ASTM E313 details the theory and practice associated with the use of a spectrophotometer to determine whiteness and yellowness. There are two reasons why it is appropriate to use the same standard method for determining both values:
Whiteness is defined as a measure of how closely a surface matches the properties of a perfect reflecting diffuser i.e. an ideal reflecting surface which neither absorbs nor transmits light but reflects it at equal intensities in all directions. For the purposes of this standard, the colour of such a surface is known as preferred white.
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.
The greater part of the standard is taken up with the theoretical aspects of colour science. Numerous equations for deriving whiteness and yellowness values from colour coordinates are discussed and recommendations are given regarding the equations that should be used.
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 American Society for Testing and Materials.
ASTM E313 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. Obviously this standard has to be used in conjunction with the operating instructions for the actual instrument used.
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.
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 needs 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.
Although PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS), we are not accredited to carry out this particular test.