ES2265225B1 - DEVICE AND METHOD FOR DIFFUSION MEASUREMENT (SCATTERING) IN OPTICAL SYSTEMS. - Google Patents

Abstract

Image polarimeter to quantitatively determine the level of scattering in any optical system, particularly in the human eye. The instrument uses as its core a double-pass ophthalmoscopic system that incorporates a polarization state analyzer unit composed of a rotating retarder sheet and a linear polarizer. Keeping the polarization fixed on the input arm, for each of the independent orientations of the fast axis of the retarder sheet, the camera or intensity detector records an image of the plane of the retina. From these images, the Stokes vector of the light emerging from the eye is calculated and with it the degree of polarization, a parameter directly related to scattering.

Description

Device and method for diffusion measurement ( scattering ) in optical systems.

Field of the Invention

The invention described herein is framed within the field of Visual Optics and Ophthalmology. In this field the most important applications are oriented to studies on the aging of the visual system and early detection of cataracts, as well as to the improvement of the diagnosis of glaucoma and to the follow-up of patients undergoing refractive surgery or implanted with intraocular lenses.

Background of the invention

Ocular aberrations and intraocular diffusion (hereinafter scattering ) contribute together to the deterioration of the retinal image in the human eye. Although the presence of scattering in the eyes of normal young subjects is low, it increases with age, the presence of ocular pathologies and after refractive surgery interventions (I. IJspeert, JK, de Waard, PW, van der Berg, TJ, de Jong, PT (1990). The intraocular straylight function in 129 healthy volunteers; dependence on angle, age and pigmentation. Vision Research , 30 (5), 699-707). Intraocular scattering increases significantly over normal values if loss of transparency of the ocular media occurs, especially in the lens (cataracts).

It has been known for more than a century that the eye suffers from aberrations. Throughout that time, different techniques, objective and subjective, have been developed to measure them, but on the contrary in the literature there is no robust method widely accepted for the objective measurement of intraocular scattering .

To date, most intraocular scattering determinations have been carried out using subjective methods that are combined with visual acuity and contrast sensitivity measures (Bailey, J., Bullimore, MA (1991). A new test for the evaluation of disability glare, Optometry and Vision Science , 68, 911-917). These types of measures require the active and prolonged participation of the observer, and may be influenced by other factors. Therefore, they are not the most appropriate in a clinical setting that requires speed and precision.

On the other hand, the ophthalmologist makes use of the slit lamp for routine observation of the cataract. Once observed, its classification and analysis are purely subjective (system called LOCS III (Chylack. LT, Wolf, JF, Singer. DM, Leske, MC, Bullimore, MA, Bailey, 1. L., Friend, J., McCarthy , D., Wu, SY (1993), The Lens Opacities Classification System III. Archives of Ophthalmology , 111,831-836)) based on parameters such as color, appearance or location. For this, the ophthalmologist must be specialized in this type of diagnosis and due to its intrinsic nature, it may differ among professionals. A characterization of the degree of cataract is of great relevance to determine the appropriate moment of surgery.

The joint contribution of optical aberrations and scattering affects retinal image quality. The double-pass (DP) technique, based on recording the image of a projected point on the retina, provides information on aberrations and scattering together. The contribution of aberrations is located in the central part of the image, so that the more aberred the eye is, the more extensive this part will be. For its part, the effect of scattering is preferably located in the most peripheral areas of the DP image, away from the central part, so that the greater the amount of light contributing to this part of the image, the greater the presence of scattering

It is possible to separate these contributions in the DP images by simultaneously measuring aberrations and DP images. The combination provides a measure of the amount and angular distribution of the light that has suffered scattering in the eye. However, obtaining satisfactory results requires a rigorous control of the experimental conditions and high levels of light to increase the signal-noise in the area of the tails of the DP images. Westheimer and Liang (Westheimer, G. and Liang, J. (1994). Evaluating diffusion of light in the eye by objective means. Invesrigative Ophthalmology and Visual Science (suppl) , 35, 2652-2657) combined subjective and objective measures proving that Intraocular scattering increases with age. In the objective part they used DP images and defined a diffusion index as the relationship between the light in the tails and the light in the central part.

It has also been proposed to use the spot size of Hartmann-Shack images for the analysis of intraocular scattering (Applegate, RA, Thibos, LN (2000). Localized measurement of scaner due to cataract. Investigative Ophthalmology and Visual Sciences (suppl ) , 41, S3). Making use of an artificial eye that can introduce different levels of scattering , Cox et al . have shown that while aberrations remain constant, the extent of the DP image increases with the level of scattering introduced (Cox, MJ, Atchison, DA, and Scott, DH (2003). Scaner and its implications for the measurement of optica] image quality in human eyes Optometry and Vision Science , 80, 58-68).

