WO2008000008A2

WO2008000008A2 - Achromatising triplet for the human eye

Info

Publication number: WO2008000008A2
Application number: PCT/AT2007/000312
Authority: WO
Inventors: Enrique Joshua Fernandez Martinez; Wolfgang Drexler
Original assignee: Medizinische Universität Wien
Priority date: 2006-06-26
Filing date: 2007-06-26
Publication date: 2008-01-03
Also published as: WO2008000008A3; WO2008000008A8

Abstract

An ophtalmoscopic achromatizing lens unit (1) having planar outer surfaces (5, 6), and comprising a first, a second and a third lens element (2, 3, 4) arranged one behind the other, as seen in the path (15) of a light beam passing through the lens unit, wherein the first and third lens elements (2, 4) comprise one and the same first glass material, and the second, inner lens element (3) comprises a second glass material.

Description

Ophtalmoscopic Achromatizing Lens Unit

The present invention relates to an ophtalmoscopic achromatizing lens unit having planar outer surfaces, and comprising a first, a second and a third lens element arranged one behind the other, as seen in the path of a light beam passing the lens unit.

Furthermore, this invention refers to an ophthalmoscopic arrangement, and a vision enhancement arrangement comprising such an achromatizing lens unit.

The optical media of the human eye exhibit a significant chromatic dispersion, the respective refractive indexes being a function of wavelength. This fact is manifested in an ocular chromatic aberration. In order to simplify its study, it is common practice to separate this particular aberration in the longitudinal or axial chromatic aberration (LCA) , and in the transverse or lateral chromatic aberration (TCA) . LCA generates a change in the focussing plane as a function of the wavelength and it is also referred to as chromatic defocus. TCA appears with off-axis illumination, essentially producing a variation of the angle of the light, referred to the image plane, as a function of the considered wavelength. As a consequence, TCA produces the effective magnification to be wavelength dependent. Therefore, this aberration is typically associated with extended objects. Measurements of LCA typically use many different objective and subjective methods in the visible range. The magnitude of the LCT is ~ 2.5 D (D - diopter) from 400 to 700 nm. LCA is known to be similar for different eyes, and essentially independent of age. LCA is also practically invariant as a function of field for small and moderate ranges. Therefore, ocular LCA can be assumed to be constant for a given spectral range, in particular as compared to monochromatic aberrations, which present a much larger variability among subjects. Only recently, the LCA of the eyes has been mathematically modeled in the near infrared (NIR) portion of the spectrum; compare D.A. Atchinson and G. Smith, ,,Chromatic dispersion of the ocular media of human eyes", J. Opt. Soc. Am. A 22, 29-36 (2005). From 700 to 900 nm the value of the ocular LCA is ~ 0.4 D. Achromatizing the human eye by using refractive elements has been accomplished in the past in the visible range, exclusively for visual applications. Different lens designs have been proposed for achromatizing the human eye. The last achromatizing lens unit reported for the visible range based on a triplet lens unit design was described in A. L. Lewis, M. Katz, and C. Oehr- lein, ,,A modified achromatizing lens", Am. J. Optom. Physiol. Opt. 59, 909-911 (1982) . All prior art lenses suffered of a rather limited field rapidly increasing the LCA off-axis. In order to overcome this important limitation, which is particularly important when testing the lenses with extended polychromatic objects, the use of a more complex design to achromatize the eye, with a triplet lens and a doublet air space, was proposed, compare I. Powell, ^Lenses for correcting chromatic aberration of the eye", App. Opt. 20, 4152-4155 (1981) .

It should also be mentioned that from US 6,879,448 B, there is already known a triplet lens unit comprising a bi-concave inner lens element between two outer lens elements having different thicknesses (so that an asymmetrical design is given) , and being comprised of three different materials. Accordingly, this known lens unit is relatively complicated in manufacture, and the costs for manufacture are relatively high. Furthermore, this achromatizing lens unit is designed for general laser beam arrangements only.

All previous lens designs were fundamentally intended to enhance vision by exactly introducing the opposite LCA found in the human eye. However, achromatizing lens units have been neither used nor proposed for retinal imaging purposes, and different spectral ranges than the visible have never been explored so far. Probably, a fundamental reason for such a lack of interest in other applications or different spectral ranges has been the extended use of monochromatic or quasi-monochromatic light sources in ophthalmoscopy, therefore avoiding chromatic aberrations. In addition, modern ophthalmoscopes preferably use infrared light, and only very recently the associated LCA in this range has been referred to; s. E. J. Fernandez, A. Unterhuber, P. M. Prieto, B. Hermann, W. Drexler, and P. Artal, ,,0cular aberration as a function of wavelength in the near infrared meas- ured with a femtosecond laser", Opt. Express 13, 400-409 (2005) .

