Patent Description:
<CIT> discloses an optical element for converting a uniform beam of light of wavelength λ into an array of illuminated spots, the optical element including a phase plate made of an array of constant phase zones. The optical element comprises further an image plane disposed parallel to and at a preselected distance from the phase plate.

From <CIT> an optical system, such as a microscope or spectroscope is know. The optical system is configured to stimulate a sample and collecting fluorescend light emitted from that sample. The system has a focusing element for focusing light onto the sample, and a collector for collecting light emitted from the sample and directing it towards a detector. To reduce the effects of background fluorescence, the focusing element is a diffractive optical element.

<CIT> discloses a diffractive optical element for generating a periodic light signal. This known element is divided in a plurality of subareas which are intended to cause a phase shift of transmitted light which amounts either <NUM> or π. This manipulated light interferes so that a periodic light signal is obtained on a target surface being arranged at a target distance from the diffractive optical element.

However, when the target surface is imaged through the diffractive optical element, the image is deteriorated considerably. Thus, this known element is usable only in transmitted light methods, i.e. the target must be a transparent object.

Therefore, it is an object of the invention to provide a diffractive optical element, a method for its design and a confocal microscope being usable for transparent and opaque objects.

The object is solved in one embodiment by a diffractive optical element for generating from an input light beam a plurality of smaller light spots on a target surface, said diffractive optical element comprising a material layer having a plurality of at least first and second subareas having any of first and second absorption coefficients or first and second thicknesses, wherein the arrangement of said first and second subareas on the material layer is obtained by defining a desired first field distribution c(x',y') of at least one single light spot on the target surface; calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane defined by the material layer for said at least one single light spot; arranging and summing up said second field distribution uw(x, y) in said plane for a predetermined arrangement of a plurality of light spots, and optionally adding a plane-wave component W to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y); defining a plurality of discrete subareas on the material layer of the diffractive optical element; determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas.

The object is solved in one embodiment by a diffractive optical element for generating from an input light beam a plurality of smaller light spots on a target surface, said diffractive optical element comprising a material layer having a plurality of at least first and second subareas having any of first and second absorption coefficients or first and second thicknesses, wherein the arrangement of said first and second subareas on the material layer is obtained by defining a desired first field distribution c(x',y') of at least one single light spot on the target surface; arranging and summing up said first field distribution c(x',y') on the target surface for a predetermined arrangement of a plurality of light spots; calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane defined by the material layer for said at least one single light spot; arranging and summing up said second field distribution uw(x, y) in said plane for a predetermined arrangement of a plurality of light spots, and optionally adding a plane-wave component W to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y); defining a plurality of discrete subareas on the material layer of the diffractive optical element; determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas.

The object is solved in one embodiment by a diffractive optical element for generating from an input light beam a plurality of smaller light spots on a target surface, said diffractive optical element comprising a material layer having a plurality of at least first and second subareas having any of first and second absorption coefficients or first and second thicknesses, wherein the arrangement of said first and second subareas on the material layer is obtained by defining a desired first field distribution c(x',y') of at least one single light spot on the target surface; arranging and summing up said first field distribution c(x',y') on the target surface for a predetermined arrangement of a plurality of light spots; calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane defined by the material layer for said at least one single light spot; optionally adding a plane-wave component W to the second field distribution uw(x, y) to get a plane-wave-combined field distribution up(x, y); defining a plurality of discrete subareas on the material layer of the diffractive optical element; determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas.

The object is solved in another embodiment by a diffractive optical element for generating from an input light beam a plurality of smaller light spots on a target surface, said diffractive optical element comprising a material layer having a plurality of at least first and second subareas having any of first and second absorption coefficients or first and second thicknesses, wherein the arrangement of said first and second subareas on the material layer is obtained by defining a desired first field distribution c(x',y') of a single light spot on the target surface; calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane defined by the material layer for a single light spot by means of a Rayleigh-Sommerfeld-Integral; arranging and summing up said second field distribution uw(x, y) in said plane for a predetermined arrangement of a plurality of light spots; adding a plane-wave component W to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y); defining a plurality of discrete subareas (<NUM>, <NUM>, <NUM>) on the material layer (<NUM>) of the diffractive optical element (<NUM>); determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas.

