WO2021008686A1

WO2021008686A1 - Ophthalmologic slit lamp microscope with a spatial light modulator

Info

Publication number: WO2021008686A1
Application number: PCT/EP2019/069076
Authority: WO
Inventors: Frank Zumkehr; Claudio DELLAGIACOMA
Original assignee: Haag-Streit Ag
Priority date: 2019-07-16
Filing date: 2019-07-16
Publication date: 2021-01-21
Also published as: DE112019007543T5

Abstract

The microscope comprises an illumination device (9) with a spatial light modulator (24). The image of the spatial light modulator (24) is projected onto the target (10). In order to correct for a non-homogeneous spatial distribution of the irradiance of the illumination device (9), the individual pixels of the spatial light modulator (24) are set to differing transmissions depending on their location and taking the uncorrected distribution of the irradiance into account.

Description

Ophthalmologic slit lamp microscope with a spatial light modulator

Technical Field

The invention relates to an ophthalmologic slit lamp microscope comprising an illumination device having a spatial light modulator and illumination imaging optics projecting the spatial light modulator onto a target plane. The micro- scope further comprises a control unit connected to the spatial light modulator for controlling it.

The invention also relates to a method for operating such an oph- thalmologic slit lamp microscope

Background Art

DE10151314 describes an ophthalmologic slit lamp microscope having a spatial light modulator instead of a mechanical slit The spatial light modula- tor can generate any pattern of light which is then projected onto the target plane by means of suitable imaging optics.

In order to have uniform brightness in the bright parts of the pattern, the light from the light source, which is spatially non-homogeneous, has to be pro- cessed such that it has uniform irradiance. Dedicated, complex beam-shaping optics, such as e.g. described in US 2005/0270487, can be used for that purpose.

Disclosure of the Invention

The problem to be solved by the present invention is to provide a microscope of this type, and a method for operating it, where the spatial inhomogene- ity of the illumination device has little or no influence on the pattern of light gener- ated by the slit lamp on the target

This problem is solved by the ophthalmologic slit lamp microscope of claim 1. Accordingly, the microscope comprises at least the following elements:

- An illumination device: This is the component generating a shaped illumination on a target plane of the microscope. The illumination device comprises a spatial light modulator with a plurality of pixels as well as imaging optics projecting the spatial light modulator onto the target plane. The spatial light modulator forms the “slit” of the slit lamp, i.e. it is the part that shapes the light into a pattern to be pro- jected onto the target plane (e.g. onto the cornea of the eye). The spatial light modula- tor has a plurality of controllable pixels. The illumination optics projects the spatial light modulator, and therefore the pattern of light defined by it, onto the target plane.

- A control unit: The control unit is connected to the spatial light modulator for controlling its pixels. It comprises a memory storing profile data de- scriptive of the“uncorrected spatial distribution” of irradiance of light fiom the illu- mination device when all pixels of the spatial light modulator are in their on-state. In other words, the profile data describes, at least in an approximation, the inhomogene- ity of the light that is generated by a light modulator that is not subject to the correc- tion of the present technique.

According to the invention, the control unit is adapted to control the spatial light modulator in dependence of the profile data.

This allows the control unit to take, at least partially, into account how the inhomogeneity of the light from the illumination device affects the bright- ness of the illumination pattern.

Hence, inherent spatial inhomogeneities of the illumination device can be corrected for, at least partially, in a simple manner.

Advantageously, the microscope further comprises at least the fol- lowing:

- A microscope optics: The microscope optics is located to generate an image of the target plane.

- A camera: The camera is located to receive the image from said microscope optics, and it is connected to the control unit such that the control unit can obtain the image.

The control unit is adapted to record the image from the camera and to derive at least part of the profile data therefrom. In other words, the microscope it- self can generate or maintain the profile data, which e.g. allows calibrating the micro- scope for different device configurations, light source ageing, device-by-device dif- ferences, etc.

In order to carry out such a calibration, a standard target may be used, e.g. a target that has homogeneous reflectivity.

