How to Use a Slit Lamp: Setup, Focusing and Illumination Controls Step by Step

How to use a slit lamp comes down to a fixed order of operations. Power the unit at a moderate brightness. Focus each eyepiece for your own eyes with the focusing rod. Seat the patient so the outer canthus lines up with the mark on the headrest. Then work the joystick, the slit controls, the illumination arm and the magnification changer in a consistent sequence. This guide is for the people who run the instrument every day: optometrists, ophthalmic technicians, new associates, and practice owners training staff on a new unit. It covers the operator procedure only. It is about the instrument, not about reading what you see through it.

The steps are drawn from published instrument manuals and teaching sources, cited at the end. Model-specific numbers come from the spec tables of the Ezer slit lamps we carry, so you can match each control to the hardware on your own table.

Before you start: know the four control groups

EyeWiki (American Academy of Ophthalmology) describes the slit lamp as "a stereoscopic biomicroscope that emits a focused beam of light with variable height, width, and angle." Every step below touches one of four groups of controls:

• Observation system: the binocular microscope, eyepieces (diopter rings and interpupillary distance) and the magnification changer.
• Illumination system: the lamp housing, brightness control, slit width, slit height and aperture, beam rotation, the filter wheel, and the illumination arm that swings around the microscope.
• Base and joystick: coarse movement of the whole instrument, fine focusing by tilting the joystick, height by rotating it, and a base lock.
• Patient frame: the chin rest with its height knob, the forehead band, and the canthus marker on the upright.

The design point that makes the procedure work is the shared pivot. Wikipedia's history of the instrument notes that "the common swivel axis for microscope and illumination system was connected to the cross-slide stage." A Zeiss patent (US5196874) puts it more precisely: the plane of the slit image and the microscope's focal plane both contain the extension line of that rotational axis. In practice, once the beam is sharp where you are looking, you can swing the illumination arm to any angle and the beam stays in focus at the same point. Most of the steps below depend on it.

If you are still choosing between optical designs, read our explainer on slit lamp microscope optics and magnification first. This article assumes the instrument is already on your table.

Step 1: Power up at a moderate brightness

1. Unlock the base. EyeWiki's sequence begins: "Unlock the carriage and turn the power switch on."
2. Set the brightness control to the middle. A slit lamp instruction manual published with a public tender says: "Switch on the main power switch and set the brightness control switch at middle position. Turn the slit width control knob to make the slit width to be 2~3mm." The MediWorks S360 manual gives the same instruction ("turn the brightness adjustment knob to the middle").
3. Don't run at maximum for long. Both of those manuals warn: "Avoid working continuously at high setting, as the service life of the bulb will be shortened." On our Ezer ESL-2600 and ESL-7800, brightness is a two-position High / Normal switch. Normal is the everyday setting.

Bulb type matters for this habit. The ESL-2600 spec table lists a 6V 20W halogen bulb, while LED units such as the Ezer Emerald-26 (offered in LED or halogen) have different service intervals. Bulb care, part numbers and reseating are covered in our slit lamp parts, bulbs and maintenance guide.

Step 2: Focus the eyepieces for your own eyes

This step is easy to skip, and an unfocused eyepiece makes the whole session harder. The microscope has to be parfocal with the slit beam for your eyes, and the focusing rod (also called the test rod) is how you set that.

1. Insert the focusing rod. The tender manual says to "Insert it into the main shaft hole with the flat surface facing the objective lens the direction of the operator." The rod sits on the pivot, so its flat face marks the plane where the beam and microscope focus coincide.
2. Narrow the slit and project it onto the rod.
3. Wind one eyepiece fully out, then bring it in. In that manual's words: "Rotate the diopter adjustment ring counter clockwise down to the end," then "Rotate the ring clockwise until a sharp slit image appears." Approach focus from one direction only, as the manual's sequence does.
4. Repeat for the other eyepiece, then "Record the diopter value on each eyepiece for future reference." Write your setting on a label at the instrument if several clinicians share it.
5. Remove the rod. The manual's caution: "After adjusting, remember to take out the rod."

The University of Iowa's EyeRounds teaching module adds: "The oculars should be set at zero unless you are trying to correct your refractive error." If you examine in your glasses or contact lenses, zero is usually your starting point. Use the rod only if the beam still isn't crisp.

Adjustment ranges differ by model. The ESL-2600 spec table lists diopter adjustment of ±5D with its 10x eyepiece. The ESL-7800 lists −5D to +3D. If your uncorrected refraction is outside your instrument's range, examine in your correction.

Step 3: Set the interpupillary distance

With the rod still in place (or on any fixed target), push the two eyepiece tubes apart or together until you see one fused image. The tender manual says to adjust "until both eyes could see the same image on the focusing test rod." The MediWorks manual adds a check worth teaching: "ensure that the two eyepieces are at the same height."

The published ranges: ESL-2600 55–82 mm, ESL-7800 55–75 mm. Note your PD setting next to your diopter values so you can restore both in seconds when you sit down after a colleague.

