Autorefractor Keratometer

Everything you need to know about Autorefractor Keratometer

Autorefractor keratometer is an advanced machine to correct common vision problems such as nearsightedness (myopia) and farsightedness (hyperopia) caused by changes in the shape of the eye. These common vision problems result in changes in the refractive index, and the patient cannot focus clearly. Autorefractor harnesses the power of the computer to measure the refractive error during an eye examination. Optometrists extensively use this equipment mainly in the examination of patients before and after surgery. Earlier, conventional techniques such as retinoscopy and baseline refraction were employed for diagnostics and prescription. Autorefractor keratometers are easy to operate and generates more reliable diagnostic data. The optometrists are now able to carry out a substantially precise vision-examination in terms of vision sharpness and clarity (20/20 vision tests).

History

The quest for precise measurement of cornea-related problems began in the early 1700s with the first-ever Keratometer invented by Jesse Ramsden, an English optician in 1779. Jesse attempted to prove the Kepler’s principle; that the eye accommodation correlated with the variation in the corneal curvature. Later, a keratometer was produced in 1728 in Paris, which could measure the different dimensions. Thomas Young, in 1881, further investigated Kepler’s theory by experimenting with his sight. Early designs were not accurate due to the constant movement of the eye and imprecise measurement of the image with poor alignment challenges. Since 1960, with the advent of minicomputers and electronics, the automatic refractometers with infrared radiations resolved those issues. They execute the more repetitive and iterative tasks in refraction measurements, including access to the posterior corneal surface of the opening. The modern machines validate on the similar principals of the last century developments.

Indications to use autorefractor keratometer

Optometrists primarily use Autorefractor Keratometers for vision examinations. The most common vision problems (myopia and hyperopia) resulting from refractive cornea errors can be diagnosed and corrected by this computer-controlled instrument. Differentiating corneal and lenticular aberrations conditions –which correlate with the diagnostic of refractive error – can be monitored and adjusted by Autorefractors Keratometers. These examinations are critical before the surgical operation of the eye. They are effective and useful in diagnosing and treating pre-presbyopes, emerging presbyopes, and patients who suffer from eye aging problems and struggle to focus in their mid to old age visually. Their ability to continually focus reduces to the point where it results in acute headaches. Prolonged rest is recommended as a temporary cure for headaches, thus deteriorating the quality of life. 

Another condition is ‘Latent’ hyperopia, for which optometrists commonly use the Autorefractor Keratometer. Patients with this disease suffer from farsightedness problems, which are quite common in 25% of adult individuals. Latent means the extent to which accommodative muscles respond to enhance the focusing power. Eyeball becomes too small or cornea-curvature flattens. This result in the following conditions:

  1. Loss of focusing and difficulty to see close objects
  2. Headaches, strains, and tiredness after performing work
  3. Tears and painful fatigues
  4. Prolonged rests and muscle fatigue
  5. Weak eye-hand coordination

Astigmatism – a disease that involves losing focus at all distances – can be diagnosed using autorefractor keratometers. Patients sticking with this disease see image distortion in different directions, e.g., a line is apparent in a vertical position while blurred in horizontal layout resulted from slight tilting of the lens. This causes headaches, severe muscle fatigue, and loss of concentration. Laser surgery, followed by Keratometer, can be an effective way to eliminate astigmatism. 

The diagnosis of most optometry conditions associated with the Autorefractor Keratometer is by measuring the light effects as it passes through the eyeball. It illuminates the light transmitted and measures the variations as it reflects from the back-of-the-eye. The autorefractor takes the results from a moving image and suggests a measurement for the entirely focused position. The optician recommends the corrective actions required for the patient’s perfect vision. Keratometers are best among vision measuring instruments. They measure the cornea-shape which is substantial in examining astigmatism and corneal-distortion diseases (cataracts, corneal opacities)

Complications of using Autorefractor Keratometer

Although the Autorefractor Keratometer offers substantial benefits in terms of speed and accuracy of data, there are some limitations associated with its use. 

  1. They do not assess the clarity of the cornea
  2. Only compute the refractive status of the small central cornea (3 to 4 mm)
  3. Their accuracy is dramatically lost while measuring the quite flat or steep cornea
  4. Focal-point distance based on the image distance 
  5. Their high-tech nature and procedures can be appealing for some patients 
  6. Cannot be used for young children with cycloplegia conditions due to proximal accommodation errors
  7. Non-portable and expensive as compared to retinoscope 

How do Autorefractor Keratometers work?

Autorefractor Keratometers work on the principle of refractometry. It determines the refractive errors by using an optical device knows as a refractor or optometer. Scheiner theory of refractive measurement is the basis for their functioning, and it follows the following steps:

  1. Double pinhole openings in front of the pupil convert the parallel rays of light reflected from a distant object; they are constrained to two small bundles finally.
  2. Two small light spots observed as myopia light bundles cross each other before they reach the retina.
  3. In hypermetropia conditions, these bundles intersect each other before reaching the retina and seen as two small light spots.
  4. Finally, these two light spots combine to a single place by displacing the double pinhole opening to the distant point of the eye. Hence, from that distant point, the refractive error can be devised.

