Colour and Optical Properties

The BS and ASTM standards mentioned here are copyright-protected documents and we are not able to provide you with copies. If required however, you can easily obtain copies from the British Standards Institution or the American Society for Testing and Materials.

Visual Colour Comparison: BS EN ISO 3668, BS 3900-D1

Scope and Field of Application

BS EN ISO 3668 and BS 3900-D1 are alternative names for the same method of carrying out a visual comparison of the colour of paint films or related products with a standard colour under natural or artificial lighting conditions.

Visual colour matching has the advantage that it provides information on how the colour is perceived. The disadvantage of visual colour matching however is it relies on the opinion of an observer and may well result in disagreement between observers.

In such cases you should consider instrumental colour matching. This has the advantage that it can provide reproducible data regarding the closeness of a colour match. The disadvantage is that the data generated may not always correlate with the way that the human eye perceives colour.

In our opinion there is no need to choose between visual and instrumental colour matching. The most satisfactory way to compare colours is to use both methods.

There are no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

Summary of Method

The standard describes how the colour vision of observers should be checked and how eye fatigue should be avoided. The point is made that observers need not only normal colour vision but also the ability to discriminate between colours. The latter is developed by both training and experience.

Colour matching in daylight is carried out with the observer facing away from the equator with the samples illuminated by diffuse daylight from a partially cloudy sky.

When colour matching under artificial illumination a colour-matching booth fitted with D65 (artificial daylight), CIE Illuminant A (tungsten filament light) and TL84 (fluorescent light) is used.

The standard gives details of the construction and illumination of the booth and the way that it should be used to view the samples. It also describes how metamerism is assessed. Metamerism defines the situation where the colour of the sample and the standard match under one illuminant but not under another.

The observer reports the results of the visual colour comparison using a six-point rating scheme given in the standard. This ranges from imperceptible to major colour differences.

Sample Requirements

Ideally the test sample and the reference colour standard should be flat panels about 150 x 100 mm wherever possible the sample and reference panels should be similar in size, gloss and surface texture.

Only one sample panel and one reference panel is required for each determination. We have collections of BS and RAL colour standards which we can use should you require your sample to be compared with one of these references.

In many cases it is possible to adapt the method to compare the colour of samples other than test panels. Examples include building components, automotive parts, electrical components and display/packaging material.

If you wish, we can prepare the test panels. The quantity of liquid paint we need depends on the method of application. We will be pleased to advise you regarding our requirements.

Gloss Measurement: BS EN ISO 2813, BS 3900-D5

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. 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.

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 standard does not cover the measurement of gloss on this type of coating.

There are no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

Summary of Method

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 need depends on the method of application. We will be pleased to advise you regarding our requirements.

Determination of Opacity: BS 3900-D4, ISO 2814

Scope and Field of Application

BS 3900-D4 and ISO 2814 are alternative names of a method for comparing the opacities of paints. It is relatively simple in operation.

There are no pass/fail criteria associated with this standard. This is a matter of agreement between the parties concerned.

Summary of Method

This standard describes how a wet film of the test coating is applied to a black and white patterned substrate. After the film has dried, a spectrophotometer is used to measure the amount of light reflected from the overcoated black areas of the substrate. This is then expressed as a percentage of the amount of light reflected from the overcoated white areas.

A paint which is highly opaque will obscure the black and white areas to an equal extent. In this situation, equal amounts of light will be reflected from the overcoated black and white areas and consequently an opacity value of 100% will be obtained.

Sample Requirements

This method is only applicable to white or light coloured paints and not applicable to paints which are textured or those which fluoresce or have a metallic finish.

We require 250ml of paint.

Determination of Opacity: BS 3900-D7, ISO 6504/1

Scope and Field of Application

BS 3900-D7 and ISO 6504/1 are alternative names of a method for determining the opacity of paints. It is mainly used for checking that coatings meet the requirements of the Ecolabel scheme.

Summary of Method

This method is based on equations (derived by Kubelka and Munk) relating the scattering and absorption coefficients of pigmented films to their opacity. In order to use these equations to obtain a value for opacity, it is necessary to know:-

• The reflectance of the paint film at infinite thickness. In practice this is taken as the reflectance at a thickness where any further increase in thickness does not change the reflectance.
• The reflectance of the paint film (of known thickness) when measured over a black background.
• The dry film thickness of the paint together with the corresponding wet film thickness

The test paint is applied to clear polyester sheet in order to produce dry films ranging from 75 to 150 microns in thickness. A spectrophotometer is then used to measure the reflectance of each film and, if necessary, additional, thicker, films are produced until the reflectance values obtained are no longer dependent on film thickness (regarded as the reflectance of the paint film at infinite thickness).

A set of paint films is then prepared at a thickness which will give less than maximum reflectance. The reflectance of these is then determined with the coated polyester sheet placed over a black background.

The dry film thicknesses of these films are then determined gravimetrically. The corresponding wet film thicknesses are then calculated from the dry film mass per unit area measured previously the density and non-volatile content of the paint.