All attempts at objective evaluation of scattering have proved to be not very robust and there is still no effective and adequate alternative for clinical applications. In this context, it is undoubtedly advantageous to propose a new method of objective measurement of intraocular scattering that is easily adaptable to clinical instrumentation. This method will be, on the one hand independent of the ocular aberrations and, on the other hand, it will not have the disadvantage of requiring low levels of light of the areas of the image that contain the information about the scattering and that in many cases are confused with the noise.

Description of the invention

The present invention relates to an apparatus (DP image polarimeter) and an objective method of scattering measurement. Since the depolarization effects of an optical system are linked to the presence of scattering , the method presented here is based on polarimetric measurements of DP. In particular, it is an object of the invention to determine the "degree of polarization (GDP)" of the light emerging from the eye as an objective and quantitative estimator of the level of intraocular scattering present. Obtaining GDP requires four PD images, associated with four independent polarization states of the elements placed in the registration arm of the experimental system.

The method has the particularity that it is independent of the ocular aberrations and that simultaneously to the determination of the level of intraocular scattering , it also provides objective information on the quality of retinal image obtained directly from the recorded DP images. In addition, it does not require the active participation of the subject as in previous devices found in the literature.

The image polarimeter allows quantitative determination of the level of scattering in any optical system, particularly in the human eye. The instrument uses as a core a double-pass ophthalmoscopic system that incorporates a polarization state analyzer unit composed of a rotating retarder sheet and a linear polarizer. Keeping the polarization fixed on the input arm, for each of the independent orientations of the fast axis of the retarder sheet, the camera or intensity detector records an image of the plane of the retina. Said fast axis is that with respect to which the phase of one of the two components of the light that falls is advanced with respect to the other. From these images, the Stokes vector of the light emerging from the eye and with this the GDP, a parameter directly related to scattering, is calculated. Although a special application in the field of ophthalmology is shown throughout the invention, it can also be applied to other types of optical systems, either in transmission, or in reflection. In addition, the optics of the system can be adjusted at will so that the plane conjugated with the registration system can be any other, being able to measure the effect of scattering in multiple circumstances.

A first aspect of the invention relates to a device for diffusion measurement ( scatrering ) in optical systems determining a parameter, "degree of polarization", characterized in that it comprises:

first linear polarization means for generate linearly polarized light;

a state analyzer unit of polarization comprising:

media generators to generate 4 independent polarization states; Y

second means linear polarization;

at least first means of registration for record an input signal for each of the four states of polarization independent.

The generating means may comprise:

at least one rotating blade; Y

orientation means configured to orient the rotating blade with a fast axis according to four orientations independent.

The device of the invention can

comprise four sheets; Y

have the orientation means configured to orient each sheet with a fast axis according to each orientation Independent.

The means of registration may comprise detectors selected from:

image detectors to register an image double step for each orientation of the sheet;

intensity detectors to register a intensity signal for each orientation of the sheet;

and combinations thereof.

The device of the invention can also comprise a plurality of beam separators and a plurality of linear polarizers.

The device of the invention can also comprise four detectors, three beam separators and two linear polarizers.

The detectors can be detectors of intensity selected from: photomultipliers, detectors Avalanche, devices capable of recording intensity signals for a certain time and combinations thereof.

The device may also comprise first focusing means comprising a first fixed lens and a second mobile lens to conjugate a plane of interest of the system optical to be measured with a plane containing the recording means.

The device of the invention can also understand second focusing means configured to allow a focus correction varying the distance of the optical path between the first lens and the second lens manually and of automatically

The device of the invention, the four Rapid axis orientations of the sheet λ / 4 may be configured to generate polarization states independent.

In a specific case, the axis orientations Rapid sheet λ / 4 can be -45 °, 0 °, 30 ° and 60 °.

A second aspect of the invention relates to a method for measuring diffusion ( scattering ) in optical systems in reflection by the device of the invention.

A third aspect of the invention relates to a method for measuring diffusion ( scattering ) in optical systems in transmission by means of the device of the invention.

With the instrument of this invention, the GDP parameter obtained can be used as an estimator of the degree of intraocular scattering of the eye in all types of applications of interest in Ophthalmology, such as, for example, evaluation of visual quality after various refractive surgeries or evaluation of the degree of cataracts

Brief description of the drawings

To complement the description that is being performing and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization of it, is accompanied as part member of said description, a set of figures where, with an illustrative and non-limiting nature, what has been represented next:

Figure 1.- Shows the general scheme of the DP polarimeter made in accordance with the purpose of The present invention.

