US20080111972A1 - System and method for the non-contacting measurements of the eye - Google Patents

Abstract

Combined equipment for non-contacting determination of axial length (AL), anterior chamber depth (VKT) and corneal curvature (HHK) of the eye, are also important for the selection of the intraocular lens IOL to be implanted, particularly the selection of an intraocular lens (IOL) to be implanted, preferably with fixation of the eye by means of a fixating lamp and/or illumination through light sources grouped eccentrically about the observation axis.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of application Ser. No. 10/916,151 filed Aug. 10, 2004, which claims the priority to German Application No. 198 57 001.5 filed Dec. 10, 1998 and German Application No. 198 57 000.7, filed Dec. 10, 1998 and International Application No. PCT/EP99/09766, filed Dec. 10, 1999, each of which is hereby fully incorporated herein by reference.
BACKGROUND OF THE INVENTION
• A longitudinal section through the human eye is shown schematically inFIG. 1. The axial length AL of the human eye is usually measured by a contact ultrasound method. Other measuring methods are described in DE 3201801, U.S. Pat. No. 5,673,096 and DE 4446183 A1. The curvature of the cornea-cornea radius HHR is determined by means of known keratometers/opthalmometers (DD 251497, U.S. Pat. No. 4,572,628, U.S. Pat. No. 4,660,946, U.S. Pat. No. 5,212,507, U.S. Pat. No. 5,325,134). The measurement of the anterior chamber depth VKT can be carried out by ultrasound or by means of a unit added to a slit lamp (anterior chamber depth gauge, adjustment via the slit lamp image).
• These measurements, which are also important for selecting the intraocular lens IOL to be implanted, must be determined particularly before a cataract operation, but also to monitor the progress of school children's myopia and to detect aniseikonia. In clinical practice, it is common to measure these quantities by means of at least two devices (e.g. ultrasonic a-scan and automatic keratometer). The measured quantities are used in formulas for calculating the optical power of the IOL. Various errors influencing the choice of IOL can occur depending on the type of device used.
OBJECT AND SUMMARY OF THE INVENTION
• It is the primary object of the invention to reduce these device-dependent measurement errors to a minimum.
• In accordance with the invention, a combination device for non-contacting measurement of axial length and corneal curvature of an eye comprises an interferometer arrangement with adjustable path length difference, a first splitter cube which images the radiation of the interferometer arrangement on the eye, a photo-diode on which partial beams reflected during axial length measurement at the cornea and retina are coupled out via a second splitter cube and a focusing element, a plurality of light sources which are arranged concentric and symmetric to the optical axis of the eye and which illuminate the eye diagonal to the optical axis with a punctiform image for measurement of corneal curvature, and a camera on which the reflections of punctiform images of the light sources occurring at the cornea during measurement of corneal curvature are imaged via the first and the second splitter cubes, wherein a part of the radiation from the interferometer arrangement that is reflected on the cornea and retina is imaged on the camera for purposes of observation.
• The invention also encompasses a method for determining the corneal curvature radius and/or the anterior chamber of a human eye.
• According to the invention, all necessary parameters of the eye are advantageously determined by means of a device arrangement and corresponding measurement methods.
• Necessary adjustments which make it possible to adjust the device to the patient are likewise realized in this arrangement.
• The calculation of the IOL is also carried out by means of this device arrangement. Accordingly, there is also no lost or corrupted data in the transmission of measurements from various devices to the computer carrying out the IOL calculation. The invention and its advantages are described in more detail in the following with reference to schematic drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• In the drawings:
• FIG. 1 schematically shows a longitudinal section through the human eye;
• FIG. 2 schematically illustrates the construction of the overall device;
• FIG. 3 shows a front view of the device in the direction of observation;
