Before advanced imaging, eye doctors still had clever ways of measuring the shape of the cornea. One of the simplest and most enduring of these is the Placido disc, a flat, circular target with bright concentric rings that is reflected off the cornea. A century later, the same idea lives on inside modern corneal topography systems, and it remains central to the diagnosis of astigmatism, keratoconus, and many other corneal conditions.
This guide walks you through what the Placido disc is, how it works, what it diagnoses, and how it compares with newer imaging like Scheimpflug tomography.
What Is a Placido Disc?
The Placido disc is a flat or domed target carrying a series of concentric illuminated rings. When held in front of the eye, the rings are reflected off the cornea. A regularly-shaped cornea produces smooth, round, evenly spaced reflections. An irregular or steeply curved cornea distorts them.
Named after Antonio Placido, a 19th-century Portuguese ophthalmologist, the disc has evolved from a simple handheld target into the core of modern topography systems.
How Does the Placido Disc Work?
The principle is reflection. Light from the concentric rings bounces off the tear film on the cornea. A camera or observer captures the reflected pattern.
- On a regularly-shaped cornea, rings appear evenly spaced and smooth
- On a steep cornea, rings appear compressed
- On a flat cornea, rings appear spread out
- On an astigmatic cornea, rings appear oval rather than circular
- On a keratoconic cornea, the pattern shows localised steepening and distortion
A computer processes this reflection into a colour-coded map of the corneal surface.
What Does the Placido Disc Diagnose?
The Placido disc and Placido-based topography help diagnose and monitor:
- Astigmatism
- Keratoconus and early forms (subclinical keratoconus)
- Pellucid marginal degeneration
- Post-refractive surgery ectasia
- Corneal scars
- Dry eye effects on surface smoothness
- Post-transplant corneal shape
- Contact lens fitting
- Pre-LASIK screening
- Intraocular lens calculation in complex eyes
What Test Uses Concentric Rings to Show Astigmatism?
The test is called Placido-based corneal topography. The pattern of reflected concentric rings is converted into a colour-coded curvature map. Steep areas are usually shown in warm colours (reds and oranges); flat areas in cool colours (blues and greens). The map visually depicts astigmatism, its axis, and its regularity.
In clinical reports, you may also see terms such as “keratometric maps”, “axial (sagittal) maps”, and “tangential maps”, each highlighting different aspects of the reflected ring data.
What Is the Difference Between Placido and Scheimpflug?
Both are methods used in modern corneal imaging, and many devices combine them.
Placido-based topography
- Works on reflection from the anterior (front) corneal surface
- Excellent for measuring the outer shape of the cornea
- Shows front-surface maps with high resolution
- Cannot image the back (posterior) surface directly
- Cannot measure corneal thickness directly
- Less effective on very irregular or tear-film-poor surfaces
Scheimpflug imaging
- Uses a rotating camera to take cross-sectional images of the cornea and anterior segment
- Images both the front and back surfaces
- Provides 3D tomographic data
- Measures corneal thickness (pachymetry) across the entire map
- Detects posterior surface ectasia earlier than Placido alone
- Requires cooperation (the patient must look straight and sit still)
Combined systems
Modern instruments combine Placido rings for surface detail with Scheimpflug cross-sections for 3D detail, providing the most complete corneal assessment.
Supportive eye treatments such as lubricating drops before imaging improve tear film quality and image accuracy.
What Is K1 and K2 in Keratometry?
Keratometry measures the curvature of the cornea at the two principal meridians.
- K1 is the flattest meridian reading
- K2 is the steepest meridian reading
- The difference between K2 and K1 represents corneal astigmatism
- The axis of K2 tells the angle of the steepest meridian
K-values are expressed in diopters (D) or millimetres. For example, K1 of 43.25 D and K2 of 44.75 D indicate a corneal astigmatism of 1.5 D in the steep meridian.
K-readings are essential for:
- Intraocular lens calculation before cataract surgery
- Refractive surgery planning
- Contact lens fitting
- Monitoring conditions like keratoconus
Placido Disc in LASIK and Refractive Surgery
Before any refractive procedure, the cornea must be mapped in detail. Placido-based topography helps:
- Rule out keratoconus and forme fruste cases
- Identify asymmetric astigmatism
- Plan wavefront-guided or topography-guided laser treatment
- Assess candidacy
- Predict outcomes
After surgery, topography tracks healing and detects any early ectasia.
