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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 5 — Mar. 5, 2007
  • pp: 2753–2761
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Simulation of realistic retinoscopic measurement

Bo Tan, Ying-Ling Chen, K. Baker, J. W. L. Lewis, T. Swartz, Y. Jiang, and M. Wang  »View Author Affiliations


Optics Express, Vol. 15, Issue 5, pp. 2753-2761 (2007)
http://dx.doi.org/10.1364/OE.15.002753


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Abstract

Realistic simulation of ophthalmic measurements on normal and diseased eyes is presented. We use clinical data of ametropic and keratoconus patients to construct anatomically accurate three-dimensional eye models and simulate the measurement of a streak retinoscope with all the optical elements. The results show the clinical observations including the anomalous motion in high myopia and the scissors reflex in keratoconus. The demonstrated technique can be applied to other ophthalmic instruments and to other and more extensively abnormal eye conditions. It provides promising features for medical training and for evaluating and developing ocular instruments.

© 2007 Optical Society of America

1. Introduction

The retinoscope is a clinical standard device for measuring the refractive state of the eye. Although similar in some respects to the classic spot retinoscope, a contemporary streak retinoscope projects a straight-filament image onto a patient’s eye at a distance of 0.5-1.0 meter. The width of the streak projection is adjustable by moving a condenser lens above the filament in the handle of the device. The retinal reflex is observed by the examiner through a peephole on the scope. When moving the streak projection across the patient’s pupil, the reflex of a myopic or hyperopic eye appears to move with or against the projection motion depending on the position of the condenser lens. Simplified geometrical-optical analysis is normally employed to describe the movement of the reflex in relation to the refractive state of the eye [16

16. R. B. Bennett , Clinical Visual Optics, 3rd ed., (Butterworth-Heinemann, Oxford, 1998).

18

18. E. Landolt, “ Methods of determination of the refraction and accommodation of the eye,” in The Refraction and Accommodation of the Eye, C. M. Culver, ed. (Pentland, Edinburgh, 1886).

]. Swaine used a simple model to predict further pupil size influence and the intensity profile of the reflex [19

19. W. Swaine, “ Retinoscopy. I. General outline and symbols, “ Optician, April, 171–179 (1945).

22

22. W. Swaine, “Retinoscopy. IV. Variation of the reflex brightness with refractive error and pupil diameter, ” Optician, August, 71–74 (1945).

]. Higher order aberration that is normally considered a difficulty in retinoscopy interpretation has been investigated more carefully in recent years [23

23. A. Roorda and W. R. Bobier, “Geometrical technique to determine the influence of monochromatic aberration on retinoscope, ” J. Opt. Soc. Am. A , 13, 3–11 (1996). [CrossRef]

, 24

24. M. T. Caballero, W. D. Furlan, A. Pons, G. Saavedra, and M. Martinez-Corral, “Detection of wave aberrations in the human eye using a retinoscopy-like technique, ” Opt. Comm . 260, 767–771 (2006). [CrossRef]

]. Rather than the spot retinoscopy, Smith included the consideration of streak orientation and brightness change in the presence of astigmatism [25

25. G. Smith and S. Haymes, “The streak retinoscopy pupil reflex in the presence of astigmatism, ” Ophthal. Physiol. Opt . 23, 295–305 (2003). [CrossRef]

]. In this paper, we simulate the streak retinoscopic observation of individual patients under both plane- and concave-mirror operations. Ambiguous observation of the so-called “anomalous with-motion” of the high myopia condition is also produced. Further, the famous scissors reflex of a keratoconus eye is simulated for the first time.