The object is solved in still another embodiment by a method for designing at least one diffractive optical element comprising a material layer having a plurality of at least first and second subareas having any of first and second absorption coefficients or first and second thicknesses, wherein the arrangement of said first and second subareas on the material layer is obtained by defining a desired first field distribution c(x',y') of at least one single light spot on the target surface (<NUM>); calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane defined by the material layer for said at least one single light spot at a given distance; arranging and summing up said second field distribution uw(x, y) in said plane for a predetermined arrangement of a plurality of light spots; adding a plane-wave component W to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y); defining a plurality of discrete subareas (<NUM>, <NUM>, <NUM>) on the material layer (<NUM>) of the diffractive optical element (<NUM>); determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas.

The object is solved in still another embodiment by a confocal microscope comprising at least one light source, being adapted to emit at least one input light beam; at least one beam splitter having a first port, a second port and a third port, and being adapted to receive said input light beam on said first port and to direct it to said second port; at least one diffractive optical element being operable to receive said input light beam from the second port of said beam splitter and generating a plurality of smaller light spots on a target surface being arranged at a target distance from the diffractive optical element; at least one focusing means and an image sensor being adapted to capture an image of said target surface from light originating from the target surface and passing through said diffractive optical element to the second port of said beam splitter, the third port of said beam splitter and said focusing means.

The accompanying drawings which are incorporated in and constitute a part of the specification, illustrate one or more embodiments described herein, and, together with the detailed description, explain aspects of these embodiments. In the drawings.

The invention relates to a diffractive optical element for generating from an input light beam a plurality of smaller light spots on a target surface. In some embodiments, the diffractive optical element may be part of a microscope, more specifically a confocal microscope. In these embodiments, the target may be an object or a material to be analyzed. In other embodiments, the diffractive optical element may be part of a data storage device, so that the target may be an optical storage means with different unit cells or pixels which may alter the intensity or polarization of transmitted or reflected light, thereby coding at least one bit. In any case, the diffractive optical element may generate a plurality of light spots with high numerical aperture so that a plurality of locations or unit cells may be illuminated and thereby analyzed or read. A high numerical aperture may amount more than <NUM>,<NUM> or more than <NUM>,<NUM> or more than <NUM>,<NUM> or more than <NUM>,<NUM> or more than <NUM>,<NUM> in some embodiments of the invention.

According to the invention, the diffractive optical element <NUM> comprises a material layer <NUM> having a plurality of at least first and second subareas <NUM>, <NUM>. Each subarea may have any of first and second absorption coefficients or first and second thicknesses. Thus, the phase or the amplitude or both is altered when light is transmitted or reflected at the material layer <NUM> of the diffractive optical element <NUM>. The light may interfere after having passed the diffractive optical element <NUM> so that a plurality of light spots <NUM> in a predefined pattern <NUM> is obtained at a target distance from the diffractive optical element <NUM>.

Turing now to <FIG>, a diffractive optical element <NUM> according to the invention is illustrated. As can be seen from the drawing, the diffractive optical element <NUM> comprises a material layer <NUM> which might be opaque if the diffractive optical element is used in a reflecting geometry. In other embodiments, when the diffractive optical element is used in a transmission geometry, the material layer <NUM> may be transparent in at least a part of the electromagnetic spectrum. In some embodiments, the material layer <NUM> may be selected from the group comprising any of a metal, an alloy, a glass, quartz, a polymer, polycarbonate, or a resin. In some embodiments, the material layer <NUM> may have a thickness being selected from <NUM> to <NUM>. In other embodiments, the material layer <NUM> may have a thickness being selected from <NUM> to <NUM>. In still another embodiment, the material layer <NUM> may have a thickness being selected from <NUM> to <NUM>.