• FIG. 4 shows the characteristic transmission path of the light in accordance with the invention;
• FIG. 5 is a block diagram showing central control of the system in accordance with the invention;
• FIG. 6ashows the transmission of the polarization splitter;
• FIG. 6bshows the transmission of the S-pol splitter;
• FIG. 7 shows how the VKT is determined based on the image on the CCD matrix; and
• FIGS. 8aand8bshow further illustrations of the optical paths in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The construction of the device is shown schematically inFIG. 2. In order to measure axial length, the light of alaser diode1 is imaged on the patient'seye14 via a Michelson interferometer (3-5) comprising a stationary reference arm R1 with areflector4, in this case a triple prism, and an adjustable reference arm R2 shown with reference to different positions of another reflector5 (triple prism), as well as a beam splitter cube3 for superposition of the beam components reflected in R1 and R2, asplitter cube8, and a diffractive-optical element DOE9. Adiode7 monitors the light output of thelaser diode1. The partial beams reflected by the cornea and retina of theeye14 are superposed on one another and are imaged, via a focusing element, in this case anachromate16, on anavalanche photodiode APD17 by means ofDOE9,splitter cube8 having a quarter-wave plate P1 for rotation of the polarization plane, andsplitter cube15 with a half-wave plate P2. In this connection, the axial length is measured according to known methods which are described, for example, in U.S. Pat. No. 5,673,096.
• In order to observe the eye and the occurring reflections, a portion of the reflected light (light coming from the eye) is imaged on aCCD camera23 by means of anachromate22 viamirror20.Achromates18,19 are swiveled out in this case. In this connection, adiaphragm21 is in the off setting.
• In order to measure the corneal curvature HHK, theeye14 is illuminated in a manner analogous to DD 251497 at an angle of approximately 18° to the optical axis A1 by means of six, preferably infrared,LEDs10, two of which are shown by way of example in the drawing plane inFIG. 1.Pinhole diaphragms10aare arranged downstream of the LEDs to generate punctiform illumination images.
• Sixlenses11 are arranged downstream of the LEDs in the illumination direction for collimating the diode light. The images of these light sources formed in the eye (as reflection of the cornea) are imaged on the CCD camera viasplitter cubes8 and15 and achromates18 and19. The DOE9 is advantageously swiveled out, but can also remain in the beam path. Theachromate22 is swiveled out.
• Each eye is illuminated slitwise at an angle of approximately 33° by means ofLED12,slide diaphragm12aandcylindrical lens13 in order to determine the VKT. The resulting scatter images of the cornea and front lens surface are imaged on theCCD camera23 viasplitter cubes8 and15 and achromates18 and19 with the DOE preferably being swiveled out. Theachromate22 is swiveled out.
• FIG. 3 shows a front view of the device in the direction of observation; a known slit lamp cross-slide stage for X-Y-Z adjustment is not shown.
• The drawing shows the DOE9 (whose center point A1 identifies the position of the optical axis in the device),lenses11 for determining the corneal curvature and, located in back of the latter and not visible,LED10,cylindrical lenses13 for the slit imaging for measuring the VKT and sixIR diodes24 for illumination and alignment of theeye14.
• The measurement tasks will be explained in more detail with reference toFIG. 4 based on the beam path A-D from theeye14 to theCCD camera23.
Beam path C: Alignment of the Device Relative to the Eye
• The eye lies within the focal length of theachromate18, is imaged to infinity and is imaged, viaachromate22, in the plane of the CCD camera.Achromate19 is swiveled out. The patient is asked to look at a fixating light using laser diode (LD) orLED1 so that the patient orients the pupil in the direction of the optical axis. It is necessary to image a larger portion of the eye14 (e.g., 15 mm) on the CCD camera. Due to its low efficiency (about 5% in the focusing part), the DOE is less suited for imaging the iris structures, so that an optical system with fixed imagescale comprising achromates18 and22 carries out the imaging. The DOE is preferably swiveled out.
