How To Measure Gloss
As the surface characteristic of an object, gloss is closely related to the specular reflection of the surface to light. The specular reflection refers to the reflection phenomenon in which the reflection angle equals the incident angle. Glossiness is a physical quantity that evaluates the ability of a material surface to reflect light under a set of geometric conditions. Therefore, it expresses the reflection property with direction selection.
In the coatings field, there are commonly super matte, matte, mercerized (eggshell light), bright (low light), semi-gloss and high light.
The gloss of paint film is one of the important indexes of its appearance quality, which is also related to the performance of paint film. When the paint film appears micro-cracks, subsidence and ageing, it begins to lose gloss.
For paint, it is generally considered that the greater the amount of reflected light, the higher the gloss, also called specular gloss.
In addition, the ratio of the reflected light intensity and the diffused light intensity of the surface is also an indicator of gloss. The gloss is closely related to the reflectivity, and the colour of the paint film, such as the black film’s gloss, is better than that of a white film with the same reflectivity.
There are three grades for the gloss of paint film:
- High gloss paint film and enamel film mainly belong to this type.
- Semi-gloss paint film and indoor emulsion paint film belong to this type.
- Matte paint film, usually the bottom film.
The reflected light intensity measured at different angles is different. Generally, it is measured at 45° or 60° . According to the results measured by 60° gloss meter, the coating has the following grades:
- Semi-gloss 70-30%
- Mercerization 30-6%
- Eggshell light 30-6%
- Matte less than 2%
The percentage of gloss is no reflectivity, but a relative value is obtained by comparing the sample plate with a standard plate. Assume the gloss of the standard plate is 100%, and then measure the sample percentage.
Factors affecting the gloss of coatings
Many factors affect the gloss of coatings, such as the size and dispersion of pigment particles, the distribution and size of latex particles and so on.
Dispersant
If it is colour paint, we must consider the influence of dispersant. Relevant tests have shown when using an inappropriate dispersant in the same condition, the pigment will flocculate, and the particles will coarsen, which will affect the gloss. Solvents also have a specific impact on the gloss. For example, if the volatile system dries too fast, it will affect the gloss and cause whitening and fog shadow.
Size of paint
Gloss is related to the pigments and fillers used. The state and particle size of pigment affect the colouring strength, transparency, colour light, weather-fastness and solvent resistance and the gloss of colour coating film. Pigments and fillers with large particle sizes usually will reduce some gloss; if the particle size is too small, it also impacts the gloss in coatings with high oil absorption. The reduction of pigment particles reduces the diffusion and scattering of light by particles in the paint film, thus increasing the gloss.
Different hues
The level of absorption and reflection of various pigments to light is different. Because black paint completely absorbs light and white paint completely reflects light, black paint shows a higher gloss than white paint.
Paint film thickness
The thickness of the same coating is closely related to the film coverage. Within a specific range, the gloss increases as the film thicken. In addition, the operation environment also impacts the film gloss, such as temperature, humidity, etc.
Surface flatness and roughness
The intensity of the reflected light on the paint film’s surface depends on the paint itself’s nature, the surface flatness and the roughness of the substrate. The incident angle of light will also affect the gloss. The larger the incident angle, the higher the intensity of the reflected light. For the same type and batch of coatings, surface flatness and roughness are the main factors affecting the film gloss. Therefore, the more flat the substrate, the more strict the coating surface treatment.
What are the types of test angles for gloss meters?
Since the gloss measured is different under different angles, we need to measure it through an appropriate angle to correctly evaluate the gloss of the object’s surface. Currently, there are three common measurement angles, 20 °, 60 ° and 85 ° for gloss meters on the market. Different measurement angles are suitable for object surfaces with different glosses.
45 ° and 75 ° measurement angles will also be used for paper industries.