According to the invention, the material layer <NUM> comprises a plurality of at least first subareas <NUM> and second subareas <NUM>. Said subareas <NUM> and <NUM> may have any of first and second absorption coefficients or first and second thicknesses. Thus, the phase or the amplitude or both of light is altered, when being transmitted or reflected at the material layer <NUM> of the diffractive optical element <NUM>. The light may interfere after having passed the diffractive optical element so that a plurality of light spots in a predefined pattern is obtained at a target distance from the diffractive optical element <NUM>. The subareas <NUM>, <NUM> may be manufactured by any of lithography, printing, or embossing.

As an exemplary example, <FIG> shows <NUM> subareas in a regular pattern of <NUM> x <NUM> subareas having a pitch px and py. This is by way of example only. It should be clear that in most embodiments, the number of subareas is much larger and may reach up to <NUM> or up to <NUM> or up to <NUM> or up to <NUM> or up to <NUM> or up to <NUM> in some embodiments.

The way how a specific subarea is assigned to be a first subarea <NUM> having first characteristics or to be a second subarea <NUM> having second characteristics is explained in greater detail below. It should be clear that the invention requires at least first subareas <NUM> having a first characteristic and second subareas <NUM> having a second characteristic to function properly. In other embodiments, a higher number of different subareas each having different characteristics may be used to increase the quality of the diffractive optical element <NUM>.

<FIG> illustrates by way of example a cross sectional view of a diffractive optical element <NUM> according to a first embodiment of the invention. The cross section shows <NUM> subareas which are shaped either as first subareas 11a, 11b, 11c, 11d having a first characteristic and second subareas 12a, 12b, 12c, 12d having a second characteristic. As can be seen from <FIG>, the material layer has a lower thickness in the first subareas 11a, 11b, 11c, and 11d and a higher thickness in the second subareas 12a, 12b, 12c, and 12d, wherein the thickness of each first subarea has an approximately constant first value and the thickness of each second subarea has an approximately constant second value.

This feature has the technical effect that the phase of a beam of light is shifted by a constant first value when passing through a first subarea and the phase of a beam of light is shifted by a constant second value when passing through a second subarea. Said first and second values may be π and <NUM>π in some embodiments. The transmitted light will interfere and form a desired light pattern behind the diffractive optical element <NUM> by diffraction.

<FIG> illustrates a cross sectional view of a diffractive optical element <NUM> according to a second embodiment of the invention. Like elements are assigned like reference numbers so that the following description is restricted to the main differences.

As can be seen from <FIG>, a third subarea <NUM> is used beside first and second subareas <NUM> and <NUM>. As can be seen from <FIG>, the material layer has the lowest thickness in the third subareas 13a and 13b, a higher thickness in the first subareas 11a, 11b, and 11c, and the highest thickness in the second subareas 12a, 12b, and 12c.

This feature has the technical effect that the phase of a beam of light is shifted by a constant first value when passing through a first subarea, a constant second value when passing through a second subarea, and a constant third value when passing through a third subarea. Said first, second, and third values may be 2π/<NUM>, 4π/<NUM> and <NUM>π in some embodiments.

<FIG> illustrates a cross sectional view of a diffractive optical element according to a third embodiment of the invention. Like elements are assigned like reference numbers so that the following description is restricted to the main differences.

As can be seen from <FIG>, the material layer <NUM> has a constant thickness over its entire surface. The first and second subareas <NUM> and <NUM> are defined by a coating which is either reflective or absorbing so that the diffractive optical element can be used in reflection or transmission geometry. Therefore, the material layer <NUM> shown in <FIG> modulates the amplitude of reflected or transmitted light. Interference of this amplitude modulated light leads to a desired light pattern at a predefined distance from the material layer <NUM>.