• In order to avoid creating any additional fixation stimuli for the patient, theeye14 is illuminated by means of IR diodes24 (FIG. 2) (e.g., 880 mm) which are preferably characterized by a wide radiating characteristic (large half-power angle). The device is adjusted to the patient by means of the known slit lamp cross-slide table which is displaceable in the X-Y-Z directions. The CCD camera can be, for example, a VCM 3405 by Phillips. Illumination of the eye is necessary in order to be able to adjust the patient to the device also in darkened rooms. This illumination should be as diffuse as possible for a field of 15 mm; however, imaging of the light source through the cornea cannot be avoided (since the cornea acts as a convex mirror).
• The basic idea in this case is to use the illuminating means for adjustment of the patient's eye at the same time.
• Sixinfrared LEDs24 with a relatively large half-power angle are arranged on the circumference of a circle (possibly the same circle circumference as in keratometer measurements). TheseLEDs24 generate six points on the cornea which are imaged on the CCD camera. The eye of the patient is shown live on an LC display or monitor; in addition, a circle or cross-hair is shown on the LCD or monitor to mark the center. In order to position the eye, the six points must be centered with respect to the displayed circle. This is done by moving the cross-slide table. The patient is correctly adjusted with respect to height, side and depth when the points are centered and sharp. The patient himself looks into the device, from which an adjustinglaser1 orLED1ais projected, and the patient must focus on the latter. The laser reflection is seen in the center of the pupil.
• Additional adjusting means are to be shown on the LC display/monitor. An avalanche photodiode APD is provided for detecting the interference signals of the device measuring axial length.
• When the patient's eye is on the optical axis of the measuring device, the adjustinglaser1 orLED1ais reflected by the front surface of the cornea; the reflected light is imaged on the APD. In this way, a direct voltage signal whose (relative) height is a measure for the centering of the patient's eye is generated by the APD. This direct voltage signal is returned to the internal computer via an A-D converter and, from the latter, is displayed in appropriate form (e.g., a bar or circle) on the LCD. Accordingly, further information about the state of adjustment of the patient's eye is conveyed to the user through the varying size of the bar or circle.
Beam Path D: ALM
• The reflections of the laser diode1 (e.g., 780 mm) are imaged on theCCD camera23 via the DOE, as parallel beam path, and theachromates22, wherein an eye portion of about 5 mm with swiveled outoptics18,19 is displayed for observation and reflection adjustment. In order to transmit maximum energy to theAPD17, a large part of the total energy on the APD, advantageously more than approximately 80 to 95%, is coupled out in thesplitter cube15 shown inFIG. 2; therefore, only approximately 20 to 5% of the light falls on the CCD camera.
Beam Path B: Keratometer
• The illumination is preferably carried out in a manner analogous to DD 251497 by means of six IR diodes10 (e.g., 880 nm) in order not to inhibit the fixation of the patient'seye14 on the fixating light of theLD1 orLED1a.
• The predetermined resolution of theCCD camera23 requires the imaging of a field no larger than approximately 6 mm on theeye14 in order to achieve a measurement accuracy of 0.05 mm. The action of the DOE is preferably canceled again by swiveling it out and the achromates18 and19 carry out the imaging of the six cornea reflection images.
• The following are used to increase a measuring accuracy which is extensively independent from the distance from the patient's eye to the device:
• • atelecentric diaphragm21 which preferably limits the aperture for measurement to less than 0.05; and
• collimators11 which are located between the LED and the patient's eye and which keep the incident angle constant independent from the axial position of the patient's eye.
• The imaging of the LED light is advantageously carried out via apinhole diaphragm10awhich enables exact alignment of the keratometer measurement points. The collimator focal length should be greater than fifty-times the effective light source extension in order to achieve the desired accuracy of radial measurement independent from position.
Beam Path A: VKT
