The EPO sets high standards on sufficiency of disclosure of measurement methods, but there are limits. It all depends on what the skilled person may be expected to understand.
[7.1] Claim 1 requires that the polymers of components (A) and (B) are such that i) the value of the refractive index of the electrostatic polymer should be in-between those of the polycarbonate and the cycloaliphatic copolyester resins and further so that ii) the refractive index of the electrostatic resin be matched within a value of 0.005 units to that of the miscible mixture of the polycarbonate and the cycloaliphatic copolyester.
According to the EPO case law the unambiguous characterisation in a claim of a product by a parameter (here the refractive index) necessarily requires that the parameter can be clearly and reliably determined (see e.g. T 555/05 [3.2.8]). This requirement is in particular necessary in order for the public to know whether they are working within the claims or not.
In the present case whilst the information provided in D6 […] which is considered by the board to be generally representative of the common general knowledge at the relevant filing date, shows that there are various methods available in the art to determine the refractive index […], it is made clear that the refractive index of a material that is quoted in the literature is the index at 23 °C or 25 °C and at the specific wavelength of the D line of the sodium emission spectrum which is 589.3 nm […]. Furthermore it is stated that the standard for the plastics industry, ASTM D 542 calls for two methods of index measurement, one of which is accurate to three decimal places and possibly four, and the second of which is only accurate to approximately two decimal places […].
It is conspicuous to the board in this connection that the refractive indices are quoted in the application in suit to three decimal places. Consequently and even though there are details given on later pages of the same document which were not cited by the appellant but which were […] discussed during the oral proceedings, of how refractive index can vary with the temperature of measurement, the wavelength of measurement and the moisture content of the atmosphere during measurement, it is nevertheless considered by the board that in spite of the absence of any precise definition in the application in suit itself of the way in which the refractive index is to be measured, this is indeed a well known and even standardised parameter […] for which there is no evidence for assuming a degree of variance in the values obtained by measuring it according to the relevant standard which would lead to an objectionable lack of clarity in the sense of A 84.
On the contrary the board takes the view that the skilled person reading the application in suit at the filing date would immediately understand that any values of the refractive index would be those measured according to ASTM D 542 to the appropriate degree of accuracy.
The fact that there may be a purely theoretical lack of mathematical precision in the values quoted owing to the environmental variables mentioned above does not in the board’s view in the present case lead to any real doubt as to whether a given composition would fall inside or outside the scope of claim 1 as regards the refractive index requirements of the latter, depending on whether it was measured at, say, 23 °C or 25 °C.
In other words the use of the term “refractive index” in claim 1 is clear in the sense of A 84.
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