Ophthalmic measuring apparatus

Abstract: In an ophthalmic measuring apparatus a laser beam ( 1 ) is projected onto a selected spot ( P ) in the eye ( 30 ) and light scattered therefrom is received by a photoelectric converter and processed for ophthalmic measurement. The photoelectric converter is realised by an array of avalanche photodiodes ( APD1 to APD6 ) or by a photomultiplier ( 16 ) provided with a shutter module which comprises an array of liquid crystal elements ( 15′ ). Signals from the avalanche photodiodes ( APD1, APD2, APD5, APD6 ) or from the liquid crystal elements disposed at lateral positions are subtracted from signals from those at the central position ( APD3, APD4 ) to remove a noise component based on the light ( BG1, BG2, BG3 ) scattered from the portions surrounding the selected spot (P) in the eye (30).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an ophthalmic measuring apparatus, and more particularly to an ophthalmic measuring apparatus for detecting ophthalmic diseases in which laser light is radiated via an optical system at one spot in the eye , particularly in the anterior chamber thereof, and the laser light scattered therefrom is analyzed to measure the ophthalmic diseases. Description of the Prior Art The measurement of protein concentration in the anterior chamber of the eye is of great importance in determining whether the eye is inflamed, that is, whether a blood-aqueous barrier functions normally or not. Conventionally, a slit lamp microscope is very often used to determine the turbidity by grading via the naked eye. On the other hand, a photographic measuring method has been developed to make a quantitative measurement. The determination by naked eye depends upon an examiner and is not reliable in data. To overcome this problem, a method has been proposed in which a laser beam is radiated in the eye and the light scattered therefrom is received and analyzed quantitatively (see, for example, U.S.P No. 4,711,542 ). In such a measuring method, the scattered or diffused light from portions in the eye such as the cornea, iris, crystalline lens, artificial crystalline lens implanted after operation of cataract etc. enters as noise into the laser beam scattered from the eye or into the measured spot in the eye in receiving the laser beam scattered. This degrades measurement accuracy and reduces the reproducibility of measured values. To reduce the noise due to the reflected or scattered light entering into the spot to be measured in the eye, the Japanese Laid-open Patent Publication No. 135128/88 or U.S.P No 4,832,043 discloses an apparatus in which a laser beam is deflected for scanning in the eye beyond the width of a slit on a mask disposed in front of the photoelectric converter. The output signal (including noise components ) obtained when the laser beam is deflected beyond the slit width is subtracted from the output signal (including effective and noise components ) when the laser beam passes across the slit width, thereby removing background noise from the cornea, crystalline and so on. In another method, the mask is displaced to measure the quantity of light obtained when the laser beam is located at upper and lower portions of the mask. In the same manner, the subtraction from the laser-scattered light is performed to remove the background noise. The utilization of such methods in the prior art makes it possible to remove noise based on the dark current in the photoelectric converter or unnecessary scattered or reflected light contained in the signal from the photoelectric converter, thus providing an improved resolving power and improved accuracy of measurement. In the first method, however, means for deflecting the laser beam are required, while needing means for displacing the mask in the second method. This disadvantageously makes the apparatus complicated and expensive. Furthermore, these methods necessitate moving parts, which eventually may degrade the reproducibility of measured data and guarantee no precise measurement of data with high accuracy. It is, therefore, an object of the invention to provide an ophthalmic measurement apparatus capable of reducing noise due to reflected or scattered light entering into the spot to be measured for improvement in accuracy of measurement. According to the present invention, there is provided an ophthalmic measuring apparatus in which a laser beam is projected at a selected spot in the eye and light scattered therefrom is received and processed for ophthalmic measurement, comprising: a laser source for producing the laser beam; means for focusing the laser beam from the laser source at the selected spot in the eye; photoelectric converting means for receiving light scattered from the selected spot in the eye and converting it into an electrical signal which is processed for ophthalmic measurement; characterised by a liquid crystal video module comprised of a plurality of liquid crystal elements arranged in a matrix at that portion in front of the photoelectric converting means which is a conjugate point relative to the selected spot in the eye, each of said liquid crystal elements being actuatable to transmit the light impinging thereon into the photoelectric converting means, said liquid crystal video module being arranged in such a manner that the liquid crystal elements at a central portion receive the light scattered from the selected spot in the eye, while the other liquid crystal elements receive light scattered from portions surrounding the selected spot in the eye, and means for reducing noise by subtracting, from the photoelectric converting means output signals corresponding to said central portion liquid crystal elements (S1), intensity signals from the photoelectric converting means corresponding to the said other liquid crystal elements except for those signals which are derived from the boundary portion between the said central portion liquid crystal elements and the said other liquid crystal elements and which, due to Gaussian distribution of the laser beam, have intermediate intensity values not belonging to the intensity values derived from the central and other liquid crystal elements. As indicated above, the liquid crystal video module is disposed in front of the photoelectric converting means at a portion which is a conjugate point relative to the predetermined spot in the eye, and light scattered from the spot is introduced through each of the liquid crystal elements onto the photoelectric converter. This eliminates the necessity of deflecting the laser beam or displacing the mask, and yet enables the measurement of the noise as in the previous embodiment. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a horizontal cross-section showing the arrangement of an ophthalmic measuring apparatus according to one embodiment of the invention; Figure 2 is a vertical cross-section of the apparatus in Figure 1; Figure 3 is an illustrative view showing the arrangement of a liquid crystal video module; Figure 4a is an illustrative view showing the light intensity distribution in the memory; Figure 4b is an illustrative view showing an image of the laser beam with the liquid crystal video module; and Figure 5 is an illustrative view showing the light intensity distribution in a liquid crystal video module having finer picture elements. The invention will now be described in detail with reference to the drawings. In Figures 1 and 2, which show an arrangement of the ophthalmic measuring apparatus according to the present invention, reference numeral 1 denotes a laser beam projector, in which a laser light source 4 such as, for example, a helium-neon or argon laser source is accommodated together with a condenser lens 5. Light from the laser light source 4 is passed through the condenser lens 5 to converge on the eye 30 under examination at a spot P in the anterior chamber 30a thereof. The laser beam projector 1 is provided with a slit light source (not shown). Light from the slit light source passes via a slit to form a slit image on the anterior chamber 30a of the eye. This slit image is for illuminating the surrounding area to facilitate confirmation of the position of the spot of converted laser light. A portion of the laser light scattered from the measuring spot P in the anterior chamber 30a of the eye passes in a light receiving section 3 through a lens 6, a beam splitter 7, a lens 14, and an interference filter F and impinges on a liquid crystal video module 15'. The interference filter F is a kind of narrow-band filter which only passes the wavelength of the laser beam therethrough. The liquid crystal video module 15' is disposed at that point in the light receiving section 3 which is a conjugate point relative to the convergence point P. The liquid crystal video module 15' produces an output signal in response to the intensity of light impinging thereon and is such that about one third thereof can be irradiated with the impinging laser beam. An observation section 2 serves to observe how the laser beam is projected in the eye, and includes a lens 6, a beam splitter 7 (the lens 6 and beam splitter 7 are also shared by the light receiving section 3), a lens 8, prisms 9 and 10, a field stop 11 and an eyepiece 12 by which an examiner 13 can carry out observations. The operation of the apparatus arranged thus will now be described. In conducting the measurement, the laser light source 4 is activated to emit a laser beam, which is converged on the measuring point P of the anterior chamber 30a in the eye. The light scattered from the measuring point P passes through the lens 6, and a portion of the scattered light is directed by the beam splitter 7 to the examiner 13 for observation through the lens 8, prisms 9 and 10, field stop 11 and lens 12. The light scattered from the measuring point P and divided by the beam splitter 7 simultaneously passes through the lens 14 and the interference filter F and impinges on the liquid crystal video module 15'. Each of the liquid crystal elements 15a' of the liquid crystal video module 15' produces an output depending upon the quantity of light impinging thereon (the intensity of light scattered from the eye). The intensity of the scattered light detected by the elements 15a is counted as numbers of pulses per unit time period. The output of the counter 18, i.e., the number of samplings or the total pulse count, is stored in the memory cells in the memory 19 allocated for each unit time period. This process is carried out for every element 15a to produce a map representing the intensity distribution in terms of all of the elements 15a. Figure 2 shows that a conventional type of photomultiplier 16 is used as light receiving means and as stated above, and that the liquid crystal video module 15' is disposed in front of the photomultiplier 16 at a portion which is a conjugate point of the convergence point P. The liquid crystal video module 15' is, as shown in Figure 3, comprised of a plurality of liquid crystal elements 15a' which are all made of liquid crystal and arranged in a matrix. The liquid crystal video module 15' is disposed at that point in the light receiving section 3 which is a conjugate point relative to the convergence point P. Each of the liquid crystal elements in the liquid crystal video module functions as a shutter and permits the laser beam impinging to pass therethrough onto the