2. Method of simulation

Figure 1 illustrates the retinoscope elements with corresponding parameters in the computation. From the light source, the elements include a filament light source (0.03 mm × 3 mm), a rectangular aperture (2 mm × 4 mm), a condenser lens (20 mm focal length), a beam splitter with window aperture (10 mm × 14 mm), and a circular peephole (3 mm diameter). The distances between each element are specified in the figure. A movable sleeve was included that allows the examiner to vertically move the lens and change the convergence of the streak projection. The wavelength was set at 555 nm and 0.5 or 0.67 meter working distance was assumed. Coordinate breaks (including coordinate shifts and rotations) were used to move or rotate the streak beam across the entrance pupil of the model eye. Double-pass image analysis through the model eye was performed under the assumption of perfect diffusive retinal reflection/scattering. Multiple reflections and scattering were omitted. An aberration-free imaging system was used to simulate the examiner’s eye behind the peephole. The focus plane of the examiner’s eye was set on the corneal surface of the model eye.

Fig. 1. Optical layout in the simulation of retinoscopic measurement. Wavelength of filament is set at 555 nm. The observation behind the peephole is simulated with a Gaussian lens that focuses on the cornea plane.

As in the real condition, four effective apertures were involved in this retinoscopic simulation. These apertures were the small aperture in front of the filament, the window on the beam splitter (along both paths), the pupil of the eye (along both paths), and the peephole of the observation. Ray aiming was applied to ensure that all of the vignetting or cut-off effects were encountered when using coordinate breaks.

3. Results and discussion

3.1 Sleeve position: Plane mirror and concave mirror operations

The retinoscope sleeve position defines the plane- and concave-mirror operations. Figure 2 shows the simulation result when the retinoscope sleeve moves vertically across a fifteen mm distance. The corresponding streak projections on the center of patient’s eye are illustrated on the left column. The illustrated eye in each image has a 3.4 mm pupil and an 11 mm iris. Each image is scaled 10 cm by 10 cm. The false colors represent the relative intensity distribution. As the sleeve moves upward, the convergence of projection increases. Because the condenser lens has a focal-length of 20-mm, the filament image is sharply focused at a sleeve height of h=21 mm. The sleeve-down position (h<21mm) corresponds to the “plane mirror” position, and sleeve-up, the “concave mirror” position. When the sleeve moves all the way up, the projection shape tends to reveal the rectangular filament window.

The measurement simulation was performed for five refractive conditions of hyperopia of +2, and myopia of -1, -2, -4, and -6 D (diopter=meter -1), as indicated at the top of each column. The pupil is 3.4 mm diameter. Since the observing distance is d ob=0.5 meter, the retina surface of the -2D eye is conjugate to the window of retinoscope. Neutralization occurs at any sleeve location for this refractive condition.

Notice that the retinal strip-reflex is often in poor contrast and hard to observe if the images are in gray-level instead of the false-color illustration. It is especially so for a refractive condition that is close to neutralization and when the sleeve location is away from h=21 mm. Streak reflex is more easily observed under concave-mirror operation for high myopia and under plane-mirror operation for hyperopia.

In the streak retinocope, the filament is imaged by a condenser lens. The location of the filament image, l, has an important effect and is indicated beside the figure. From the patient’s viewpoint, the light source (filament image) changes with the sleeve position. When the lens is located at the lowest position, 10-mm above the filament, the image of the filament is about 47 mm behind the peephole. As the sleeve moves upward, the light source image rapidly moves farther away from the patient. The light source moves to infinity as the sleeve glides into the 20-mm height position. When the sleeve moves above the 20-mm position, i.e. into the concave-mirror condition, the filament image appears to be on the patient side of the peephole. At h=21 mm, the filament image is at about 10 cm in front of the patient’s eye. When the sleeve is pushed farther upward, the light source image moves toward back to the retinoscope. This light source location, in relation to the peep-hole position, determines the reflex motion, the direction, and speed of the reflex movement.

