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Holographic Refraction

Overview:
The term denotes the use of a volume phase hologram to subjectively determine the spherical refractive error of the human eye. Holographic refraction is a misnomer because the process of viewing a hologram is a diffractive process.
The term refraction is commonly used to describe the process of determining the refractive error of the eye. There are numerous techniques in the literature, but they can be broadly described as either objective refraction or subjective refraction.
Holographic refraction is a subjective technique that can determine the spherical refractive error of the human eye without the need for an objective starting point.
Spherical refractive error of the eye may either be myopic or hyperopic. Myopia is when the eye incorrectly focuses the image of a distant object in front of the retina, whereas hyperopia is when the image is focused behind the retina.
To determine the magnitude of the focusing error, an objective technique such as retinoscopy or autorefraction is usually performed first. Refinement of this tentative prescription by subjective refraction, either through a phoropter or with trial lenses, is necessary for an accurate prescription.
Unlike conventional techniques holographic refraction can be performed without the need for an objective starting point, trial lenses or letter charts. Refinement is usually not necessary, as with other objective techniques.

Hologram setup
hologram setup

Actual setup for holographic recording.
hologram setup


A 633 nm Helium- Neon laser is used for the recording of the hologram. A multivergence target consisting of a 4x4 three dimensional arrays of characters is used as the object for the holographic recording. This target is located at a specific distance from an imaging lens so that rays coming from the multivergence target passing through the lens would be imaged at difference vergences. This image of the multivergence target is then combined with the reference beam to form a complex interference wavefront and is recorded onto the hologram (Avudainayagam & Avudainayagam 2003).


hologram setup hologram setup

Hologram viewing:
For holographic refraction, the reverse phase conjugated reconstruction of the hologram is used to determine the refractive error. Using the phase conjugated beam, individual characters from the multivergence image can be simultaneously seen but at different vergences, obviating the need for letter chart or lenses to determine the refractive error.



(The same recording beam but in the reverse direction towards the hologram will result in a phase conjugated object wavefront that the observer can see).
reverse wave conjugation



Black and white photography of a simulated emmetropic view through the hologram.
The ‘0’ is located at optical infinity, and if the observer sees this as being the most clear, there is little or no refractive error.

simulated emmetropic view of hologram


Black and white photograph of a simulated hyperopic view through the hologram. The positive integers are virtual images located behind the observer’s eye. Seeing positive integers is an indication that the refractive error is hyperopic. The magnitude of the refractive error can be determined by the clearest positive integer seen.

simulated hyperopic view of hologram


Black and white photography of a simulated myopic view of the multivergence image through the hologram. The negative integers are real images located in front of the observer’s eye. Seeing negative integers is an indication that the observer is myopic, provided the accommodative ability of the eye is relaxed.
Again, the magnitude of the myopic can be determined by the clearest negative integer seen.

simulated myopic view of hologram




Accuracy of Holographic Refraction:

In a pilot study of 22 subjects with refractive errors between -5.00DS and +2.50DS, the results from holographic refraction were within ±0.19DS of the results from subjective refraction through a phoropter.
The coefficient of accuracy was found to be 0.78D, and the technique compares well with subjective refraction through a phoropter.

 reliability plot of holographic refraction versus subjective refraction



Holographic refraction was found to be more repeatable than subjective refraction through the phoropter.
The coefficient of repeatability for holographic refraction was found to be 0.69D from the study. The known coefficient of repeatability for subjective refraction is 0.76D. A value closer to zero indicates better performance.
R. B. Rabbetts, Bennett and Rabbetts’ Clinical Visual
Optics (Butterworth Heinemann, 1998).

 repeatibility plot of holographic refraction




Applications:
Spherical subjective refraction perform with holographic refraction is quick, simple to setup and safe to perform. Furthermore, the technology is adaptable as different targets can be recorded into the hologram for different applications, such as sunburst patterns to determine astigmatism.
The visual optics team at the School of Optometry and Vision Science at the University of New South Wales, headed by Dr Avudainayagam is at the forefront of this technology. They are investigating potential applications for holographic refraction, including the technology’s ability to relax accommodation because of the monochromatic coherent laser used to illuminate the hologram.

sunburst patterns for astigmatic measurement by hologram

Simulated sunburst pattern imaged at different vergences as seen through a hologram.
This is one method that the hologram could be adapted to measure astigmatism.



References:
Avudainayagam K. V. and Avudainayagam C. S.,
“Holographic multivergence target for subjective measurement of the spherical power error of the human eye,”
Opt. Lett. 28, 123–125 (2003)

Avudainayagam KV, Avudainayagam CS, Nguyen N, Chiam KW, Truong C.
J Opt Soc Am A Opt Image Sci Vis. 2007 Oct;24(10):3037-44.
Performance of the holographic multivergence target in the subjective measurement of spherical refractive error and amplitude of accommodation of the human eye.

K. V. Avudainayagam and C. S. Avudainayagam,
“Holographic multivergence target for subjective measurement of the astigmatic error of the human eye,”
Opt. Lett. 32, 1926–1928 (2007).

Bland J. M. and Altman D. J., “Statistical methods for assessing agreement between two methods of clinical measurement,” Lancet 1, 307–310 (1986).

External Links:
Performance of the holographic multivergence target in the subjective measurement of spherical refractive error and amplitude of accommodation of the human eye.

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