The EPO's normal approach to questions of obviousness is first to determine the closest single piece of prior art and then ask whether the solution to the problem of leaping from that piece of art to the claimed invention was obvious to one skilled in the art. This can create difficulties in determining the art in which the person must be skilled if the closest art is in a different field from that in which the necessary step must be taken. The issue was considered in Luminescent Security Fibers/Jalon.

The claim in question related to the production of security fibers, etc., which were luminescent by virtue of the presence of rare earth or similar chelates. The invention was characterized by introducing such luminescent chelates after the production of the fibers by use of a dyeing process. The closest prior art disclosed incorporation of such chelates into security threads and the like during the spinning process that produced the threads. The Opposition Division had held the claimed invention obvious on the basis that additional prior art showed that dyeing of security fibers was known and that one skilled in the dyeing art would know that luminescent chelates could be used in such dyeing techniques.

The Appeal Board disagreed. It held that the Opposition Division had been wrong to focus on what was obvious to the skilled dyer. Selection of a dyeing technique was part of the solution to the problem posed by the prior art. Therefore one could not use the dyer's knowledge as a basis for a finding of obviousness. The proper test was "what was obvious to one skilled in the art of marking and protection of security documents and similar materials?" Once the nature of the skilled person had been identified, the Board found that selection of dyeing was a non-obvious alternative to what had been done before and so the invention was patentable.