The optometer or refractor follows the optometer principle, a term initially used by Porterfield in 1759. It follows the following steps:

  1. A converging lens is mounted in front of the pupil at its focal length in the device.
  2. Based on the target position, light from the target object - with varying vergences such as zero, plus, or minus – enters the aperture.
  3. The position of the target is related to the light vergence in a linear manner.
  4. Finally, a scale for focusing power (diopters correction) generates with previous measurements.

General Specifications

The general specs for an Autorefractor Keratometer are shown in the following table:

Technical Specs

Objective spherical refractive error

-30 to +25 D (VD = 12 mm) (with 0.01/0.12/0.25 D increments)

Corneal refractive power

25.96 to 67.50 D (n = 1.3375) (0.01/0.12/0.25 D increments)

Cylindrical refractive error

0 to ±12.00 D (with 0.01/0.12/0.25 D increments)

Corneal cylindrical power

0 to ±12 D (with 0.01/0.12/0.25 D increments)

Cylinder axis

0 to 180° (with 1°/5° increments)

Corneal cylinder axis

0 to 180° (with 1°/5° increments)

Minimum measurable pupil diameter

ø2 mm

Pupillary distance measurement

30 to 85 mm (with 1 mm increments)

Subjective visual acuity measurements

<0.1 / 0.1 / 0.25 / 0.32 / 0.4 / 0.5 / 0.63 / 0.8 / 1.0 / 1.25 <20/200 / 20/200 / 20/80 / 20/60 / 20/50 / 20/40 / 20/30 / 20/25 / 20/20 / 20/16

Corneal size measurement

10 to 14 mm (with 0.1 mm increments) Fixation Target

Spherical refractive error

-20 to +20 D (VD = 12 mm) (with 0.25 D increments)

Pupil size measurement

1 to 10 mm (with 0.1 mm increments)

Cylindrical refractive error

0 to ±8 D (Max.) (with 0.25 D increments)

Accomodation measurement

0 to 10 D (with 0.01/0.12/0.25 D increments)

Cylinder axis

0 to 180° (with 1°/5° increments)

Observation/display method

6.5 - 8-inch color LCD

Near addition power

0 to +9.75 D (with 0.25 D increments)

Printer

Thermal line printer with auto cutter

Corneal curvature radius

5 to 13 mm (with 0.01 mm increments)

Interface connectors

RS-232C: Double ports USB: Single port LAN: One port

The power supply is AC 100 to 240 V ±10% 50/60 Hz with a 100 VA power rating. They vary in dimensions following 26 cm width, 50 cm depth, and 46 cm height in general with a weight of 15-20 kg.

Alternative Instruments

Autorefractor Keratometer is quite rigorous, fast, and produces highly accurate results. However, there are other instruments which are still useful for substantial refraction tests such as,

  1. Retinoscopes
  2. Ophthalmoscope
  3. Phoropter
  4. Snellen chart
  5. VT 1 Vision Screener
  6. Retinal Camera

Market Leaders

A list of popular brands of Autorefractor Keratometers is as in the following table. However, there is quite an extensive list of brands in the market.

List of popular brands

Company

Brand

Features

Marco

ARK-1s Autorefractor & Keratometer

Includes autorefraction, keratometry, and glare-test combinations

Nidek Co. Ltd.

Auto Ref/Kerato/Tono/Pachymeter TONOREF™ III

Space-saving and combination of a Space-saving design that is a comfortable and efficient upgrade to your practice

Topcon

Auto Kerato-Refractometer / Auto Refractometer KR-1/RM-1

A new compact and ergonomic design

Tomey USA

RT-7000 Multifunctional Auto Ref/K/Topo

Refractometer, keratometer, and Topographer all-in-one unit

Zeiss

ZEISS VISUREF 150

Autorefractor and Keratometer

Intuitive and modern with extensive data management

Medizs

SMART RK 11 - AUTOREFRACTOR / KERATO

Performance - accuracy - simplicity - powerful - generation - better - faster

Briotweco 

Visionix L67 Autorefractor / Keratometer

Economical, reliable, and accurate

Conclusion

Autorefractor Keratometer is the user-friendly medical equipment used by opticians to diagnose eye problems and prescribe corrective actions –including contact lens, spectacles, pre-and-post surgical operations, and management, and more. The equipment is effective for myopia, pre-presbyopes, emerging presbyopes, latent hyperopia, cataracts, corneal opacities, and vision-related diseases in young children. The Autorefractor Keratometers is becoming an essential need for most optometrists. Most of the brands offer standard technical specs and features with little variations in their shape, components, and sizes. They work based on the Scheiner principle of autorefraction and Porterfield theory of optometry. Although they offer highly acute diagnostic results, they also exhibit some limitations related to cornea clarity and in young children with certain conditions.