Placido Disc in Contact Lens Fitting
For complex fits, particularly in keratoconus and post-surgical eyes, topography helps:
- Choose the right base curve
- Select the right lens type (RGP, hybrid, scleral, specialty)
- Avoid corneal compression
- Monitor fit over time
- Improve comfort and vision
How Is a Placido Topography Done?
The test is quick and painless.
- You sit in front of the device
- Chin on a rest, forehead against the bar
- You fixate on a target
- The device flashes or projects rings
- A camera captures the reflection
- The computer generates the map
- Total time: 1-3 minutes per eye
No drops, no contact with the eye. Dry eye and tear film disturbance can reduce image quality; doctors often instil a lubricating drop a few minutes before the test.
Limitations of the Placido Disc
- Depends on the tear film; dry eye reduces image quality
- Only maps the front surface
- Cannot directly measure corneal thickness
- Cannot detect posterior ectasia alone
- Requires patient cooperation
- Needs a trained interpreter
Combining with Scheimpflug or OCT tomography removes most of these limitations.
When Should You See a Doctor?
Book a visit if:
- You are considering LASIK or other refractive surgery
- You have progressive astigmatism or suspected keratoconus
- You have been diagnosed with keratoconus and need monitoring
- You are planning cataract surgery with a toric lens
- You are being fitted for complex contact lenses
- You have a family history of keratoconus
- You have had a corneal transplant
A corneal topography evaluation at an eye hospital gives precise data before any surgical or lens decision.
Placido Disc Care at Vasan Eye Care
Vasan Eye Care has been looking after patients across India since 2002, now as part of ASG Enterprises. With more than 150 super-speciality centres, 500+ ophthalmologists, and over 5,000 trained eye care staff, the team uses Placido-based topography along with Scheimpflug imaging for comprehensive corneal assessments. A specialist eye specialist hospital in the network uses these tools daily to guide surgical decisions.
Key Takeaways
- The Placido disc maps the front surface of the cornea using reflected concentric rings.
- It helps diagnose astigmatism, keratoconus, and many cornea-based conditions.
- Modern topography combines Placido rings with Scheimpflug imaging for complete mapping.
- K1 and K2 represent the flattest and steepest meridians of corneal curvature.
- The test is quick, painless, and essential in refractive and cataract surgery planning.
- Dry eye can reduce image quality, so lubricating drops may be used before the test.
Frequently Asked Questions (FAQs)
The Placido disc is used to map the shape of the cornea. It helps diagnose astigmatism, keratoconus, pellucid marginal degeneration, post-refractive surgery ectasia, corneal scars, dry-eye-related surface changes, and irregularities that affect contact lens fitting. It is a cornerstone in pre-LASIK screening and in planning toric intraocular lenses for cataract surgery.
Placido-based topography measures reflection from the front corneal surface using concentric rings. Scheimpflug imaging uses a rotating camera to capture cross-sectional images of both front and back surfaces of the cornea and the anterior segment. Scheimpflug provides corneal thickness and posterior shape data, which Placido alone cannot. Modern devices often combine the two for a complete assessment.
Placido-based corneal topography uses reflected concentric rings to generate a colour-coded curvature map of the cornea. The map visually depicts astigmatism, its axis, and its regularity. It is widely used in refractive surgery planning, keratoconus diagnosis, toric IOL calculation, and specialty contact lens fitting.
K1 is the flattest meridian reading of the cornea, and K2 is the steepest. The difference between K2 and K1 indicates the amount of corneal astigmatism, and the axis of K2 tells the angle of the steepest meridian. These values are measured in diopters or millimetres and are used in intraocular lens calculation, refractive surgery planning, and monitoring corneal diseases.
References
- American Academy of Ophthalmology. Corneal Topography. https://www.aao.org/eye-health/diseases/what-is-keratoconus
- National Center for Biotechnology Information. Corneal Topography. https://www.ncbi.nlm.nih.gov/books/NBK585068/
- National Eye Institute. Eye Tests. https://www.nei.nih.gov/learn-about-eye-health
- WebMD. Corneal Topography. https://www.webmd.com/eye-health/corneal-topography