Nevertheless, a relatively new ophthalmic imaging modality has been developed in recent years, where the use of poychromatic light sources is absolutely mandatory, namely optical coherence tomography (OCT) . This non-invasive ophthalmic technique is based on white light interferometry. In OCT, axial resolution is governed by the spectral bandwidth of the light source. The use of broad bandwidth pulsed lasers, emitting continuous Gaussian spectra in the near infrared range of 130 nm FWHM (FWHM - full with half maximum) , has enabled up to 2-3 μm of axial resolution in the living retina in ultra high resolution (UHR) OCT. By using spectral light sources of ~ 50 μm FWHM, and UHR OCT, transverse resolution however remains in the order of 20 μm. In order to increase transverse resolution in this ophthalmic modality, the expansion of pupils and beams in the system with the simultaneous use of adaptive optics to compensate the monochromatic aberrations of the eye has been proposed and demonstrated, in both standard OCT and in UHR OCT. In this context, an interesting paradox has recently been reported when correcting monochromatic aberration through a 6 mm pupil size, according to E. J. Fernandez, B. Povazay, B. Hermann, A. Unterhuber, H. Sattmann, P. M. Prieto, R. Leitgeb, P. Ahnelt, P. Artal, and W. Drexler, ,,Three Dimensional Adaptive Optics Ultrahigh-Resolution Optical - Coherence Tomography using a liquid crystal spatial light modulator", Vision Res. 45, 3432-3444 (2005), as it was not possible to find any increase in the signal-to-noise ratio (SNR) of the acquired retinal images as compared to the uncorrected case. Following this work, mathematic studies have shown that ocular LCA might reduce the amount of light received at the detector, and consequently the SNR of the corresponding retinal images, up to 60 % when using light sources centered at 800 nm and 130 nm FWHM, the case of UHR OCT; s. E. Fernandez and W. Drexler, influence of ocular chromatic aberration and pupil size on transverse resolution in ophthalmic adaptive optics optical coherence tomography", Opt. Express 13, 8184-8197 (2005).

Therefore, the correction of the LCA in OCT could be fundamental to increase the quality of retinal images, especially when large pupils and broad bandwidth spectra are used. Accordingly, it is an object of the invention to provide an improved ophtalmoscopic achromatizing lens unit which is specifically suitable for the correction of the ocular LCA in the NIR for retinal imaging, and to provide its practical implementation in different arrangements, in particular in ophthalmoscopic and vision enhancement arrangements.

A number of mathematical eye models have been proposed with diverse degrees of complexity, although none of those has been designed for spectral ranges different from the visible range (from 400 to 700 ran) . However, the eye model proposed in R. Nav- arro, J. Santamaria, and J. Bescόs, ,,Accommodation-dependent model of the human eye with aspherics", J. Opt. Soc. Am. A 2, 1273-1281 (1985) could be used in order to design the achromatizing lens for the NIR. Here, aspheric surfaces are used for the surface of the anterior cornea and crystalline lens in order to better match experimental data from real eyes .

For the practical design and analysis of the performance of the lens commercial programs available for optical design as ,,Zemax" may be used.

To solve the given problem, a lot of commercially available optical glasses were tested, and searches for the optimum geometrical parameters of the lens were carried out, thereby testing different combinations of optical glasses. The optical functions selected to be minimized by the optimization were the LCA, the total thickness of the lens and the size of the final point image through the system, in order to obtain a lens unit with an acceptable optical quality. For every given pair of optical glasses, the degrees of freedom during the optimization process were the radii of curvature of the different refractive surfaces and their thicknesses. According to a preferred design, the lens was also constrained to be symmetrical.

Thus, according to the invention, to solve the above-mentioned problems, an ophtalmoscopic achromatizing lens unit as defined in claim 1 as well as an ophthalmoscopic arrangement as defined in claim 13, and a vision enhancement arrangement as defined in claim 18 are provided. Specific embodiments and further developments are defined in the dependent claims.

The ophtalmoscopic achromatizing lens unit according to the invention is easy and cheap in manufacture, and is totally insensitive with respect to the intensity of the used light (laser) beam, thus achromatizing the wavefront irrespectively of the beam. Therefore, in all cases when the achromatizing lens unit according to the invention is used, an at least near to achromatic beam may be supposed. Furthermore, the present achromatizing lens unit is specifically suited for correction of longitudinal chromatic aberration (LCA) of the human eye. Moreover, in connection with the present achromatizing lens unit, three spectral ranges are considered to be useful: visible (VIS) light (400 to 700 run); near infrared (NIR) light (700 to 900 ran); and infrared (IR) light (950 to 1150 nm) .

Although ocular chromatic aberration has been known in both the visible and the near infrared portion of the spectrum, its correction has not been taken into account for retinal imaging, since traditional ophthalmoscopes use monochromatic light only. However, as mentioned above, ophthalmic optical coherence tomography (OCT) , a relatively novel imaging technique, is based on white light interferometry. In this imaging modality the use of polychromatic light is intrinsically mandatory. Moreover, the spectral bandwidth of the light source determines the axial resolution. Chromatic aberration limits the effective spectral bandwidth projected onto the retina, and thus, the axial resolution of retinal images obtained by OCT. In addition, chromatic aberration also affects the size of the spot of light scanned on the retina, consequently impairing transverse resolution. The present achromatizer lens unit, in its different ranges of use, increases both axial and transversal resolution of retinal images, enabling the use of ultrabroad spectral bandwidth light sources for retinal imaging.

Chromatic aberration prevents the different colors to be accurately focused onto the observer's retina. A visual system which is adapted to chromatic aberration produces a visual perception where this aberration is normally neglected. Correcting chromat- ic aberration, by using the present achromatizing lens unit in the visible light range, produces a sharper image perception under photopic conditions, particularly noticeable in colorful scenes. The visual system also uses in some cases chromatic aberration as an optical clue to accommodate vision at different distances. Therefore, the use of the present achromatizing lens unit in the visible light range is particularly suitable for presbiopic eyes (those with no accommodative response, naturally occurring in individuals over 50 years) .

With respect to the effect of the achromatizing lens on the resolution for retinal imaging in optical coherence tomography (OCT) , it should further be noted that correcting ocular longitudinal chromatic aberration produces all wavelengths compounding the illuminating beam to be perfectly focused at the retina, exactly at the same plane. The practical consequence is a more concentrated spot on the patient' s retina allowing an improved transverse resolution of the retinal images.