<FIG> illustrates a cross sectional view of a confocal microscope <NUM> according to a first embodiment of the invention. The confocal microscope <NUM> comprises at least one light source <NUM>, being adapted to emit at least one input light beam <NUM>. The input light beam <NUM> may have a wavelength or a range of wavelengths being selected from the visible range. In other embodiments, the input light beam <NUM> may have a wavelength or a range of wavelengths being selected from the infrared or ultraviolet range of the spectrum. The input light beam <NUM> may comprise coherent light. Accordingly, the light source <NUM> may comprise a laser. Such a laser may be selected from any of a semiconductor laser, a gas laser, a dye laser or an excimer laser. The light source <NUM> may be composed from a plurality of individual light sources such as a diode array.

Furthermore, the microscope <NUM> comprises at least one beam splitter <NUM> having a first port <NUM>, a second port <NUM> and a third port <NUM>, and being adapted to receive said input light beam <NUM> on said first port <NUM> and to direct it to said second port <NUM>.

During operation of the microscope, light from said second port <NUM> of the beam splitter <NUM> is directed to at least one diffractive optical element <NUM> according to the invention being operable to receive said input light beam <NUM> and generating a plurality of smaller light spots <NUM> on a target surface <NUM>.

In some embodiments, the individual light spots <NUM> on the target surface <NUM> may cause fluorescence light. In other embodiments, at least a part of the incoming light of the individual light spots <NUM> may be reflected or scattered at the target surface <NUM> in the direction of the incoming light beam. Any of fluorescence light, reflected light, or scattered light emerging from the target surface <NUM> is referred to as "emerging light" in the following.

The emerging light propagates from the target surface <NUM> and passes the diffractive optical element <NUM> in the opposite direction to the second port <NUM> of the beam splitter <NUM>. The emerging light enters the beam splitter <NUM> by this second port <NUM> and exits the beam splitter by the third port <NUM>. The emerging light is then focused by an optional focusing means <NUM> to a focusing plane. It should be apparent that the invention is not restricted to a special embodiment of said focusing means <NUM>. The focusing means may comprise at least one lens or at least one mirror. In some embodiments, the focusing means may comprise a plurality of lenses or a plurality of mirrors or a combination of at least one mirror and at least one lens which may be selected from any of focusing and defocusing elements and which are arranged on an optical axis to form the focusing means. After passing the optional focusing means <NUM>, the emerging light is detected by an image sensor <NUM> being adapted to capture an image of said target surface from the emerging light. The image <NUM> sensor may comprise any of a CCD-Sensor, a CMOS-Sensor, an image intensifier tube or the like in order to capture a two dimensional image of the target surface <NUM>.

<FIG> illustrates a cross sectional view of a confocal microscope according to a second embodiment of the invention. Like elements are assigned like reference numbers so that the following description is restricted to the main differences.

In the embodiment shown, said beam splitter <NUM> comprises a dichroic mirror <NUM>. Such a dichroic mirror is reflective for some wavelength and transparent for other wavelength. This is in particular useful when sensing fluorescence light emerging from the surface <NUM> of the target <NUM> which may have another wavelength as the incoming radiation.

In some embodiments of the invention, said microscope <NUM> comprises further an emission filter <NUM> being arranged between the third port of the beam splitter and the image sensor. This feature may reduce the intensity of the scattered or reflected light from the target surface <NUM> at the image sensor so that fluorescence light may be detected with increased sensitivity.

<FIG> illustrates a cross sectional view of a confocal microscope <NUM> according to a third embodiment of the invention. Like elements are assigned like reference numbers so that the following description is restricted to the main differences.

In the embodiment shown, said beam splitter <NUM>, the focusing means <NUM> and the image sensor <NUM> are arranged in a Scheimpflug geometry, i.e. the planes defined by the target <NUM>, the focusing means <NUM> and the image sensor <NUM> respectively meet at a line. This is in particular useful when sensing targets <NUM> having a target surface <NUM> not being parallel to the image sensor <NUM>.