Step 4: Seat the patient and align the canthus

1. Set the table height first so the patient can lean in without straining. The OPTO 314 lecture notes from King Saud University's optometry program say to "Adjust the height of the slit lamp table to the comfort position for patient and examiner." See our exam chair and stand guide for lane layouts.
2. Chin in the cup, forehead against the band. EyeWiki calls the forehead-band step "crucial to ensure that the eye is within the focal range of the slit lamp." If the patient's forehead drifts back, you will run out of focusing travel.
3. Raise or lower the chin rest until the canthus meets the marker. EyeWiki: "Turn the chin height adjustment knob until the lateral canthus is aligned with the canthal indicator, usually a black demarcating line on the patient frame." Review of Optometry gives the same step: "Position the patient's chin and forehead so that the canthus aligns with the eye mark on the post."

Getting the canthus on the mark puts the patient's eye in the middle of the joystick's vertical range. That leaves room to move up and down without re-seating. Both the S360 and the tender-manual unit publish 80 mm of chin-rest travel, which is why the knob, not the joystick, does the coarse height work.

Step 5: Use the joystick for coarse, fine and vertical movement

The joystick does three jobs:

• Coarse positioning: "Keep the joystick upright and move the base to make the slit lamp move horizontally to aim at the object roughly" (tender manual). Do this while watching from the side, not through the eyepieces.
• Fine focus: "Tilt the joystick to make the slit lamp move slightly on the horizontal surface while watching through the eyepieces" (same source). EyeWiki adds: "maintain the joystick in an upright position to preserve the full range of fine back-and-forth control." Center the base under your hand before you start fine work.
• Height: on most units, rotating the joystick raises or lowers the optics. EyeWiki and the tender manual both say clockwise raises and counter-clockwise lowers. Confirm on your own unit before you train staff, though. The MediWorks S360 manual's text and its own figure label disagree on the direction.

Our ESL-2600 and ESL-7800 spec tables list base travel of 90 mm forward and back, 100 mm left and right, 15 mm of fine movement and 30 mm vertically. Once you are aligned for photography or a tonometry reading, the base can be locked so it cannot slide.

Step 6: Shape the beam: width, height, rotation and angle

This is where you set up the view you want. Learn each control's range on your instrument:

For comparison, the Haag-Streit BQ 900 brochure lists slit width "0 – 14 mm continuous" and a swivel of "Horizontal ± 90°." Wide-beam ranges vary by platform, so check your own table before following a technique recipe written for a different instrument.

The illumination arm angle is the main variable. EyeWiki: "The illumination arm sits on a swivel such that the beam can be directed from any direction in the horizontal plane 90° nasal and temporal to the microscope focal point." Swing the arm, not the microscope, and the beam stays focused at the pivot point.

Decentration breaks that link on purpose. EyeWiki warns that it "results in decentering, a loss of the parfocality between the illumination system and the viewing system." The tender manual reserves it mainly for indirect and retro-illumination work. Re-center the beam afterwards, or the next operator starts with a beam that is no longer focused at the microscope's focal point.

Step 7: Change magnification

Slit lamps change magnification with a rotating drum, a lever, or continuous zoom. EyeWiki notes that "most slit lamps offer stepwise magnification between 6X and 40X." On our models:

• ESL-7800 (Galilean, five-step drum): 6x (Ø33 mm field), 10x (Ø22.5 mm), 16x (Ø14 mm), 25x (Ø8.8 mm) and 40x (Ø5.5 mm). You select by twisting a single knob, with no eyepiece change.
• ESL-2600 (cross-angle converging, two-step lever): 10x (Ø18 mm) and 16x (Ø14.5 mm) with the 10x eyepiece.
• Emerald-26: five objectives (6x, 10x, 16x, 25x, 40x) by drum rotation.

Work from low to high. Scan at low magnification with its wide field, find the area of interest, then step up. Each step narrows the field, as the figures above show. The optics explainer covers why the same instrument can be quoted with different magnification numbers.

Step 8: Choose the filter

The ESL-2600 and ESL-7800 each have four built-in illumination filters on a selection lever: heat absorption, red-free, cobalt blue and grey. Their spec text describes the heat-absorption filter as one "that aids in patient comfort during lengthy exams" and the cobalt blue filter as "for use in fluorescein assessment." The tender manual agrees: "Usually the heat absorption filter is used for patient comfort."

Two habits are worth building:

• Return to the default filter when you finish. The MediWorks manual says "Other filters should be placed in the position of the heat-absorbing filter after use." That way the next operator doesn't start a session in the wrong filter.
• Know whether you have a barrier filter. The BQ 900 brochure describes its optional yellow filter as a "yellow barrier filter [that] provides additional contrast to fluorescein images." It sits on the observation side, not the illumination side. Check your configuration before you promise this capability to a doctor joining the practice.

Step 9: Set up the standard illumination techniques

The slit lamp selection criteria guide explains what each technique is for. The table below covers only how to set the instrument for each one. It uses the settings published in Review of Optometry's "Sharpen Your Slit Lamp Technique" unless another source is named.