In addition, axial resolution in OCT is also enhanced by the use of the present achromatizing lens unit. This may be accomplished by employing an ultrabroad bandwidth laser that is not limited by the chromatic aberration of the investigated eye. In OCT the axial resolution OCT is decoupled from the transverse one, being mainly dependent of the optical bandwidth of the used light source. Due to the limited number of available photons reflected or back scattered by the retina, the increase of the signal at the detector in OCT produced by the achromatizing lens also improves contrast, therefore allowing increasing the effective axial resolution.

The present achromatizing lens unit is especially suitable for those ophthalmoscopes whose retinal images are obtained by scanning the illuminating beam onto the retina, as scanning laser ophthalmoscopes, including the confocal version, and primarily in the majority of the optical coherence tomography (OCT) apparatus .

In scanning laser ophthalmoscopes, the use of polychromatic light might reduce the speckle typically generated with light of high coherence, increasing the signal-to-noise ratio. In OCT, the use of polychromatic light sources is mandatory, therefore the use of the achromatizing lens is even more suitable for this latter modality. For both imaging techniques the use of the achromatizing lens conjugated with the eye's enter pupil and before the scanning system is preferred. Placing the scanning system between the eye and achromatizing lens renders it possible that both the illuminating beam and the light reflected by the retina pass through the achromatizing lens with normal incidence, avoiding the possible lateral color introduced by the achromatizing lens when using a tilted beam (as would be the case when the achromatizing lens is placed in front of the eye, and the illuminating beam is scanned) . The longitudinal chromatic aberration of the eye is practically constant for small and moderate fields, so the effect of the achromatizing lens benefits the retinal images obtained in these ranges. In addition, the proposed practical implementation of the achromatizing lens maintains the lateral color unaffected, being only produced by the patient's eye.

Preferred embodiments of the invention will be described in the following in more detail, with reference to the drawings, with further features and advantages of the invention being perceptible therefrom. In particular,

Fig. 1 shows a schematic sectional side view of an ophtalmoscop- ic achromatizing lens unit for NIR use;

Fig. 2 illustrates the chromatic focal shift vs. the wavelength (in the NIR range) , in the case of a natural eye and of the combination of the eye and the achromatizing lens unit of Fig. 1;

Fig. 3 depicts retinal images and different wavelengths (in the NIR range) for a natural eye;

Fig. 4 is an illustration similar to Fig. 3, but now for the case that an achromatizing lens unit according to the invention is used in combination;

Fig. 5 is a diagram showing the effect of the achromatizing lens unit by illustrating the measured defocus and mean defocus (in D) vs. the wavelength (in nm) for a natural eye and for the combination of the eye with the achromatizing lens unit according to the invention;

Fig. 6 is a schematic sectional side view similar to that of Fig. 1 of an achromatizing lens unit used for the visible light range;

Fig. 7 illustrates the chromatic focal shift for this visible light range, similar to Fig. 2, for the case of a natural eye and of the combination of the eye and the achromatizing lens unit of Fig. 6;

Figs. 8 and 9 are similar illustrations as Figs. 1 and 2, or Figs. 6 and 7, but now for the case of the IR light range;

Fig. 10 depicts a diagram showing the refractive index of a plurality of crown glasses and flint glasses in accordance to the well-known Abbe number;

Fig. 11 is a diagram showing the refractive index of two different known glasses, namely a specific flint glass (F2) and a specific crown glass (namely N-SK4) ;

Fig. 12 schematically shows the passage of spectral light through the achromatizing lens unit according to the invention, thereby illustrating the focussing and diverging effect on different wavelengths;

Figs. 13 and 14 schematically show different ophthalmoscopic arrangements in schematically manner, with the use of achromatizing lens units according to the invention;

Figs. 15 to 17 schematically show different vision enhancement arrangements, with the use of the achromatizing lens unit according to the invention;

Fig. 18 schematically illustrates an arrangement for testing, or measuring, respectively, the effect of the achromatizing lens _ Q _ unit according to the invention on the human eye; and

Figs. 19 and 20 illustrate several diagrams showing the effect of incorrect alignment of an achromatizing lens unit with respect to an eye.

Fig. 1 depicts an ophtalmoscopic achromatizing lens unit 1 according to the invention, as proposed for the human eye, with dimensions given as may be used in the NIR range. The achromatizing lens unit 1 comprises three lens elements 2, 3 and 4, with an overall symmetric design, as may be seen from Fig. 1. The first lens element 2 and the third lens element 4 are made from the same glass material, in particular a flint glass material, as for instance the flint glass which is available on the market under the name F2 (Schott catalogue) ; the second or inner lens element 3 is comprised by a crown glass, as for instance a glass type known as N-SK4 glass (Schott catalogue). The interfaces 7, 8 between the first and second lens element and between the second and third lens element are spherical, with a given radius Rl, and R2, respectively. The shown symmetric design results in a cost-effective lens unit. In a practical embodiment, the shown achromatizing lens unit 1, in the following also referred to as ,,triplet", had a diameter of about 12,7 mm (half inch), thus being easy to mount in regular holders.

The achromatizing lens unit 1 according to Fig. 1 has planar outer surfaces 5, 6 so that the lens unit 1 has a cylindrical overall shape with plane outer front and back surfaces 5, 6. Then, it should be mentioned that known techniques can be used to connect the three lens elements 2, 3, 4, for instance by using optical cement or glue, or by applying the technique of optical contacting, which is known per se for instance from US 5,846,638 A, US 5,441,803 A and US 3,565,508 A.

