FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of measurement of aberrations of optical systems, and particularly to the measurement of chromatic aberrations of optical systems. 
       BACKGROUND 
       [0002]    Optical systems are widely used in science and technology. They are an indispensable part of vision and measurement systems, lithography and metrology systems, projection systems and others. Optical systems assist in capturing data, in generating enlarged or reduced images of objects that should be tested, measured or just viewed. They work with monochromatic (or single color, or single wavelength light), or with polychromatic light, which is a mix of a plurality of wavelengths or colors. The term “monochromatic” as used in the present disclosure includes illumination that may be characterized by a narrow band spectrum, or quasi-monochromatic illumination. The term “light” as used in the present disclosure includes electromagnetic radiation with wavelength of several nanometers to tens microns. 
         [0003]    Generally, optical systems have aberrations. Aberrations are artifacts of the optical systems. The index of refraction of lens material varies with the wavelength of light, i.e., lens material, which may be glass, bends different colors or wavelengths by different amounts. This phenomenon is called dispersion and among others is a reason for the chromatic aberration. 
         [0004]    Optical systems may have different types of chromatic aberrations. Longitudinal chromatic aberration (CA) that causes light of different wavelengths (colors) to be focused in different planes. Lateral color aberration (LCA), also known as transverse chromatic aberration (TCA) or “lateral color”, which is generally defined as the difference in the image magnification as function of the wavelength, although in some existing optical systems, lateral color aberration is present also on the optical axis. Angular color aberration (ACA), which is the angular deviation of rays of different wavelengths. Knowledge of the magnitude of these aberrations is especially important in measurements that require sub-micron accuracy. 
         [0005]    Known prior art includes U.S. Pat. No. 5,204,535 to Mizutani. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]      FIG. 1  is a schematic illustration of an optical set-up for measurement of the lateral color aberration; 
           [0007]      FIGS. 2A and 2B  are respectively schematic illustrations of the measurement target and the image of the target generated by the optical set-up of  FIG. 1 ; 
           [0008]      FIG. 3  is a schematic illustration of an optical set-up for measurement of the lateral color aberration at a number of wavelengths; 
           [0009]      FIGS. 4A ,  4 B and  4 C are schematic illustrations of the image plane of an additional embodiment of the optical set-up for measurement of the lateral color aberration; 
           [0010]      FIG. 5  is a schematic illustration of the principles of measurement of the edges of a stripe of the measurement target; 
           [0011]      FIGS. 6A ,  6 B and  6 C are exemplary embodiments of the measurement targets; 
           [0012]      FIG. 7  shows an example of measurement of lateral color aberrations for different spectral bands as compared with a reference wavelength band; 
           [0013]      FIG. 8  is a schematic illustration of an additional embodiment of an optical set-up for measurement of the lateral color aberration; 
           [0014]      FIG. 9  is a schematic illustration of a farther embodiment of an optical set-up for measurement of the lateral color aberration; 
           [0015]      FIGS. 10A and 10B  are schematic illustrations of additional exemplary embodiments of illumination systems for measurement of the lateral color aberration; 
           [0016]      FIG. 11  is a schematic illustration of an embodiment of an optical set-up utilizing reflective optical elements for measurement of the lateral color aberration; 
           [0017]      FIG. 12  is a schematic illustration of an additional embodiment of a set up for measurement of the lateral color aberration; and 
           [0018]      FIG. 13A  is a schematic illustration of a special filter used to measure lateral color aberration.  FIG. 13B  shows a schematic illustration of an image of an edge that is illuminated by optical system using the filter shown in  FIG. 13A . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a schematic illustration of an optical set-up suitable for the measurement of the lateral color aberration. Optical set-up  50  typically includes an optical path along which are arranged an illumination system  54 , a target  64 , an optical system to be tested  70  and an image plane  72 . Illumination system  54  further includes a source of illumination  56 , which may be a polychromatic source such as an incandescent or flash lamp; the source of illumination  56  operates in continuous or flash mode. A color filter  58  positioned near or in the field conjugate of the optical system  60  covers part of the field of the illumination system and transforms at least a part of the polychromatic illumination into illumination with narrower spectral bands or monochromatic illumination. In the part of the field conjugate not covered by filter  58  another filter may be positioned. Although filter  58  is shown covering the field in the direction parallel to the X-axis it may be oriented in any direction. 
