The Placido disk is acknowledged to be one of the standard units of the modern ophthalmologic clinic. Every student of the eye soon becomes acquainted with this unique device, if not from practical use at least from the pages of his textbook. One can safely state, however, that most clinics use it for a very limited scope of their activities.
Certain European investigators, notably, Amsler,1 of Lucerne, Hartinger,2 of Jena, and Fischer,3 of Leipzig, have felt the need of improving on the Placido disk ; so there are electrically lighted disks, which require a camera for photographing the image reflected from the cornea. The principle used in the designing of these photokeratoscopes is found in most ophthalmoscopic instruments ; namely, precision is effected by contrasting light with a dark background. These European instruments have proved to be an added refinement in the study of the irregular cornea and
Howard WH. A NEW KERATOSCOPE. Arch Ophthalmol. 1936;15(4):709–711. doi:10.1001/archopht.1936.00840160133011
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