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Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 4, Iss. 2 — Feb. 10, 2009
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Ocular aberrations up to the infrared range: from 632.8 to 1070 nm

Enrique J. Fernández and Pablo Artal  »View Author Affiliations


Optics Express, Vol. 16, Issue 26, pp. 21199-21208 (2008)
http://dx.doi.org/10.1364/OE.16.021199


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Abstract

Ocular aberrations were measured by using a Hartmann-Shack wavefront sensor in the visible and infrared portions of the spectrum. In the latter, wavelengths 1030, 1050 and 1070 nm were used for the first time for the study of the optical quality of the eye. In this spectral range the retinal photoreceptors barely respond, so the radiation is virtually invisible for the subject. The results were confronted with those obtained by the same system at 780 and 632.8 nm. Monochromatic aberrations were found to be similar from the visible to the infrared. Longitudinal chromatic aberration was experimentally obtained, being approximately 1 D from 632.8 to 1070 nm. The feasibility of using the infrared for studying the eye was demonstrated. The employment of the infrared has an enormous potential for the better understanding of the impact and influence of the aberrations in vision with adaptive optics. It allows for measuring and controlling aberrations whilst the subject might eventually perform visual tests, with no interference from the beacon light.

© 2008 Optical Society of America

1. Introduction

2. Methods

The experimental apparatus for measuring ocular aberrations was essentially compounded by a Hartmann-Shack wavefront sensor, three different light sources and appropriate optics for conjugating the eye’s exit pupil onto the sensor. It is conceptually similar to a wavelength tunable wavefront sensor for the eye, recently used for the visible part of the spectrum [19

19. S. Manzanera, C. Canovas, P. M. Prieto, and P. Artal, “A wavelength tunable wavefront sensor for the human eye,” Opt. Express 16, 7748–7755 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-11-7748. [CrossRef] [PubMed]

]. Figure 1 depicts the set-up showing the main components. The wavefront sensor was formed by a CCD camera (Hamamatsu C5999, Japan) placed at the focal length of an array of microlenses. The camera’s CCD chip was based on Silicon with sensitivity typically up to the 1300 nm according to the manufacturer, and enhanced quantum efficiency in the NIR (peak at 800 nm). The camera video format was RS-170 at 30 Hz frame rate, with internal electronic exposure time of 1/500 s. The microlenses were square cells of 0.3 mm and 6 mm focal length. They were implemented in the mount of the camera for simplicity. A telescope formed by two positive achromatic doublets, L1 and L2 of 250 and 150 mm of focal length (-0.6 total magnification), conjugated the eye’s exit pupil with the array of microlenses. Lenses had broad antireflection coating, reducing losses in the visible and IR part of spectrum.

Fig. 1. Experimental set-up. Three light sources illuminated consecutively the eye. Ocular aberrations were obtained by a Hartmann-Shack (H-S) wavefront sensor at wavelengths 632.8, 780, 1030, 1050 and 1070 nm. Fixation test was employed in the system during the runs.

Due to the large spectral range employed in the experiment, almost 440 nm, chromatic aberration of the system was first calculated. That was of practical interest, especially for the possible estimation of the eye’s chromatic refraction. In order to obtain the chromatic aberration of the apparatus, the system was modeled by using the optical design program Zemax (Zemax Development Corporation, USA). The maximum chromatic aberration was found to occur from 780 to 1070 nm, with a value of 0.025 D. The rest of monochromatic aberrations in the system were negligible, being diffraction limited.

3. Results

Fig. 2. H-S images obtained from different subjects at 632.8 and 1050 nm showing the augment of scattering with wavelength. Below images, average peak and pedestal value are presented.

Due to the different spectral response of the detector and the distinct irradiance applied for each wavelength, the images presented in Fig. 2 provided solely a qualitatively comparison of the existing change of scattering across wavelengths. In order to obtain numerical estimation, further analysis of the images was performed. For doing so, a systematic procedure was adopted, described in the following.

