An auto refractor gives an objective estimate of the eye’s refractive error: sphere, cylinder and axis, usually with pupillary distance. A keratometer measures the curvature of the front surface of the cornea: corneal radius or power, corneal astigmatism and its axis. An auto refractor keratometer (ARK) does both in one tabletop instrument, which is why many clinics and optical practices choose an ARK rather than a refraction-only unit or a separate manual keratometer.
What an auto refractor measures
The patient looks at a fixation target inside the instrument while it measures how light is focused by the eye. The result is an objective starting point for refraction: sphere, cylinder and axis for each eye, normally at a selectable vertex distance such as 0, 12 or 13.75 mm. Fogging of the target helps the eye relax its focus during the measurement. The auto refractor result is a starting point; the final prescription is decided by subjective refraction with a phoropter or trial lenses.
What a keratometer measures
Keratometry measures the curvature of the central cornea along its two principal meridians. Results are shown as a radius in millimetres or as corneal power in diopters, together with corneal astigmatism and its axis. These readings are used, for example, when fitting contact lenses, when comparing corneal astigmatism with total astigmatism from refraction, and as one of the inputs to intraocular lens (IOL) power calculation before cataract surgery.
Manual vs automatic keratometer
With a manual keratometer the operator looks through an eyepiece, aligns illuminated mires reflected from the cornea and reads the curvature from dials, one meridian at a time. It needs little more than illumination but depends on operator skill and takes longer. An automatic keratometer, built into an auto refractor keratometer, captures the corneal reflection electronically and calculates radius, power and axis in moments, with the results printed or sent on together with the refraction. ENOCHTEC supplies automatic keratometry as part of its auto refractor keratometers.
Auto refractor vs auto refractor keratometer at a glance
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| Auto refractor (refraction only) | Auto refractor keratometer (ARK) | |
|---|---|---|
| Refraction: sphere, cylinder, axis | Yes | Yes |
| Corneal curvature (radius / power) | No | Yes |
| Corneal astigmatism and axis | No | Yes |
| Contact-lens fitting support | No corneal readings | Corneal readings available; some models add a contact-lens base-curve mode |
| Typical setting | Screening or refraction rooms where corneal data come from another instrument | Optometry clinics, ophthalmology practices, optical shops, pre-test rooms |
What to compare when buying an auto refractor keratometer
- Measurement modes: combined refraction and keratometry, refraction only, keratometry only, and extras such as contact-lens base curve or axial length.
- Ranges: sphere, cylinder, corneal radius and minimum pupil diameter, for the patients you see.
- Alignment: joystick alignment with automatic capture, automatic tracking, or fully automatic positioning.
- Display and controls: screen size, touch or physical buttons, and whether the screen rotates for your room layout.
- Output: built-in printer, and data interfaces such as RS-232C, USB, LAN or Bluetooth, matched to the receiving system.
- Power and footprint: the electrical standard for your country and the space on the instrument table.
When axial length is worth adding
For myopia-management programmes, some clinics also track axial length, the length of the eye from front to back. A combined instrument such as the ENOCHTEC FRS-810 measures refraction, keratometry and axial length (11.5–42.0 mm) in one tabletop unit and stores patient-linked records for follow-up. For IOL planning, a dedicated optical biometer such as the AL-VIEW Lite is used instead.
ENOCHTEC auto refractors and auto refractor keratometers
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| ARK-900 | ARK-710 | ARK-8900 | RMK-8100 | FRS-810 | |
|---|---|---|---|---|---|
| Measurements | FR: refraction only; FKR: refraction + keratometry | Refraction + keratometry (K&R / REF / KRT) | Refraction + keratometry + contact-lens base curve (R&K / REF / KRT / CLBC) | Refraction + keratometry (REF / KER / K&R) | Refraction + keratometry + axial length |
| Sphere | −25.00 to +25.00 D | −25.00 to +25.00 D | −25.00 to +22.00 D (VD 12 mm) | −20.00 to +20.00 D (VD 12 mm) | −25.00 to +25.00 D |
| Cylinder | −10.00 to +10.00 D | −10.00 to +10.00 D | Confirmed with quotation | 0.00 to ±6.00 D | −10.00 to +10.00 D |
| Corneal radius | 5–10 mm (FKR) | 5–10 mm | 5.00–10.00 mm | 6.5–9.4 mm | 5–10 mm |
| Minimum pupil | Ø2.0 mm | Ø2.0 mm | Ø2.0 mm | Confirmed with quotation | Ø2.0 mm |
| Alignment | Automatic vertical tracking + joystick | Automatic eye tracking | Joystick with automatic capture | Joystick; motorized chinrest | Automatic 3D positioning |
| Display | 6.5-inch LCD | 9-inch touch LCD, rotates 360° | 5.7-inch TFT color LCD | 9-inch touchscreen | 10.1-inch touch LCD |
| Output | Built-in thermal printer | Auto printing and cutting; Bluetooth | Built-in thermal printer | 57 mm printer; RS-232C / LAN / USB | Thermal printer, network A4 printing; Wi-Fi, Bluetooth, USB, HDMI, serial, LAN |
| Weight | 18 kg | 20 kg | 18 kg | 18 kg | 22 kg |
Specifications are for reference. The exact configuration, accessories and documentation for your market are confirmed in your quotation.
See every model in the auto refractor and keratometer buying guide, or how objective refraction fits into a refraction room setup.
Related guide: Keratometer vs Corneal Topographer. Keratometry samples only the central cornea; the guide explains when topography or tomography is needed as well. For full corneal maps, see the SW-6000 corneal topographer.
Frequently asked questions
What is the difference between an auto refractor and a keratometer?
An auto refractor measures the refractive error of the whole eye (sphere, cylinder and axis). A keratometer measures only the curvature of the front surface of the cornea. An auto refractor keratometer combines both measurements in one instrument.
Does an auto refractor replace subjective refraction?
No. The auto refractor gives an objective starting point. The final prescription is decided by a qualified eye-care professional using subjective refraction with a phoropter or trial lenses.
Is a manual keratometer still useful?
A manual keratometer can still measure central corneal curvature, but readings take longer and depend on the operator. Automatic keratometry in an auto refractor keratometer gives radius, power and axis in moments and records them with the refraction.
Which ENOCHTEC model measures contact-lens base curve?
The ARK-8900 has a dedicated CLBC mode for measuring the base curve of contact lenses, in addition to refraction and keratometry.
Which ENOCHTEC model also measures axial length?
The FRS-810 measures refraction, keratometry and axial length (11.5–42.0 mm) in one tabletop instrument, with patient-linked records for follow-up.
Choosing an auto refractor keratometer?
Tell us where it will be used, how many patients you measure per day and whether you need keratometry, contact-lens base curve or axial length. We will recommend a model and send a quotation.
Request a quoteThis guide is general information for equipment buyers. Refraction, keratometry and their interpretation are the responsibility of qualified eye-care professionals.
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