As already mentioned, the interfaces 7, 8 between the three lens elements 2, 3, 4 are spherical, having a radius of 9.85 mm ( in the present NIR case) . In the optical axis, the thickness of the inner lens element 3 amounts 5 mm in the embodiment of Fig. 1, whereas the thicknesses of the outer lens elements 2, 4 in the optical axis amount 1.5 mm. Fig. 2 shows the chromatic focal shift in the NIR range at the retina of the eye, where the chromatic focal shift in μm is shown vs. wavelengths in μm. In Fig. 2, curve 9 shows the chromatic focal shift for an eye model, that is for a natural eye, whereas curve 10 illustrates the case where an achromatizing lens unit 1 according to Fig. 1 has been used, so that this curve 10 represents a situation for the combination of the lens unit 1 and the natural eye. The dashed vertical line 11 indicates the ideal case with no focal shift at the retinal plane. According to Fig. 2, the proposed achromatizing lens unit 1 corrects LCA in the selected spectral range almost perfectly. (The graph according to Fig. 2 is based on a pupil of 6 mm diameter and marginal rays meaning that paraxial rays were not considered. Furthermore, the wavelengths range of 700 to 900 nm was used, with a 200 nm spectral bandwidth.)

To show the chromatic aberration in the human eye, Fig. 3 illustrates some retinal images (geometrical point spread functions (PSFs) ) at different wavelengths, were a noticeable change of the PSFs may be seen as a function of the wavelengths. Contrary to this, Fig. 4 shows similar PSFs, but now in the case of using an achromatizing lens unit according to Fig. 1; accordingly, Fig. 4 illustrates the effect of the achromatizing lens unit 1 on the images projected on the retina, and the use of the lens unit 1 forces the different PSFs to be almost identical.

The effect of the achromatizing lens unit 1 is again illustrated in Fig. 5 where the defocus (D) is shown vs. wavelength (nm) . More in detail, measurements have been obtained from five normal subjects, and curve 12 represents the average chromatic aberration (defocus) of the natural eye whereas curve 13 presents average results obtained when using the achromatizing lens unit 1. Perfect correction would correspond to zero defocus within the entire wavelength range, as shown by the dashed line 14. It may be seen from Fig. 5 that the line 13 is a good approximation of the ideal case, line 14.

The Figures 6 and 7 are representations similar to that of Figures 1 and 2, but now for the case of the achromatizing lens unit 1 to be used in the visible (VIS) light range, that is the wavelength range of 400-700 nm. Similar materials (F2; N-SK4) as at the achromatizing lens unit 1 of Fig. 1 have been used in practice, and the spherical interfaces 7, 8 between the lens elements 2/3 and 3/4 have a radius of 15.3 mm. Furthermore, the thicknesses of the lens elements 2, 3, 4 in the optical axis 15 are 2 mm/3.5 mm/2 mm. Moreover, the front and back surfaces 5, 6 of the lens unit 1 are plane, too.

In Fig. 7, again the chromatic focal shift is shown in graph similar to that of Fig. 2, with the respective curves denoted by 9' (natural eye), 10' (combination of lens unit and eye) and 11' (ideal case) .

Then, with respect to the IR range, corresponding illustrations of the achromatizing lens unit 1, and of the retinal focal shift without and with such a lens unit in this IR range are shown in Figs. 8 and 9. Here, it may be seen from Fig. 8 that the inner lens element is a sphere, with the radius of 4 mm, which is enclosed between the two outer lens elements 2, 4 which have a thickness of 1 mm measured in the optical axis 15. The optical thickness of the inner lens element 3 amounts 8 mm.

The respective curves for the chromatic focal shift are denoted in Fig. 9 with 9'¹ (natural eye) and 10'' (combination of lens unit 1 and eye) .

The two specifically preferred materials used in the proposed triplet lens units 1 to correct the human longitudinal chromatic aberration are F2 and N-SK4 (Schott catalogue) . More generally, these materials are regular Flint and a Dense Crown optical glasses (F2 and N-SK4 respectively) However, these two materials F2, N-SK4 are not the only possible combination, and a plurality of optical glasses can be used (in appropriated pairs) with similar performance. The majority of the optical glasses available in the market are schematically represented in Fig. 10, known as the Abbe diagram, by means of circles. In this graph, the refractive index of various glasses is shown as a function of the Abbe number, the latter giving the chromatic dispersion of a given material as a function of the refractive index of the ma- terial at certain wavelengths: 589.2, 486.1 and 656.3 nm. The points 16, 17 in Fig. 10 indicate the two materials preferably- used for the present achromatizing lens unit 1. The optical glasses are divided,, according to their chromatic dispersion, into Flint and Crown glasses (the dashed line 18 in Fig. 10 separates the glasses) . For the design of the achromatizing lens unit 1 it is important to select a Flint and a Crown glass type. In general, possible Flint glasses for the current purposes are: Special Short Flint, Dense Flint, Flint, Light Flint, Very Light Flint, Barium Dense Flint, Barium Flint, Lanthanum Flint, Lanthanum Dense Flint, and Barium Light Flint. Suitable Crown glasses to be combined with any of the previously mentioned Flint glasses are: Fluorite Crown, Phosphate Crown, Dense Phosphate Crown, Barium Crown, Dense Crown, Crown, Lanthanum Crown, Very Dense Crown and Zinc Crown. (The classification, the given names, of the optical glasses is done according to the position of the materials in the Abbe diagram) .