Fig. 3. Average pedestal-peak ratio (PPR) from all subjects as a function of wavelength. Error bars corresponded to the standard deviation.

Hartmann-Shack images obtained at every wavelength were averaged. The central lines of spots from these images were taken. Corresponding sub-images from the latter, saved with equal width than the squared microlenses, were then processed. Average intensity values from columns were obtained. These data allowed for clearly identifying the spots and the space across them. Maxima from the spots peaks and average values from the baseline, i.e. inter-spots space, were manually retrieved, and analyzed with the help of some routines written in Matlab (Matlab 7.6, The MathWorks, USA). Averaging was performed in order to obtain two single values from each Hartmann-Shack image: the average peak value and the pedestal or baseline average value. As an estimator of the relative scattering we defined the pedestal-peak ratio (PPR, the quotient between these two averaged parameters). The PPR provided a simplified method to numerically compare the degree of scattering from the Hartmann-Shack images. The procedure was systematically applied to all subjects and wavelengths. The results were presented in Figure 3. The PPR from all subjects is depicted as a function of wavelength. The error bars represent the standard deviation from all subjects. Fig.3 shows a clear jump in the PPR from 632.8 to 780 nm, indicating a significant change in the scattering. The scattering estimated at 1030, 1050 and 1070 nm was found to be very similar to that at 780 nm. These results could be of importance for modeling the reflectance and diffusion of the retina. The use of light with wavelengths longer than 780 nm for retinal illumination does not seem to produce a significant increase in the scattering.

Fig. 4. Zernike coefficients from two subjects at 632.8, 780, 1030, 1050 and 1070 nm, together with aberration maps. The mean value from all wavelengths was also included (pink color).

Fig. 5. RMS, excluding defocus, of ocular aberration from each subject for a 5.8 mm pupil size at different wavelengths. Average RMS was shown in pink color. Error bars corresponded with standard deviation.

An alternative way to evaluate the change of aberrations with wavelength, using a single parameter, is calculating the root mean square (RMS) of the wavefront. Figure 5 shows the RMS from all subjects and wavelengths. The mean value was also included. The latter incorporated an error bar which corresponded to the standard deviation. The error bars were similar across subjects. From the figure, it could be also noticed the lack of any trend in the variations of the RMS with wavelength. The term corresponding to defocus was not included in the calculation of the RMS. Once the results concerning monochromatic aberrations have been shown, longitudinal chromatic aberration will be studied in the following.

The longitudinal chromatic aberration was estimated as the difference of defocus obtained from the wavefront sensor for each wavelength. The results from all subjects are presented in Fig. 6, where defocus is shown in diopters. Since the subjects presented different refractions was necessary to adopt a reference for comparing the chromatic defocus. The shortest wavelength 632.8 was taken as the origin for defocus. The experimental results were depicted with black points. The errors bars corresponded to the standard deviation obtained from each subject. As expected, there was a change in defocus connected with the variation of refractive index of the different ocular media. In order to better understand the impact of longitudinal chromatic aberration in the IR, the figure incorporated the theoretical curve of chromatic defocus, in blue color, proposed by Atchison and Smith 2005 [18

18. D. A. Atchison and G. Smith, “Chromatic dispersions of the ocular media of human eyes,” J. Opt. Soc. Am. A 22, 29–37 (2005). [CrossRef]

]. The curve was also forced for taking 632.8 nm as origin. In the aforementioned reference, the authors compiled a vast set of published experimental measurements of chromatic defocus for finding the best theoretical fit to the data. The spectral range employed was from 400 up to 900 nm. The mathematical function selected for modeling the data was the Cauchy equation. The figure shows that points corresponding with 1030, 1050 and 1070 nm are located away from the theoretical curve. In particular, the distance between the curve and the point corresponding with 1050 nm was 0.15 D. Assuming the parameters of the Cauchy equation are still valid in the IR, a possible explanation for the origin of this effect might be the different penetration of the beam at these wavelengths as compared with that in the visible. For visible light it is usually accepted that most of the light recorded at the wavefront sensor, backscattered from the fundus, originates at the retinal pigment epithelium. Likely, and due to the difference of refractive indexes, the diffusion of light could occur in the border between retinal pigment epithelium and outer segments of photoreceptors. Employing longer wavelengths might allow for deeper penetration into the retinal pigment epithelium. Consequently, light backscattering might happen more efficiently at deeper layers, even though the beam was slighted defocused.