<FIG> illustrates a design method for a known diffractive optical element. The method starts with the desired field distribution of the light spot on the target surface. The field distribution on the diffractive optical element is calculated by an angular spectrum method. In order to obtain a plurality of light spots on the target, the field distribution is periodically reproduced on the surface of the diffractive optical element. Finally, the phase is extracted and binarized. Each phase shift is assigned one of two thicknesses so that the whole surface is covered by first and second subareas. In a last step, the diffractive optical element is produced, e.g. by a lithography process.

<FIG> illustrates the performance of a confocal microscope having a known diffractive optical element. When using such a known diffractive optical element in a confocal microscope as shown in <FIG> and <FIG>, the problem arises that the target surface <NUM> has to be imaged through the diffractive optical element used for beam spot generation, i.e. emerging light passes the diffractive optical element <NUM> on its way to the image sensor <NUM>.

<FIG> shows a hypothetical intensity distribution on the target surface <NUM> as it can be generated by the illumination with discrete light spots <NUM>. <FIG> illustrates what is seen by the image sensor <NUM> after the emerging light from the target surface has passed the diffractive optical element <NUM>, the beam splitter <NUM> and the optional focusing means <NUM>. It is apparent that the signal is completely degraded on its way from the target surface to the image sensor so that the microscope <NUM> is unusable. The invention solves this problem by an alternatively designed diffractive optical element <NUM> which is explained in more detail below making reference to <FIG>.

<FIG> illustrates a design method for a diffractive optical element according to the invention. <FIG> illustrates the field distribution during different method steps and <FIG> illustrates one method steps during the design of a diffractive optical element according to the invention in greater detail.

According to the invention, the arrangement of said first and second subareas on the material layer of the diffractive optical element is obtained by the following method:
In a first step A, a first field distribution c(x',y') of at least one single light spot <NUM> on the target surface <NUM> is specified. The field distribution may have a high intensity contrast factor, i.e. the ratio of the intensity on the center and the mean intensity is close to <NUM>, or at least higher than <NUM> or higher than <NUM> or higher than <NUM>. The number of said at least one light spots may be selected to be one in some embodiments. In other embodiments, more spot may be initially used which may be arranged in a line, in a triangle or any other polygonal form. The invention does not rely on using exactly one initial light spot <NUM>.

Next, in step B, a second field distribution uw(x, y) in a plane <NUM> defined by the material layer <NUM> at a specified target or working distance for this single light spot is calculated. In some embodiments, calculation of the field distribution uw(x, y) is done by means of a Rayleigh-Sommerfeld-Integral. In other embodiments, any other method may be used such as an angular spectrum method or solving the Maxwell equations.

In some embodiments of the invention, calculating the second field distribution uw(x, y) for a single light spot in the plane defined by the diffractive optical element is done by solving the following equation: <MAT> wherein c(x',y') denotes the first field distribution of the light spot on the target surface and x, x', y, and y' are the coordinates used on the target and on the material layer of the diffractive optical element, respectively.

In some embodiments of the invention, calculating the second field distribution uw(x, y) for a single light spot in the plane defined by the diffractive optical element is done by solving the following equation: <MAT> wherein c(x',y') denotes the first field distribution of the light spot on the target surface and x, x', y, and y' are the coordinates used on the target and on the material layer of the diffractive optical element, respectively. The result C is shown in <FIG>.

In another embodiment of the invention, calculating the second field distribution uw(x, y) for a single light spot in the plane defined by the diffractive optical element is done by solving the following equation: <MAT>.