Taking glasses with identical refractive indexes at a given wavelength makes, in addition, the achromatizing lens unit 1 to have zero power at that particular wavelength, meaning that light is not affected at the selected wavelength. Therefore, a parallel beam entering the lens would exit equally parallel for that wavelength, with zero defocus. The more similar the refractive indexes are the less affected the light is, meaning that only pure chromatic aberration is corrected, introducing no additional defocus. In the particular case of F2 and N-SK4 glass materials, the refractive indexes in the whole range of interest of the patent (covering visible, near infrared and infrared) are plotted in Fig. 11.

For specific designs or purposes, there are pairs of glasses (Flint and Crown) available which have identical refractive index at certain wavelengths. This situation corresponds in Fig. 10 to a couple of glasses located on a line parallel to the Abbe number axis (the dotted horizontal line in Fig. 10 shows an example with F2 and N-SK4) .

In case of selecting a different pair of materials for the achromatizing lens unit 1, the radii of curvature of the interi- or surfaces, as well as the thicknesses of the glasses should be adapted accordingly, keeping the correction of the longitudinal chromatic aberration of the human eye.

According to Fig. 12, spectral light enters the achromatizing lens unit 1 at 19 from the left side. The central wavelength λ₀ (central frequency, e.g. green color), is virtually unaffected by the achromatizing lens unit 1, passing through the optics with no change of vergence. Spectral light with longer wavelengths λ₀ + Δλ, (e.g. red color), depicted at 20, (or generally, lower frequencies) is focused by the achromatizing lens unit 1 with the effective converging power continuously increasing with the wavelength. Spectral components with shorter wavelengths λ_o - Δλ (e.g. blue color) (or generally, higher frequencies) depicted at 21, are diverged by the achromatizing lens unit 1, which is acting here as a negative refractive lens. The design of the achromatizing lens unit 1 assures that the magnitude of the lens unit 1 for each particular wavelength or frequency is near the opposite found in the human eye, for each considered spectral range. (Of course, scales in Fig. 12 do not correspond to any presented spectral range (IR, Visible or NIR) , and the angles are exaggerated in order to provide schematic description of the achromatizing lens unit 1 effect.)

In use, the achromatizing lens unit 1 should be placed in a plane P', P'' which is optically conjugated with the eye's enter/exit pupil plane P (compare Figs. 13 to 17). In Figs. 13 to 17, the arrows show the direction of the light. Optical conjugation can be induced in the setup of the respective arrangement e.g. by using telescopic subsystems including e.g. pairs of lenses, spherical mirrors, parabolic mirrors and combinations thereof (lens and spherical mirror, parabolic mirror and lens, parabolic mirror and spherical mirror) . Consequently, lenses Ll and L2 as shown in Figs. 13, 14, and 15 can be replaced by spherical mirrors or parabolic mirrors. Focal lengths and diameters of the different optical elements depend on the required beam diameter and the magnification between conjugated planes.

At the ophthalmoscopic arrangements 22 of Figs. 13 and 14 which are used to inspect an eye 23, it is preferred to place a scan- ning system 24 between planes P and P' ; this scanning system 24 can comprise polygonal mirrors, galvanometric mirrors, or any other subsystem known per se which is able to scan the incident beam 25 onto the subject's retina 26. Placing the scanning system 24 in this particular position enables the continuous use of normal incident light at the achromatizing lens unit 1 in the plane P' , even during scanning system operation. The main advantage of this configuration is the absence of lateral color and other off-axis and wide-field optical aberrations, like astigmatism or coma-aberration, induced by the achromatizing lens unit 1 (AL) (and in general induced by any optical element which has not been designed specifically to correct these aberrations) in the case of non-normal incidence of the beam 25. In particular, lateral color, i.e. transverse or lateral chromatic aberration, could highly degrade the optical performance and/or the quality of the retinal images achieved through the achromatizing lens unit 1. Therefore, the use of the achromatizing lens unit 1 in combination with the scanning system 24 as shown is quite advantageous, particularly when scanning wide angles.

In principle, it is also possible to place the achromatizing lens unit 1 directly in front of the eye 23, close to the cornea 27 of the eye 23 (although a distance of some centimeters, 3-4 cm, is acceptable) . In this situation, the performance of the achromatizing lens unit 1 regarding the correction of the ocular chromatic aberration is still high. However, for those systems including or using scanning beams into the retina 26, especially with wide angles, the use of the achromatizing lens unit 1 directly in front of the cornea 27 might reduce the optical quality of the image projected onto the retina 26.

Fig. 13 shows the preferred implementation of the achromatizing lens unit 1 for an ophthalmic optical coherence tomography system 28 (also known as OCT, including the modality of ultrahigh resolution, UHR OCT, in both time and frequency or spectral domain, as is known per se) comprising a laser light source 29. The imaging system including the achromatizing lens unit 1 can be connected to an interferometer 30 for retinal imaging via optical fibers 31, as Fig. 13 depicts. Light back-reflected or back-scattered from the retina 26 can be coupled into the fiber 31 through a collimator 32. Plane P', where the achromatizing lens unit 1 is located, can be optically conjugated with plane P'', the plane containing the collimator 32, by means of an optical relay 33, using lenses, converging mirrors, or a combination of both (presented in Fig. 13 as ,,Telesl") . Depending on the dimensions of the reguired beam 25, and the characteristics of the fiber collimator 32, the optical relay 33 can act as a beam expander or beam condenser, in addition to conjugate optically the planes P' and P' ' .

