Ophthalmic instruments and appliances- Discuss the Principle, Optics, Parts and Uses of Auto-refractometer, Tonometer and Direct ophthalmoscope

1. AR, TONOMETER & DIRECT OPHTHALMOSCOPE
Ms. Khushi Kansal
Assistant Professor
SWAMI VIVEKANAND SUBHARTI UNIVERSITY

2. Auto-refractometer
Auto Refractometer is an instrument by which we can asses spherical, cylinder and
sphero cylinder power easily. In this instrument, Near infrared Rays (NiR) is used to
determined Refractive Error.
NiR is used due to more reflection from the fundus is happen compare to normal
Radiation, and another reason is in presence of Nir, patient does not feel photophobia,
pupil contriction and accommodation is unaffected.

3. The Scheiner principle
 It consists of opaque disc in which two apertures are present and these aperture are
equidistant from the centre of the disc.
 In case of Emmetropia, NiR are coming from the near object, that will be passing
through these two holes and are focused as a point on the retina, and accommodation
is in rest condition.
 In case of a Myopia, whatever the NiR are coming from the near object, that will be
passing through these two holes and the beam would illuminate two areas of fundus,
and it is focused in front of the retina and accommodation is in rest condition.
 In case of Hypermetropia, whatever the NiR are coming from the near object, that will
be passing through these two holes and the beam would illuminate two areas of
fundus, and it is focused behind the retina and accommodation is in rest condition.

5.  By adjusting the position of the object (performed optically by the
autorefractometer) until one focus of light is seen by the patient, the far point of
the patient’s eye and the refractive error can be determined.

7. Tonometer
 Tonometer is a tool used to measure the pressure of the eye.
 Tonometry is a procedure performed to determine the intra-ocular pressure
(IOP).
 IOP: is a medical term for the pressure of the fluid (Aqueous humor) inside
the eye.
 IOP is an important aspect in the evaluation of patients at risk from
glaucoma (high eye pressure alone does not cause glaucoma. However, it is a
significant risk factor.
 Eye pressure is measured in millimeters of mercury (mmHg).
 Normal eye pressure ranges from 12-22 mmHg.

9. Manometry
 It is measured by inserting a cannula, directly into the anterior chamber which is
connected with a manometer.

10. Digital tonometry
 Response of eyeball to the pressure applied by the pulp of finger.
 Method: patient look down, Rest both hands on the patient’s forehead
and alternately apply just enough pressure on the globe (above the
superior plate) to indent it slightly with 1 index finger while feeling the
compliance with the other.
 If IOP is raised- fluctuation produced is feeble or absent and the eyeball
feels firm to hard.
 If IOP is low- eye feels soft like a partially filled balloon.

11. Instrumental tonometry
 Contact tonometer
 Indentation tonometer : Schiotz
 Applanation tonometer: Goldmann, Parkin’s, Tonopen
 Non-contact tonometer

12. Indentation tonometer
 Schiotz tonometer:
 Free floating barrel with a footplate of 10.1mm.
 Principle: based on the principle, that a plunger will indent a soft eye more than a
hard eye.

13. Procedure

14.  Advantage:
 Portable
 Easy to use
 Reasonably priced
 No need of slit lamp or power supply
 Disadvantage:
 Corneal abrasion
 Done only in supine position

15. Goldmann applanation tonometry
 Based on the principle of Imbert Fick Law.
 it is stated that the pressure inside a sphere (P) filled with liquid is equal to force (F) required to
flatten a given area (A).
 P= F/A
 Standard area of cornea of 3.06mm diameter has been flattened.

16. Procedure
 Patient counseling
 Dry, clean tonometer probe should be inserted
 Slit lamp should be adjusted and the patient comfortably positioned.
 Topical anesthetic
 Place a fluorescein strip in the lower fornix
 Room light should be dimmed.
 Illumination system should be at approximately 60 degree to the lateral side of the eye
to be measured.
 Cornea and biprism is illuminated by cobalt blue light from slit lamp
 Biprism is then advanced until it just touches the apex of cornea.
 At this point 2 semi circles are viewed.

18. Error in measurement
 patient related:
 Thin cornea- produces underestimate
 Thick cornea- gives overestimate
 Astigmatism
 Irregular cornea
 Squeezing of eyelids
 Technical:
 Out of calibration
 Repeated tonometry

19.  Advantages:
 Highly accurate
 Do not required supine position
 Disadvantages:
 Not portable
 Costly
 Reading error if scars on cornea

20. Tono-pen
 It is computerized pocket tonometer
 It convert IOP into electric waves
 Fully portable
 Held at any position
 Operated by index finger
 Reading displayed as a digital located on the handle
 Are not affected by corneal astigmatism

21. Non-contact tonometer
 Air puff tonometer: in this central part of the cornea is flattened by a
jet of air.
 Only for screening purpose
 An optical system measures the time that it takes for the air puff to
flatten the cornea.
 Advantage: the instrument is beneficial in mass glaucoma screenings
because it does not required topical anesthetic and, with proper use,
there is no risk of injuring the cornea.

22. Direct Ophthalmoscope
 It is a hand held instrument used to examine fundus of the eye.
 It is used monocularly.
 The modern direct ophthalmoscope works on the basic optical principle of glass
plate ophthalmoscope introduced by von Helmholtz.
 A convergent beam of light is reflected into the patient’s pupil. The emergent rays
from any point on the patient’s fundus reach the observer’s retina through thew
viewing hole in the ophthalmoscope. The emergent rays from the patient’s eye are
parallel and brought to focus on the retina of emmetropic observer when
accommodation is relaxed.

24. Direct Ophthalmoscope
 It consist of two systems:
 Illuminating system:
 Tungsten bulb
 Condensing lens
 A reflector /prism
 Apertures and filtres
 Viewing system:
 Sight hole
 Focusing system

27. Specifications
Image Virtual, erect
Field of view 10 degree
Magnification 15X
Working
distance
1-2cm

28. USES
 Examine Optic Disc: shape, color, cup disc ratio, clarity of margins.
 Macula and foveal reflex: the fovea is located approximately 3mm temporal. The
fovea act like a concave mirror & gathers the light from the ophthalmoscope,
presents as a sharp reflex called foveal reflex.
 Vessels : examine artery to vein, branching of vessels in all four quadrants.