Fig. 6. Averaged chromatic defocus from all subjects as a function of wavelength. Error bars showed the standard deviation.

We theoretically estimated the magnitude of the 0.15 D mismatch. The penetration into the retinal tissue required for placing the experimental point 1050 nm over the theoretical curve was calculated. By using the Liou and Brennan eye model [20

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

], the 0.15 D corresponded to an absolute displacement on the image plane, i.e. the retina, of 0.55 µm.

4. Conclusions

In this work we have demonstrated the feasibility for measuring ocular aberrations in the IR, at 1030, 1050 and 1070 nm, with a standard CCD camera coupled in a Hartmann-Shack wavefront sensor. The camera used in the experiment had a regular Silicon-based CCD detector, making the apparatus cost effective. Other cameras operating in the IR with enhanced performance are commercially available. Specifically, CCD cameras with detectors based on InGaAs offer higher sensitivity in the IR. However, their cost is at least one order of magnitude superior to regular Silicon CCD. For research purposes they might offer advantages as improved signal-to-noise ratio. Nevertheless, we have demonstrated that standard Silicon CCD still offers enough sensitivity, allowing a significant reduction of the total cost of the experimental set-up. This fact might have a positive impact in the potential use of this new spectral range in clinical applications. In addition, the cost of the system could be further reduced if super luminescent diode laser is employed as light source.

From the analysis of the Hartmann-Shack images it was found that light scattering at 1050 nm was similar to that measured at 780 nm. That indicated that the signal-to-noise ratio appeared comparable, so there were no particular constraints or need for refinement in the Hartmann-Shack algorithms for locating the centroids from the images of the sensor. The use of IR produces in general a similar estimation of the wavefront as compared with that in the visible from the point of view of stability of the measurements. The variance of the RMS, obtained from the variance of the individual Zernike coefficients, shows similar values across wavelengths. As an example showing the typical trend found in the measurements, the error for the RMS from subject AB for wavelength 632.8, 780, 1030, 1050 and 1070 nm was 0.09, 0.13, 0.10, 0.08 and 0.09, respectively. Regarding the PPR, it should be noted that solely changing the wavelength should produce a slight variation in the aforementioned parameter. The cause could be understood in terms of the broadening of the point spread function caused by increasing the wavelength. In the experiment presented in this work, irradiance was not kept constant, so practical evaluation of the effect was not possible. Moreover, even under such ideal conditions, retina would not backscatter light from different wavelengths with equal efficiency to the sensor. We think the possible effect of broadening the Hartmann-Shack spots associated to wavelength is modest and would not change significantly the trend exhibited in Fig 3.

The chromatic difference of focus was near 1 D from 632.8 to 1070 nm. That was a modest difference as compared to that expected for the same spectral width in the visible. In the latter, a displacement from 400 to 800 nm will generate a chromatic defocus of approximately 3 D [18

18. D. A. Atchison and G. Smith, “Chromatic dispersions of the ocular media of human eyes,” J. Opt. Soc. Am. A 22, 29–37 (2005). [CrossRef]

,21

21. R. E. Bedford and G. Wyszecki, “Axial chromatic aberration of the human eye,” J. Opt. Soc. Am. 47, 564–565 (1957). [CrossRef] [PubMed]

24

24. L. N. Thibos, M. Ye, X. Zhang, and A. Bradley, “The chromatic eye: a new reduce-eye model of ocular chromatic aberration in humans,” Appl. Opt. 31, 592–599 (1992). [CrossRef]