3.2 Streak rotation: observation of cylinder

In retinoscopy, astigmatism is often observed by rotating the streak projection. When rotating the streak, two distinctive astigmatic appearances are the variation in reflex brightness and strip thickness. When the streak projection is aligned with one of the two major meridians, the thickness and the brightness appear to be either optimized or minimized. Illustrated in gray scale and, more clearly, in false color, Fig. 3 shows the retinal reflex of an eye with the prescription of (S+1.00, C+2.00, X90). The dotted arrow line in each image indicates the orientation of the projection. The sleeve location was set at 18 mm. The reflex thickness and intensity variations are obtained. A third astigmatic appearance is the skew or break phenomenon, which shows the misaligned motions between the projection and reflex streak. This is also clearly shown in the simulated images where the beams are not aligned with meridians at either 180 or 90 degrees. The streak reflex appears to be misaligned to the streak projection.

Fig. 2. Simulation results of the retinoscopic observation as the condenser lens moves from a height of 10 mm (indicated as h=10 mm) to 25 mm above the filament. The left most column shows the streak projections on the surface of the examinee’s eye. The 5 columns on the right illustrate the appearances of retinal reflex of 5 eyes with +2, -1, -2, -4, and -6 diopter of refractive errors.
Fig. 3. Streak retinoscopic reflex of an astigmatic eye, (S+1.00, C+2.00, X90). Both gray level and false color illustrations are scaled from 5% to 95% of the maximum intensity. The orientations of streak projections are indicated by dotted arrow lines.

3.3 With and against motion in ametropia

Figures 4 and5 show the retinal reflex motion under the plane mirror (sleeve located at h=19 mm) and the concave mirror (h=21 mm) operations, respectively. The upper row in each simulation illustrates the projection that moves across the pupil along one major meridian of the eye. Three personalized eye models [11

11. Y.-L. Chen, B. Tan, K. Baker, J. W. L. Lewis, T. Swartz, Y. Jiang, and M. Wang, “Simulation of keratoconus observation in photorefraction,” Opt. Express 14, 11477–11485 (2006). [CrossRef] [PubMed]

] are used in the simulation. In upper Fig. 4 is presented the reflex of a mild myopic eye, MY1 with prescription of (S-1.50D, C+0.25D, X180) and a best-correction RMS wavefront aberration (WA) of 0.111 μm in the 3.4 mm pupil. The streak projection moves along the -1.5D meridian at a working distance of 667 mm. The neutralization or reversal appearance is clearly seen. The intensity variation shows the larger high-order aberration of this eye.

In middle Fig. 4 and upper Fig. 5 are illustrated measurements of an hyperopic eye, HY2, of (S+2.55D, C+0.5D, X10) and best-correction RMS WF of 0.079 μm. The streak projection moves along the 10-degree meridian. The characteristics of with-motion in the plane-mirror setting and against -motion in concave-mirror operation are clearly shown. Similarly, in lower Figs. 4 and 5, the observations of a myopic eye model, MY3, with prescription of (S-6.0D, C+0.75D, X70) and best-correction RMS WF of 0.117 μm are predicted and the against-motion and with-motion behaviors, respectively, in plane- and concave-mirror operations are clearly demonstrated. One observation to be noticed is the so-called “cut-off” phenomenon that occurs when the edge of projection falls inside the pupil. This effect is present in the against-motion cases in lower Fig. 4 and upper Fig. 5. In these two sets of images, the appearance of the edge doesn’t affect the judgment on reflex movement. However, at certain conditions, anomalous reflex motions occur.

The anomalous motion is often observed at high myopic or accommodative conditions in infants or patients with large pupils. This phenomena was first reported by Borish in 1970 [29

29. I. M. Borish, Clinical refraction, 3rd ed. (Professional press, Chicago, 1970).

] and named the cut-off phenomena. Later, Howland in 1978 [30

30. H. C. Howland, “Retinoscopy of infants at a distance: limits of normal and anomalous reflexes, ” Vision Res . 18, 597–599 (1978). [CrossRef] [PubMed]

] and Mutti in 2004 [31

31. D. O. Mutti, “Sources of normal and anomalous motion in retinoscopy, ” Opotom. Vision. Sci . 81, 663–672(2004). [CrossRef]

] investigated the geometric causes of this anomalous motion. Figure 6 shows the simulation of such observation in the myopic eye, MY3, with 5.65 mm pupil. Under plane-mirror operation, the myopic eye should be against-motion, but because of the edge-effect that occurs at sideways, 5, 7, and 9 mm, the reflex motion appears as with-motion. If one looks carefully at the center images without edge influence, the movement of reflex, although not clear, is against-motion as it should be. This is more evident from the false-color images.

