ZABBYS KERATOMETER JAVAL SCHIOTZ TYPE Z-KERATO-JS:

The Javal-Schiotz keratometer is a two position installment which uses a fixed image and doubling size and
adjustable object size to determine the radius of curvature of the reflective surface. It uses two self illuminated mires
(the object), one a red square, the other a green staircase design, which are held on a circumferential track in order to
maintain a fixed distance from the eye. The object size is adjusted by maneuvering the mires along this track,
changing the distance between them. The reflected image is doubled through a Wollaston prism, which then allows
either side of the doubled image to be aligned, and any eye movement to cancel out as both images move with the
same magnitude and direction, the relative separation remaining constant. A Wollaston prism uses the polarizing
property of light in order to split a single image into two separate, visually identical but oppositely polarized images.
Once the mires are focused, the only variable remaining is object size, which is calibrated to a measurement of
reflective surface radius (and sometimes dioptric power using an estimation of refractive index). This gives the
curvature of the meridian along the path of the circumferential arms, the axis of which can be read from a scale
around which the arms rotate. The axis can be manipulated to any axis, giving a distinct advantage over a single
position keratometer in cases of irregular astigmatism.

In order to get repeatable, accurate measurements, it is important that the instrument stays focused. It uses the
Scheiner principle, common in autofocus devices, in which the converging reflected rays coming towards the
eyepiece are viewed through (at least) two separate symmetrical apertures. As the rays passing through each aperture
will have the same vergence, they should, meet at the same point. By adjusting the distance between the object and
the reflective surface, the vergence of the rays can be altered until a crisp focus is obtained, correlating to the fixed
focal point of the telescopic eyepiece.