What is Keratometer? Why is a Keratometer Important for Eye?

A keratometer is a tool that ophthalmologists use to measure the curvature of the cornea, which is the clear, dome-shaped surface that covers the front of the eye. The cornea is responsible for bending (refracting) light that enters the eye. The keratometer is used to determine the cornea’s degree of curvature (also called the refractive power). This information helps the eye doctor prescribe the correct eyeglass or contact lens prescription for each patient.

keratometer is also known as an ophthalmometer. It’s used to measure the curvature of the anterior cornea surface and, in particular, determine the extent and axis of astigmatism. The German physiologist Hermann von Helmholtz invented it in 1851. However, Everard Home and Jesse Ramsden had an earlier model in 1796.

A keratometer provides the following information:-

  • The curvature radius of the corneal surface.
  • The direction of the eye’s principal meridian. (With-the-rule or against-the rule astigmatism)
  • The axis and degree of corneal astigmatism.
  • The occurrence of any kind of corneal distortion.

Use of Keratometry

  • Measurement of corneal astigmatism.
  • Estimating the radius of curvature of the cornea helps in the contact lens fitting.
  • Assess the integrity of the cornea and tear film.
  • Detection of irregular astigmatism – keratoconus/pterygium/corneal scarring.
  • Assess refractive error in cases with hazy media (Rough estimate, comparison of two eyes).
  • Establish baseline data – should be done in all patients.
  • The patient may later want a Contact lens or develop an injured/diseased cornea.
  • IOL Power calculation (Pre-op Cataract Surgery workup).
  • Pre & post-surgical astigmatism.
  • D/D of axial versus curvatural anisometropia.
  • Detect Rigid Gas Permeable lens flexure.
  • Progressive myopia.

Keratometry (K) refers to the measuring of corneal curvature. It determines the strength on the cornea. Differences in force across the cornea (opposite meridians) result in astigmatism; therefore, keratometry measures astigmatism.

It can be acquired with various instruments, either manually or via automated methods.

Measurements can be very sophisticated, such as with topographers that measure a cornea across a broad number of points, or it can be measured in a finite area of the cornea, such as by using a manual keratometer. Or using an IOL master (Carl Zeiss Meditec).

IOL Master IOL Master can also measure length of the axial as well as other Ocular variables (such like anterior chamber thickness, white-to-white measurements) and also K readings.

Types of keratometer :- Javal-Schiotz Keratometer

Javal-Schiotz is a principle that uses an image that is fixed with a doubling size and an adjustable size to obtain the cornea’s curvature. It’s a two-position device and utilizes two self-illuminated objects. One of them is a square in red, while the other one is a green design that resembles a staircase.

They are made to stay in the circular track to keep a certain distance from the eye. The device must remain concentrated on achieving exact results. Like other autofocus devices, operates using the Scheiner principle. This is where the inbound refracted light beams (converging in the natural world) are seen by two or more separate apertures that are symmetrical.

Methods for using the Javal-Schiotz Keratometer

  • Eyepiece Adjustment: Firstof all, the eyepiece has to be completely turned counterclockwise. A piece of white newspaper can be placed on top of the lens. The eyepiece is turned clockwise until it reaches an exact focus on the target object of your choice.
  • The device’s height must be in line with the level of the patient’s vision sign.
  • The eye that is not used for testing of the patient needs to be removed.
  • The patient will then be instructed to focus their eyes on the central point in the apparatus. The patient will be able see the image of the circles within their eye.
  • The device should be adjusted to provide an accurate image of the rectangle and staircase targets.
  • The instrument is able to be rotated in order to align the line on the staircase target to the the target rectangle.
  • The knob for measuring is able to be turned until the rectangle and staircase targets meet.
  • The results will be shown on the keratometer scale.

Bausch as well as Lomb Keratometer

It is the Bausch and Lomb Keratometer is built on the Bausch and Lomb principle that uses an object fixed instead of an image that is fixed. It is a keratometer with one position and the image’s size can be altered. In this case, the beams of light (converging in the natural world) are pushed by a Scheiner disk with four apertures. Two prisms are perpendicular to each the other. The power of the minor and major axes are independent, without altering the instrument’s orientation.

Methods of using the Bausch as well as a Lomb Keratometer

  • Eyepiece Adjustment : Eyepiece Adjustment has to be turned completely counterclockwise. A sheet of white paper can be put on top of the telescope. The eyepiece is turned clockwise until it reaches an exact focus on the target object of your choice.
  • The device’s height must be in line with the height of the patient’s visual sign.
  • The eye that is not used for testing of the patient must be shut off.
  • The patient will then be directed to stare directly at the central point on the gadget. The patient should be able to clearly see the circle-shaped image through their eyes.
  • The patient needs to change the knob of focusing until they see a single picture of their eye’s center circular.
  • The top and side circles must coincide with the center circle. This can be accomplished by turning on the device’s body.
  • Horizontal alignment knob in the instrument is utilized to connect the cross-sections closer to one another.
  • A vertical aligner knob in the instrument is utilized to align the cross-sections closer to one another.
  • The results will be shown on the keratometer scale.

