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Optical instruments in ophthalmology
Anatomy & Optics
Key Points
- •Slit lamp: Koehler illumination + Galilean telescope viewing; provides 6–40× magnification with stereoscopic, parfocal examination of the anterior segment
- •Direct ophthalmoscope: 15× magnification, ~5° field, erect virtual image, monocular - high magnification but very narrow field
- •Indirect ophthalmoscope: magnification = 60/D (D = condensing lens power); image is real, inverted, laterally reversed; higher lens power → lower magnification but wider field
- •Gonioscopy eliminates total internal reflection at the cornea-air interface (critical angle ~46°); indirect gonioscopy (Goldmann) views the opposite angle via a mirror
- •Goldmann tonometry uses the Imbert-Fick principle with a 3.06 mm applanation diameter where surface tension and corneal rigidity cancel; correct for CCT (~0.5 mmHg per 10 μm from 555 μm)
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