]. The parameters of the Cauchy equation proposed by Atchison and Smith [18

18. D. A. Atchison and G. Smith, “Chromatic dispersions of the ocular media of human eyes,” J. Opt. Soc. Am. A 22, 29–37 (2005). [CrossRef]

] for describing chromatic aberration were obtained with experimental data up to 900 nm. Consequently, the equation did not incorporate the possible changes in refractive index occurring in wavelengths beyond. We have assumed in Fig. 6 that the change in chromatic defocus followed a similar trend, so extrapolation of the Cauchy equation up to 1070 nm was possible. Nevertheless, this fact must be taken carefully until experimental confirmation is reported in the future. Possibly, the discrepancy between theoretical curve and experimental points around 1050 nm arouse as a combination of two factors: the parameters describing Cauchy equation might be slightly different, and there might be also a deeper penetration into the retina. The relative contribution of each of these causes is to be determined with further studies.

A result of practical importance is the constancy of the monochromatic aberrations from the visible range up to the IR. This fact allows for effectively measuring the ocular aberrations with IR light, inferring the optical quality of the eye in the visible. The advantage employing IR is, in addition of safety, the lack of any retinal response at the irradiance levels required for using the wavefront sensor. This could have an impact in a number of experiments. In particular, the use of adaptive optics for studying the role and influence of ocular aberrations [25

25. E. J. Fernández, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26, 746–748 (2001). [CrossRef]

31

31. K. M. Hampson, C. Paterson, C. Dainty, and E. A. H. Mallen, “Adaptive optics system for investigation of the effect of the aberration dynamics of the human eye on steady-state accommodation control,” J. Opt. Soc. Am. A 23, 1082–1088 (2006). [CrossRef]

] in different aspects of vision could greatly benefit from this novel concept. So far, appropriate instructions must be given to the subjects participating in such experiments, indicating that the beacon light must be neglected and attention must be focused solely in the visual target. Some subjects find difficulties following such instructions, being even unable for performing the experiment. For these subjects the use of IR is particularly suitable. Moreover, visual experiments under mesopic or scotopic conditions, where the stimuli must be kept a low level of irradiance, including experiments of accommodation, are currently impossible to accomplish with simultaneous ocular aberration measurements. In those experiments, the beacon light is more intensely perceived for the subject that the visual stimulus itself. Employing IR illumination overcomes the problem.

In addition, the results presented in this work might also serve as the basis for combining adaptive optics for ophthalmoscopy in the IR. Recently, the utility of using wavelengths in the surroundings of 1050 nm have been demonstrated in the context of optical coherence tomography [32

32. A. Unterhuber, B. Považay, B. Hermann, H. Sattmann, A. Chavez-Pirson, and W. Drexler, “In vivo retinal optical coherence tomography at 1040 nm - enhanced penetration into the choroid,” Opt. Express 13, 3252–3258 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-9-3252. [CrossRef] [PubMed]

35

35. S. Makita, T. Fabritius, and Y. Yasuno, “Full-range, high-speed, high-resolution 1-µm spectral-domain optical coherence tomography using BM-scan for volumetric imaging of the human posterior eye,” Opt. Express 16, 8406–8420 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-12-8406. [CrossRef] [PubMed]

]. In this ophthalmoscopic modality, retinal images obtained in vivo show a deeper penetration into the choroids region, as compared to those obtained in the visible or NIR. Measuring and correcting ocular aberrations in this spectral range might allow for resolving subtle structures of the retina with important clinical application.

Acknowledgments

The authors thanks Wolfgang Drexler for early discussions on employing IR light in the eye and the subjects participating in the study. This research has been supported by the Spanish Ministry of Science (grant FIS2007-64765) and from “Fundación Seneca”, Región de Murcia, Spain (grant 4524/GERM/06).

References and links

1.

M. S. Smirnov, “Measurement of the wave aberration of the human eye,” Biophys. J. 7, 766–795 (1962).

2.

F. Berny and S. Slansky, “Wavefront determination resulting from Foucault test as applied to the human eye and visual instruments,” in Optical Instruments and Techniques, H. Dickson, 375–386 (Oriel, London, 1970).