Figures 2A and 2B.- They show two alternatives for correct the spherical ametropia of the observer (myopia or hyperopia), dispensing with two mirrors (figure 2A) and three (figure 2B).

Figure 3.- Shows a front view of four λ / 4 sheets mounted on the same support for Provide 4 independent polarization states. This stand together with the linear polarizer of Figure 1 form a unit alternative analyzer.

Figure 4.- Shows four DP images in one real eye Each image corresponds to an independent state in the analytical unit of the experimental system.

Figure 5.- Shows the scheme of a divider of amplitude used as an analyzer unit in a polarimeter.

Figure 6.- Shows the procedure scheme experimental used to calculate the GDP in the images of DP.

Figure 7.- Shows the radial profiles of intensity corresponding to four DP images.

Figure 8.- Shows the radial profile of the GDP corresponding to the images in Figure 7, calculated using the equations [4] and [5].

Figure 9.- Shows the general scheme of the polarimeter in transmission performed in accordance with the purpose of the present invention

Description of a preferred embodiment of the invention

The invention described herein provides an objective method for the determination of eye scattering , or of any other optical system in reflection or in transmission. The information obtained is quantitative and differs greatly from the methods known in the state of the art.

More specifically, the experimental system of the present invention is based on illuminating the eye (or any system optical under study) using linearly polarized light and shown in Figure 1. The light from a laser diode 1 (or any other type of suitable light source) is filtered spatially, it expands and collimates with a converging lens before of passing the first linear polarizer 2 (or any other device that by mechanical, magnetic, ferroelectric methods or electro-optical produce linearly polarized light) and of reflected in the separator sheet 3, to enter the eye 4. Said first linear polarizer 2 on the input arm converts a generic polarization state of the light emitted by the laser diode 1 in linearly polarized light.

This input beam has a diameter of 1.5 mm, limited by a fixed circular opening 5. Circular opening 5 can be replaced by a diaphragm that allows resizing of the incident beam over the eye 4. The choice of 1.5 mm has been made based on that the polarization properties of the cornea for that Pupil size can be considered constant. Other sizes of This opening could also be used. If the source of lighting used emits a collimated beam, the system it would dispense with the spatial filter and the collimating lens.

Although the polarization state of the beam incident has been chosen vertical, this linear polarization can have any orientation between 0º and 180º.

The system in question uses light from a laser diode 1 of wavelength corresponding to] infrared close (780 nm). However you could use any other wavelength of the visible spectrum (between 360 and 780 nm), either from a coherent source such as a laser or similar, or from any other type of quasi-chromatic source (diode super luminescent or similar).

Eye optics causes the beam to hit over eye 4 converge on the retina and form the image of a bright spot Some of the light that reaches the retina is absorbed and part is reflected back. A CCD camera 6 registers said light. The 50mm focal length 7 lens forms the image of the bright spot of the retina (called DP image or image aerial) on camera CCD 6. On this exit path the beam It is reflected in mirrors 8, 9 and 10 (oriented at 45º with respect to to the optical axis of the system), passes through the first lens 11 and the second lens 12 and passes through the analyzer unit 13 of states of polarization formed by a sheet 130 quarter retarder wave (λ / 4) that rotates on itself to orient its axis fast 130a in the right directions for the purpose of the invention, and a linear polarizer 131.

The artificial pupil 14 functions as a pupil of effective exit and is conjugated with the real pupil of the eye by middle of the first lens 11 and the second lens 12. In the invention described here the focal lengths of these lenses have been 190 mm and 200 mm respectively. These focal points may be different. of those used here, but always keeping in mind that the quotient between them provides lateral increase between pupils. Although the artificial pupil 14 has a fixed size of 5 mm, of again an iris diaphragm of variable diameter can be used to be able to change the size of the effective exit pupil to choice of the experimenter. The focal length of objective 7 does not it is only limited to 50 mm, but it can be changed by any other (with the corresponding camera displacement CCD 6), always taking into account that this focal mark marks the extension of the registered retinal image.

Mirrors 9 and 10 are mounted on a mobile support forming the so-called focus corrector 15, which allows to vary the distance (optical path) between the first lens 11 and the second lens 12 to compensate for the spherical ametropia of the subject (myopia or farsightedness). Focus correction can be do it manually or automatically, in which case the support mobile must be coupled to a computer controlled engine. AND] the same effect would be achieved if the lens 12 could move to along the optical axis of the system (manually or automatically), case in e] which mirrors could be dispensed with 8-10 or 9 and 10 as indicated in Figure 2A and 2B.