Fig. 4. Predicted retinal reflex motion of neutralization (top), with motion (middle), and against motion (bottom). Sleeve of retinoscope is located at 19 mm above the filament (plane-mirror).
Fig. 5. Predicted retinal reflex motion of with motion and against motion under concave-mirror operation. Sleeve of retinoscope is located at 21 mm above the filament.
Fig. 6. Anomalous retinal reflex of a myopic eye from a streak etinoscope.The top row shows the streak beam swiping from the left to the center of pupil. Sleeve position of retinoscope is 19 mm above the filament (plane mirror).

3.4 Scissors reflex in the keratoconus patient

The simulation results of the keratoconus (KC) eye are shown in Fig. 7.This KC eye has a protruding cone of about 60 μm in the lower left quadrant in its topography [11

11. Y.-L. Chen, B. Tan, K. Baker, J. W. L. Lewis, T. Swartz, Y. Jiang, and M. Wang, “Simulation of keratoconus observation in photorefraction,” Opt. Express 14, 11477–11485 (2006). [CrossRef] [PubMed]

]. The manifest refraction is (S-6.00D, C+6.00D, X135), and the best-correction RMS WA is 1.994 μm. The upper set of images in Fig. 7 shows the result of rotating the retinoscope projection at a distance of 0.5 meter. Although the refraction of -6.00D is significant, the strip-shaped reflex is not observed. Instead, a typical keratoconus “shadow” appears in the retinoscopic reflex. The irregular intensity distribution shows the significant high-order aberration and especially the coma of this eye. The lower set of images shows the so-called scissors reflex of KC eye as the projection moves along the meridian of 135 degree. The opening and closing movements of a pair of scissors is clearly shown

Fig. 7. Simulated retinoscopic observation of a keratoconus eye. The upper images show the observation when the streak rotates along the pupillary axis. The lower images show the observation when streak swipes across the pupil in 135 degree angle. The scissors reflex that indicates the irregular cornea surface is clearly shown.

4. Summary

The realistic simulation and illustration of retinoscopic measurement are presented. We demonstrated the theoretical prediction of the ophthalmic measurement using three-dimensional ray tracing and anatomically accurate eye models. Both the personalized eye model and general symmetric eye models are employed. The technique can be applied to predict the ophthalmic measurements of eyes of various conditions. The application can be directly used for medical training and for evaluating performance of ophthalmic instruments.

Acknowledgment

This study was partially supported by ARMY TATRC Grant W81XWH-05-1-0409, the University of Tennessee Space Institute’s Center for Laser Applications, and a NASA Space Grant Fellowship.

References and links

1.

A. Gullstrand, “The Optical System of the Eye, ” in Physiological Optics, 3rd ed., H. von Helmholtz (Hamburg, Voss, 1909), Vols. 1 and 2, pp.350–358.

2.

H. Von Helmholtz, Physiological Optics, 3rd ed. (Hamburg, Voss, 1909), Vols. 1 and 2, pp. 91–121.

3.

Y. Le Grand, Optique physiologique. T. 1. Dioptrique de l’oeil er sa correlations. English translation by S. G. el Hage. (Berlin, Springer-Verlag, 1980), pp. 64–67.

4.

W. Lotmar, “Theoretical eye model with aspherics, ” J. Opt. Soc. Am . 16, 1522–1529 (1971). [CrossRef]

5.

R Navarro, J. Santamaria, and J. Bescos, “Accommodation-dependent model of the human eye with aspherics, ” J. Opt. Soc. Am. A 2, 1273–1278 (1985). [CrossRef] [PubMed]

6.