3.

H. Howland and B. Howland, “A subjective method for the measurement of monochromatic aberrations of the eye,” J. Opt. Soc. Am. 67, 1508–1518 (1977). [CrossRef] [PubMed]

4.

G. Walsh, W. N. Charman, and H. Howland, “Objective technology for the determination of monochromatic aberrations of the human eye,” J. Opt. Soc. Am. A 1, 987–992 (1984). [CrossRef] [PubMed]

5.

D. R. Williams, D. H. Brainard, M. J. McMahon, and R. Navarro, “Double-pass and interferometric measures of the optical quality ofthe eye,” J. Opt. Soc. Am. A 11, 3123–3135 (1994). [CrossRef]

6.

P. Artal, S. Marcos, R. Navarro, and D. R. Williams, “Odd aberrations and double-pass measurements of retinal image quality,” J. Opt. Soc. Am. A 12, 195–201 (1995). [CrossRef]

7.

I. Iglesias, E. Berrio, and P. Artal, “Estimates of the ocular wave aberration from pairs of double-pass retinal images,” J. Opt. Soc. Am. A. 15, 2466–2476 (1998). [CrossRef]

8.

J. Liang, B. Grimm, S. Goelz, and J. F. Bille, “Objective measurement of WA’s of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A 11, 1949–1957 (1994). [CrossRef]

9.

J. Liang and D. R. Williams, “Aberrations and retinal image quality of the normal human eye,” J. Opt. Soc. Am. A 14, 2873–2883 (1997). [CrossRef]

10.

P. M. Prieto, F. Vargas-Martín, S. Goelz, and P. Artal, “Analysis of the performance of the Hartmann-Shack sensor in the human eye,” J. Opt. Soc. Am. A 17, 1388–1398 (2000). [CrossRef]

11.

D. R. Griffin, R. Hubbard, and G. Wald, “The Sensitivity of the human eye to infra-red radiation,” J. Opt. Soc. Am. 37, 546–553 (1947). [CrossRef] [PubMed]

12.

P. L. Walraven and H. J. Leebeek, “Foveal sensitivity of the human eye in the near infrared,” J. Opt. Soc. Am. 53, 765–766 (1963). [CrossRef] [PubMed]

13.

D. H. Sliney, R. T. Wangemann, J. K. Franks, and M. L. Wolbarsht, “Visual sensitivity of the eye to infrared laser radiation,” J. Opt. Soc. Am. 66, 339–341 (1976). [CrossRef] [PubMed]

14.

American National Standard Institutes “For the safe use of lasers,” ANSI Z136.1-2000, Laser Institute of America, (Orlando, Fla., 2000).

15.

F. C. Delori and K. P. Pflibsen, “Spectral reflectance of the human ocular fundus,” Appl. Opt. 28, 1061–1077 (1989). [CrossRef] [PubMed]

16.

Jan van de Kraats, T. J. M. Tos , Dirk Berendschot, and van Norren, “The pathways of light measured in fundus reflectometry,” Vision Res. 36, 2229–2247 (1996). [CrossRef]

17.

E. J. Fernández, A. Unterhuber, P. Prieto, B. Hermann, W. Drexler, and P. Artal, “Ocular aberrations as a function of wavelength in the near infrared measured with a femtosecond laser,” Opt. Express 13, 400–409 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-2-400. [CrossRef] [PubMed]

18.

D. A. Atchison and G. Smith, “Chromatic dispersions of the ocular media of human eyes,” J. Opt. Soc. Am. A 22, 29–37 (2005). [CrossRef]

19.

S. Manzanera, C. Canovas, P. M. Prieto, and P. Artal, “A wavelength tunable wavefront sensor for the human eye,” Opt. Express 16, 7748–7755 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-11-7748. [CrossRef] [PubMed]

20.

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

21.