         [0020]    The illumination system  54  directs illumination to target  64 , which is typically a chrome-on-glass target bearing certain pattern. At least a section of the pattern should be “color neutral”, i.e., the ratio of transmission of the different features within the “color neutral” section should not depend on the spectral composition of the illumination. Similarly, for a reflective target, as needed for instance in optical setup as illustrated in  FIG. 8 , the ratio of reflectivity for different features of the “color neutral” section should not depend on the spectral composition of the illumination. Optical system to be tested  70  is inserted in the optical path in such a way that target  64  would be positioned in the object plane of system  70  and the image  64 ′ of target  64  would be in image plane  72  of system  70 . A detector  74  such as a CCD camera or another illumination spot position sensitive detector captures the content of image plane  72 . 
         [0021]      FIG. 2A  is a schematic illustration of target  64  for measuring LCA in one direction at a certain region in the field of view, which in its simplest form may be a chrome-on-glass target having an opaque  82  section with at least one edge  84  and a transparent section  88 . As used in the present disclosure the term “edge” means any object feature that could be imaged and according to which the image position may be defined. “Edge” could be a phase object also. In order to measure one of the chromatic aberrations and in particular the lateral chromatic aberration (LCA) of optical system  70 , part  78  of target  64 , shown as the upper part, is illuminated by the reference or “first” illumination. The other part  80 , shown as lower part of target  64  is illuminated by the “second” illumination. This method of illumination causes a section of edge  84  located in part  78  of target  64  to be “colored” by light from the “first” illumination. This part can be used as reference for the measurement. The other section of edge  84  located in part  80  of target  64  is illuminated (“colored”) by the “second” illumination. Actually, it is not necessary to flood illuminate the whole target. It is enough to illuminate edge  84  by spots of first illumination  86  and second illumination  90 . It should be emphasized here, that the edge sections illuminated by the first and second illumination do not have to belong to the same edge. Only for reasons of simplicity of the description only one edge ( 84 ) is shown here. The separating of the target illumination into “first” and “second” illumination can be achieved for instance by separating the field conjugate in the illumination system into two or more parts. An example for this can be seen in  FIGS. 1 and 3 . 
         [0022]      FIG. 2B  is a schematic illustration of image  64 ′ of target  64 . Optical system  70  to be tested generates image  64 ′. Image  86 ′ of the target illuminated by spot  86  of the first illumination includes image  86 ′-A of respective section of edge  84 . Image  90 ′ of target  64  illuminated by spot  90  of second illumination includes image  90 ′-B of respective section of edge  84 . Image section  90 ′-B of edge  84  illuminated by the second illumination may include optical artifacts and particularly lateral chromatic aberration. 
         [0023]    If the system to be tested has LCA the position of image  90 ′-B of section of edge  84  may depend on the spectral composition of the second illumination. Image locations  90 ′-B 1 ,  90 ′-B 2  or any other, shown in phantom lines, represent the different positions of image  90 ′-B of edge  84 , depending on the chosen wavelength band for the second illumination. Camera  74  captures the image of target and allows determination of the positions  86 ′-A and  90 ′-B of the different sections of edge  84  illuminated by “first” and “second” illuminations. The position of the edges (or other features) can be done using common algorithms of image processing. In order to measure the LCA, at least two images should be taken, each with a different wavelengths band for the second illumination. The method defines LCA with respect to the image produced by the first or reference illumination. The spectral composition of the first illumination, illuminating the reference area  86  of the target  64 , must remain constant for all the images. If the value of LCA has to be measured for a number of selected wavelengths, it could be measured for these particular wavelengths by illuminating section of edge  84  located in part  80  with respective wavelengths. Illumination with these selected wavelengths may be produced by transmitting the “white” illumination through a set of exchangeable filters  94  as shown in  FIG. 3 . For example, LAC for illumination produced by filters E and F may be found according to the following formula: 
         [0000]        LCA   E-F  (at the position defined by edge  84  and by the area  90 )=[Edge position (second illumination, filter E, image 1)−Reference edge position (first illumination, image 1)]−[Edge position (second illumination, filter F, image 2)−Reference edge position first illumination, image 2)]
 
         [0024]    Use of the image of a section of edge  84  as reference for the measurement helps to reduce measurement errors that may be caused by undesired changes, such as target shift in the time between two successive images of different “colors” e.g. because of vibrations, thermal expansion, etc. The fact that the LCA is measured by comparing the edge position of different “colors” to the position of the edge of the reference area, which is illuminated by a light with constant “color”, is the reason for this. 
