haze-gloss
What
is Gloss?
Gloss is an optical phenomenon caused when evaluating the appearance of a surface. The
evaluation of gloss describes the capacity of a surface to reflect directed light.
Why is gloss measured?
Gloss is often used as a criterion to evaluate the quality of a product, especially in
the case of products where the aesthetic appearance is of importance. This includes
products such as automotive coatings, furniture coatings, plastics, metals and paper. A
visual gloss evaluation includes many subjective sources of error and is not sufficient.
Therefore, to be objective, it is necessary to put a measured value on the degree of
gloss. A complete evaluation of gloss is dependent on several factors. Since the 1930's,
measuring instruments have been used to associate reflection behavior and a defined
measurement value under defined conditions. How this is done and what has to be considered
will be explained on the following pages.
How is Gloss Perceived?
Gloss when perceived by the human eye is a subjective evaluation. However, visually
observed differences can not always be measured physically by using, for example,
glossmeters.
The appearance of gloss is influenced by several factors.
Surface properties
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- Material
(e.g. glass, coatings, metals)
- Surface profile (e.g. flat, structured)
- Transparency and substrate
Type of illumination
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For gloss
evaluation it is required to have direct illumination. A diffuse illumination causes
diffuse reflection resulting in decreased gloss impression.
Observer
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Visual
evaluation is dependent on the eyesight -> physiology
and the mood -> psychology.
As the perception of gloss is a sensation which is not a mere physical measurement, it is difficult to describe with physical parameters.
Reflection Behavior of Surfaces
Different types of surfaces and their reflection behavior under direct illumination
are evaluated in the following chapter.
Glossy surface
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Gloss
=> direct reflection on the 1st surface.
In case of high gloss surfaces, light reflected from the surface follows the reflection law (angle of illumination = angle of reflection). The intensity of the reflected light is dependent on the angle of illumination and material properties.
Metals: very high intensity, hardly angle dependent
Coatings: low intensity, angle dependent
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Color => diffuse reflection from within the sample.
Part of the illumination penetrates the surface and is selectively absorbed and scattered internally by pigment particles and other bodies. It then is diffused from within the first surface. This is how the impression color is caused. This diffuse reflection can be measured with BYK-Gardner color measuring instruments.
Glossy surface
with haze
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The dominant part of light is reflected in the main direction of reflection (specular). A small amount of light is scattered in directions adjacent to the direction of specular reflection. This scattered light of low intensity causes haze. The surface seems to be glossy, yet has a milky appearance.
Medium to mat
surfaces
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In this case light is not only reflected in the direction of specular reflection but also in other directions. The capacity of a surface to reflect a light source or other images is strongly reduced. The more evenly the intensity is distributed in all directions, the less glossy a surface will appear.
The differences between high, semi and low-gloss surfaces can be determined with a regular glossmeter (e.g. micro-gloss, micro-TRI-gloss). The visually perceived phenomenon "haze" can hardly be detected with a regular glossmeter, and to date, only very complicated instruments have been available to measure this effect. Due to these facts, measurement of reflection haze is performed only on rare occasions. Consequently, supplier and vendor were often faced with disputes concerning the appearance quality as they were not able to measure haze.
By developing the haze-gloss, BYK-Gardner allowed the practical measurement of gloss and reflection haze with one laboratory instrument. Now it is possible to measure reflection haze as easily and as fast as you can measure gloss.
How is Gloss Measured?
By using reflectometers reflected light of a surface is measured in an angle range
which is limited by aperture dimensions.
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Light beam in a reflectometer
The light source is projected over the sample surface onto the opening of aperture 2. A photoelectronic detector measures the light passing through the aperture.
The measurement results are influenced by various factors:
- Type of measuring instrument
- Angle of illumination
- Calibration of the instrument
- Surface characteristics
In order to obtain comparable measurement results apparatus and measurement procedure were internationally specified.
What are
the conventions of an international specification?
- Measuring instrument (optics)
- Calibration of the instrument
- Surface of the sample
- Measurement procedure
The most important specifications
ISO 2813
ASTM D 523
DIN 67530
These different specifications agree in the essential points.
Standardization
of the instrument
- Angle of
illumination I reflection 20°/60°/85°
- Dimensions of the source and receptor aperture
- Light source
- Receptor response (sensitivity).