The“uncorrected spatial distribution” is advantageously the distri- bution of the irradiance at the target plane (which is the conjugate plane (in respect to the illumination imaging optics) of the plane of the spatial light modulator) when all its pixels are in their on-state. In this case, the present technique can account for non- uniform spatial transmission properties not only of the light source(s) and the optics before the spatial light modulator but also of the illumination imaging optics, which allows to use simpler components for all these parts.

Alternatively, the“uncorrected spatial distribution” may be the spa- tial distribution of the light at the camera of the microscope in case that all pixels of the spatial light modulator are in their on-state and the target is a standard target of homogeneous reflectivity located in the target plane. If the transmission of the map- ping of the target onto the camera is spatially uniform and the illumination imaging optics is properly positioned to map the spatial light modulator onto the target, this uncorrected spatial distribution is the same as the uncorrected spatial distribution ac- cording to the previous definition.

In an advantageous embodiment, the control unit is adapted to set, for at least a subset of the pixels, a transmission T(x, y) of a pixel at a location x, y of the spatial light modulator proportional to I(x, y) / Io(x, y), with I(x, y) being a desired irradiance of the light at said location x, y and Io(x, y) being the uncorrected distribu- tion of the irradiance from the illumination device at location x, y as e.g. obtained from the profile data. This allows generating an effective irradiance that corresponds to the desired irradiance I(x, y), inherently compensating for the non-homogeneous spatial properties of the illumination device.

Advantageously, while recording an image frame with the camera, the pixel of the spatial light modulator at location x, y is switched between its trans- mitting on-state and non-transmitting off-state, with the (total) duration of the on- state being proportional to the transmission T(x, y).

The invention can also be formulated as a method for operating such an ophthalmologic slit lamp microscope comprising the step of setting a trans- mission of individual pixels of said spatial light modulator in dependence of the irra- diance profile.

Advantageously, this method comprises the various steps that the control unit is adapted to carry out.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following de- tailed description thereof. This description makes reference to the annexed drawings, wherein: Fig. 1 shows an embodiment of an ophthalmologic microscope, Fig. 2 shows an embodiment of the slit lamp components,

Fig. 3 shows an example of the uncorrected spatial distribution of the irradiance as well as the corrected transmission of the spatial light modulator along direction x,

Fig. 4 shows an example of the corrected effective irradiance along x,

Fig. 5 shows an example of a desired irradiance along x,

Fig. 6 shows the uncorrected spatial distribution of the irradiance as well as the corrected transmission of the spatial light modulator for the desired irradi- ance of Fig. 5, and

Fig. 7 shows an example of the corrected effective irradiance along x for the example of Figs. 5 and 6.

Modes for Carrying Out the Invention

Definitions

“Irradiance” is the light power per area, e.g. at the target plane.

The“transmission” T(x, y) of a pixel of the spatial light modulator at a position corresponding to the coordinates x, y in the target plane is the percentage of incoming irradiance that is converted into outgoing irradiance propagating through the illumination imaging optics. For example:

- If the spatial light modulator is a DMD, the transmission T(x, y) corresponds to the percentage of light reflected into the illumination imaging optics in a given time interval (such as in the time interval for recording a camera image).

- If the spatial light modulator is an array of light sources, the trans- mission T(x, y) is to be understood as the ratio between the energy said pixel effec- lively emits in a given time interval (such as in the time interval for recording a cam- era image) to the maximum energy said pixel could emit in said time interval. For ex- ample, if the light sources are operated with pulse-width modulation, the transmission would be the ratio of the on-time of the pixel to the interval time.

A“DMD” is a digital micro-mirror device, which is a device having an array of movable mirrors. Typically, the mirrors are small in the sense that they have a diameter of less than 100 pm, and they are arranged in a two-dimensional ar- ray. DMDs are usually manufactured as MEMS devices. Overview

Fig. 1 shows an embodiment of an ophthalmologic microscope, in particular a slit lamp microscope.

The microscope has a base 1 resting e.g. on a desk, a translationally displaceable stage 2 mounted to base 1 , a first arm 3, and a second arm 4.

Stage 2 can be linearly displaced along horizontal directions x and z in respect to base 1.

