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Ophthalmology Instruments

Source: Kanski's Clinical Ophthalmology: A Systematic Approach, 10th Edition

1. Slit Lamp (Biomicroscope)

The slit lamp is the cornerstone instrument of ophthalmic examination. It combines a binocular stereoscopic microscope with a variable-intensity light source that can be shaped into a thin "slit" beam to provide a cross-sectional view of the eye.

Illumination techniques used with the slit lamp:

Direct illumination - a narrow oblique slit beam creates an optical cross-section of the cornea, allowing depth estimation of lesions
Scleral scatter - the beam is decentered to the limbus; light undergoes total internal reflection within the cornea, illuminating any stromal haze or infiltrates
Retroillumination - reflected light from the iris or fundus illuminates the cornea from behind, detecting fine epithelial/endothelial changes
Specular reflection - shows endothelial abnormalities such as guttata and reduced cell density

Fig. 1.27 - Slit lamp biomicroscopy techniques: Direct illumination, Scleral scatter, and Retroillumination

Fig. 1.27 Techniques of slit lamp biomicroscopy of the anterior segment

2. Portable (Hand-held) Slit Lamp

The Keeler PSL Classic is a hand-held slit lamp that allows high-quality anterior segment examination in patients who cannot sit at a standard slit lamp - useful for bedridden patients, children, and outreach settings.

Fig. 1.28A - Portable slit lamp in use on a patient

Fig. 1.28A Portable slit lamp technique (Keeler PSL Classic)

Fig. 1.28B - Keeler PSL Classic portable slit lamp instrument

Fig. 1.28B Keeler PSL Classic instrument

3. Direct Ophthalmoscope

A hand-held instrument providing a magnified (x15), monocular view of the fundus. Key features:

Can be used at the bedside
Retroillumination with +15 D lens detects lens/vitreous opacities
Cobalt blue filter detects corneal abrasions after fluorescein instillation
Limitation: No stereopsis and small field of view

4. Binocular Indirect Ophthalmoscope (BIO)

The head-mounted BIO provides a wide-field, stereoscopic, inverted view of the fundus through a hand-held condensing lens. It offers superior visualization through media opacities and allows scleral indentation.

Fig. 1.37 - Head-mounted binocular indirect ophthalmoscope in use

Fig. 1.37 Head-mounted binocular indirect ophthalmoscope (BIO)

Condensing lens powers used with BIO:

Lens Power	Magnification	Field of View	Main Use
20 D	×3	~45°	Standard fundus examination
28 D	×2.27	53°	Small pupils
40 D	×1.5	65°	Children / broad scan

5. Goldmann Three-Mirror Contact Lens

A contact lens placed on the anaesthetized cornea using coupling fluid. It contains three angled mirrors that allow different zones of the fundus to be examined at the slit lamp:

Large trapezoid mirror (75°) - equatorial retina
Rectangular mirror (67°) - peripheral retina
Dome-shaped (smallest) mirror (59°) - extreme periphery, pars plana, and gonioscopy

6. Goldmann Applanation Tonometer

The gold standard for measuring intraocular pressure (IOP). Based on the Imbert-Fick principle: IOP = Force / Area of applanation. The tonometer head contains a double prism and is mounted on the slit lamp. Corneal rigidity and capillary attraction cancel out when the flattened diameter = 3.06 mm.

Technique: Topical anaesthetic + fluorescein dye is instilled; the tonometer prism is gently applied to the cornea under cobalt blue illumination.

Fig. 1.43A - Physical principle of Goldmann tonometry (applanation force diagram)

Fig. 1.43A Physical principles of Goldmann applanation tonometry

Fig. 1.43B - Goldmann tonometer (double-prism device mounted on slit lamp)

Fig. 1.43B The Goldmann tonometer instrument

7. Other Tonometers

Fig. 1.45 - (A) Pulsair pneumotonometer; (B) Perkins tonometer; (C) Tono-Pen; (D) iCare IC100

Fig. 1.45 Portable tonometers

Tonometer	Principle	Key Feature
Non-contact (Pulsair)	Air-jet applanation	No anaesthesia needed; community screening
Perkins Applanation	Goldmann prism + portable light source	Hand-held; use in anaesthetized/bedridden patients
Tono-Pen AVIA	Electronic indentation/applanation	Useful for distorted/oedematous corneas; portable
iCare (Rebound)	Deceleration of a 1.8 mm probe ball on corneal contact	No anaesthesia; suitable for self-monitoring at home
PASCAL (DCT)	Dynamic contour (solid-state sensor)	Measures IOP more independently of corneal properties
Schiotz	Indentation (plunger of known weight)	Seldom used today; historical importance

8. Gonioscopy Lenses

Used to visualize the anterior chamber angle (which cannot be seen directly due to total internal reflection at the tear film-air interface). Goniolenses have a refractive index similar to the cornea, eliminating this reflection.

Two types:

Goldmann / Haag-Streit Three-Mirror Lens (Indirect gonioscopy)

Requires coupling fluid (methylcellulose)
Provides an inverted, magnified view of the opposite angle
Suitable for laser trabeculoplasty

Zeiss / Posner / Sussman Four-Mirror Gonioprism (Indirect, indentation gonioscopy)

Contact surface is flatter than the cornea - no coupling fluid needed
Four mirrors allow 360° angle assessment without rotating the lens
Allows indentation (dynamic) gonioscopy to distinguish appositional from synechial closure

Fig. 1.49A Zeiss four-mirror goniolens

9. Summary Table

Instrument	Function	Key Principle
Slit lamp	Anterior + posterior segment examination	Variable-beam biomicroscopy
Portable slit lamp	Bedside/field anterior segment exam	Hand-held slit beam
Direct ophthalmoscope	Fundus (monocular, magnified)	Retroillumination, x15 magnification
Binocular indirect ophthalmoscope	Wide-field fundus exam, scleral indentation	Condensing lens, inverted image
Goldmann three-mirror lens	Peripheral fundus + angle (slit lamp)	Three mirrors at different angles
Goldmann applanation tonometer	IOP measurement (gold standard)	Imbert-Fick; flattening diameter 3.06 mm
Non-contact tonometer	IOP screening	Air-jet applanation
Perkins tonometer	IOP in non-seated patients	Portable Goldmann prism
Tono-Pen	IOP in irregular corneas	Electronic indentation
iCare	IOP; self-monitoring	Rebound (probe deceleration)
Zeiss goniolens	Angle examination + indentation	Four-mirror; no coupling fluid
Goldmann goniolens	Angle + peripheral fundus	Three-mirror; coupling fluid required

All figures and clinical content sourced from Kanski's Clinical Ophthalmology: A Systematic Approach, 10th Edition, the standard ophthalmic reference text.

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