I. Escudero-Sanz and R. Navarro, “Off-axis aberrations of a wide-angle schematic eye model, ” J. Opt. Soc. Am. A, Opt. Image Sci. Vis . 16, 1881–1891 (1999). [CrossRef]

7.

S. Marcos, S. A. Burns, P. M. Prieto, R. Navarro, and B. Baraibar, “Investigating sources of variability of monochromatic and transverse chromatic aberrations across eyes, ” Vision Res . 41, 3861–3871 (2001). [CrossRef] [PubMed]

8.

I. H. Al-Ahdali and M. A. El-Messiery, “Examination of the effect of the fibrous structure of a lens on the optical characteristics of the human eye: a computer-simulated model, ” Appl. Opt . 34, 5738–5745 (1995). [CrossRef] [PubMed]

9.

H. Liou and N. Brennan, “Anatomically accurate, finite model eye for optical modeling, ” J. Opt. Soc. Am. A 14, 1684–1695 (1997). [CrossRef]

10.

R. Navarro, L. Gonzalez, and J. L. Hernandez-Matamoros, “On the prediction of optical aberrations by personalized eye models, ” Optom Vision Sci . 83,371–381 (2006). [CrossRef]

11.

Y.-L. Chen, B. Tan, K. Baker, J. W. L. Lewis, T. Swartz, Y. Jiang, and M. Wang, “Simulation of keratoconus observation in photorefraction,” Opt. Express 14, 11477–11485 (2006). [CrossRef] [PubMed]

12.

S. MacRae, J. Schwiegerling, and R. W. Snyder, “Customized and low spherical aberration corneal ablation design, ” J. Refract. Surg . 15, S246–S248 (1999). [PubMed]

13.

M. Mrochen, C. Donitzky, C. Wullner, and J. Loffler, “Wavefront-optimized ablation profiles: theoretical background, ” J. Cataract Refractive Surg . 30, 775–785 (2004). [CrossRef]

14.

J. B. Almeida and A. M. Garcia, “Theoretical calculation of a contact lens thickness designed to correct the eye’s monochromatic aberrations, ” Optom. Vision Sci . 82, 59–63 (2005).

15.

P. R. Preussner, J. Wahl, and D. Weitzel, “Topography-based intraocular lens power selection, ” J. Cataract Refractive Surg . 31, 525–33(2005). [CrossRef]

16.

R. B. Bennett , Clinical Visual Optics, 3rd ed., (Butterworth-Heinemann, Oxford, 1998).

17.

J. I. PascalModern Retinoscopy (Hatton, London, 1930).

18.

E. Landolt, “ Methods of determination of the refraction and accommodation of the eye,” in The Refraction and Accommodation of the Eye, C. M. Culver, ed. (Pentland, Edinburgh, 1886).

19.

W. Swaine, “ Retinoscopy. I. General outline and symbols, “ Optician, April, 171–179 (1945).

20.

W. Swaine, “Retinoscopy. II. Basic theoretical principles, ” Optician, June, 335–339 (1945).

21.

W. Swaine, “Retinoscopy. III. Variation of illumination across the reflex, ” Optician, August, 35–40 (1945).

22.

W. Swaine, “Retinoscopy. IV. Variation of the reflex brightness with refractive error and pupil diameter, ” Optician, August, 71–74 (1945).

23.

A. Roorda and W. R. Bobier, “Geometrical technique to determine the influence of monochromatic aberration on retinoscope, ” J. Opt. Soc. Am. A , 13, 3–11 (1996). [CrossRef]

24.

M. T. Caballero, W. D. Furlan, A. Pons, G. Saavedra, and M. Martinez-Corral, “Detection of wave aberrations in the human eye using a retinoscopy-like technique, ” Opt. Comm . 260, 767–771 (2006). [CrossRef]

25.

G. Smith and S. Haymes, “The streak retinoscopy pupil reflex in the presence of astigmatism, ” Ophthal. Physiol. Opt . 23, 295–305 (2003). [CrossRef]

26.