Manual Vs. Automated

Manual keratometry since it gives an accurate understanding of the quality of the precorneal tear film and a live image of the cornea’s surface. Using manual keratometry, you can see the exact nature of the reflections created from the film. You can identify areas that have corneal surface irregularities or damage. When the film of your tear is oily or damaged, or the cornea displays mild dystrophy or degeneration, it will show up in the quality of tests. It is easier to comprehend the accuracy and precision of the measurements.

Acquiring automated measurements is not static. The measurements are taken at a particular moment and are computer generated. As an instrument’s operator, there is no sense of the exactness of measurements “measured time.”

Keratometry is an actual test in cataract surgery as the measurement errors are linked 1:1 to refractive results. If there is 1.00 D off from the K readings, you’ll experience a 1.00 D refractive shock. If your readings are incorrect and you’re unsure, you’ll get an unexpected refractive shock after surgery. In the modern era of cataract surgery and the best intraocular lens (IOLs), a refractive missed result is extremely disappointing to the doctor and patient. Resolving residual ametropia for postoperative cataract patients might necessitate a second surgery, whether IOL swap, piggybacking an additional IOL, or an operation on the surface.

If you are unsure of any doubt about your K results, the readings must be rechecked on a different day. We’ll do a second set of K readings if there are any signs of a problem or in cases where it is hard to obtain. I generally test the right eye before the left, after which I repeat to confirm.

Suppose I’m not happy with any of my measurements-for instance. In that case, if the patient suffers from a dry-eye condition or is wearing contact lenses, for example, I will require the patient to come back after treatment for dry eyes and wear no contact lenses for a period of 1 up to three weeks based on the type of lens. The first measurement you take may differ from the measurement you’ll need to perform surgery. When you’re taking measures to prepare for cataract surgery, this is the first thing you’ll need to perform before instilling drops, conducting gonioscopy, or doing anything else that might damage the cornea’s surface.

While it’s not difficult to learn, it takes some time to make certain that you are taking precise measurements using manual K. It is important to be confident about the measurements you take before you write your findings on paper. In practice at Omni Eye Surgery, only doctors, not including residents, can perform keratometry on patients considering cataract surgery.

Some surgeons opt for the latest surgical procedure that is automated. It’s a good idea to contrast manual and automated methods to understand the difference since each technique measures different cornea regions. However, measurements must be correlated. In addition, you should expect a correlation between the corneal cylinder and the patient’s eyeglasses.

Controlling the corneal surface before Ocular surgery is vital. Patients are sometimes referred for cataract surgery, but the cataract isn’t the main issue; its corneal surface has to be. In some instances, patients treated for cornea problems have chosen to avoid cataract surgery because the issue was more related to the cornea’s surface than cataract. The clinical signs are spotted through keratometric tests that can assist in identifying subtle cornea diseases that are not obvious, like basement membrane dystrophy. It is more difficult to spot and is easy to overlook with the slit lamp exam. Patients with corneal dystrophy must be aware that even though their vision may improve after cataract surgery, there is a cornea-related issue that could impact their post-operative sight.
When an operator experiences difficulties generating measurements, Try to pinpoint the cause. It could be because of dry eye or cornea damage, meibomian gland dysfunction, corneal disorders, incorrect positioning of the patient, or fixation in the exam.
It is of the greatest importance to have a good grasp of Keratometry. As the person responsible for those measurements, the surgeon puts the outcome of surgery for the patient in your control. You are responsible for ensuring that they’re in good health and alerting you when they’re not.
The measurements of keratometry are crucial when fitting contact lenses, especially gas permeable lenses, for the monitoring of corneal pathology as well as for the identification of Keratoconus. It is common to identify corneal Keratoconus by examining them with keratometry.

The curvature radius of the cornea can be determined by :- 

R = 2 d (I/O)

R : The radius of curvature of cornea measured in meters.

d : Distance of object from cornea

I: Size of the image.
O: Size of the object.

The cornea’s refractive power can be calculated using the formula

D = (n-1)/R

D: Dioptric refracting strength of corneal surface.

n: refractive index of instrument (n= 1.3375 generally).

Keratometers display the cornea’s curvature in terms of diopters or power, or millimeters as well as diopters. When the measurements are presented in millimeters, dioptric power can be calculated using the equation above.


Keratometers are a great way to measure the curvature of your cornea, and can be very helpful in diagnosing and treating certain eye conditions. They’re quick, simple to utilize, and cost-effective.. If you think you may have an eye condition that could benefit from a keratometer reading, talk to your eye doctor today.

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