For the diffuse setup, the tender manual specifies "about 30 degrees" between the microscope and the illumination unit, with the slit "at full aperture." For sclerotic scatter, Review of Optometry describes a narrow beam, while Wikipedia describes "a wide light beam" at a very low angle of incidence. Either way, it is the limbus placement and the low angle that define the technique.

Step 10: Shut down, clean, cover

1. Return the controls to neutral: heat-absorption filter, moderate brightness, beam re-centered, joystick upright, base locked.
2. Power off. The tender manual says: "After examining, turn off the main power."
3. Clean patient-contact surfaces between every patient. EyeWiki says to make sure "that the forehead band, chin rest, and handles have been thoroughly cleaned with alcohol." The tender manual specifies a soft cloth with "soluble detergent or water" followed by medicinal alcohol for the plastic parts.
4. Clean the optics only as the manual allows. The tender manual says to use a brush first, then soft cotton dipped in absolute alcohol. The MediWorks manual suggests cleaning the eyepiece, lens and mirror areas "every 2 months."
5. Cover it once it has cooled. In the tender manual's words: "Cover the instrument with the dust-proof cover after the lamp cap has been cool." On halogen units, never touch the housing straight after a long session. The same manual warns, "The bulb is very hot."

A one-page checklist for training new staff

• Base unlocked, brightness at Normal or middle, slit at 2–3 mm.
• Focusing rod in; each eyepiece wound out, then in to sharp; values recorded; rod out.
• PD set to one fused image; eyepieces level.
• Table height comfortable; chin in the cup; forehead on the band; canthus on the mark.
• Coarse position from the side; fine focus through the eyepieces with the joystick upright.
• Low magnification first, then step up.
• Beam re-centered and default filter selected before you leave.
• Power off, surfaces cleaned, dust cover on once cool.

Choosing a slit lamp your team can learn quickly

A few hardware choices make training faster:

• A labeled slit-width scale (the ESL-7800 has "a helpful scale on the knob").
• A brightness control with a clear everyday position.
• A five-step drum if your doctors use the higher magnifications.
• Fixed inclination stops if you fit contact lenses.

Our conventional tower slit lamps (browse the collection) start with the two-step ESL-2600 and go up to the five-step ESL-7800. The Made-in-Japan Emerald line includes the LED or halogen Emerald-26. For a ranked comparison, see our best slit lamps buying guide.

Every Ezer unit is backed by US Ophthalmic's in-house technical and spare-parts department and by US warranty and service. Replacement bulbs, filters and service come from the same team that supplies the instrument. Request a quote and tell us which instrument you use today, and we'll help you compare its controls with the model you're considering.

Frequently asked questions

How do you focus a slit lamp?

First set each eyepiece for your own eyes. Insert the focusing rod at the pivot, wind each eyepiece's diopter ring fully counter-clockwise, then turn it clockwise until the slit image on the rod is sharp, and remove the rod. During the session, focus by tilting the joystick toward or away from the patient while you look through the eyepieces.

Where should the patient's eye be on a slit lamp?

Chin in the chin cup and forehead against the band. Then adjust the chin rest height until the lateral canthus lines up with the black canthus mark on the headrest upright. That centers the eye in the instrument's vertical travel.

Which way do you turn the slit lamp joystick to go up?

EyeWiki and a published instrument manual both say turning clockwise raises the optics and counter-clockwise lowers them. At least one manufacturer's manual is inconsistent on this, so confirm on your own unit.

What brightness should I use?

Start at the middle or Normal setting. Manuals warn that working continuously at the high setting shortens bulb life. Go to maximum only briefly, for techniques such as the conical beam.

What are the filters on a slit lamp for?

The common built-in set is heat absorption (for patient comfort and routine use), red-free, cobalt blue (for fluorescein viewing) and a grey or neutral-density filter. Some instruments add a yellow barrier filter on the observation side.

How often should I clean a slit lamp?

Clean the chin rest, forehead band and handles with alcohol between patients. Clean the optics only with the method your manual specifies: one published manual suggests every two months for the eyepiece, lens and mirror areas. Put the dust cover on once the lamp has cooled at the end of the day.

What magnification should I start at?

Start at the lowest step for the widest field of view, then increase. On the Ezer ESL-7800, the five steps run from 6x (a Ø33 mm field) to 40x (Ø5.5 mm).

Sources

• EyeWiki (AAO), "Slit Lamp Examination": eyewiki.org/Slit_Lamp_Examination
• University of Iowa EyeRounds, "Slit Lamp Examination": eyerounds.org/atlas-video/slit-lamp-examination.htm
• Review of Optometry, "Sharpen Your Slit Lamp Technique": reviewofoptometry.com
• King Saud University, OPTO 314 lecture notes (slit lamp biomicroscopy)
• MediWorks S360 / S360S / S390L user manual (mediworks.biz)
• Slit lamp instruction manual filed in a public tender (storage.mtender.gov.md)
• Haag-Streit BQ 900 brochure, 11th edition, 2023
• Wikipedia, "Slit lamp"; US Patent 5,196,874 (Carl Zeiss)
• US Ophthalmic product specification tables: ESL-2600, ESL-7800, ESL-Emerald-26