T 0092/06 29-10-2008

1. The appeal is admissible.

2. Technical background

2.1 The application is directed to a resistor containing carbon black and ground carbon fibre particles in a resistor base material, wherein the particle size distribution of the carbon fibres, as measured by a laser diffraction-diffusion method, has the properties specified in claim 1.

2.2 Laser diffraction-diffusion measurements are a standard tool when determining particle size distribution. The sample to be measured is prepared so that a homogeneous suspension of the particles in a suitable medium is obtained. When the suspension is exposed to a monochromatic laser beam a diffraction pattern arises. This pattern gives the light scattering intensity as a function of the diffraction angle, where the diffraction angle depends inter alia on the particle size. In the case of a distribution of particle sizes a complicated pattern is obtained which may be considered in a first approximation as the superposition of the patterns for each particle size (this approximation is essentially the Fraunhofer hypothesis). The Mie theory is the more rigorous optical solution but requires that the refractive index of the suspending medium and that of the particles as well as the particle's absorbency with respect to the irradiating light source be known. More detailed explanations are given in documents D3 and D4.

2.3 When calculating the particle size distribution from the light diffraction pattern the Fraunhofer approximation and the Mie theory give different results for particles smaller than about 50 µm (D4, page 8, "Adequacy of the optical model"). It is therefore recommended in ISO 13320 that the Fraunhofer approximation be used for particles larger than 50 µm while the Mie theory should be used for particles smaller than that (D2, page 46, rightmost column, 2nd paragraph).

3. Consequently, in order to determine whether the feature of claim 1 that "80 % by volume or more carbon fiber of the whole carbon fiber is included in the particle size range from 1 to 20 µm" is fulfilled or not the optical model underlying the calculation has to be known, since for this particle size range (< 50 µm) the Fraunhofer approximation and the Mie theory produce different particle size distributions according to ISO 13320.

4. The examining division found that the definition of the resistor of claim 1 was unclear (Article 84 EPC 1973) for the reason that at the claimed (Paris Convention) priority date the skilled person would have been unable to decide unambiguously whether a given resistor fell within the definition, since the priority document did not disclose which optical model (Fraunhofer approximation or Mie theory) was to be employed for interpreting the data.

5. The decision under appeal does not indicate any express legal basis for its conclusion that a lack of clarity at the priority date entails refusal. However, the observation: "A clarification of claim 1 by amendment is not possible, since the whole application does not contain any information on the physical model used" (decision under appeal, point 5), suggests a reasoning based on a lack of clarity at the priority date which had not been remedied at the filing date - since the originally filed application documents were identical to the priority document - and which could only be remedied subsequently by an impermissible subject-matter adding amendment.

6. This reasoning, albeit plausible, is unsound.

6.1 In the present quite unusual circumstances the issuing of ISO 13320, published in November 1999 according to D5 (ie after the convention priority date but before the filing date of the application), is not prejudicial, but rather beneficial evidence of the common general knowledge in the art at (at least) the filing date, that for particle sizes below 50 µm the Mie theory should be applied, thus disambiguating the alleged lack of clarity in the definition of the resistor in claim 1. In effect the publication of ISO 13320 at a date intermediate between the priority and the filing dates expanded the state of the art, including the common general knowledge in the art, so that the understanding of the invention by the person skilled in the art differed at the respective dates. In effect the change in the state of the art in the priority interval added the subject-matter (ie the information) which on the basis of the evidence available was required to clarify the claim - by amending the interpretation of an unamended text.

6.2 In order to apply the Mie theory the complex refractive indices of the medium and of the particles have to be known. The examining division objected therefore in the decision under appeal that, as no standardisation of these indices existed, the subject-matter of claim 1 could not be determined. However, the board cannot recognize any reason that would hinder the skilled person to determine these complex refractive indices at the specific wavelength of the laser light, as the particles are identified in the claim as ground carbon fibres and there are only a limited number of adequate suspension mediums. The board therefore concludes that claim 1 of the application is clear within the meaning of Article 84 EPC 1973 in this respect also.

6.3 It follows that the refusal ground of (irremediable) lack of clarity was not well founded.

7. On the face of the evidence presently available it appears that claim 1 was not clear at the claimed convention priority date. This would in turn cast doubt on the priority claimed being for the same invention within the meaning of Article 87 EPC and Article 88 EPC (G 2/98, OJ 2001, 413). Absent any intervening prior art prejudicial to novelty or inventive step it would be premature to decide this question now. It need hardly be said that an invalid priority claim is not as such a bar to the grant of a patent.

8. As the examination of the further requirements of the EPC has not yet been carried out, remittal of the case for further prosecution pursuant to Article 111(1) EPC 1973 is appropriate.