In practical tests, and mainly due to the dispersion of the light produced by the ocular chromatic aberration, the bandwidth of the light source 29 employed in UHR OCT system 28 has been limited to 130 nm in the near infrared (approximately centered at 800 nm) . The use of the achromatizing lens unit 1 enables broader spectra to be efficiently focused at the retina 26, and efficiently coupled back into the fiber 31 connecting the system to the interferometer 30. The axial resolution of retinal images in OCT is directly given by the spectrum of the light source 29. Therefore, the achromatizing lens unit 1 increases the transverse resolution by reducing the size of the spot projected into the retina 26, as well as the axial resolution.

The achromatizing lens unit 1 can also be used in a scanning laser ophthalmoscope system 34 (SLO) , one embodiment therefor being shown in Fig. 14, including the confocal version (CSLO) , in combination with a broad bandwidth spectral light source 29'. Using a spectral light source 29' in SLO reduces speckle formation, therefore increasing the contrast and resolution in the retinal images. These benefits when using spectral sources are however drastically reduced, if not even vanished, when ocular chromatic aberration correction is not simultaneously accomplished. Therefore, the use of the achromatizing lens unit 1 in this case is convenient, especially with ultrabroad bandwidth spectral sources 29'. The optical configuration for this particular SLO system is shown more in detail in Fig. 14. The location of the achromatizing lens unit 1 is similar as in the case of OCT, previously described with reference to Fig. 13. The optical relay 33 (Fig. 13) connecting the plane P' , where the achromatizing lens unit 1 is located, with both the detector 30' and the light source 29' should include a beam splitter 33' (also termed BS in Fig. 14), or a flipper mirror, so that the light 25 can be distributed or sent to these two different planes.

The achromatizing lens unit 1 can also be used in arrangements 35 (cf. Figs. 15, 16 and 17) to enhance vision, with higher benefits in colored scenes. Polychromatic visual acuity is increased by placing all wavelengths of the visible spectrum in a single retinal plane, avoiding the chromatic defocus appearing as a consequence of longitudinal chromatic aberration. For this application, the achromatizing lens unit 1 can be placed either in a plane P¹ conjugated with eye's enter pupil plane P, see Fig. 15, or directly in front of the cornea 27, see Fig. 16. The use of a scanning system 24 between the eye 23 and the achromatizing lens unit 1 is also possible, compare Fig. 17, avoiding possible parasitic aberrations typically occurring with wide angles and off-axis illumination. This last option is particularly suitable for accurate and controlled visual testing and evaluation of visual performance.

In all cases presented previously, lenses Ll, L2 compounding a telescope or optical relay conjugating the eye's enter pupil plane P' and the achromatizing lens unit 1 plane can be replaced by converging mirrors (spherical or parabolic) or any combination between them (i.e. spherical mirror and lens; spherical mirror and parabolic mirror; lens and parabolic mirror) .

Furthermore, it is referred to a monitor or display 36 in Figs. 15 to 17 as a target means for the visual testing.

In Fig. 18, a relatively simple optical system 37 is shown hich has been implemented to measure the effect of the achromatizing lens unit 1 on the human eye 23. The system 37 incorporates a Hartmann-Shack wavefront sensor 38 allowing the objective estimation of the aberrations with and without the achromatizing lens unit 1. The system 37 conjugated the enter pupil of the eye 23, the achromatizing lens unit 1 and the Hartmann-Shack sensor 38 by means of two telescopes. The wavefront sensor 38 used a CCD camera 39 with quantum efficiency optimized for the NIR and an array of square microlenses, 0.3 mm size and 7.6 mm focal length, mounted on the camera 39. The small size of the mi- crolenses allowed to achieve high sampling in the incoming wave- front, enabling an accurate estimate of the aberration. Since the achromatizing lens unit 1 was designed to exactly compensate for the LCA of the eye 23, absolute magnification of 1 was required between the eye's enter pupil and the achromatizing lens unit 1, yielding that the two first lenses of the system, both labeled in Fig. 1 as Ll' (fi' = 50 mm), have to be necessarily identical (in this case magnification -1 is obtained) . Locating the achromatizing lens unit 1 directly in front of the eye 23 is also possible, although perfect conjugation with the eye's pupil is then not possible and alignment during the measurements is more complicated due to the other optical elements. The second telescope, formed by lenses Ll and L2 (f₂' = 100), comprised the pupil size to match the appropriate diameter at the wavefront sensor 38. All the lenses implemented in the set-up were off- the-shell achromatic doublets designed for the NIR. Subjects were fixed and accurately centered to the system 37, with the help of a bite-bar with the corresponding dental impression for each subject, mounted on a three dimensional positioning stage, increasing the stability during the measurements. In order to assure an optimum alignment of the achromatizing lens unit 1 with respect to the rest of the system 37, the achromatizing lens unit 1 was mounted on a special holder allowing micrometric control of tilts as well as movements in the XY plane (perpendicular to the direction of the incoming light) .