The arms 3 and 4 are mounted to stage 2 and pivotal about a com- mon vertical pivot axis 5, i.e. an axis parallel to vertical direction y.

The device may further include a headrest 7 mounted to base 1 for receiving the patient’s head.

Arm 3 carries a microscope device 8, and arm 4 carries an illumina- tion device 9, such as a slit lamp.

Microscope device 8 has an optical axis 12. It comprises micro- scope optics 14, 15, such as an objective 14 and zoom optics 15, which project an im- age of eye 10 onto a camera 16 and/or an eyepiece 18. A beam splitter 20 may be ar- ranged to spilt light between these components.

Illumination device 9 adapted to project a shaped light beam onto the eye 10 to be examined. It comprises a light source 22, a spatial light modulator 24, and illumination imaging optics 26.

Light source 22 can e.g. comprise several units emitting different wavelengths, e.g. in the red, green, blue, and infrared range of the optical spectrum. These units can be controlled separately in order to change the color of light source 22. Illumination imaging optics 26 projects the light from modulator 24 onto the ante- rior surface of eye 10, e.g. via a mirror 28 mounted to arm 4. The anterior surface of eye 10 is assumed to be located at a target plane 11, which is the optically conjugate plane of spatial light modulator 24 in respect to illumination imaging optics 26.

Illumination device 9 can be arranged above or below mirror 28.

A control unit 32 controls the components of the microscope. In particular, it may e.g. comprise a microprocessor 34 and a memory 36. Microproces- sor 34 is programmed to cany out the steps of the method as described below, and memory 36 contains the data and/or instructions to do so.

Illumination Device

Fig. 2 shows a more detailed embodiment of illumination device 9. It is designed to project an illumination field of defined, sharp contours onto target, such as the eye 10. The illumination field may e.g. be round, rectangular, or slit- shaped. Even though the illumination device is called a“slit-lamp” herein, the illumi- nation field does not need to be slit-shaped at all. It can take any shape.

In the present embodiment, illumination device 9 comprises four light sources 22a - 22d of different spectral emission characteristics. For example, they may include an infrared light source, a red light source, a green light source, and a blue light source. Advantageously, the light sources are LEDs. In particular, each light source may be a single LED.

The light from each light source is substantially collimated by means of collimation optics 40a - 40d.

Three dichroic mirrors 42a, 42b, 42c are used to combine the light from the light sources 22a - 22d to become coaxial.

The combined light is passed through homogenization optics 44, such as a fly-eye lens array, e.g. as described in US 6507434.

Two cylindrical lenses 46a, 46b, a further lens 46c as well as the homogenization optics 44 together also widen the light beam along one direction, e.g. giving it an elongate cross-section, e.g. having a width-to-height ratio of 16:9, for bet- ter matching the typically available form factor of spatial light modulators.

A mirror 48 deflects the light into an assembly of two prisms 50a, 50b with a gap 52 between them.

The light beam passes prism 50a, gap 52, and prism 50b and arrives at spatial light modulator 24.

In the shown embodiment, spatial light modulator 24 is a DMD with an array of micro-mirrors. Control unit 32 is adapted to control the alignment of each micro-mirror, e.g. between a first and a second position.

For the micro-mirrors being in the first position, the light is re- flected back into prism 50b along a direction denoted by 54 in Fig. 2. Light traveling along this direction 54 is subject to total internal reflection at the interface of second prism 50b to gap 52 and reflected into a direction denoted by 56 in Fig. 2.

For the micro-mirrors being in the second position, the light is still reflected back into prism 50b, but along a different direction (not shown in Fig. 2), along which it does not fulfil the conditions for total internal reflection at the interface to gap 52. The small fraction still reflected at this interface will travel in a direction different from direction 56 and not be further processed by illumination imaging op- tics 26 described in the following.

Hence, control unit 32 is able to individually set each pixel (each micro-mirror) of spatial light modulator 24 into an on-state and an off- sate, thereby defining the contour and shape of the light field at target 10 (which is assumed to be located in the target plane 11 of the illumination device).

