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| Foucault's Knife Edge Test |
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In the production of telescopes in particular, special significance was given to what is known as Foucault’s knife edge technique for testing the surface quality of lens elements and mirrors. Foucault invented it as early as 1856, but did not actually mention it until he published a paper on the construction of telescopes in 1859. This technique was used for over a century for quality testing and quality assurance in the building of astronomical instruments. Today, advanced interferometric measuring techniques are used to measure the quality of all kinds of optics.
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| Laboratory model of a knife edge instrument. | DIRECT® 100 Laser Interferometer from Carl Zeiss. | Electrical arc lamp with Foucault’s automatic regulator. |
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The optics to be tested – e.g. a concave mirror or an objective lens – images a point of light lying at infinity in its focal point. If a knife edge located in this focal point is then moved at right angles to the optical axis and the lens aperture is then observed by bringing the eye directly behind the knife edge, all rays are covered by the knife edge simultaneously if the optics are perfect. This means that the lens aperture darkens evenly over the entire surface. If, however, the optics contain areas with aberrations so that rays are combined in front of or behind the knife edge, uneven illumination is observed, the extent of which is dependent on the position of the knife edge.
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