearance including whiteness, brightness, color, opacity and gloss. They’re important for the paper and board themselves and their conversion. For example, increasing gloss by varnishing the highlights of a selected area causes it to stand out from the background.
For 100 years, we’ve been making machines to test paper and paperboard so that these optical properties can be understood. Our instruments measure more than 50 discrete paper quality properties based on international testing standards. In this blog, we talk about some of the ways we help our customers with their paper testing.
Changes in process affect paper quality
In terms of the paper constituents, optical properties are affected by all stages of the pulp and paper production process. Changing parameters to enhance one property of paper quality may be detrimental to another, so there’s trade-offs to consider. For example, increasing bleaching to improve brightness will reduce opacity.
The pulping and bleaching processes are responsible for the whiteness of the fibers. The ultimate brightness of a paper or board is governed by the light scattering effects of its constituents, formation, loadings, coatings and any optical brightening agents applied.
Which optical properties affect paper quality?
There are a number of paper quality tests that can be carried out to determine the optical properties of paper samples, which include:
Paper quality and color
Color may appear as a shade of whiteness due to chemicals remaining in the fibers after processing, or as a color from dyes added to the pulp or applied to the surface.
With the TQC SHeen X-Rite RM200QC spectrocolorimeter in your bag, you’ll be ready to swiftly and accurately evaluate paper color – anytime, anywhere.
Paper quality and opacity
Opacity is initially governed by the translucency of the fiber furnish, where the needle shaped hardwood fibers are more opaque than the ribbon shaped softwood fibers. Formation has an influence, but loading and coating are the dominant factors in the final opacity of the sheet.
If its opacity testing you need, check out the Technidyne PROFILE/Plus® TAPPI Opacity. It’s our fast and accurate, fully automated paper opacity measurement device.
Paper quality and gloss
Gloss is a complex optical property, it’s not easy to define accurately or measure in terms of its appearance to the naked eye. Physically, it’s connected with the reflection of a collimated light beam that falls upon a paper surface.
In uncoated papers, some of the light is reflected from the fibers in the surface as if they were an assembly of miniature mirrors. This is called specular refraction. The rest of the light penetrates more deeply into the paper and is reflected and refracted in many directions. This is called diffuse reflection. The total light reflected is composed of specularly and diffused reflected components, which are not easy to differentiate.
Fiber formation influences gloss
The two most important contributors to the appearance of a paper are fiber formation and gloss, and the former can influence the latter.
When considering the appearance of paper, changes in the treatment applied to the base paper result in a wide variation from a matte finish (minimal light reflectance) to the high gloss finish of cast coated, lacquered, metallic and plastic laminated papers. The options in between are:
- Wash and lick coated papers
- Various medium and heavily coated papers
- Papers with loadings ranging from chalk to titanium dioxide
- Papers subject to calendering and super calendering.
The appearance resulting from these variables has to be measured to ensure conformity with the specification and consistency with the product. Equally important is the need for appearance in the printed paper. Print gloss depends on:
- Fiber formation
- The properties of ink supply
- The interaction between ink and paper, where color and intensity can vary according to the penetration and absorption of the ink vehicle.
Due to the changes in specular and diffuse reflection in different types of paper and the difficulty in correlating with the human eye, it’s necessary to apply different geometries to glossmeters when paper testing. This is to obtain the best correlation with specific groups of products.
For papers with very high gloss (e.g wax papers), an angle of 20° to the vertical, is the standard applied since there is a high proportion of specular reflection. The angle used for the greater majority of papers, however, is 60 or 75°, according to the chosen standard. This is to account for the greater amount of diffuse reflection in these types of paper.
The design of glossmeters varies according to the purpose and the paper test parameters.
Glossmeter paper testing options
A gloss testing device like a glossmeter will help you understand the optical properties of your paper and board samples. The most important features of a glossmeter include reliability, ease of use, reproducibility and traceability of calibration.
With these requirements in mind, we have developed two precision glossmeters in our product portfolio – the SOLOGloss® and the POLYGloss®.
The SoloGloss® is our single-angle gloss tester. It’s suitable for most paper testing applications and is preferred for measurements in the semi-gloss range. The light source and the detector are positioned under an angle of 60° of the surface to be measured.
The PolyGloss® is our most advanced gloss tester. It’s a triple angle paper testing device that inspects across the entire gloss spectrum from angles of 20°, 60° and 85°. For the most reliable gloss measurements on high gloss surfaces the 20° angle is ideal. When inspecting low gloss levels with high diffuse reflection, or matt surfaces, the 85° angle suits best.
Other gloss testing devices we make include the Technidyne TEST/Plus® Gloss, designed for paper, cardboard, corrugated, inks and coatings, paper, paperboard and tissue. Also from Technidyne, check out the PROFILE/Plus® Gloss 75 Degree.
Paper testing – the angle of incidence
The relative components will also change according to the angle of incidence at which the light strikes the paper during the paper test.