• Due to the fact that light scattering is the decisive factor in the observation of light sections in the human eye, a light source with the shortest possible wavelength (e.g., 400-600 nm) must be selected for illumination of theeye14. Also, in determining the VKT, a field no greater than approximately 6 mm at theeye14 may be imaged on theCCD camera23 to achieve the required measuring accuracy of 0.1 mm. This is accomplished by means of theachromates18 and19 while circumventing the DOE effect or when the DOE is swiveled out.Achromate22 is swiveled out. Thetelecentric diaphragm21 which is swiveled in or adjusted must have a greater diameter (for an aperture which is preferably greater than 0.07, e.g., 13 mm) in order that the light intensity of the low-light scatter images occurring in the VKT measurement is reduced only minimally. Accordingly, it is adjustable in at least two positions or can be exchanged for a second diaphragm.
• The eye of the test subject is illuminated from the side at a fixed angle through the bright light slit. The light sections occurring on the eye are imaged on the CCD camera by anoptical system18,19,21. Illumination and observation form a fixed angle, preferably approximately 33°.
• FIG. 8a, bis a schematic view of the arrangement for determining the VKT in the illumination direction inFIG. 8aand in the detection direction inFIG. 8b.
• The light slit is formed by a line of light-intensive LEDs12 which are at a defined distance from aslit12aof fixed width.
• Theslit12awhich is illuminated in this manner is imaged on the test subjects eye through acylindrical lens13 as slit image S.
• The utilized LEDs typically last at least 10,000 hours (compared to 100-200 hours in halogen lamps).
• There are no wear phenomena due to temperature load as in halogen lamps.
• Imaging of the eye of the test subject is preferably carried out with the relevant image sections viaimaging optics18,19, shown schematically, preferably on aCCD sensor23. The imaging is carried out telecentrically—telecentric diaphragm21—in order to minimize the influence of the alignment of the test subject. The video signal is displayed on a monitor or LC display, so that the user can carry out the adjustment of the test subject and the measurement in a natural posture.
• The measurement process is not based on the measurable displacement of partial images; therefore, the pupil division can be dispensed with.
• The signal of theCCD camera23 is received in the memory of the computer C by means of a frame grabber FG.
• Suitable image processing software is used for determining distances in the section image from which the VKT (accuracy 0.1 mm) is calculated.
• An improvement in the relevant image contents (e.g., by eliminating ambient light) is achieved in that the illumination LEDs in suitable form are turned on and off cyclically synchronous to the video fields.
• An achromate having a defined focal length is sufficient for imaging the eye on the CCD camera. The focal length is determined depending on the desired image section at the eye to be imaged.
• Thediaphragm23 satisfying the condition of telecentrism is arranged in the image-side focal length of the achromate.
• This simple construction of the imaging system ensures the possibility of uncomplicated integration in other systems.
• The fixatinglight1,1a(LED) is blended in viabeam splitter8 inFIG. 8b.
• A light source (e.g.,LED1aor laser diode1) on which the subject fixes his gaze is integrated in the observation system.
• The video signal of the camera is displayed on a monitor or LC display.
• The user can confirm during adjustment and measurement of the subject that the subject's gaze is correctly fixed and, therefore, that the measurement results are not corrupt.
• The slit which is illuminated in this manner is imaged on the test subject's eye (4) through a cylindrical lens.
• Imaging—diverging slightly from a 1:1 imaging—of a slit with a width of 0.3 mm and aperture greater than 0.1 and the use of white-light LEDs have proven particularly advantageous.
• Imaging of the eye of the test subject is preferably carried out with the relevant image sections viaimaging optics18,19, preferably on aCCD sensor8. The imaging is carried out telecentrically in order to minimize the influence of the alignment of the test subject. The video signal is displayed on a monitor or LC display, so that the user can carry out the adjustment of the test subject and the measurement in a natural posture.
