Huvitz CRK-1, Руководство по обслуживанию, Страница: 33 / 88

32 Auto Ref/Keratometer CRK-1 ------------------------------------------------------------------------------------------------
Measurement principle of keratometry
When the ring light source of the keratometry is projected on to the cornea, it i
s reflected as a circle or an ellipse in the case of the eye with corneal astigmatis
m, 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 imag
e of the reflected light (mire image) from cornea be captured on TV camera and t
hen, read it’s coordinates, processor unit in this instrument will calculate the diamet
er 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 cap
tured image, the projection angle of ring light (
 ). The correlation is as follows. The
refore, 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 p
ower of cornea (D), and the refraction index (n). From this equation, strong and wea
k 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)