Lasers in Ophthalmology Made Easy: Time, Colour and Use
Last updated: October 2026
Ophthalmic lasers seem like a zoo of names and wavelengths. Two questions sort them all: how long and how hard does the laser hit (which decides what it does to tissue) and which colour it is (which decides what absorbs it).
Three rules
1. Time and intensity decide the effectThe same energy delivered slowly heats (photocoagulation); delivered in nanoseconds or femtoseconds it tears tissue with plasma (photodisruption). Ultraviolet breaks molecular bonds (photoablation); low-power light plus a drug triggers chemistry (photochemical).
2. The colour picks the targetMelanin (RPE, choroid, trabecular pigment) absorbs green to near-infrared. Haemoglobin absorbs green and yellow, not red. Xanthophyll at the macula absorbs blue, so blue light is avoided there. Water absorbs infrared; corneal protein absorbs far ultraviolet (193 nm).
3. The name is the lasing mediumArgon and krypton are gases, Nd:YAG is a crystal (1064 nm; halved to 532 nm by frequency doubling), diode lasers are semiconductors (810 nm), excimer is argon fluoride (193 nm). The femtosecond laser is a solid-state laser near 1053 nm.
The lasers you will meet
| Laser | Wavelength | Mechanism | Main uses |
|---|---|---|---|
| Argon (blue-green / green) | 488 / 514 nm | Photocoagulation | PRP, focal laser, ALT, iridoplasty (largely replaced by 532 nm) |
| Frequency-doubled Nd:YAG | 532 nm | Photocoagulation; Q-switched for SLT | PRP, focal and grid, retinopexy; SLT |
| Yellow (dye or solid-state) | 577 nm | Photocoagulation | Macular work: haemoglobin absorbs it well, xanthophyll hardly at all |
| Krypton red | 647 nm | Photocoagulation | Through cataract or vitreous haemorrhage; less absorbed by blood |
| Diode | 810 nm | Photocoagulation (deep) | Transscleral cyclophotocoagulation, ROP, micropulse, TTT |
| Nd:YAG (Q-switched) | 1064 nm | Photodisruption | Posterior capsulotomy, peripheral iridotomy, vitreolysis |
| Femtosecond | ≈1053 nm | Photodisruption (tiny bubbles) | LASIK flaps, SMILE, laser-assisted cataract surgery, keratoplasty cuts |
| Excimer (ArF) | 193 nm | Photoablation | PRK, LASIK, PTK |
| Diode for PDT | 689 nm | Photochemical (verteporfin) | Polypoidal choroidal vasculopathy, chronic CSCR |
| UVA (not a laser) | 365–370 nm | Photochemical (riboflavin) | Corneal cross-linking |
Memory hook: Cook, Cut, Carve, Chemistry: coagulation (argon, 532, diode), cutting by plasma (Nd:YAG, femto), carving by UV (excimer), chemistry (PDT, cross-linking).
Common procedures, simply
| Procedure | Laser | Key settings and points |
|---|---|---|
| Panretinal photocoagulation | 532 nm or 577 nm (or pattern lasers) | Moderate grey-white burns one burn-width apart, sparing the macula and disc; destroys ischaemic retina so VEGF falls. Effective spot size depends on the contact lens magnification |
| Focal / grid macular laser | 532 or 577 nm, or subthreshold micropulse | Light burns to leaking microaneurysms (modified ETDRS); now mostly after or with anti-VEGF |
| Selective laser trabeculoplasty | Q-switched 532 nm, 3 ns, 400 µm spot | Targets pigmented trabecular cells only (selective photothermolysis); first-line option (LiGHT trial); repeatable |
| Argon laser trabeculoplasty | Argon, 50 µm, 0.1 s | Thermal scarring of the meshwork; largely replaced by SLT |
| Peripheral iridotomy | Nd:YAG (± argon or 532 first in thick brown irides) | Usually superior or temporal; watch for IOP spike, bleeding, dysphotopsia |
| Posterior capsulotomy | Nd:YAG, about 1–2 mJ per shot | Focus just behind the capsule; risks: IOP spike, IOL pitting, CME, retinal detachment (higher in high myopes) |
| Laser retinopexy | 532 nm or diode | Two to three rows of confluent burns around a tear; scar becomes strong in about 2 weeks |
| ROP treatment | Diode 810 or 532 nm | Near-confluent laser to avascular peripheral retina (ETROP type 1); anti-VEGF is an alternative |
| Cyclophotocoagulation | Diode 810 (transscleral, continuous or micropulse); endoscopic | Reduces aqueous production; for refractory glaucoma |
| Photodynamic therapy | 689 nm with IV verteporfin | Standard fluence for PCV; half-dose or half-fluence for chronic CSCR |
Why the colour matters at the slit lamp
- Green 532 nm: well absorbed by melanin and haemoglobin; the workhorse for retina and SLT.
- Yellow 577 nm: near the haemoglobin peak and barely absorbed by xanthophyll, so it is gentler near the fovea.
- Red and near-infrared (647, 810 nm): pass through blood and cloudy media and reach deeper into the choroid; burns are less visible and more painful.
- Blue (488 nm): absorbed by xanthophyll and scattered by the ageing lens, so avoided at the macula.
Safety essentials
- Most ophthalmic treatment lasers are Class 4: everyone in the room needs eyewear rated for that wavelength.
- Know the aiming beam and focus carefully: the commonest serious error is a foveal burn.
- After Nd:YAG procedures, check IOP at 1 hour, especially in glaucoma patients.
- Write down the laser, wavelength, spot size, duration, power and number of spots: repeat treatments depend on it.
Where residents slip
- Thinking the Nd:YAG “burns” the capsule: it disrupts with plasma, not heat.
- Using a blue wavelength at the macula: xanthophyll absorbs it.
- Forgetting that the contact lens changes the spot size on the retina: always know your lens's magnification.
- Assuming SLT is the same as ALT: SLT uses nanosecond pulses that spare the meshwork structure and can be repeated.
- Calling corneal cross-linking a laser treatment: it uses UVA light with riboflavin.
Test yourself: flip cards
Quick quiz
Related: Basic sciences hub, Glaucoma hub, Refractive surgery for residents, Clinical optics made easy.
Key sources
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation; Section 12: Retina and Vitreous (current editions)
- Boulnois JL. Photophysical processes in recent medical laser developments: a review. Lasers Med Sci. 1986;1:47-66
- Gazzard G, Konstantakopoulou E, Garway-Heath D, et al. Selective laser trabeculoplasty versus eye drops for first-line treatment of ocular hypertension and glaucoma (LiGHT). Lancet. 2019;393:1505-1516
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