The light from the pixels enters illumination imaging optics 26, which may include one or more lenses. From there, it may pass mirror 28 to arrive at target 10.

The illumination imaging optics images spatial light modulator 24 onto target 10, i.e. target 10 is at the target plane 11, which is the conjugate plane, in respect to illumination imaging optics 26, of the plane 62 of spatial light modulator

24.

Irradiance Distribution

Fig. 3 shows the“uncorrected spatial irradiance distribution” Io(x, y) at the location of plate 11 if all pixels of spatial light modulator 24 are in their on- state.

As can be seen, in the shown embodiment, the irradiance Io(x, y) is at its maximum close to the optical output axis of illumination device 9 (i.e. at loca- tion x = 0 in Fig. 3), and it may e.g. be substantially independent of x at that region, but then it starts to decrease for larger values of |x|. This is due to the limitations of the optics and the light sources in illumination device 9.

The inhomogeneity of the uncorrected irradiance is advantageous for small slits (i.e. if spatial light modulator 24 is e.g. controlled to create a slit illumi- nation between x = -1 and x = 1, with all its other pixels being in their off-state) be- cause small slits can profit from the high brightness in the center of the light field.

However, if the target is to be illuminated over a larger area, this in- homogeneity may become noticeable and affect the quality of the pictures observed in microscope device 8. Even though it is possible to modify the optics for better homo- geneity at the outer regions, this would come at the cost of decreasing the brightness in the center of the field, i.e. the brightness that can be exploited for small slits.

Hence, control unit 32 can be set to operate in a corrected mode of operation, the features of which are described in the following.

In particular, control unit 32 stores“profile data” in memory 36.

This profile data is descriptive of the uncorrected spatial distribution Io (x, y) of the irradiance when all pixels of the spatial light modulator are in their on-state.

Now, when illuminating target 10, control unit 32 is adapted to pulse-width modulate the light by means of spatial light modulator 24 as a function of this profile data. In other words, control unit 32 switches a pixel corresponding to a position x, y between its on and off-states, e.g. for time periods t_on and toff, thereby generating an averaged transmission T(x, y) = t_on(t_on + t_off) in the time interval ton + toff. For the pixels that should be bright, the averaged transmission is e.g. chosen such that, at least within a certain region around the optical axis of illumination device 9, the following condition holds:

where I is a constant that is the same for all pixels in the on-state.

The total interval time ton + toff advantageously corresponds to the time for recording an image frame with camera 16. Hence, in general, each pixel (that should not have maximum transmission) is switched off during part of the time inter- val during which the image frame is recorded.

Fig. 3 shows an example of T(x, y) in a dotted line, with I = 50 in the arbitrary irradiance units of the graph.

As can be seen, obviously, condition (1) can only be maintained for a subset of the pixels, namely for this where T(x, y) of Eq. (1) is smaller or equal to 100%.

T(x, y) of Fig. 3 illustrates the situation where all pixels should be bright.

The effective (time-averaged) irradiance I_eff(x, y) at target plane 11 is proportional given by

with ~ denoting proportionality. A plot of I_eff(x, y) as a function of x for the example of Fig. 3 is shown in Fig. 4.

As follows from Eqs. (1) and (2), Ieff(x, y) is constant (proportional to I) for those pixels where Eq. (1) can be maintained. Outside said range (i.e. outside |x| > 10 in Fig. 4), I_eff(x, y) drops to lower values.

As a comparison of Io(x, y) and I_eff(x, y) of Figs. 3 and 4 illustrates, this technique allows to extend the range where a homogeneous illumination can be achieved.

Figs. 3 and 4 show an embodiment where illumination device 9 is used to generate an illumination pattern of maximum size. In many situations, the user limits the size of the illuminated area at target plane 1 1. This is illustrated in Figs. 5 to 7.

Here, the device should generate a desired irradiance I(x, y) at target plane 11 as shown in Fig. 5. In many applications, this desired irradiance is a binary pattern defining the pixels of the illumination field that are to be bright and dark, e.g. with the bright pixels having a value of 1 and the dark pixels having a value of 0.

