Keratometry uses the anterior cornea as a mirror to measure corneal radius. It focuses the corneal reflection using objective lenses and splits the image using a half-field prism. By adjusting the prism position, it can provide a measure of the anterior corneal radius. The keratometer provides an estimate of total corneal power based on the measured anterior corneal radius and assuming a standard refractive index, though it cannot directly measure the posterior corneal radius or thickness. This estimated total corneal power is used in applications like estimating corneal astigmatism and intraocular lens power calculations.

3. OBJECTIVE h  ½ h C MIRE F CORNEA ½ h PRISM (P  ) IMAGE PLANE Fig 13.17, Page 13.18 Moving prism toward image plane decreases image displacement ( x ) Previously doubled images are no longer doubled (now overlap) What new corneal radius would this prism position “suit”? What happens if we move the prism?  x h  < h  P

5. Fig 13.22, Page 13.27 ILLUMINATED MIRE HORIZONTAL & VERTICAL PRISMS EYEPIECE OBSERVER APERTURE PLATE CORNEAL MIRE IMAGE P H P V OBJECTIVE LENS Schematic View of the B & L Optical System OBJ GRATICULE PLANE Two prisms means two deviated images

6. Topcon Keratometer What the Clinician Sees V 90 / H 180

7. B & L: Oriented to Measure r 90 and r 18 0 Question: If most corneas are aspheric, what is one drawback with a keratometer? Answer: only measuring radius at one location (annulus) on cornea; and it is NOT central radius OBJ h' 90 Corneal vertex h' 180

8. B & L: Oriented to Measure r 60 and r 15 0 h' 60 Corneal vertex h' 150

10. B & L: Oriented to Measure r 90 and r 18 0 Question: What does the above appearance indicate? Answer: anterior corneal astigmatism. What type? Against-the-rule OBJ h' 90 Corneal vertex h' 180

13. Basis of Corneal Power Estimate - Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D Page 13.23 n cornea 1.376

14. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only n cornea 1.376 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D

15. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D

16. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Keep true anterior corneal radius - this is what keratometry measures Want single surface cornea to give same +43.05 D as the Exact Eye cornea Using n  = 1.3315, the +7.7 mm Exact Eye anterior corneal radius yields correct total corneal power +43.05 D Why is new n  < 1.336? Only option is to change n aqueous