Light emitted by a pulsed broad bandwidth laser 40 was launched to the system 37 through a 100 m monomode optical fiber 41, stretching the pulses generated by the broad bandwidth source 40, avoiding the existence of peaks of high intensity on the retina 26. An achromatic lens collimator 42 collimated the beam out of the fiber 41, whose final diameter was selected by using a diaphragm 43 mounted in front of the collimator 42. This diaphragm 43 was mounted in a holder allowing the change of position in the XY plane. It enabled to avoid corneal reflections easily by slightly displacing the beam out of the corneal apex. The final illuminating beam 44 entering the eye was 1 mm diameter. A set of interference filters 45 (each of 10 nm bandwidth, indicated in Fig. 18 also by IF) were used to select different wavelengths within the range of interest: 700, 725, 750, 775, 800, 825, 850, 875 and 900 run. A neutral filter 46 (also NF in Fig. 18) was also located in the illuminating arm to adjust the flux intensity finally sent to the eye 23. Intensity was always kept under 15 μW/cm² , near to two orders of magnitude below the maximum exposure limit for the retina 26. Five subjects, familiar with the purpose of the experiment, with ages ranging from 29 to 39 (mean age was 32.2 years with standard deviation of 4) participated in the measurements. In order to minimize changes in accommodation, affecting the subsequent estimation of the LCA through the defocus, cyclopentolate (0.5 %) was instilled in the eye 23 (left eye was systematically selected) of the subjects immediately before the measurements. Ocular aberrations were recorded in two different runs, each of them at 30 Hz during 2 seconds for every subject. The procedure was repeated for each interference filter 45, with and without the achromatizing lens unit 1 implemented in the system. The filters were randomly implemented for every subject, avoiding any established order. With such a measuring system 37, measurements referred to above, in particular with respect to Figs. 3 and 4, were carried out.

In practice, the achromatizing lens unit 1 can only be approximately aligned with respect to the eye's enter pupil. Moreover, a hypothetical perfect alignment with the enter pupil would not assure that the optical axes of the eye 23 and lens unit 1 are coincident, not even parallel to each other. Therefore, it is useful to analyze the performance of the achromatizing lens unit 1 in presence of tilts and misalignments, covering realistic ranges. This is particularly important for the case of NIR illumination, where subjective alignment can not be performed by the subject. Although the human retina is still active in the range of 700 to 900 nm, subjects perceive the light from different wavelengths in this portion of the spectrum as a single color (red), only distinguishing changes in the intensity.

Figs. 19 and 20 show the results of the performance of the lens unit 1 with the eye model at different situations (calculated through a pupil of 6 mm and using marginal rays) . The diagrams of Fig. 19 present the results from simulations obtained for the case of pure tilt around the X axis. Tilts in the Y axis must produce similar results due to the symmetry of the problem. The change of defocus at any wavelength in the considered NIR range as a function of the tilt is presented at the top of Fig. 19. The simulation covered up to 12 degrees of tilt, which was clearly including both the values one should expect after a careful alignment of the optical system and the reasonable changes in the position of the eye when using a bite-bar to stabilize the subject. Fig. 19, top, shows that defocus monotonic- ally increases with the induced tilt. For tilts of 12 degrees the defocus error is larger than 0.12 D. The main cause of the change in defocus is the total thickness of the achromatizing lens unit 1. Since the experimental protocol in the real measurements allowed subjects to rest between consecutive runs, this source of variation could potentially play a role in the obtained LCA. At the bottom of Fig. 19, the lateral color is presented as a function of the tilt. Lateral color was obtained as the distance (in μm) between the marginal rays of the two extreme wavelengths of the considered range (900 and 700 nm) at the retinal plane. The curve shows in this case a modest effect over the lateral color, barely surpassung the size of the diffraction image (~ 5 μm) .

Another potential source of errors is the incorrect centering of the achromatizing lens unit 1, compare Fig. 20. Assuming there are no tilts, still the achromatizing lens unit 1 could be misaligned with respect to the eye's enter pupil. Fig. 20 shows the corresponding results for this particular situation. At the top of Fig. 20, the defocus is presented as a function of the displacement of the lens unit from the center along the Y axis. The symmetry of the problem assures similar results, in absolute values, if the X axis would be considered. As expected, the error in this case is smaller, an order of magnitude below the case of tilts. The small variations in defocus found, even for rather noticeable misalignments like 2 mm (which actually is a very severe value accounting for the fact that the studied pupil is 6 mm diameter) , indicate that the achromatizing lens unit 1 does not introduce spherical aberration. In total absence of spherical aberration the defocus error should be strictly zero. As in the case of tilts, lateral color is also presented at the bottom of Fig. 20 as a function of the change in position. In this situation, the calculations show that lateral color is increasing with the error in the centering of the achromatizing lens unit 1. Although the increase for the studied range is more important here as compared to the case of tilts, the lateral color is still within reasonable values.

The mathematical calculations and experimental results referred to above showed that the proposed achromatizing lens unit 1 corrects the LCA of the human eye, leaving the rest of the aberrations unaffected. The design of the lens unit 1 is relatively simple, based on a triplet design using only regular optical glasses combined symmetrically, resulting in a cost-effective and easy to implement achromatizing lens unit 1. Extended range of use for the achromatizing lens unit 1 was also explored in simulations, from 600 to 1000 nm, showing a ~ 90 % reduction in the chromatic focal shift at the retina (321.13 μm in the natural eye and 39.67 μm using the achromatizing lens unit 1) .

The achromatizing lens unit 1 has been designed to be conjugate with the eye's enter pupil, with a magnification of ± 1. To modify the proposed design to have an achromatizing lens unit 1 operating with a different magnification with the eye's pupil seems to be unnecessary. This can be of importance for practical implementation of the achromatizing lens unit 1 in clinical system, where compactness is always advantageous. The lens can also be placed directly in front of the eye, close to the cornea. This way might simplify the optical system, although this is not convenient for retinal imaging as has been discussed.

