The slit lamp and tonometer

The slit lamp is a microscope with a thin, bright slit of light. With it, an ophthalmologist sees the eye in layers, as if sliced.

A patient with her chin on the rest of a slit lamp, the applanation tonometer beside her

It shows the front of the eye in fine detail, layer by layer.

With a hand-held lens it looks at the back of the eye too.

The tonometer measures eye pressure, the standard test for glaucoma.

How it works

You rest your chin and forehead on a support, so that your head is still. The doctor looks through two eyepieces, one for each of their eyes, so the view is three-dimensional. A slit of light, which can be widened, narrowed and swung to the side, lights a thin slice of the eye. That is how the lids, the white of the eye, the cornea, the iris and the lens are seen one layer at a time.

A patient's eye lit by the narrow beam of a slit lamp, seen close up — A slit of light across the cornea and lens.

What it shows

Cataract, and how dense it is.
Corneal ulcers, scars and swelling.
Inflammation inside the eye.
Dry eye, and the state of the tear film.
The drainage angle of the eye, with a special lens.
The back of the eye, with a hand-held lens.

The tonometer

An applanation tonometer is fitted to the slit lamp, as you can see in the picture. After a drop to numb the surface, its small probe touches the cornea very lightly, and the force needed to flatten a tiny area of it gives the pressure inside the eye. It is the standard test for glaucoma.

An examination room at Arihant Netralaya: a slit lamp and auto ref-keratometer on one table, and a chair unit

Slit lamp with applanation tonometer

Appasamy Associates, at Netralaya. An Indian-made slit lamp, on the same instrument table as an auto ref-keratometer.

Binocular slit lamp for a three-dimensional view.
Applanation tonometer fitted, for pressure in the same visit.
Auto ref-keratometer on the same table, for glasses power and corneal curvature.
Ophthalmic chair unit, so every test happens in one place.