Increased coating weight and calendering causes the surface to become smoother and more reflective and the specular reflection increases accordingly. When light falls on a matte surface, the reflected light is diffused in all directions. At near glancing incidence, some of the light is specularly reflected, causing a lustre sometimes called ‘sheen.’
The angle of incidence used in the measurement of gloss therefore varies according to the paper quality types being measured and the chosen standard method applied.
Paper quality and brightness
To satisfy the industry’s need for a single value to denote brightness, a very specific spectral distribution of light has been chosen by standards organizations to illuminate the paper under test.
Most white, or near white, papers have a reflectance curve which is relatively flat – from 550 to 700 nanometers – but which slopes down in the blue region of the spectrum. So, the area of the spectrum to indicate the whiteness of a sheet has been chosen in the blue band, specifically 547 nanometers, with a spectral range of 400 to 500 nanometers. As the sheet becomes more yellow during the paper test, the blue reflectance increases.
To achieve the single value measurement of brightness, the yellow and red area of the visible spectrum has been ignored. So, colored or tinted papers cannot be fully described by this single number. For example, a paper with a reddish tint could have the same brightness as a neutral white paper.
Brightness testing controls
The complete and absolute characterization of a tinted sheet can therefore only be achieved using a three-color paper testing method of measurement. However, brightness measurements are more easily understood and provide a quick and easy method of checking the optical appearance under most manufacturing conditions. It should be noted that the actual spectral response of the instrument used to measure brightness is affected by:
- The lamp used as a light source
- The characteristics of the photo sensor
- The focusing lens
- The filters.
Changing any components, such as the lamp, during the paper quality test can change the spectral response.
Testing brightness and whiteness
In some applications, measured brightness does not correlate well with the visual assessment of whiteness. This is due to variation in areas outside the blue band of the spectrum. It may therefore be necessary to obtain whiteness and yellowness indices using a colorimeter paper test. The values obtained are not the tri-stimulus values needed for the complicated measurement of color that use formulas to assess the degree of whiteness and yellowness.
Brightness is often enhanced by the introduction of optical brightening agents. These cause the paper to fluoresce. This is a process where light energy of one spectral region is absorbed and re-emitted at other (usually longer) wavelengths. For some materials the level of reflected light is higher than the incident light at certain wavelengths. The measurement of the fluorescent factor is made by inserting a UV filter in the incident light path and subtracting the resultant brightness from the value obtained without the additional filter.
This most critical of measurements in the pulp and paper industry deserves the most advanced and standardized paper tester on the market. Check out Technidyne’s PROFILE/Plus® Brightness device for automated measurement of top and/or bottom brightness and fluorescence.
Three color testing paper quality systems
There are several systems available for measuring color. All use numbers measured by a tristimulus colorimeter based on the internationally accepted CIE standard observer. The three most commonly used color testing paper quality systems are:
- CIE, L*a*b* (CIELAB or STARLAB)
- Hunter L, a, b system
- CIE, dominant wavelength, purity and luminosity
The L value relates to lightness / darkness of the color
The a factor relates to chromaticity on the red/green axis
The b-factor refers to chromaticity on the blue/yellow axis
These systems are mathematical in nature and don’t specify the use of a particular instrument. However, correlation between them needs to consider the geometry, photometry and spectral response. Using these paper testing systems, the magnitude and direction of color differences between a sample and standard can be determined and understood.
TAPPI and ISO color paper testing
There are fundamental differences in the optical geometry of instruments used for the determination of optical properties according to TAPPI or ISO. But in either method, the sensor views the sample perpendicular to the surface.
The TAPPI paper test method illuminates the sample with a light beam angle to 45° to the surface. The disadvantage of angled beam illumination is that the amount of light reflected varies according to the orientation of the fibers in the sheet. This requires sample measurements to be taken in both machine and cross-direction.
The ISO paper test method uses diffuse illumination. The lack of any directional effect in ISO instruments means that they are more readily adapted to automated strip feed systems than equivalent TAPPI instruments.
How to test opacity to determine paper quality
The opacity of paper is particularly of interest for manufacturers of printing paper and can be determined by:
- Measuring the reflectance of a single sheet of paper at a certain wavelength (usually four 457 nm) when held on a non-reflective black background
- Measuring the reflectance of a single sheet of paper when held against a standard white background (TAPPI method), or a pad of the same paper – thick enough to prevent light transmission. The opacity is the ratio of the two reflectance values, expressed as a percentage.
All optical measurements during this paper test are normally carried out on a single instrument, such as a brightness and colorimeter. The necessary wavelengths are controlled by a series of filters to facilitate brightness and opacity measurement.
One hundred years of paper testing experience
We’ve specialized in the design and manufacture of testing instruments for pulp, paper, and packaging industries for almost 100 years. We apply this experience to every part of our development and selection process to bring you the best paper testing solutions.
Check out our full range of paper testing products, or get in touch!