The results presented in Fig. 19 show that there is change in defocus as a function of tilt. This can be reduced by using a more compact design, making the achromatizing lens unit 1 thinner, although in this case more expensive materials would be required, incrementing the cost of the lens unit 1. The tolerance of the achromatizing lens unit 1 to misalignments in both tilts and displacements in the transverse plane, when the lens unit 1 is intended to be used in normal incidence, is quite high. The results presented in Figs. 19 and 20 have demonstrated that lateral color appears as a consequence of existing misalignments, although errors in centering the achromatizing lens unit 1 con- tribute more than those produced by tilts. In practice, it is relatively easy to assure a good matching between pupils, being the correct tilt more difficult to achieve. In addition, calculations and real measurements have demonstrated that the amount of spherical aberration introduced by the proposed achromatizing lens unit 1 is negligible.

The designed achromatizing lens unit 1 can be used for retinal imaging purposes. In particular, the achromatizing lens unit 1 is especially suitable for ophthalmoscope arrangements.

Another advantage of the present achromatizing lens unit 1 is that monochromatic aberrations, excluding defocus, are not affected by using the achromatizing lens unit 1, which is of a huge practical importance for a hypothetical correction of these aberrations by using adaptive optics. In both standard OCT and UHR OCT, adaptive optics has been demonstrated for increasing the transverse resolution, revealing intraretinal features in the living retina otherwise undetectable. The implementation of the achromatizing lens unit 1 in such an adaptive optic system might produce a full correction of the ocular aberrations with a large benefit on the retinal images' quality. The use of the achromatizing lens unit 1 could also solve the apparent paradox occurring in adaptive optics UHR OCT when correcting the monochromatic aberrations in large pupils, probably related to the chromatic aberration of the eye.

Claims

Claims :

1. An ophtalmoscopic achromatizing lens unit (1) having planar outer surfaces (5, 6) , and comprising a first, a second and a third lens element (2, 3, 4) arranged one behind the other, as seen in the path (15) of a light beam passing through the lens unit, wherein the first and third lens elements (2, 4) comprise one and the same first glass material, and the second, inner lens element (3) comprises a second glass material.

2. The achromatizing lens unit according to claim 1, wherein the first glass material is a flint glass.

3. The achromatizing lens unit according to claim 2, wherein the flint glass is a glass chosen from the group comprising Special Short Flint, Dense Flint, Flint, Light Flint, Very Light Flint, Barium Dense Flint, Barium Flint, Lanthanum Flint, Lanthanum Dense Flint and Barium Light Flint glass.

4. The achromatizing lens unit according to any one of claims 1- 3, wherein the second glass material is a Crown glass.

5. The achromatizing lens unit according to claim 4, where in the Crown glass is a glass chosen from the group comprising FIu- orite Crown, Phosphate Crown, Dense Phosphate Crown, Barium Crown, Dense Crown, Crown, Lanthanum Crown, Very Dense Crown and Zinc Crown glass.

6. The achromatizing lens unit according to any one of claims 1 to 5, wherein the two glass materials have at least essentially the same refractive indexes at a wavelength with which the lens unit is to be used.

7. The achromatizing lens unit according to any one of claims 1 to 6, wherein the second, inner lens element (3) is a bi-convex lens element.

8. The achromatizing lens unit according to claim 7, wherein the outer surfaces of the bi-convex lens element (3) which form interfaces (7, 8) to the first and third lens elements (2, 4) are spherical surfaces .

9. The achromatizing lens unit according to any one of claims 1 to 8, wherein the first, second and third lens elements (2, 3, 4) define a symmetrical lens unit design.

10. The achromatizing lens unit according to any one of claims 1 to 9, which is arranged to be used in a visible spectral range comprising wavelengths of 400 to 700 nm.

11. The achromatizing lens according to any one of claims 1 to 9, which is arranged to be used in a near infrared spectral range comprising wavelengths of 700 to 900 nm.

12. The achromatizing lens, according to any one of claims 1 to 9, which is arranged to be used in an infrared spectral range comprising wavelengths of 950 to 1150 nm.

13. An ophthalmoscopic arrangement (22) comprising an achromatizing lens unit (1) according to any one of claims 1 to 12, a light beam source (29, 29')_/ a scanning system (24) for scanning a light beam emitted by the light beam source onto a retina of an eye and detector means (30, 30') for receiving and detecting the light beam.

14. The ophthalmoscopic arrangement according to claim 13, wherein the scanning system (24) comprises moveable, e.g. tilt- able, mirrors.

15. The ophthalmoscopic arrangement according to claims 13 or 14, wherein the light beam source (29) is a laser beam source.

16. The ophthalmoscopic arrangement according to any one of claims 13 to 15, wherein the detector means (30) comprises an OCT- interferometer.

17. The ophthalmoscopic arrangement according to any one of claims 13 to 16, wherein the achromatizing lens unit (1) is placed in a plane (P') which is optically conjugated with the eye's enter/exit pupil plane (P).

18. Vision enhancing arrangement (35) comprising an achromatizing lens unit (1) according to any one of claims 1 to 12, wherein the achromatizing lens unit (1) is placed either in a plane (P') conjugated with the eye's enter pupil plane (P) or directly in front of the cornea (27) of the eye (23).

19. The vision enhancement system according to claim 18, wherein a scanning system (24) is placed between the eye (23) and the achromatizing lens unit (1) in the conjugated plane (P').

20. The use of the achromatizing lens unit (1) according to any one of claims 1 to 11 in ophthalmic optical coherence tomography.