In this case, only the pixels where I(x, y) is non-zero are set to a transmission as given by Eq. (1) while all others are maintained in their off-state.

Since, in the example of Fig. 5, the desired irradiance I(x, y) is only non-zero where the uncorrected spatial distribution Io (x, y) is smaller than I of Eq.

(1), the resulting effective irradiation Iefi(x, y), as shown in Fig. 7, has constant irradi- ance within the lighted parts of the pattern.

It must be noted that the constant I can be set depending on the de- sired irradiance I(x, y) by determining the minimum I_min of the effective irradiance at any pixel that is bright in the desired irradiance I(x, y) and then e.g. use I = I_min or, more generally, chose the constant I as a function of Lin, e.g. I = c-Lin with c being a constant value for all pixels. This allows obtaining an optimally bright but uniform il- lumination

Profile Data

As mentioned, memory 36 comprises profile data descriptive of the uncorrected spatial distribution Io(x, y) of the irradiance. For example, memory 36 can store any of the following:

- Io(x, y) for all pixels of spatial light modulator 24.

- T(x, y) for all pixels of spatial light modulator 24 as obtained by

Eq. (1).

- Any other transform of the uncorrected spatial distribution Io(x, y)

- Any parameters that allow calculating any of the above infor- mation at least in approximation, e.g. parameters of a model function approximating Io(x, y) or T(x, y).

The profile data can be obtained in a calibration measurement, e.g. at the site of the manufacturer of the microscope when manufacturing or servicing the microscope. The calibration measurement may, however, also be carried out at the user’s site.

For maximum flexibility, control unit 32 is advantageously adapted to carry out such calibration. In particular, such calibration can be carried out by recording an im- age by means of camera 16 and processing this image to derive information about the uncorrected irradiance distribution Io(x, y).

Advantageously, a standard target is placed in plane 11. This stand- aid target has homogeneous non-specular reflection along directions x and y. Then, all pixels of spatial light modulator 24 are set to the same transmission, e.g. placed in their full on-state.

Then, microscope device 8 is focused onto the standard target and an image is recorded by means of camera 16.

Assuming that the imaging optics of microscope device 8 has negli- gible spatial transmission inhomogeneities over the whole illuminated area, the rec- orded image represents the uncorrected spatial distribution Io(x, y) of the irradiance. From this, the profile data can be calculated and stored in memory 36.

The steps of controlling spatial light modulator 24, of recording the image by means of camera 16, and of deriving the profile data can be carried out au- tomatically by control unit 32.

Notes

In the examples above, the desired irradiance I(x, y) is a binary pat- tern defining pixels that are bright or dark, with no state in between. It must be noted, though, that the technique described here can be used for desired irradiances I(x, y) of more than two possible values. In this case, the transmission T(x, y) may be calcu- lated, as

For example, the desired irradiance I(x, y) may be a gradient, or im- age processing of the current image recorded by camera 16 may be used to increase the irradiance at a certain location of the eye, e.g. at a location of particular interest, while maintaining a medium illumination at other parts of the eye and suppressing the illumination at yet further parts of the eye.

In the embodiment above, spatial light modulator 24 is a DMD. It may, however, also rely on other spatial light modulation techniques. For example, it may be a TFT or LCoS device.

Further, the spatial light modulator may comprise an array of light sources as its pixels, in which case the light sources and the spatial light modulator are combined into a single device. If illumination device 9 comprises several light sources 22a - 22d whose light is spatially modulated by spatial light modulator 24, and the light sources 22a - 22d have differing spatial emission characteristics, memory 36 may comprise profile data for each one of them. In that case, control unit 32 may operate at least some of the light sources 22a - 22d sequentially or at least separately and apply the corresponding profile data for each of them. For example, while first light source 22a is on, tiie pixels may be pulse-width modulated individually to generate the desired transmission T(x, y) for first light source 22a.