J.-M. Gorrand, A. Alfieri, and J.-Y Boire, “Diffusion of the retinal layers of the living human eye, ” Vision Res . 24, 1097–1106 (1984). [CrossRef] [PubMed]

27.

G. J. van Blokland, “Directionality and alignment of the foveal receptors, assessed with light scattered from the human fundus in vivo, ” Vision Res . 26, 495–500 (1986). [CrossRef] [PubMed]

28.

R. Röhler and F. Schmeilau, “Properties of isolated frog retinae in reflecting non-polarized and polarized light, ” Vision Res . 16, 241–246 (1976). [CrossRef] [PubMed]

29.

I. M. Borish, Clinical refraction, 3rd ed. (Professional press, Chicago, 1970).

30.

H. C. Howland, “Retinoscopy of infants at a distance: limits of normal and anomalous reflexes, ” Vision Res . 18, 597–599 (1978). [CrossRef] [PubMed]

31.

D. O. Mutti, “Sources of normal and anomalous motion in retinoscopy, ” Opotom. Vision. Sci . 81, 663–672(2004). [CrossRef]

OCIS Codes
(170.4460) Medical optics and biotechnology : Ophthalmic optics and devices
(170.4580) Medical optics and biotechnology : Optical diagnostics for medicine
(330.4060) Vision, color, and visual optics : Vision modeling
(330.4460) Vision, color, and visual optics : Ophthalmic optics and devices

ToC Category:
Vision, Color, and Visual Optics

History
Original Manuscript: December 4, 2006
Revised Manuscript: February 4, 2007
Manuscript Accepted: February 12, 2007
Published: March 5, 2007

Virtual Issues
Vol. 2, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Bo Tan, Ying-Ling Chen, K. Baker, J. W. Lewis, T. Swartz, Y. Jiang, and M. Wang, "Simulation of realistic retinoscopic measurement," Opt. Express 15, 2753-2761 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-5-2753


