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Bausch and Lomb Keratometer

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Jaime Juarez

on 20 May 2015

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Transcript of Bausch and Lomb Keratometer

Bausch and Lomb Keratometer
Introduction
What is the Bausch and Lomb Keratometer?
What is its function?
What are the optical principles behind it?
Design of the B&L Keratometer
One position keratometer
Consists of: an in-built illumination system
Circular mire of fixed size
Objective lens
Aperture diaphragm
Variable distance doubling prism
Eyepiece lens
Function
1. Measure radii of curvature (mm)
2. Measure corneal astigmatism (in dioptres)
3. Directions of principle meridians
4. Detect presence of corneal distortion
Optical principles
1. Illumination system
In-built lamp with an internal mirror reflecting rays axially towards the patient's cornea through fixed size mires
Image by goodtextures: http://fav.me/d2he3r8
Fig 1
. Keratometer of Bausch and Lomb
Image sourced from http://www.bibonline.co.uk/

Fig 2.
Optical system of the Bausch and Lomb Keratometer
I
mage sourced from ‘Theory and Practice of Optics and Refraction’, by AK Khurana, p.162 (2)

Limitations
Only measures central 3mm of cornea (most spherical in shape)
Only measures anterior surface
Assumes the surface is a spherical convex mirror
Can only measure meridians orthogonally in a fixed position; but may measure irregular astigmatism if the position is changed.
2. Imaging system
The illuminated mires become the target object
The mires are circular with + symbols laterally, and - symbols superior and inferiorly
Fig 3.
Mires used in a Bausch and Lomb Keratometer

Image sourced from ‘Theory and Practice of Optics and Refraction’, by AK Khurana, p.163 (2)

3. Image formation
The first Purkinje image is used. It is a virtual, erect image, slightly posterior to the anterior surface of the cornea
Ray diagram construction:
1) A ray parallel to the principal axis, reflected off the surface from principal focus
2) Ray from the top of the object (O) travelling toward centre of curvature (C), reflected along the same path
3) Ray from top of the object (O) travelling toward F, reflected parallel to principle axis
3. Image formation
The transverse magnification (m) of the curved mirror is the ratio of the image size (h') to the object size (h)
Using similar triangles, the ratio h'/h is proportional to the ratio of the distances of the image (v) and object (u) from the anterior surface of the cornea
Magnification = I / O = h'/h = v/u
For practical purposes, the image (I) is located close to the focal point (F), which is midway between C and the surface of the cornea i.e bisecting the radius, thus:
v = r/2
The radius of curvature can be calculated by substituting into the above equation:
r = 2u.h’/h
This equation is known as the
approximate keratometer equation
(3)
4. Calculations
The dioptric power (D) can be calculated using (2) (4):
D = (n' - n) / r
A simplification of the keratometric power (K) of the cornea in dioptres is thus:
K = 337.5/r
Most keratometers approximate n' to be 1.3375 (may vary with instruments)
5. Aperture diaphragm and Scheiner disk
The aperture diaphragm consists of a disk with four circular holes arranged in a cross, situated just behind the objective lens (Fig 5)
Fig 5
. Aperture diaphragm with Scheiner disk
The Scheiner disk helps with focusing: the mires appear doubled when not in focus
6. Doubling prisms
Fig 6.
Doubling prisms in the calculation of image height
Imaged sourced from ‘Optometry: science, techniques and clinical management’, by Mark Rosenfield et al (3)
A base up prism focuses light from the left aperture, while a base out prism focuses light from the right aperture.
This produces a doubling of the images laterally and vertically, 90° from another.
Used to estimate image height and to overcome the nystagmoid movements of the eyes
When the symbols of the mires overlap, the displacement induced by the prism is equal to the height of the image (Fig 6) . (3)
Steps
(1) The patient is positioned correctly and given instructions
(2) The observer adjusts the instrument and focuses the central mires
(3) The principal meridians are located by aligning mires
(4) The mires are overlapped by adjusting the prism distances to find the corneal curvature
(5) The scale can then be read to determine the corneal curvature at each meridian
Image sourced from Catherine Care Group website: http://optometry.catherinecaregroup.com/keratometry/ (6)
References
1. Katz, M. Introduction to geometrical optics. s.l. : World Scientific, 1994.
2. Khurana, AK. Theory and practice of optics and refraction. s.l. : Elsevier India, 2008.
3. Mark Rosenfield, Nicola Logan. Optometry: science, techniques and clinical management. s.l. : Butterworth-Heinemann, 2009.
4. Zia Chaudhuri, Murugesan Vanathi. Postgraduate Ophthalmology. New Delhi : Jaypee Brothers Medical Publishers, 2012. Vol. 2.
5. T Swartz, M Wang et al. Corneal topography in the wavefront era: a guide for clinical application. Thorofare; NJ : Slack Inc, 2006.
6. Goh, Janice. Toric Contact Lenses: Keratometry. Optometry by Catherine Care Group. [Online] [Cited: 18 5 2015.] http://optometry.catherinecaregroup.com/keratometry/.

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