Monday, May 22, 2006

21

Schematic Eye (Figure 33)


A schematic eye helps to conceptualize the optical properties of the human eye. The reduced schematic eye treats the eye as if it were a single refracting element consisting of an ideal spherical surface separating two media of refractive indices of 1.00 and 1.33. The reduced schematic eye assumes an eye power at the corneal surface of +60.00D (actual power of the Gullstrand’s schematic eye is +58.60D). The anterior focal point is approximately 17mm (1/-60 = -16.67mm in front of the cornea) and the eye is 22.6mm in length with the nodal point 5.6mm behind the cornea.

The nodal point is the point in the eye where light entering or leaving the eye and passing through the nodal point, is undeviated. This allows similar triangles to be used to determine the retinal image size of an object in space. For example, to determine the retinal image size of a Snellen letter (viewed at 6 meters), the following formula would be used: Retinal image height/Snellen letter height = 17mm/6000mm

Click on image to enlarge.
Schematic Eye


Question: Assume a disc diameter of 1.7 mm. What is the diameter of the blind spot when plotted on a tangent screen 2 meters from the eye?


Answer: Using similar triangles, 1.7/17 = X / 2,000. Rearranging X = 1.7/17 x 2,000 = 200 mm or 20 cm. In general, you can use the formula: object height/retinal image height = distance from the point of reference/17 mm. 17mm is the distance from the internal focal point of the eye to the retina.


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