An additional mirror 16 and a video camera with Its objective is the so-called auxiliary control system pupil 17 which allows the observer's pupil to align with the beam of entry at all times.

The analyzer unit 13 consists of a sheet λ / 4 130 rotating followed by a second linear polarizer 131 parallel to the first polarizer 2. The first polarizer 2 and the second polarizer 131 may not be parallel, but if they are optimizes the signal that reaches the registration system. The generation of 4 independent polarization states in the unit analyzer 13 depends on the orientation of the rapid axis 130a of the sheet λ / 4 130. Previous tests in the laboratory of inventors have shown that the orientations -45º, 0º, 30º and 60º (taking as reference a horizontal axis parallel to the optical table on which the system is mounted) are optimal for produce 4 independent states in the analyzer unit 13. Yes the second polarizer 131 is not in horizontal or vertical position the optimum orientations of the sheet λ / 4 130, must be the same, but referred to the orientation of the transmission shaft of the second polarizer 131.

The rotation of the sheet λ / 4 130 in its support to place the shaft in the proper position, you can Perform manually or automatically. In the latter case the This bracket can be attached to a gear and this in turn to a motor, so that the rotation can be performed controlled by software on the computer through the corresponding card. Four λ / 4 130 sheets can also be placed on a Revolver type support 132a so that each one has the proper orientation and the stand rotate to place the sheet that it is required each time (Figure 3). As an alternative, the sheet λ / 4 130 can be replaced by any device that allow analysis of the polarization state of a light beam: pairs of liquid crystals, Pockels cells, modulators photoelastic, ferroelectric retarders or any other device that can produce 4 polarization states independent.

For each state of independent polarization generated in the analyzer unit 13 a DP image is registered 1 second duration (exposure time can be timely modified). Figure 4 shows an example of four corresponding polarimetric PD images in a young subject for the four orientations of the sheet λ / 4 130 before cited.

Images are recorded using a CCD camera. The plane of the CCD is conjugated with the plane of interest (the retina in the case of the eye). Said camera integrates the energy that comes to it during the exposure time during which the shutter is open. Any camera or image recording device that can integrate in a defined time (by software or hardware ) the light that comes to it, can serve as a recording system.

In the present invention the set "analyzer unit 13+ registration system" by the called amplitude dividing system (Figure 5). This consists of Four CCD cameras (CCD-1 18, CCD-2 18, CCD-3 18 and CCD-4 6), three beam separators (two non-polarizers -19a and 19b- and a dichroic -20-), one λ / 4 130 sheet and two linear polarizers (second polarizer 131 and third polarizer 21). In this case the dichroic beam separator 20 transmits and reflects polarized light horizontal and vertical linear, respectively. The third polarizer 21 makes light fall on CCD-3 linear polarized at 45º. Finally the fast shaft 130a of the blade λ / 4 130 and the second polarizer 131 form 45 °, which indicates that the set works as a circular polarizer.

In the invention described herein the system of Image registration can be changed by a registration system intensities This can be done using any type Intensity detector: photomultiplier, photodiode avalanche or similar, so that it integrates the intensity that it reaches the active area in the corresponding exposure time. In In the case of using a detector, the polarimeter would stop provide spatially resolved information so that only information from an "extensive point" would be obtained conjugate of the retina, whose size will depend on the focal points of the lenses placed in the system. If the system incorporated the amplitude divider described in the previous paragraph, would be four detectors needed to replace the four cameras

The detailed operation of the invention is the following (a general outline of the procedure is shown in the Figure 6). The linearly polarized beam of light in the direction vertical per share of P1 has an associated Stokes vector S_ {IN} = [1, -1, 0, 0] T, where the T index indicates transposed. This light performs a double step in the eye, going through the ocular means and reflected in the retina. The properties of polarization of the eye in DP are contained in a matrix of Mueller 4x4 (M = m_ {ij}) whose elements are real numbers. Thus, the  Stokes vector of the light emerging from the eye, S_ {OUT}, will be result of:

one

This Stokes vector contains information on the polarization properties of the eye and in particular on depolarization, directly linked to scattering . This light coming from the eye passes through the analyzer unit 13 formed by the λ / 4 130 sheet and the vertical polarizer (with Mueller, Mα and MP matrices), so that the Stokes vector that finally reaches the registration system, S_ {F}, is:

2

where c = cos2 \ alpha_ {i} and s = sen2α_ {i}; and α_ {i} (i = 1, 2, 3, 4) are the angles forming the rapid axis 130a of the sheet λ / 4 130 with the horizontal reference. For the invention presented here, These angles have been chosen as: -45º, 0º, 30º and 60º. But nevertheless these orientations could be any others that allow generate four independent states in the analyzer unit 13 (-45º, 0º, 45º and 90º; -60º, -30º, 0 and 45º; etc).