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References

  1. A. Gullstrand, "The Optical System of the Eye," in Physiological Optics, 3rd ed., H. von Helmholtz (Hamburg, Voss, 1909), Vols. 1 and 2, pp. 350-358.
  2. H. Von Helmholtz, Physiological Optics, 3rd ed. (Hamburg, Voss, 1909), Vols. 1 and 2, pp. 91-121.
  3. Y. Le Grand, Optique physiologique. T. 1. Dioptrique de l’oeil er sa correlations. English translation by S. G. el Hage. (Berlin, Springer-Verlag, 1980), pp. 64-67.
  4. W. Lotmar, "Theoretical eye model with aspherics," J. Opt. Soc. Am. 16, 1522-1529 (1971). [CrossRef]
  5. R Navarro, J. Santamaria, and J. Bescos, "Accommodation-dependent model of the human eye with aspherics," J. Opt. Soc. Am. A 2, 1273-1278 (1985). [CrossRef] [PubMed]
  6. I. Escudero-Sanz and R. Navarro, "Off-axis aberrations of a wide-angle schematic eye model," J. Opt. Soc. Am. A,  16, 1881-1891 (1999). [CrossRef]
  7. S. Marcos, S. A. Burns, P. M. Prieto, R. Navarro, and B. Baraibar, "Investigating sources of variability of monochromatic and transverse chromatic aberrations across eyes," Vision Res. 41, 3861-3871 (2001). [CrossRef] [PubMed]
  8. I. H. Al-Ahdali, M. A. El-Messiery, "Examination of the effect of the fibrous structure of a lens on the optical characteristics of the human eye: a computer-simulated model," Appl. Opt. 34, 5738-5745 (1995). [CrossRef] [PubMed]
  9. H. Liou and N. Brennan, "Anatomically accurate, finite model eye for optical modeling," J. Opt. Soc. Am. A 14, 1684-1695 (1997). [CrossRef]
  10. R. Navarro, L. Gonzalez, and J. L. Hernandez-Matamoros, "On the prediction of optical aberrations by personalized eye models," Optom Vision Sci. 83, 371-381 (2006). [CrossRef]
  11. Y.-L. Chen, B. Tan, K. Baker, J. W. L. Lewis, T. Swartz, Y. Jiang, and M. Wang, "Simulation of keratoconus observation in photorefraction," Opt. Express 14, 11477-11485 (2006). [CrossRef] [PubMed]
  12. S. MacRae, J. Schwiegerling, and R. W. Snyder, "Customized and low spherical aberration corneal ablation design," J. Refract. Surg. 15, S246-S248 (1999). [PubMed]
  13. M. Mrochen, C. Donitzky, C. Wullner, and J. Loffler, "Wavefront-optimized ablation profiles: theoretical background," J. Cataract Refractive Surg. 30, 775-785 (2004). [CrossRef]
  14. J. B. Almeida and A. M. Garcia, "Theoretical calculation of a contact lens thickness designed to correct the eye's monochromatic aberrations," Optom. Vision Sci. 82, 59-63 (2005).
  15. P. R. Preussner, J. Wahl, and D. Weitzel, "Topography-based intraocular lens power selection," J. Cataract Refractive Surg. 31, 525-33 (2005). [CrossRef]
  16. R. B. Bennett, Clinical Visual Optics, 3rd ed., (Butterworth-Heinemann, Oxford, 1998).
  17. J. I. Pascal, Modern Retinoscopy (Hatton, London, 1930).
  18. E. Landolt, "Methods of determination of the refraction and accommodation of the eye," in The Refraction and Accommodation of the Eye, C. M. Culver, ed. (Pentland, Edinburgh, 1886).
  19. W. Swaine, "Retinoscopy. I. General outline and symbols," Optician, April, 171-179 (1945).
  20. W. Swaine, "Retinoscopy. II. Basic theoretical principles," Optician, June, 335-339 (1945).
  21. W. Swaine, "Retinoscopy. III. Variation of illumination across the reflex," Optician, August, 35-40 (1945).
  22. W. Swaine, "Retinoscopy. IV. Variation of the reflex brightness with refractive error and pupil diameter," Optician, August, 71-74 (1945).
  23. A. Roorda and W. R. Bobier, "Geometrical technique to determine the influence of monochromatic aberration on retinoscope," J. Opt. Soc. Am. A,  13, 3-11 (1996). [CrossRef]
  24. M. T. Caballero, W. D. Furlan, A. Pons, G. Saavedra, and M. Martinez-Corral, "Detection of wave aberrations in the human eye using a retinoscopy-like technique," Opt. Commun. 260, 767-771 (2006). [CrossRef]
  25. G. Smith and S. Haymes, "The streak retinoscopy pupil reflex in the presence of astigmatism," Ophthalmic Physiol. Opt. 23, 295-305 (2003). [CrossRef]
  26. J.-M. Gorrand, A. Alfieri, and J.-Y. Boire, "Diffusion of the retinal layers of the living human eye," Vision Res. 24,1097-1106 (1984). [CrossRef] [PubMed]
  27. G. J. van Blokland, "Directionality and alignment of the foveal receptors, assessed with light scattered from the human fundus in vivo," Vision Res. 26,495-500 (1986). [CrossRef] [PubMed]
  28. R. Röhler and F. Schmeilau, "Properties of isolated frog retinae in reflecting non-polarized and polarized light," Vision Res. 16,241-246 (1976). [CrossRef] [PubMed]
  29. I. M. Borish, Clinical refraction, 3rd ed. (Professional press, Chicago, 1970).
  30. H. C. Howland, "Retinoscopy of infants at a distance: limits of normal and anomalous reflexes," Vision Res. 18, 597-599 (1978). [CrossRef] [PubMed]
  31. D. O. Mutti, "Sources of normal and anomalous motion in retinoscopy," Opotom. Vision.Sci. 81, 663-672 (2004). [CrossRef]

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