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To fully understand the power of the iTrace system, one must first understand the basics of wavefront aberrometry. Traditionally we have described the optical properties of the eye in terms of a single set of refraction numbers: sphere, cylinder and axis. Refraction can be measured with a variety of instruments, however these measurements represent an average over the entrance pupil. If we wish to measure the spatially resolved optical properties of the eye, we must use another terminology to describe these properties. One method to describe these properties is by characterizing the wavefront of the eye’s optical system.
What is Wavefront?
Wavefront is defined as an imaginary surface joining all points in space that are reached at the same time by a lightwave propagating through a medium. A beam of parallel light rays entering the eye would have a flat or plane wavefront, that is, each point in the lightwave arrives at the imaginary surface in front of the eye at the same time. For an emmetropic eye, the light would now have a curved wavefront, because light travels different distances in reaching the optics (medium) and at different speeds (refraction) resulting in this imaginary surface that represents the points reached at the same time being curved.
For a myopic eye of the same length, the wavefront would be more curved because of the greater power (curvature).
What are Zernike Modes?
The deviations between the actual wavefront and an ideal wavefront are referred to as aberrations or errors: a deviation from the normal or expected course. |
The Emmetropic Wavefront
Myopic Wavefront
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In 1934, Fritz Zernike published a paper describing a set of polynomials that could be used to expand the aberration function. Each polynomial represents a particular mode of optical aberration. Thus our irregular wavefront can be described by coefficients (multipliers for each of the modes) which when taken as a whole reconstruct the wavefront map, but individually describe the relative amount of each aberration mode. Shown here are the Zernike modes (aberrations) for an expansion through 6th order terms.
The 2nd order terms represent sphere and cylinder.
The 3rd order terms and higher represent higer order aberrations, which are those aberrations which cannot be corrected by conventional refractive treatments. The different Zernike modes allow us to visualize the primary types of aberrations which contribute to the overall deviation.
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