The angle of illumination highly influences the measurement results. In order to
evaluate the whole range from high-gloss to mat surfaces, three different angles Of
illumination (which means three different measuring ranges) are defined for the paint and
coatings industry:
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20° high
gloss surfaces
60° medium gloss surfaces
85° mat surfaces other industry specifications:
45° ceramic industry
75° paper industry (TAPPI specification)
Instrument
calibration
The
measurement of the reflectometer value R' is a relative measurement. Results are related
to a highly polished black glass with a refractive index of 1.567. The glass has an
assigned specular gloss value of 100 for each geometry. As instrument and standard
tolerances are tightly controlled, the measurement error should not be more than ± 1
unit.
Surface
properties
In order
to achieve highly accurate and repeatable results, the test specimen should be
- flat
- free of structures
- similar in color and lightness
- non-luminescent material
Measurement
procedures and measurement results
In order
to differentiate gloss of samples, it is necessary to select the appropriate measuring
geometry.
- First the test specimen is measured with the 60° geometry. The 60° geometry should
be used if the gloss reading is between 10 and 70 units.
- If the 60° gloss is higher than 70 units the 20° geometry will be advantageous for
comparison.
- If the 60° gloss is lower than 10 (30*) units, the 85° geometry should be used.
*recommendations according to DIN.
In some cases the 60° geometry will be advantageous for very mat test specimens.
At least three readings should be taken on a test specimen. If the range is greater than five gloss units, additional readings have to be taken. For correct measurement results, the mean gloss reading is to be calculated and the geometry used is to be reported. e.g. 20° reflectometer value R'20 = 55 units.
Why do specific gloss levels require different measuring geometries?
The reflection on paint and plastic surfaces basically follows the Fresnel equation.
The amount of light that is reflected on the 1st surface or penetrating the material is dependent on the illumination angle. Since, in the case of a mat surface, the amount of reflected light is distributed over a wide-angle range only a small part is detected by the aperture of the reflectometer.
Therefore it is necessary to choose the correct measurement geometry to guarantee high measurement accuracy.
Which measurement geometry is required for which gloss level?
In order to illustrate the proper selection for a measuring geometry, the following
test was performed:
Thirteen black glass tiles (sample number 1-13) were visually ranked from mat (1) to
high-gloss (13) and were measured with 20°, 60° and 85° geometries. By graphing the
measurement results (0-100 gloss units) against sample number 1-13 each geometry has a
different curve. It is remarkable that the measured gloss difference between two
successive tiles is different on each curve. For example, sample number 10 and 11 show the
largest difference in the 20° geometry and confirm the use of the 20° geometry for high
gloss samples. While in the low gloss area, for example sample number 2 and 3, the largest
differences can be achieved with an 85° geometry.
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Peculiarities
when measuring gloss
Different phenomena can cause problems when measuring gloss.
Calibration
Mistakes during calibration will result in wrong measurement results.
- Calibration standard is not clean (fingerprints, dust)
- Calibration standard is damaged (scratches)
- Calibration standard has changed because of other influences (aging)
- Usage of semi-gloss standards as calibration standards.
In order to avoid the above mentioned sources of errors international specifications recommend a yearly re-calibration of the calibration standard.
These types of errors are to a large extent excluded with BYK-Gardner micro-gloss, micro-TRI-gloss and haze-gloss. In the case of the micro-gloss and micro-TRI-gloss the calibration standard is securely housed in a holder. The calibration is performed automatically.
Due to the long-term stability of the haze-gloss, re-calibration is only necessary every two months, which protects the standard from routine use.
Surface properties
Gloss measurement is based on the detection of the spatial reflection behavior of a surface, which is strongly influenced by a curvature (distorted image). Depending on the geometry and the degree of gloss, a curvature < 100 cm (dia.) can contribute an influence on the measured value. In addition, the risk of tilting the sample and the influence of ambient light has to be considered.
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Regular
structures on surfaces
will result in different gloss values dependent on the measuring direction. If gloss
measurement is used as a quality control criterion for those types of surfaces, it is
recommended to indicate the measuring direction.
(App 1-12)
Irregular
structures
(artificial leather) show
relatively high variations.
