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To see clearly, the cornea and the lens must bend - or refract - light rays so they focus on the retina - a layer of light-sensing cells that line the back of the eye. The retina converts the light rays into impulses that are sent to the brain, where they are recognized as images. If the light rays don't focus on the retina, the image you see is blurry. This is called a refractive error. Glasses, contacts and refractive surgery attempt to reduce these errors by making light rays focus on the retina.
Refractive errors are caused by an imperfectly shaped eyeball, cornea or lens, and are of three basic types:
Geekspeak TranslatedAt our Los Angeles LASIK Institute
Murnnerlyn formula -Calculates the amount of tissue to be removed by the excimer laser in order to achieve the desired correction.
Nyquist limit - This is the smallest spatial frequency that the eye can discern. Limit of foveal vision by density of cones.
Oblate surface - As referenced to a sphere, the surface would be flatter. Very common curvature of a cornea after refractive procedures.
OPD - Optical path difference
OPL - Optical path length
Point spread function - This is a representation of the real way an infinite point object would appear to the visual system.
Prolate shape - a curve having a steeper ( higher) center than its periphery (similar to the curve at the tip of an egg).
Tscherning aberrometry - A method of ray tracing. A grid is projected onto the retina and a highly sensitive CCD camera photographs this formed image of the spot pattern. Each real spot position is compared to the ideal spot position so aberrations can be determined.
Aberration - The theoretical focus of light is not where it should be, a faulty focus. This is also called irregular astigmatism by tradition, but now we can define and describe irregular astigmatism.
Adaptive optics - Optics used to correct aberrated light rays.
Astigmatism-irregular - when the primary meridians are not orthogonal (90 degrees apart). "Secondary and higher order aberrations" Expressed in RMS values.
Point spread function
Point spread function (PSF) is a measure of how well one object point is imaged on the retina through the optical system of the eye. The PSF and image of an ideal optical system of the eye is zero (a point in the foveola).
Axial radius of curvature map - Called the "Axial map" topography map showing the corneal curvature with respect to the optical axis of the cornea.
| Hartmann-Shack Aberrometry | Tscherning aberrometry | Ray tracing aberrometry | Ray tracing Scanning slit refractometer |
| currently the most commonly used. | |||
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analyze an outgoing light that emerge or is reflected from the retina and passes through the optical system of the eye.
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analyzes the ingoing light, which forms an image on the retina. | measures an ingoing light that passes through the optical system of the eye and forms an image on the retina. | Scanning slit refractometer is based on retinoscopic principles. |
| Light of specific characteristics is projected into the eye onto the macula. Rays that emerges from a single point in the foveola and pass through the optical system of the eye are analyzed. Multiple tiny lenslets in newer devices simulate apertures in the Hartmann disc located in front of the eye that isolate a narrow pencil of light emerging through different parts of the pupil. A charge-coupled device (CCD) camera registers the true position of each ray and compares it to the calculated reference position of such a ray for a perfect optical system of the eye without aberrations. This difference enables calculation of the aberrations of the true optical system of the eye. | A grid pattern formed by multiple spots is projected through the optical system of the eye and forms an image on the retina. This image is observed and evaluated by a method similar to indirect ophthalmoscopy and captured on CCD camera. The distortion of the grid pattern enables calculation of the aberrations of the optical system of the eye. | It measures one ray at a time in the entrance pupil rather than measuring all the rays at the same time like previously mentioned devices. This decreases the chance of crossing the rays in highly aberrated eyes. The total time of scanning is 10-40 milliseconds. | Both the projecting system consisting of an infrared light source and the receiving system rotate at high speed around the optical axis synchronously, and 360° meridians are measured in 0.4 seconds. A group of photodetectors is located above and below the optical axis at 2.0, 3.2, 4.4, and 5.5 mm, which detect the time of its stimulation by reflected light. The time difference depends on the type and amount of refractive error and is converted into the refractive power. . |
| Examples Wavefront analyzer made by Adaptive Optics, 20/10 Wavefront system by VISX, Wavefront analyzer made by umphrey/Zeiss & Alcon/LADAR, and Bausch & Lomb aberrometer based on Technolas design. | Dresdner Wavefront Analyzer distributed by Technomed, Wavelight, and Schwind. | TraceyScan distributed by Tracey Technologies. | This principle is used in ARK 10000 distributed by Nidek |
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Santa Barbara LASIK Disclaimer: Dr. Khanna serves eyecare patients in Los Angeles and surrounding areas. This site is for informational purposes only. It is not intended to be medical advice for those seeking LASIK surgery, but to provide information about the Los Angeles, California LASIK Surgeon, Doctor Rajesh Khanna, M.D. Dr. Khanna proudly serves patients throughout Southern California and Orange County including: Alhambra, Altadena, Anaheim, Arcadia, Baldwin Park, Bakersfield, Beverly Hills, Burbank, Camarillo, Carson, Cerritos, Chino, Chino Hills, Compton, Corona, Costa Mesa, Diamond Bar, Downey, East Los Angeles, Fontana, Gardena, Glendale, Hacienda Heights, Hawthorne, Huntington Beach, Huntington Park, Inglewood, La Habra, Laguna Niguel, Lake Forest, Lakewood, Lancaster, Long Beach, Los Angeles, Lynwood, Mission Viejo, Montebello, Monterey Park, Newhall, Newport Beach, Norwalk, Ontario, Oxnard, Palmdale, Palos Verde, Paramount, Pasadena, Pico Rivera, Pomona, Redondo Beach, Riverside, Rosemead, Santa Barbara, Santa Clarita, Santa Monica, Santa Rosa, Simi Valley, South Gate, Thousand Oaks, Torrance, Trabuco, Upland, Ventura, and West Covina. |
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