photomultiplier 16. The output from the photomultiplier 16 is passed through an amplifier 17 and is fed to a counter 18 connected to a controller 20 and the intensity of the scattered light detected by the photomultiplier 16 is counted as numbers of pulses per unit time period. The output of the counter 18, i.e., the number of samplings or the total pulse count, is stored in memory cells in a memory 19 allocated for each unit time period. As will be described later, this process is carried out for every liquid crystal element 15a' in the liquid crystal video module and repeated for all the liquid crystal elements to measure the intensity distribution on the liquid crystal video module, which is stored in the form of map data in the memory 19. The measured data stored in the memory 19 are processed in the controller 20 to evaluate the concentration of protein in the anterior chamber. In operation, each of the liquid crystal elements in the liquid crystal video module 15' functions as a kind of shutter and is actuated sequentially one from another on and off to permit the light scattered in the eye to pass therethrough. The quantity of light (scattered light intensity) passing through each liquid crystal element is received by the photomultiplier 16 and stored in the memory 19 in the form of time-course data to produce a map of data representing the intensity distribution. As described earlier, when the laser beam is projected at the measuring point P, the scattered light BG1 is generated when the laser beam hits the cornea 30b, and the scattered light BG2 and BG3 when the laser beam passes through the front and rear surfaces of the crystalline lens 30c. These rays function as noise and impinge on the photomultiplier 16 together with the effective component from the measuring point P. Thus, the memory 19 includes a distribution region S2 based on the background (noise) components and a distribution region S1 based on the total of the effective and noise components, as shown in Figure 4a. It will be noted that the liquid crystal video module is selected so that it is much greater in size than an image P1 of the laser beam having a diameter d (Fig. 4b). The distribution region S1 in Figure 4a contains the signal component corresponding to the protein concentration in the anterior chamber of the eye and the noise components based on the reflection and scattering at the surrounding area. Assuming that the average intensity of the values in this region of the memory 19 be X, the controller 20 subtracts the average intensity Y of the noise in the region S2 from the average intensity X to extract the effective signal component and calculate the protein concentration in the anterior chamber. Actually, the liquid crystal elements of the liquid crystal video module are so fine as shown in Figure 5 that the intensity distribution of the laser beam shows a Gaussian distribution with a region having an intermediate value between the values in the regions S1 and S2, as shown by shaded portions. These shaded portions are taken neither as the region S2 nor as the region S2, so that the intensity values in these regions are omitted from the calculation. In the embodiment above described, it is to be noted that it is preferable to perform the measurement again because of a high possibility of mis-alignment if there is a difference between the values in the upper portion S2 and the lower portion S2 of the laser beam image in Figures 4a and 5. While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention should not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claim.

Claims: An ophthalmic measuring apparatus in which a laser beam is projected at a selected spot (P) in the eye and light scattered therefrom is received and processed for ophthalmic measurement, comprising: a laser source (4) for producing the laser beam; means (5) for focusing the laser beam from the laser source (4) at the selected spot (P) in the eye, and photoelectric converting means (16) for receiving light scattered from the selected spot in the eye and converting it into an electrical signal which is processed for ophthalmic measurement; characterised by a liquid crystal video module (15') comprised of a plurality of liquid crystal elements (15'a) arranged in a matrix at that portion in front of the photoelectric converting means (16) which is a conjugate point relative to the selected spot (P) in the eye, each of said liquid crystal elements (15'a) being actuatable to transmit the light impinging thereon into the photoelectric converting means, said liquid crystal video module (15') being arranged in such a manner that the liquid crystal elements (S1) at a central portion receive the light scattered from the selected spot (P) in the eye, while the other liquid crystal elements (S2) receive light scattered from portions surrounding the selected spot (P) in the eye, and means for reducing noise by subtracting, from the photoelectric converting means output signals corresponding to said central portion liquid crystal elements (S1), intensity signals from the photoelectric converting means corresponding to the said other liquid crystal elements (S2) except for those signals which are derived from the boundary portion between the said central portion liquid crystal elements (S1) and the said other liquid crystal elements (S2) and which, due to Gaussian distribution of the laser beam, have intermediate intensity values not belonging to the intensity values derived from the central and other liquid crystal elements (S1 and S2).

Inventors: AKIYAMA KOICHI, AKIYAMA, KOICHI
Applicants: KOWA CO, KOWA COMPANY LTD.
Application year: 1989
Main classification: A61B3
IPC codes: A61B3