Peculiarities
for high-gloss surfaces
When visually observed,
high-gloss surfaces can differ in their appearance, while readings taken with a 200
glossmeter are equal or are very close. The reason for this phenomenon is scattered light
being observed by the human eye. A regular glossmeter is not capable of measuring this
scattered light.
Scattered
light can be caused by:
- Long term surface structures
--> Flow/leveling defects, orange peel leading to a distortion of the reflected image
on the surface
- Short term surface structures
--> Reflection haze: Microscopic defects on or just beneath the surface cause scattered
light of low intensity adjacent to the direction of specular reflection (see
"scattered light with gloss measurement").
Scattered light with gloss measurement
When describing the appearance of a surface with a reflectometer, the limits of this
test method are often reached.
How is visual appearance evaluation performed?
The image of a light source or another illuminated object (e.g. an overhead
fluorescent light) reflected by the sample surface is visually evaluated. By using three
different types of surfaces the following phenomena will be described:
- quality of a reflected image
- theoretical reflection behavior
- measured reflection distribution
On surfaces that exhibit flow/leveling defects or orange peel, the reflected image will
be distorted.
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This type of image distortion, caused by long term waviness, results in an enlargement of reflection peak.
In the case of an undisturbed high-gloss surface, the reflected image appears as clean
and distinct as observing it directly.
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How is
reflection haze measured?
Reflection haze is caused by microscopic defects, which result in scattered light
adjacent to the direction of specular reflection, and is visually observed. With the
haze-gloss, it is now possible to measure and differentiate both the directly reflected
(gloss) and diffusely scattered (haze) light. With the haze-gloss, the measurement of
reflection has become as easy and as fast as the measurement of gloss.
On surfaces with reflection haze the reflected image is surrounded by a halo.
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Reflection haze caused by scattered light of low intensity, adjacent to the direction of specular reflection, can be measured with the haze-gloss.
Portable glossmeters from BYK-Gardner
The ideal solution to measure directly at the object.
micro-gloss micro-TR I-gloss mirror-TR I-gloss
micro-gloss and micro-TRI-gloss are the small portable glossmeters. They fit easily into a lab coat pocket.
The micro-TRI-gloss is equipped with three geometries (20°, 60° and 85°) in a single unit and can be used for a wide range of applications. If the application can be restricted to a single geometry, we also offer the micro-gloss as a single angle unit in 20°, 60°, 85°, 45°, or 75° geometries.
Advanced electronic engineering provides operator ease and high accuracy at a level not
previously achieved:
- rapid automatic calibration that occurs within a specially designed holder
- calculation of average readings within the instrument
- storage of measured data
- micro-processor controlled operation
- display in English, French and German
- battery powered
- RS-232 interface for data transfer to a PC
- optional software package "gardner-soft gloss and haze" for numerical and
graphical data evaluation. The PC interface is a comfortable solution that allows
extensive evaluation of measured data, while having the flexibility of a portable unit.
BYK-Gardner haze-gloss
Complete and rapid gloss measurement in the laboratory.
- 20°, 60° and 85°
- mirror reflection
- reflection haze
all in a single unit.
As described in the previous pages, classical specular gloss measurement is to completely evaluate the aesthetic appearance of a surface. In those cases the measurement of reflection haze offers vital information and is an indirect method to describe material properties such as dispersion degree, flocculation behavior or the influence of manufacturing processes on the surface. Up to this point, reflection haze could only be measured with time intensive and expensive methods. The haze-gloss offers an easy and fast solution to measure gloss and haze. The haze-gloss is a rugged, stationary laboratory instrument, which was developed with special emphasis on measuring ease.
- special sample support table permits fast sample handling
- sample support table optionally motorized and controlled using a footswitch
- illuminated target for accurate sample positioning
- high accuracy and long term stability due to reference beam principle
- operation without warm up time
- long term calibration (every two months)
- automatic calibration
- microprocessor controlled operation guarantees high measurement confidence
- display in English, French, German, Spanish and Italian
- statistic mode includes average, mm/max, standard deviation and coefficient of variance
- RS-232 interface for data transfer to PC and control of haze-gloss via PC.
- optional software package "gardner-soft gloss and haze"