R. E. Bedford and G. Wyszecki, “Axial chromatic aberration of the human eye,” J. Opt. Soc. Am. 47, 564–565 (1957). [CrossRef] [PubMed]

22.

W. N. Charman and J. A. Jennings, “Objective measurements of the longitudinal chromatic aberration of the human eye,” Vision Res. 16, 999–1005 (1976). [CrossRef] [PubMed]

23.

P. A. Howarth and A. Bradley, “The longitudinal chromatic aberration of the human eye and its correction,” Vision Res. 26, 361–366 (1986). [CrossRef] [PubMed]

24.

L. N. Thibos, M. Ye, X. Zhang, and A. Bradley, “The chromatic eye: a new reduce-eye model of ocular chromatic aberration in humans,” Appl. Opt. 31, 592–599 (1992). [CrossRef]

25.

E. J. Fernández, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26, 746–748 (2001). [CrossRef]

26.

E. J. Fernández, S. Manzanera, P. Piers, and P. Artal, “Adaptive optics visual simulator,” J. Refract. Surg. 18, 634–638 (2002).

27.

P. Artal, L. Chen, E. J. Fernández, B. Singer, S. Manzanera, and D. R. Williams, “Neural compensation for the eye’s optical aberrations,” J. Vision 4, 281–287 (2004). [CrossRef]

28.

E. J. Fernández and P. Artal, “Study on the effects of monochromatic aberrations in the accommodation response by using adaptive optics,” J. Opt. Soc. Am. A 22, 1732–1738 (2005). [CrossRef]

29.

P. Piers, E. J. Fernández, S. Manzanera, S. Norrby, and P. Artal, “Adaptive optics simulation of intraocular lenses with modified spherical aberration ,” Invest. Ophthalmol. Vis. Sci. 45, 4601–4610 (2004). [CrossRef] [PubMed]

30.

L. Chen, P. B. Kruger, H. Hofer, B. Singer, and D. R. Williams, “Accommodation with higher-order monochromatic aberrations corrected with adaptive optics,” J. Opt. Soc. Am. A 23, 1–8 (2006). [CrossRef]

31.

K. M. Hampson, C. Paterson, C. Dainty, and E. A. H. Mallen, “Adaptive optics system for investigation of the effect of the aberration dynamics of the human eye on steady-state accommodation control,” J. Opt. Soc. Am. A 23, 1082–1088 (2006). [CrossRef]

32.

A. Unterhuber, B. Považay, B. Hermann, H. Sattmann, A. Chavez-Pirson, and W. Drexler, “In vivo retinal optical coherence tomography at 1040 nm - enhanced penetration into the choroid,” Opt. Express 13, 3252–3258 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-9-3252. [CrossRef] [PubMed]

33.

E. C. Lee, J. F. de Boer, M. Mujat, H. Lim, and S. H. Yun, “In vivo optical frequency domain imaging of human retina and choroid,” Opt. Express 14, 4403–4411 (2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-10-4403. [CrossRef] [PubMed]

34.

R. Huber, D. C. Adler, V. J. Srinivasan, and J. G. Fujimoto, “Fourier domain mode locking at 1050 nm for ultra-high-speed optical coherence tomography of the human retina at 236,000 axial scans per second,” Opt. Lett. 32, 2049–2051 (2007). [CrossRef] [PubMed]

35.

S. Makita, T. Fabritius, and Y. Yasuno, “Full-range, high-speed, high-resolution 1-µm spectral-domain optical coherence tomography using BM-scan for volumetric imaging of the human posterior eye,” Opt. Express 16, 8406–8420 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-12-8406. [CrossRef] [PubMed]

OCIS Codes
(010.1080) Atmospheric and oceanic optics : Active or adaptive optics
(330.4460) Vision, color, and visual optics : Ophthalmic optics and devices
(330.5370) Vision, color, and visual optics : Physiological optics

ToC Category:
Vision, color, and visual optics

History
Original Manuscript: October 17, 2008
Revised Manuscript: November 28, 2008
Manuscript Accepted: December 2, 2008
Published: December 8, 2008