• The signal of the CCD camera is sent to the memory of a computer, e.g., by means of a frame grabber. Suitable image processing software is used for determining distances in the section image from which the VKT (accuracy 0.1 mm) is calculated. An improvement in the relevant image contents (e.g., by eliminating ambient light) is achieved in that the illumination LEDs in suitable form are turned on and off cyclically synchronous to the video fields.
• In the following, it will be shown with reference toFIG. 7 how the VKT is determined based on the image on the CCD matrix. The Figure shows the image of the eye that is detected by the CCD camera with the reflection image FI of the adjusting laser and fixating LEDs, and the scatter light SH of the cornea and lens SL withillumination1 switched on.
Determination of the Distance of the Front Edges of the Scatter Images of the Cornea and Lens in Digitized Recordings
• The starting point for image processing is (n times) a pair of images recorded in immediate succession:image1 with switched on slit illumination (“bright image”) andimage2, without slit illumination, with image of the fixating lamp (“dark image”). Processing is carried out through the following essential steps:
• detection of the pupil in the dark image: histogram-based selection of a threshold value for binarization taking into account boundary constraints; determination of an ellipse circumscribing the pupil by evaluating the covariance matrix of the binary image;
• detection of the fixation point in the pupil in the dark image: determination of all contiguous regions whose gray values are above the 0.9-quantile of the gray value distribution in the dark image; determination of a probability quantity for every region which depends on surface, shape and distance from the center point of the pupil; selection of the centroid of the likeliest region as fixating point;
• calculation of the differential image (bright image minus dark image) and noise suppression in the differential image by means of median filtering;
• determination of the edge shape of the scatter images of the slit illumination in the differential image: histogram-based selection of a threshold value for binarization taking into account boundary constraints; rough determination of the edges as location where threshold value is exceeded in given region around the fixating point; fine-tuned detection of the edges as location of the reversal point of the gray value shape in the line profile which comes closest to the roughly detected position; elimination of reflection edges by detection of mavericks or freak values in the edge shape (distance of a given proportion of points furthest from the average edge shape);
• determination of the distance X of the front edges of cornea scatter image and lens scatter image SH, SL (in pixels): approximation of the edge shape through ellipses (restricted minimization of the square error sum); calculation of the distance of the intersections of these ellipses with the horizontal through the fixating point.
Calculation of the Anterior Chamber Depth from the Above-Mentioned Distance:
• Conversion of the distance K in pixels into mm (including imaging scale of optics and pixel size of CCD matrix)
• r=corneal radius
  n=index of refraction of the aqueous humor
  ω=angle between illumination and observation
• $\mathrm{VKT}=r+\frac{x}{\tan \left[\mathrm{arc}\sin \frac{\sin \left(\omega -\mathrm{arc}\sin \frac{x}{r}\right)}{n}+\mathrm{arc}\sin \frac{x}{r}\right]}-\sqrt{{\left\{\frac{x}{\tan \left[\mathrm{arc}\sin \frac{\sin \left(\omega -\mathrm{arc}\sin \frac{x}{r}\right)}{n}+\mathrm{arc}\sin \frac{x}{r}\right]}\right\}}^2-{\left\{\frac{x}{\sin \left[\mathrm{arc}\sin \frac{\sin \left(\omega -\mathrm{arc}\sin \frac{x}{r}\right)}{n}+\mathrm{arc}\sin \frac{x}{r}\right]}\right\}}^2}+{\mathrm{<figure-callout\; label="r"\; filenames="US20080111972A1-20080515-D00000.png,US20080111972A1-20080515-D00002.png"\; state="\{\{state\}\}">r</figure-callout>}}^2$
• This formula applies exactly when the image of the fixating lamp is located at the front edge of the lens scatter image as is shown inFIG. 7; otherwise, the distance of the fixating lamp image from the front edge of the lens scatter image can be determined and a correction value can be determined for the anterior chamber depth from the amount of this “decentering” based on known imaging formulas. The corneal radius is preferably measured by means of the keratometer device described above.
• Following is an overview of characteristic settings to be considered when combining the three required measurements and the adjusting process:

• 			Kerato-
  	Adjustment	ALM	meter	VKT

  Field size	approx.	approx.	approx.	approx.
  	15mm	5mm	6mm	6 mm
  Wavelength	IR (e.g.,	e.g.,	IR (e.g.,	VIS (e.g.,
  	880 nm)	approx.	880 nm)	400-600 nm)
  		780 mm
  Beam	removed	removed	approx.	approx.
  diaphragm			6mm	13 mm
  DOE	no effect	active	no effect	no effect
  	(removed)		(removed)	(removed)

• As this overview shows, different wavelength ranges are used with the different measurement tasks. Thesplitter cubes8 and15 are very important in this connection, since the illumination beam path, observation beam path and measurement beam path are separate from one another.
• This task is performed by special splitter layers taking into account the linear polarization of thelaser diode1.
Splitter Cube8:
• The laser light coming from the interferometer should be reflected at a maximum in the direction of theeye14; the laser light coming from theeye14 should have maximum transmission. In addition, the splitter layer in thecube8 must have maximum transmission for the IR light component and VIS light component of the keratometer measurement and VKT measurement. Since the LD1 (for example, LT 023, Sharp) involves linearly polarized light, a dielectric multiple layer with a polarizing effect can preferably be used. The characteristic transmission path is shown inFIG. 4. The vertically polarized light (s-pol, 780 nm) coming from 1 is reflected as much as possible (approximately 98%).
• Circularly polarized light is generated by the quarter-wave plate. The light reflected by theeye14 is accordingly linearly polarized again after passing the quarter-wave plate; however, the polarization direction is rotated by 90° (parallel-polarized, p-pol). For this oscillation direction, the splitter layer has approximately 100% transmission at 780 nm. The IR and VIS LEDs emit unpolarized light. As will be gathered fromFIG. 6, the transmission of the splitter layer in the wavelength range of 420 to 580 nm and in the range of 870 to 1000 nm is greater than 90% for unpolarized light.
Construction of Layer: Splitter Cube8:
• Aside from its normal function—high polarization splitter effect in a defined wavelength range—this polarization splitter cube fulfills additional requirements of high transmission in the visual wavelength range (420 . . . 560 nm) and in the near infrared range (870 . . . 1000 nm). The layer design meets these requirements for a narrow incident angle range of 46°. The utilized materials are adapted to one another with respect to the refractive index of the substrate, cement and coating substances. The following materials were selected for this special use:

• 	Substrate:	SF2	n = 1.64
  		cement	n = 1.64
  		H	n = 1.93
  		L	n = 1.48

The design is formed of 17 alternating layers of H L.HFO2 is H, SIO2 is L.For comparable splitters, suitable splitters can be fabricated by suitable selection of the index of refraction of the substrate and coating substances and angle of incidence.Parameters:
• high transmission of 420 . . . 560 nm, unpolarized
• high transmission of 870 . . . 1000 nm, unpolarized
• polarization splitting 780±20 nm
Example

• 1	HFO2 156.8nm
  2	SIO2 118.1 nm
  3	HFO2 166.4nm
  4	SIO2 95.8nm
  5	HFO2 160.2nm
  6	SIO2 147.3nm
  7	HFO2 145.6nm
  8	SIO2 151.0nm
  9	HFO2 144.9nm
  10	SIO2 148.2nm
  11	HFO2 149.2nm
  12	SIO2 139.9nm
  13	HFO2 161.3nm
  14	SIO2 103.9nm
  15	HFO2 179.5nm
  16	SIO2 64.9nm
  17	HFO2 170.9 nm

Splitter Cube15:
• The laser light coming from thesplitter cube8 should be reflected by about 80 to 95% with a transmission of approximately 20-5%, The splitter layer should have maximum transmission for the IR and VIS light components. This layer is likewise realized by a polarization splitter which resembles the splitter layer in8 with respect to its characteristics. The half-wave plate arranged on thesplitter cube15 rotates the polarization direction of the arriving light by 90° so that the s-pol component again impinges onsplitter cube15. The splitter ratio mentioned above is adjusted by modifying thelayer8. For unpolarized light in the IR and VIS range, the transmission is greater than 90%.
Construction of the Layer: Splitter15:
• Apart from the requirements of the s-pol reflection of 80 . . . 95% with a wavelength of 780 nm±20 nm, the splitter cube meets the additional requirements of high transmission in the visual wavelength range (420 . . . 560 nm) and in the near infrared range (870 . . . 1000 nm) (FIG. 6b). The layer design satisfies these requirements for a narrow incident angle range of 46°. The utilized materials are adapted to one another with respect to the refractive index of the substrate, cement and coating substances. The following materials were selected for this special use:

