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E. Acosta, S. Chamadoira; Low–Cost, Robust Interferometer for the Characterization of Progressive Lenses . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1208.
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The use of interferometric methods for the optical characterization of ophthalmic components has been avoided mainly due to the following reasons: They are thought to be too complex for implementation in a robust device, they produce interferograms which are difficult to interpret and they are too sensitive to allow for the testing, for instance, of progressive lenses. This work shows how a small modification of the simplest of the common path interferometers, the point diffraction interferometer (PDI), is able to overcome the three mentioned drawbacks.
The esential part of a PDI is a semi–transparent region surrounding a clear pinhole, the diameter of which is smaller than the Airy radius of the unaberrated optics illuminating the set up. By increasing the size of the pinhole diameter and adapting the optical density of the semitransparent region in order to acquire interference fringes with high contrast, the interferometer losses sensitivity at the expense of increased dynamic range. Thus, progressive lenses with additions of approximately 3D (or less) can be measured. Since the size of the pinholes has been substantially increased , we have called this device a "hole diffraction interferometer (HDI)".
We measured different kinds of progressive lenses by obtaining the phase map of a significant region of the lens. However, it must be stressed that the interferometer is also able to scan small zones over any region of the lens. The phase maps represent iso–phase levels of the wavefront at the lens plane and therefore, the interpretation of the fringes is simple.
A hole diffraction interferometer (HDI) permits the design of an interferometric device capable of measuring not only progressive lenses but also any other kind of ophthalmic components. The fact that it is a common path interferometer together with facility of alignment (due to the increase in the size of the pinhole) make it a very compact and robust device. On the other hand, the fringes obtained represent the iso–phase levels at the output of the lens (very useful for inspection). Accuracy is adequate for the characterization of ophtalmic lenses and it can be produced very inexpensively.
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