While there are shown and described presently preferred embodi- ments of the invention, it is to be distinctly understood that the invention is not lim- ited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. An ophthalmologic slit lamp microscope comprising

an illumination device (9) having

- a spatial light modulator (24) with a plurality of pixels and

- illumination imaging optics (26) projecting said spatial light mod- ulator (24) onto a target plane (11), and

a control unit (32) connected to said spatial light modulator (24), wherein said control unit (32) comprises a memory (36) storing pro- file data descriptive of an uncorrected spatial distribution of irradiance of light from the illumination device (9) when all pixels of the spatial light modulator (24) are in an on-state, and

wherein said control unit (32) is adapted to control said spatial light modulator (24) in dependence of the profile data.

2. The microscope of claim 1 further comprising a microscope optics (14, 15) and

a camera (16) located to receive an image from said microscope op- tics (14, 15) and being connected to said control unit (32),

wherein said control unit (32) is adapted to record said image and to derive at least part of the profile data therefrom.

3. The microscope of claim 2 wherein said control unit (32) is adapted for recording said image with all pixels of said light modulator set to equal transmission, in particular set to a full on-state.

4. The microscope of any of the preceding claims wherein said con- trol unit (32) is adapted to set, for at least a subset of the pixels, a transmission T(x, y) of a pixel at a location x, y proportional to I(x, y) / Io(x, y), with I(x, y) being a de- sired irradiance of the light at said location x, y and Io(x, y) being the uncorrected spa- tial distribution of the irradiance at location x, y.

5. The microscope of claim 4 wherein said desired irradiance I(x, y) is a binary pattern defining pixels that are bright and dark, and wherein said control unit (32) is adapted to determine a minimum I_min of the uncorrected spatial distribution Io(x, y) of the irradiation at any pixel that is bright and

set the transmission T(x, y) at the pixels that are on to c I_min/I_o(x, y), with c being a constant.

6. The microscope of any of the preceding claims wherein said con- trol unit (32) is adapted to pulse-width modulate the light from said illumination de- vice (9) by means of said spatial light modulator (24) in dependence of the profile data.

7. The microscope of any of the claims 2 or 3 and of any of the claims 4 or 5 and of claim 6 wherein, said control unit (32) is adapted to, while re- cording an image frame with said camera (16), switch a pixel of the spatial light mod- ulator (24) at said location x, y between an on-state and an off-state, wherein a dura- tion of said on-state is proportional to the transmission T(x, y).

8. The microscope of any of the preceding claims wherein said un- corrected spatial distribution is the distribution of the irradiance at the target plane (11) when all pixels are in their on-state.

9. The microscope of any of the preceding claims, wherein said illu- mination device (9) comprises several light sources (22a - 22d) and wherein said memory (36) comprises individual profile data for said light sources (22a - 22d), and in particular wherein said control unit (32) is adapted to operate at least some of said light sources (22a - 22d) separately and to apply the corresponding profile data for each of them.

10. A method for operating the ophthalmologic slit lamp micro- scope of any of the preceding claims comprising the step of setting a transmission of the individual pixels of said spatial light modulator (24) in dependence of the irradi- ance profile.

11. The method of claim 10 further comprising the steps of recording an image of a standard target through microscope optics (14, 15) and a camera (16) and

using said image for deriving at least part of the profile data.

12. The method of claim 11 wherein, for recording said image, all pixels of said light modulator are set to equal transmission, in particular set to a full on-state.

13. The method of any of the claims 10 to 12 comprising the step of setting a transmission T(x, y) of a pixel at a location x, y of said light modulator pro- portional to I(x, y) / Io(x, y), with I(x, y) being a desired irradiance of the light at said location x, y and Io(x, y) being the irradiance from the illumination device (9) at loca- tion x, y as obtained from said profile data.

14. The method of any of the claims 10 to 13 comprising the step of pulse-width modulating the light from said illumination device (9) by means of said spatial light modulator (24) in dependence of the profile data.

15. The method of any of the claims 11 or 12 and of the claims 13 and 14 comprising the step of, while recording an image frame with said camera (16), switching a pixel of the spatial light modulator (24) at said location x, y between an on-state and an off-state, wherein a duration of said on-state is proportional to the transmission T(x, y).