The 75 ° gloss measurement angle is mainly used for coated paper and paperboard, uncoated paper and paperboard or paper and paperboard samples with low printing gloss;
The 20 ° gloss measurement angle is mainly used for high gloss paper and paperboard such as cast coated paper, wax gloss paper and polishing paper, high gloss printing paper and paperboard samples, high gloss ink and paint, etc.;
The 45 ° gloss measurement angle is mainly used for the aluminium foil paper, vacuum aluminized paper and other metal composite paper and paperboard;
The 60 ° gloss measurement angle is mainly used for paper, coating, paint, etc.
The 85°gloss measurement angle is mainly used for camouflage coating, low gloss paint and so on.
How to choose the measuring angle of the gloss meter?
For different object surfaces, the measurement angle is also different.
For typical materials, we can choose a 60 ° gloss meter.
For high gloss materials, we can choose a 20 ° gloss meter.
For low gloss materials, we can choose an 85 ° gloss meter.
How to measure gloss?
A gloss meter is an instrument used to measure the surface gloss of material, which is suitable for ceramics, paints, inks, plastics, marble, aluminium, hardware and other materials.
The Principle of gloss meter
The measuring head of the instrument is composed of a transmitter and a receiver. The transmitter is composed of an incandescent light source and a group of lenses, which produce a certainly required incident beam. The receiver is composed of the lens and photosensitive element, which is used to receive the cone-beam reflected from the sample surface.
Specular gloss is a relative measure value. The reference standard is black glass with a refractive index NP = 1.567. Assuming that its plane is in the state of ideal polishing, the natural beam is reflected in the mirror direction by the plane, and the gloss value at this time is defined as 100 units. The gloss board is divided into high, medium and low according to the gloss value. The high gloss board is made of black optical glass or other materials. The medium and low gloss boards are made of glazed ceramic or black optical glass frosted. The gloss meter measures the gloss of the sample by using the Principle of light reflection:
Irradiate the sample under the specified incident angle and specified beam conditions to get the reflected light in the direction of the mirror reflection angle.
Measuring tips:
1. Please wait a moment before measuring from the place with lower temperature to the place with higher temperature (outdoor to indoor in winter). Do not measure the tested object in the direct sunlight.
2. When measuring curved objects with a small hole gloss meter, to ensure that the small holes are thoroughly covered, we can slowly cover the measured object along the tangent of the measured curved surface in a small amount until we can not find the light spot on the measured object.
How to use gloss meter?
Press the switch, place the instrument on the black glass standard plate, and rotate the adjustment knob to make the displayed value the same as the nominal value of the standard scale.
Then place the instrument on the white ceramic standard plate and ensures the error between the displayed value and the nominal value of the white ceramic standard plate is less than ± 1.2 gloss unit.
We can measure the sample after calibration.
How to choose a gloss meter?
Constant source and temperature compensation
The stability of the instrument’s light source and temperature compensation are also important factors for the instrument’s stability. We know that the luminous efficiency of the light source is different under different temperatures and power supply currents.
Therefore, the light source of gloss must have a constant source and temperature compensation to ensure a good result.
If the ambient temperature changes after calibration, it will cause a change in the luminous efficiency of the light source, resulting in an error in measurement data. If there is a temperature compensation, it can ensure the same luminous efficiency of the light source.
Linearity of measurement data
From the principle of gloss meter, we know that the linearity of instrument measurement data is an important factor when choosing the instrument. Suppose the gloss standard plate is accurate, for example. In that case, the gloss of the standard plate is 95.5, the general measurement data of the materials with glossiness near 95.5 is relatively accurate, but the accuracy of the measurement data of the materials with glossiness far away from the standard plate depends on the quality of linearity of the instrument measurement system.
Automatic detection
The gloss meter must have the self-diagnosis of standard plate contamination. If the standard plate is not clean, the instrument can recognize and tell the user to clean the stock plate. Otherwise, it will cause the inaccurate result.
Well protected for the standard plate of gloss meter
The measurement of the gloss meter must take the data of the gloss standard plate as the reference standard. If the standard scale is not accurate, the instrument must be inaccurate. Moreover, the gloss of the standard plate is effortless to be affected, so a suitable protection device must be used on the standard plate of the instrument.