The opacity of the paint can then be expressed as the spreading rate (in terms of square metres per litre) needed to give an opacity value of 98%.

Sample Requirements

This method is only applicable to white or light coloured paints and not applicable to paints which are textured or those which fluoresce or have a metallic finish.

We require 250ml of paint

Determination of Opacity: BS 3900-D11, ISO 6504/3

Scope and Field of Application

BS 3900-D11 and ISO 6504/3 are alternative names of a method for determining the opacity of paints. In our experience it is most widely used method.

There are no pass/fail criteria associated with this standard. This is a matter of agreement between the parties concerned.

Summary of Method

It has been shown that, over a restricted range of film thicknesses, opacity is inversely proportional to spreading rate. Fortuitously, this restricted range of film thicknesses includes those commonly used for white and light-coloured paints. This standard utilises the relationship.

The methodology is similar to that used in BS 3900-D7: ISO 6504/1 in so much as the paint is applied to clear polyester sheet and the reflectance of the film measured. The same methods as those described in BS 3900-D7: ISO 6504/1 are used to determine the wet and the dry film thicknesses of the films.

Duplicate paint films at three different film thicknesses are prepared and their reflectance is measured when the film is placed over both a black and a white background. The reflectance over the black background is then expressed as a percentage of the reflectance over the white background. This value is the opacity of the film.

The spreading rate of each film (in terms of square metres per litre) is then calculated and a plot of opacity versus spreading rate is constructed. A linear relationship is assumed and the opacity at a spreading rate of 20 square metres per litre is read from the plot.

Sample Requirements

This method is only applicable to white or light coloured paints and not applicable to paints which are textured or those which fluoresce or have a metallic finish.

We require 250ml of paint

Determination of Opacity: in-house method

Scope and Field of Application

In addition to these standard methods, we also carry out an in-house method whereby paint is applied to a surface which includes a contrasting background. This gives a measure of both the opacity of the coating and its application properties.

There are no pass/fail criteria associated with this method. This is a matter of agreement between the parties concerned.

Summary of Method

The method of apply paint using laboratory equipment to small areas of smooth non-absorbent substrates may not give a true indication of the opacity obtained when the paint is used for its intended purpose.

The PRA in-house method involves an experienced painter applying the coating, using both a brush and a roller, to 1200 x 1200 mm panels. Depending on the nature of the paint, the panels can be either plasterboard or hardboard. Black and white cards are fixed to the panel before painting.

After the paint has dried, the cards are removed and a visual assessment is made of how well the paint has obliterated the black and white pattern.

The advantage of this method is that it gives a measure of opacity obtained at the natural spreading rate of the coating. It also has the ability to show up any tendency of the paint to produce patchy or uneven films, either of which will have an adverse effect on its opacity.

Sample Requirements

Provided the paint can be applied by brush or roller to yield an air-drying film, then we can test it.

We require 2.5 litres of paint

Determination of Whiteness and Yellowness: ASTM E313

Scope and Field of Application

In many cases the ageing of white paints in service is accompanied by yellowing.

The standard details the theory and practice associated with the use of a spectrophotometer to determine whiteness and yellowness. It is appropriate to use the same standard method for determining both values as the same equipment and experimental techniques are used and only the methods of calculation differ.

Whiteness is defined as a measure of how closely a surface matches the properties of a perfect white diffuser i.e. an ideal reflecting surface which neither absorbs nor transmits light but reflects it at equal intensities in all directions.

Yellowness is defined as a measure of the degree to which the colour of a surface is shifted from preferred white (or colourless) towards yellow.

There are no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

Summary of Method

The method is not specifically a paint standard and consequently it does not define how test panels should be prepared. Some guidance is given regarding how the spectrophotometer should be used to obtain the colour coordinates needed to carry out the calculations.

We carry out the determinations using a GretagMacbeth Color-Eye 7000A Spectrophotometer. This enables us to measure both in transmission and reflectance. The software includes the equations for whiteness and yellowness that are recommended in ASTM E313 and the calculations are done automatically.

Sample Requirements

We need to be able to press the sample against the port of the spectrophotometer in such a way that no light can leak past the sample and interfere with the measurement. Ideally the test sample should be a flat panel about 150 x 100 mm. The paint surfaces should be smooth or lightly textured and the substrates should not be perforated. Provided the other requirements apply, we can work with samples as small as 10 x 10 mm and as large as 300 x 300 mm.

If you wish, we can prepare the test panels. The quantity of liquid paint we need depends on the method of application. We will be pleased to advise you regarding our requirements.

We have also used this method to measure the whiteness of pigments and extenders. About 100 grams of sample will suffice for this purpose.

It is also possible to use the spectrophotometer in the transmission mode to measure the yellowness of transparent solids or liquids. In the case of solids the sample size required is 50 x 50 x not more than 25 mm. In the case of liquids, 50 ml will suffice.

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