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Gloss Measurement
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Scope and Field of Application
BS EN ISO 2813 and BS 3900-D5 are alternative names for the same method for measuring specular gloss of non-metallic paint films.
The word "specular" means mirror-like and "specular gloss" is defined as the perception by an observer of the mirror-like appearance of a surface. In truth this appearance cannot be quantified: all that can be done instrumentally is to measure the amount of incident light that is reflected at a defined angle (or range of angles).
Glossmeters are simply devices that illuminate a test surface at a defined angle of incidence and measure the amount of light at a defined angle of reflection. Since basic physics tells us that the angle of incidence is equal to the angle of reflection, it is convenient to refer to one angle only and this is generally referred to as the "geometry" of the instrument.
In practice only a perfect mirror will give rise to a precise angle of reflection: a paint film will tend to scatter the incident light and reflect it over a range of angles. It is for this reason that the standard defines geometry-related ranges of angles over which the reflected light is measured.
Specular gloss is quantified by measuring the amount of light reflected from the sample and comparing it with the amount of light reflected when a polished black glass calibration standard is measured under the same conditions. The glass standard is assigned a value of 100 gloss units and in practice the highest attainable gloss values for non-metallic paints tend towards 95 gloss units. It follows from this definition that any surface that reflects more light than the black glass standard will produce a glossmeter reading of greater than 100 gloss units. Metallic finishes (of the type used as automotive paints) will generally produce glossmeter readings well in excess of 100 gloss units and this BS standard does not cover the gloss measurement of coatings of this type.
The standard covers the measurement of specular gloss using 20, 60 and 85 degree geometries. The 60 degree geometry is applicable to the majority of paint films. The 85 degree geometry is used to differentiate between low-gloss finishes (less than 10 gloss units) while the 20 degree geometry is useful in differentiating between the glossier paints (greater than 70 gloss units).
There are no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.
The actual standard is a copyright-protected document and we are not able to provide you with copies. If required however, you can easily obtain copies from the British Standards Institution.
Summary of Method
A significant proportion of the standard is taken up with details of the way that a glossmeter should be constructed and how primary and secondary gloss standards should be prepared. Gloss measurements are sensitive to minor imperfections in the coating and the standard describes how to measure surfaces with discernible brush marks or raised wood grain. Obviously this standard has to be used in conjunction with the operating instructions for the actual instrument used. We use a Rhopoint Novogloss instrument which has 20, 60 and 85 degree geometries and can carry out a statistical analysis of stored values.
Sample Requirements
We require one coated panel about 150 x 100 mm. The paint surface should be smooth or lightly textured and the substrate should not be perforated. We can work with samples other than test panels provided these restrictions apply and provided we have about 150 x 100 mm of flat surface available.
If you wish, we can prepare the test panels. The quantity of liquid paint we needs depends on the method of application. We will pleased to advise you regarding our requirements.
Accreditation
PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS) to carry out this test.
Please contact Peter Collins for further details.
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