• 	Substrate:	BK7	n = 1.52
  		cement	n = 1.52
  		H	n = 1.93
  		L	n = 1.48

The design is formed of 13 alternating layers of H L.For comparable splitters, suitable splitters can be manufactured by suitable selection of the index of refraction of the substrate and coating substances and angle of incidence.Parameters:
• high transmission of 420 . . . 560 nm, unpolarized
• high transmission of 870 . . . 1000 nm, unpolarized
• reflection s-pol approximately 80 . . . 0.95%780±20 nm
Example

• 1	HFO2 130.2nm
  2	SIO2 215.4 nm
  3	HFO2 130.6nm
  4	SIO2 17.8nm
  5	HFO2 160.7nm
  6	SIO2 241.6nm
  7	HFO2 136.6nm
  8	SIO2 240.0nm
  9	HFO2 156.4nm
  10	SIO2 18.0nm
  11	HFO2 135.1nm
  12	SIO2 214.1nm
  13	HFO2 131.3 nm

• According toFIG. 5, a central control is provided for adjusting and controlling all adjustable units and optical elements such asoptics18,19,22,diaphragm21, etc.
• The different imaging scales taking into account the action of the DOE call for reversing processes in the device. These reversing processes are preferably carried out by means of motor and in a program-controlled manner.
• A compact device has been realized in which the essential electronics building blocks are integrated. The heart of the device is an embedded Pentium controller C to which are connected a display D (showing theeye14 being examined and a menu for the user), keypad, mouse, foot switch and printer as peripheral equipment.
ALM
• The control of thelaser diode1 and interferometer slide IS (movable prism5, connected with measuring system) is carried out via the controller C. To reduce the influence of eye movements, a short measuring time (less than 0.5 sec) must be realized. The signal generated by theAPD17 reaches a signal processing unit SE, is amplified depending on the signal size, then amplified in a frequency-selective manner and converted from analog to digital at a sampling frequency corresponding to approximately four-times the frequency of the useful signal. The digital sampling values are taken over by the high-speed port HS of the Pentium platform. Digital signal processing is carried out in the latter by means of Fourier transformation without externally generated reference frequency. The signal is shown on the display; the path measurement system delivers the respective axial length quantity.
Keratometer
• The controller C is connected with the control of theCCD camera23 and thediodes10. In the adjusting process for measuring the corneal curvature, thediodes10 are preferably operated in continuous light mode in order to prevent flickering of the corneal reflected images shown on the LCD. During the measuring process, these diodes are switched on and off imagewise; for this purpose, the controller C controls thediodes10 synchronously with the image pulse of theCCD camera23, i.e., the diodes are switched on in one image and switched off in the next image. After subtraction of two successive images, only the reflections of the cornea that were generated by theLEDs10 are obtained in image pairs, and interfering reflections of ambient light are eliminated. The reflection images occurring on thecamera23 are digitized by means of frame grabbers FG and are stored in the working memory of the Pentium platform (controller C).
• Subsequently, the centroid positions of the reflection images of the diodes are determined by means of image processing and calculation of the corneal radii by means of the approximation formulas described in DD 251497. To improve the reproducibility of the measurement results, approximately 5 image series (each comprising two fields or half-images, with and without exposure by the synchronized LEDs) are recorded per measurement process.
VKT
• Further, the controller C is connected with thediodes12. During the adjusting process (alignment), thediodes12 are preferably operated in continuous light mode in a manner analogous to the keratometer.
• During the measuring process, the illumination diodes for the left and right eye are selectively cycled (similar to the keratometer) by the controller.
Depending on the user preset, the device is displaced to the left or to the right and is aligned with the center of the eye by appropriate means.
• The edge position of the scatter images is determined by means of image processing.
• The VKT is calculated from the distance of the cornea and lens scatter images as was already described.
• Again, approximately 5 image series are recorded per measurement process.
Illumination
• The controller C is connected with thediodes24. TheIR diodes24 for illuminating the eye can be switched on at a desired time by means of the controller (so as to be controlled within the program or by the user). Further, the controller is connected (not shown) with the controls for swiveling in and swiveling out or adjusting theDOE9,lenses18,19,22 anddiaphragms21.
• The calculation of the IOL from the determined measurement values AL, HEIR, VKT is carried out by means of internationally recognized calculation formulas which are stored in the device memory so as to be retrievable and is printed out by the printer.
• While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.

