Huvitz HRK-9000A, Руководство по обслуживанию

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---------------------------------------------------------------------------------------------- Auto Ref/Keratometer HRK-9000A 25
3-5-2. Measurement principle of keratometry
When the ring light source of the keratometry is projected on to the cornea, it is
reflected as a circle or an ellipse in the case of the eye with corneal astigmatism, the size
and the shape of reflected light are different according to the radius of curvature of the
cornea because the cornea is not a sphere. By making the image of the reflected light
(mire image) from cornea be captured on TV camera and then, read it
’s coordinates,
processor unit in this instrument will calculate the diameter of major (2a) and minor axis
(2b), and then, the rotational angle (

, axis on the minor meridian). Furthermore, there is
a correlation between the radius of cornea (R), the height of the projected ring image (H)
which can be obtained from the captured image, the projection angle of ring light (

). The
correlation is as follows. Therefore, small and large radius of curvature of cornea can be
calculated with respect to the minor and major axis.
And there is the following relation between the radius of cornea (R), the refractive power of
cornea (D), and the refration index (n). From this equation, strong and weak refractive
powers of cornea (D) are calculated in regard to large and small radius of curvature (R).
D
＝
1000 (n-1)
R(mm)
R
＝
H
sin(

/2)
The following 3 values can be selected
for corneal equivalent refractive index (n)
n
＝
1.3375 , 1.336 , 1.332