The slit lamp (biomicroscope) combines a binocular compound microscope with a slit illumination system
The two systems share a common focal plane and pivot about the same vertical axis (they are isocentric), so the slit stays in focus wherever the arm is swung

## The microscope

The objective forms a real, inverted, magnified image. The eyepiece acts as a loupe on that image
The final image of a compound microscope is inverted. The slit lamp and operating microscope use Porro prisms (or similar erecting prisms) to make it erect
Total magnification = objective × magnification changer × eyepiece. A Galilean magnification changer (a rotating drum of small telescopes, or a zoom) gives about ×6 to ×40
The two eyepieces converge slightly (about 10–15°), giving stereopsis; adjust each for the examiner's refraction and interpupillary distance

## Illumination

A bright source with condenser lenses projects an image of an adjustable slit onto the eye (Köhler-type illumination gives even brightness)
Filters: cobalt blue (fluorescein), red-free (vessels, nerve fibre layer), heat-absorbing and neutral density
The slit width, height and angle can be varied, and the beam can be decoupled from the microscope's focus for indirect methods

Technique	Method	Use
Diffuse	Broad, defocused beam, low magnification	General survey of the lids, conjunctiva and cornea
Direct focal: optic section	Narrow slit at an angle, focused on the tissue	Depth of corneal lesions, anterior chamber depth, lens layers
Direct focal: parallelepiped	Slit 1–2 mm wide	A three-dimensional block of cornea: scars, foreign bodies
Conical beam	A small spot of light across a dark pupil, high magnification	Cells and flare in the anterior chamber (the Tyndall effect)
Specular reflection	Microscope and beam at equal and opposite angles to the surface	Endothelial mosaic, guttata, tear film
Retroillumination	Light reflected from the iris or fundus behind the structure	Keratic precipitates, corneal vessels, iris transillumination defects, posterior capsule opacification
Sclerotic scatter	Beam on the limbus; light travels through the cornea by total internal reflection	Subtle corneal haze, oedema and opacities glow against the dark pupil
Indirect (proximal) illumination	Beam beside the area being viewed	Lesions seen in light scattered from the neighbouring tissue

## Fundus lenses

The slit lamp cannot focus on the fundus unaided, because the eye's +60D of power puts the retina's image at infinity. A lens is needed
Non-contact indirect lenses (+60D, +78D, +90D)

A high-power condensing lens, used like a small indirect ophthalmoscope (see Indirect Ophthalmoscope)
Image: real, inverted and laterally reversed, formed in front of the lens for the slit lamp to focus on
Magnification ≈ 60 ÷ lens power, times the slit lamp magnification: about ×1 for 60D, ×0.8 for 78D and ×0.7 for 90D
The 60D lens is best for detail of the disc and macula; the 90D gives a wider field and works through smaller pupils

Hruby lens: a powerful plano-concave lens (about −58.6D) held just in front of the cornea

It neutralises the eye's power, forming a virtual, erect, diminished image anterior to the retina, within the slit lamp's focusing range
The field is small. It is used for the posterior pole and vitreous, and has largely been replaced by the 90D lens

A contact lens neutralises the cornea's power. Its central lens gives a virtual, erect view of the posterior pole with excellent stereopsis
Goldmann three-mirror: the central lens (posterior pole), a 73° mirror (from the posterior pole to the equator), a 66° mirror (equator to the periphery) and a 59° mirror (the far periphery, ora serrata and angle). The smaller the mirror's angle, the more peripheral the view
A mirror shows the part of the eye opposite to it: the mirror at 12 o'clock shows the inferior fundus or angle. The image is upside down but not reversed side to side
Wide-field indirect contact lenses (e.g. panfundoscope types) give a real, inverted image with a large field, for laser treatment

## Gonioscopy optics

Light from the angle meets the tear–air interface at more than the critical angle (about 46° for the cornea, n = 1.376), so it is totally internally reflected back into the eye (see Refraction at Surfaces)
A goniolens of similar refractive index, coupled with tears or a viscous solution, removes this interface. Light then enters the lens and is either viewed directly (e.g. Koeppe) or reflected by a mirror (e.g. Goldmann, Zeiss) (see Gonioscopy)
The same total internal reflection stops the far peripheral retina being seen without indentation or a mirrored lens

The van Herick test estimates the peripheral anterior chamber depth with an optic section at 60° to the limbus, compared with the corneal thickness
Applanation tonometry and pachymetry use slit lamp attachments (see Tonometry)
Laser delivery systems are mounted on slit lamps, often with a contact lens to stabilise the eye and focus the beam