Claims (16)

1. An arrangement for determining the anterior chamber depth of an eye, comprising:

a) a slit-shaped illumination radiating on the eye at an angle to the observation axis, and imaging optics and comprising at least one light source or LED;

b) a receiver arrangement which detects scatter images coming from structures of the eye located substantially on the observation axis, the receiver arrangement, the slit shaped illumination and the imaging optics being angularly fixed relative to each other;

c) a signal processing device which receives signals from the receiver arrangement and processes the signals to determine distances between portions of the scatter images that represent the structures of the eye and calculates anterior chamber depth from the distances; and

d) a fixating light source emitting a fixating light on which the patient fixes his gaze, the fixating light being coupled along the observation axis, wherein the observation axis substantially coincides with the patient's viewing axis.

2. The arrangement according toclaim 1, wherein the angle to the observation axis under which angle the slit-shaped illumination radiates on the eye is fixed, preferably at 33 degree.

3. The arrangement according toclaim 1, wherein a slit is arranged after the light sources or LEDs in the illumination direction.

4. The arrangement according toclaim 1, wherein imaging optics are constructed anamorphotically or include a cylindrical lens.

5. The arrangement according toclaim 1, wherein the receiver arrangement comprises a CCD camera.

6. The arrangement according toclaim 1, in which the slit-shaped illumination comprises a series of white light LEDs.

7. A method for determining the anterior chamber depth of a human eye, comprising the steps of:

illuminating the eye;

imaging relevant image sections on a CCD camera;

switching the illumination on and off so as to be synchronized with image pulses of the CCD camera;

recording a bright image of the CCD camera when the illumination is switched on and recording a dark image of the CCD camera when the illumination is switched off;

subtracting successive bright images and dark images to generate a differential image to suppress noise;

illuminating the eye by a slit image;

detecting the eye pupil and a fixation point within the pupil in the dark image;

determining an edge shape of scatter images of the slit-image illumination in the differential image;

determining a distance X from the edges of a cornea scatter image and a lens scatter image; and

calculating the anterior chamber depth from the distance X.

8. The method ofclaim 7, wherein the generation of the differential image comprises further noise suppression by means of median filtering.

9. The method ofclaim 7, wherein the determination of the edge shape comprises a histogram-based selection of a threshold value for image binarization.

10. The method ofclaim 7, wherein the determination of the edge shape further comprises rough detection of the edges as such locations where a threshold value is exceeded in a given region around the fixating point.

11. The method ofclaim 10, wherein the determination of the edge shape additionally comprises fine detection of the edges as such locations where a reversal point of a gray value shape in a line profile which is next to the roughly detected position occurs.

12. The method ofclaim 7, wherein the determination of the edge shape further comprises elimination of reflections by detecting of maverick or freak values in the edge shape.

13. A device arrangement which determines the axial length and corneal curvature radius of a human eye, comprising a controller which carries out a central control of adjustable units and optical elements of the device arrangement and calculates the optical action of an intraocular lens to be implanted in the eye.

14. The device arrangement according toclaim 13, with a device storage in which calculation formulas are stored such that they can be called up.

15. The device arrangement according toclaim 13, which additionally determines anterior chamber depth.

16. The device arrangement according toclaim 15, which initially determines the axial length, then determines the corneal curvature radius and then the anterior chamber depth, or which initially determines the corneal curvature radius, then the anterior chamber depth and then the axial length, or which initially determines the corneal curvature radius, then the axial length and then the anterior chamber depth.

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