As the diffractive optical element shall generate a predefined pattern <NUM> of light spots <NUM> during operation, said second field distribution uw(x, y) of a single light spot is arranged in said plane in this predetermined pattern in step D. In some embodiments, this pattern may be rectangular or quadratic with a pitch p between adjacent light spots. In other embodiments, another pattern may be selected such as a Nipkow disk or a statistical pattern. The resulting field distribution u<NUM>(x, y) of the arrangement is calculated by overlapping or summing up the field distributions uw(x, y) of the single light spots. As an example, the field distribution u<NUM>(x, y) for a plurality of light spots having a rectangular arrangement with the pitch p may be calculated by <MAT>.

The result E is again shown in <FIG>, third part from the left. It has to be noted that this step may be omitted if only one ligth spot is needed or a plurality of light spots has been selected initially in step A. That is to say, the step of overlapping in an arrangement (<NUM>) doesn't necessarily to be only used on uw in step A. it can also be used on c(x',y') at first, or even on both, i.e. overlap two times.

In the next step F, a plane-wave component W is added to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y). This plane-wave component W may alter the transparency of the diffractive optical element. In some embodiments, the transparency may be increased. In other embodiments, the transparency may be reduced.

The result G obtained after this method step F is shown in <FIG>, fourth part from the left.

In some embodiments of the invention, said plane-wave component W is constant. This allows for an easy calculation of the plane-wave-combined field distribution up(x, y).

In some embodiments of the invention, said plane-wave component W is determined by optimizing any of a transmitted intensity, a signal-to-noise-ratio, a numerical aperture of the spot, and a spot size. Optimization can be done either experimental or by computer simulations by stepwise altering the plane-wave component W and watching the selected variable.

In a further step, a plurality of discrete subareas is defined on the material layer of the diffractive optical element. In other words, the surface of the material layer <NUM> is discretized in a predefined pattern. Said pattern may comprise a number of rectangular subareas as shown in <FIG>. In other embodiments of the invention, other patterns may be used. Defining discrete subareas on the material layer may be done before carrying out step H or at any time before the steps A to G.

In the next method step H, any of the phase ϕB(x, y) or the amplitude of the plane-wave-combined field distribution up(x, y) is determined at least in the subareas <NUM>, <NUM>, <NUM>. This means that the field distribution up(x, y) on the material layer is discretized at the locations of said subareas and a phase shift for incoming light is calculated so that the interference of this light behind the diffractive optical element results in the desired light pattern to be generated. Such a phase shift may be easily generated by the subareas having different thicknesses so that the time of travel is delayed in the high index material layer of the diffractive optical element compared with the surrounding atmosphere. The same effect may be achieved in other embodiments by masking some of the subareas as to affect their transmission. This intensity-modulated light originating from different subareas may interfere in the same manner to generate the desired pattern <NUM> of light spots <NUM>. The result I of this method step is a binarized phase distribution as shown exemplarily in <FIG>, fifth part from the left. In other words, each discrete subarea defined previously is assigned a first characteristic or a second characteristic, thereby forming first and second discrete subareas <NUM>, <NUM> on the material layer <NUM> of the diffractive optical element.

In some embodiments of the invention, determining the phase ϕB(x, y) of the plane-wave-combined field distribution up(x, y) is done by solving the following equation: <MAT> wherein n denotes the number of thicknesses of the material layer or the number of different characteristics a subarea may have. As the number of thicknesses is directly related to the number of different phase shift values, it can be appreciated that a higher value for the parameter n may result in an improved optical quality of the diffractive optical element and a lower value for the parameter n may result in a diffractive optical element being easier to manufacture.

In some embodiments of the invention, the value for the parameter n may be selected between <NUM> and <NUM>. In other embodiments, n is selected between <NUM> and <NUM>. In still another embodiment, n is <NUM>. These values may result in a good compromise between manufacturing effort and optical quality.

In some embodiments of the invention, when determining the phase ϕB(x, y) from the plane-wave-combined field distribution up(x, y), the parameter B may be constant. In some embodiments, the value for the parameter B is determined by optimizing any of a transmitted intensity, a signal-to-noise-ratio, a numerical aperture of the spot, and a spot size. Optimization can be done either experimental or by computer simulations by stepwise altering the parameter B and watching the selected variable.