Let M_ {UA} be the auxiliary 4x4 matrix in which each row corresponds to the first row of the matrices M_ {Pα} corresponding to each of the four sheet orientations λ / 4 130. Then it verify:

3

where I_ {i}; are the intensities of each pixel of the DP images registered for each blade orientation λ / 4 130.

The objective of the invention is to estimate the contribution of intraocular scattering by calculating the GDP parameter that depends directly on the elements of the Stokes vector, S_ {OUT}.

Known the intensities of the four Registered DP images, the four elements spatially Stokes vector solved can be calculated as:

4

Finally the spatially resolved GDP (map) It will be obtained as:

[5] GDP = frac {S2 <1> + S2 <2> S2 {3}) {1/2} {S_ {0}}

which takes values between 0 (depolarized light, which has suffered a lot of scattering ) and 1 (fully polarized light, which has not suffered any scattering ).

Another option is to calculate the radial profile of intensity of each DP image (Figure 7). From them and using [4] and [5] the GDP for each point of the radial profile (Figure 8).

The third option is to calculate the intensity bright in an area of the central area of the image. Then, intensity values for each image in the series would correspond to the I_ {i} mentioned above. Behind them the Equations [4] and [5] would allow obtaining GDP.

If instead of using a CCD camera you use a detector, the registered I_ {i} do not correspond to 4 images, but at 4 intensity values. In this way, there is no spatial resolution and therefore GDP goes from being a map 2-D at a simple numerical value.

In the case of using as an analyzer unit and registration system the amplitude divider system of Figure 5, the auxiliary matrix M_ {UA} will be:

5

This setting makes about four cameras affect independent polarization states and therefore Stokes vector can be reconstructed using equation [4] and considering [6]. With equation [5] it would be calculated as New GDP.

Although the present invention has focused on the measure of scattering present in the eye, it can also be used in other samples and systems, both in reflection and in transmission. In both cases, the procedure would be the same as described. The scheme corresponding to the device in transmission is shown in Figure 9.

Claims (11)

1. Device for diffusion measurement ( scattering ) in optical systems determining a parameter, "degree of polarization", characterized in that it comprises: first linear polarization means (2) for generate linearly polarized light; an analyzer unit (13) of states of polarization comprising:

media generators to generate 4 independent polarization states; Y

second means linear polarization (131);

at least first means of registration (6) for record an input signal for each of the four states of polarization independent. 2. The device of claim 1, characterized in that the generating means comprise: at least one rotating blade (130); Y orientation means (132) configured to orient the rotating blade (130) with a fast shaft (130a) according to Four independent orientations. 3. The device of claim 2, characterized in that it comprises four sheets (130); Y the orientation means (132) are configured to orient each sheet (130) with a fast axis (130a) according to each independent orientation. 4. The device of any of claims 2-3, characterized in that the recording means (6) comprise detectors (18) selected from: image detectors to register an image double pass for each orientation of the sheet (130); intensity detectors to register a intensity signal for each orientation of the sheet (130); and combinations thereof. 5. The device of claim 4, characterized in that it further comprises a plurality of beam separators (19a, 19b, 20) and a plurality of linear polarizers (131, 21). The device of claim 5, characterized in that it comprises four detectors (18), three beam separators (19a, 19b, 20) and two linear polarizers (131, 21). 7. The device of any of claims 4-6, characterized in that the detectors (18) are intensity detectors selected from: photomultipliers, avalanche detectors, devices capable of recording intensity signals for a certain time and combinations thereof. 8. The device of any of claims 2-7, characterized in that it further comprises first focusing means comprising a first fixed lens (11) and a second mobile lens (12) to conjugate a plane of interest of the optical system to be measured with a plan containing the recording means (6). 9. The device of any of claims 2-8, characterized in that it further comprises second focusing means (15) configured to allow a focus correction by varying the distance of the optical path between the first lens (11) and the second lens (12 ) manually and automatically. 10. The device of any of claims 2-9, characterized in that the four orientations of the fast axis (130a) of the sheet λ / 4 (130) are configured to generate independent polarization states. 11. The device of any of claims 2-10, characterized in that the orientations of the rapid axis (130a) of the sheet 214 (130) are -45 °, 0 °, 30 ° and 60 °.