The merits and shortcomings of bibliometric evaluation techniques are well known; the reliability of the techniques varies according to the discipline. For technology the reliability is small. The electron microscope is a clear case of extreme mismatch between the number of citations received and the impact of the instrument in a wide area of science. The instrument is comparable to a scientific publication in the way in which it is used and referred to in the literature. In this paper we estimate the size of the citation gap, i.e. the number of citations an author misses because the results of his research are made public in the form of an instrument instead of via an article in a journal.