<FIG> illustrates the performance of a confocal microscope having a diffractive optical element according to the invention. When using such a known diffractive optical element in a confocal microscope as shown in <FIG> and <FIG>, the problem arises that the target surface <NUM> has to be imaged through the diffractive optical element used for beam spot generation, i.e. emerging light passes the diffractive optical element <NUM> on its way to the image sensor <NUM>.

The left part of <FIG> shows a hypothetical intensity distribution on the target surface <NUM> as it can be generated by the illumination with discrete light spots <NUM>. The right part of <FIG> illustrates what is seen by the image sensor <NUM> after the emerging light from the target surface has passed the diffractive optical element <NUM> according to the invention, the beam splitter <NUM> and the optional focusing means <NUM>. When comparing the right part of <FIG> with <FIG>, it is apparent that the signal is much less degraded and the target surface may be imaged through the diffractive optical element according to the invention without the image suffering from strong degradation. This proof of concept shows that, when using the diffractive optical element according to the invention in a confocal microscope, opaque targets may be examined. This effect is mainly due to the plane wave component added to the field distribution when designing the diffractive optical element according to the invention.

<FIG> illustrates a cross sectional view of a confocal microscope according to a third embodiment of the invention. Like elements of the invention are denoted with like reference numbers so that the following description may be limited to the main differences of the third embodiment compared to the first and second embodiment shown above.

The third embodiment uses a Scheimpflug principle to allow for an off-axis imaging of the target surface <NUM>. As can be seen in <FIG> and <FIG>, the focusing means <NUM> and the image sensor <NUM> are parallel, and the plane of focus (PoF) is parallel to the focusing means and the image sensor. If a planar subject such as the target surface <NUM> is also parallel to the image sensor <NUM>, it can coincide with the PoF, and the entire target surface <NUM> can be rendered sharply. If the target surface <NUM> is not parallel to the image sensor <NUM>, it will be in focus only along a line where it intersects the PoF.

However, when said focusing means <NUM> are tilted with respect to the image sensor <NUM>, an oblique tangent extended from the image sensor 5and another extended from the focusing means <NUM> plane meet at a line through which the PoF also passes. With this condition, a planar subject that is not parallel to the image sensor can be completely in focus. Thus, the third embodiment has the potential to improve the axial resolution of the confocal microscope <NUM>.

Claim 1:
Diffractive optical element (<NUM>) for generating from an input light beam (<NUM>) a plurality of smaller light spots (<NUM>) on a target surface (<NUM>),
said diffractive optical element comprising a material layer (<NUM>) having a plurality of at least first and second subareas (<NUM>, <NUM>, <NUM>) having any of first and second absorption coefficients or first and second thicknesses, characterized in that the arrangement of said first and second subareas (<NUM>, <NUM>, <NUM>) on the material layer (<NUM>) is obtained by defining a desired first field distribution c(x',y') of at least one single light spot on the target surface (<NUM>); calculating from the first field distribution c(x',y') a second field distribution uw(x, y) in a plane (<NUM>) defined by the material layer (<NUM>) for said at least one single light spot (<NUM>);
arranging and summing up said second field distribution uw(x, y) in said plane (<NUM>) for a predetermined arrangement (<NUM>) of a plurality of light spots (<NUM>);
adding a plane-wave component W to the overlapped field distribution u<NUM>(x, y) to get a plane-wave-combined field distribution up(x, y),
defining a plurality of discrete subareas (<NUM>, <NUM>, <NUM>) on the material layer (<NUM>) of the diffractive optical element (<NUM>) ;
determining the phase ϕB(x, y) and/or the amplitude of the plane-wave-combined field distribution up(x, y) at least in the subareas (<NUM>, <NUM>, <NUM>).