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

Citation
Enrique J. Fernández and Pablo Artal, "Ocular aberrations up to the infrared range: from 632.8 to 1070 nm," Opt. Express 16, 21199-21208 (2008)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-16-26-21199


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References

  1. M. S. Smirnov, "Measurement of the wave aberration of the human eye," Biophys. J. 7, 766-795 (1962).
  2. F. Berny and S. Slansky, "Wavefront determination resulting from Foucault test as applied to the human eye and visual instruments," in Optical Instruments and Techniques, H. Dickson, 375-386 (Oriel, London, 1970).
  3. H. Howland and B. Howland, "A subjective method for the measurement of monochromatic aberrations of the eye," J. Opt. Soc. Am. 67, 1508-1518 (1977). [CrossRef] [PubMed]
  4. G. Walsh, W. N. Charman, and H. Howland, "Objective technology for the determination of monochromatic aberrations of the human eye," J. Opt. Soc. Am. A 1, 987-992 (1984). [CrossRef] [PubMed]
  5. D. R. Williams, D. H. Brainard, M. J. McMahon, and R. Navarro, "Double-pass and interferometric measures of the optical quality ofthe eye," J. Opt. Soc. Am. A 11, 3123-3135 (1994). [CrossRef]
  6. P. Artal, S. Marcos, R. Navarro, and D. R. Williams, "Odd aberrations and double-pass measurements of retinal image quality," J. Opt. Soc. Am. A 12, 195-201 (1995). [CrossRef]
  7. I. Iglesias, E. Berrio, and P. Artal, "Estimates of the ocular wave aberration from pairs of double-pass retinal images," J. Opt. Soc. Am. A. 15, 2466-2476 (1998). [CrossRef]
  8. J. Liang, B. Grimm, S. Goelz, and J. F. Bille, "Objective measurement of WA’s of the human eye with the use of a Hartmann-Shack wave-front sensor," J. Opt. Soc. Am. A 11, 1949-1957 (1994). [CrossRef]
  9. J. Liang and D. R. Williams, "Aberrations and retinal image quality of the normal human eye," J. Opt. Soc. Am. A 14, 2873-2883 (1997). [CrossRef]
  10. P. M. Prieto, F. Vargas-Martín, S. Goelz, and P. Artal, "Analysis of the performance of the Hartmann-Shack sensor in the human eye," J. Opt. Soc. Am. A 17, 1388-1398 (2000). [CrossRef]
  11. D. R. Griffin, R. Hubbard, and G. Wald, "The Sensitivity of the human eye to infra-red radiation," J. Opt. Soc. Am. 37, 546-553 (1947). [CrossRef] [PubMed]
  12. P. L. Walraven and H. J. Leebeek, "Foveal sensitivity of the human eye in the near infrared," J. Opt. Soc. Am. 53, 765-766 (1963). [CrossRef] [PubMed]
  13. D. H. Sliney, R. T. Wangemann, J. K. Franks, and M. L. Wolbarsht, "Visual sensitivity of the eye to infrared laser radiation," J. Opt. Soc. Am. 66, 339-341 (1976). [CrossRef] [PubMed]
  14. American National Standard Institutes "For the safe use of lasers," ANSI Z136.1-2000, Laser Institute of America, (Orlando, Fla., 2000).
  15. F. C. Delori and K. P. Pflibsen, "Spectral reflectance of the human ocular fundus," Appl. Opt. 28, 1061-1077 (1989). [CrossRef] [PubMed]
  16. J. van de Kraats, T. T. J. M. Berendschot, and D. van Norren, "The pathways of light measured in fundus reflectometry," Vision Res. 36, 2229-2247 (1996). [CrossRef]
  17. E. J. Fernández, A. Unterhuber, P. Prieto, B. Hermann, W. Drexler, and P. Artal, "Ocular aberrations as a function of wavelength in the near infrared measured with a femtosecond laser," Opt. Express 13, 400-409 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-2-400. [CrossRef] [PubMed]
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