         [0025]      FIGS. 4A and 4B  are schematic illustrations of image plane of an additional embodiment of the optical set-up and method for measurement of chromatic aberrations and in particular the lateral color aberration. The first or reference illumination may illuminate two or more sections of edge  84  of target  64  and accordingly to produce two or more reference images  86 ′- 1  and  86 ′- 2  of edge  84 . Should target  64  move or rotate during the measurements, as shown in  FIG. 4C , the images of sections  86 ′- 1  and  86 ′- 2  illuminated by reference illumination will shift. This target rotation can be measured and a correction of the measurement error of the LCA, caused by the target rotation, may be introduced. If no color aberration would be present, the image of the section of edge  84  illuminated by spot  90  would be on the rotated edge  84 R′ (Suffix R marks the rotated images.) and it could be observed as image  90 ′-B 0 -R. Presence of the color aberration causes the image of the section of edge  84  illuminated by spot  90  to be located in position  90 ′-B 1  -R. Thus by measuring the target rotation angle for each of the images it is possible to correct the measurement errors caused by target  64  rotation between successive image measurements. The ability to correct the results of the measurements for the errors caused by the target rotation makes the method immune to shifts and other movements of target position and/or of other components in the setup that may occur between successive measurements. 
         [0026]    The accuracy of aberration measurement may be further increased if instead of measuring a single edge position, such as edge  84  of target  64 , position of two edges of a strip would be measured and the result of the measurement would be averaged. Reference is now made to  FIG. 5 , which illustrates this method of measurement. Illumination spot  100  could be selected to exceed the width of stripe  102 . Position of stripe  102  edges  104  and  106  may be measured and accordingly determination of the position of stripe  100  becomes more accurate than that of the position of a single edge.  FIG. 6A  shows an exemplary embodiment of a multiple stripe target  104 . Such targets allow LCA measurement in one direction, e.g. X-direction and at different places in the field of view.  FIG. 6B  illustrates an additional embodiment of measurement targets  112 . Performing measurements on orthogonal stripes  114  and  116  of target  112  of  FIG. 6B  enables LCA determination of the optical imaging system in both X and Y directions.  FIG. 6C  illustrates yet another embodiment of measurement target  150  having a pattern  152 ,  154 . Alternatively, openings  118  may be made opaque with their edges providing two orthogonal measurement directions. Targets  104 ,  112  and  150  as measurement targets may replace target  64 . 
         [0027]      FIG. 7  shows an example of results of measurement of lateral color aberration for  3  colors within a field of view (in pixels of an imager) as compared to a reference spectral band. 
         [0028]      FIG. 8  is a schematic illustration of an additional embodiment of a set up for measurement of the aberrations (lateral color aberration) of an optical system. Illumination  120  is provided by a source of illumination (not shown) that may have certain beam forming optics. The source of illumination may be a polychromatic source or an assembly of Light Emitting Diodes (LEDs), lasers, or other light sources; it may be positioned in the optical system or at a remote place and the light might be brought to the optical system using light guides, fiber optics or any appropriate mean. Plane  122  could be an infinite conjugate of illumination system  126  and an optical conjugate of the plane where measurement target  136  is located. If the source of illumination is a polychromatic source, a narrow band filter  128  may be used to provide the reference illumination. A set of exchangeable filters  130  may provide a number of monochromatic illuminations for performing the measurement. Each of filters  130  provides monochromatic (or narrow band) illumination of a different wavelength (or color, accordingly). Alternatively, a group of LEDs emitting at different wavelengths may be used. Filter  128  and the particular operable filter of set of filters  130  are located in plane  122 . Illumination system  126  directs illumination  120  to optical system  134  to be tested. Optical system  134 , eventually together with tube lens  144  forms monochromatic and polychromatic images of sections of target  136 . Target  136  may be a reflective target similar in its structure to the earlier described targets. Optical system  134  collects the reflected by target  136  illumination and directs it with the help of beam splitter  140  and tube lens  144  or other auxiliary optics to a camera  146  where image  148  of target  136  is formed. The principles of measurement of image  148  have been explained earlier. Beam splitter  140  is typically a 50%-50% beam splitter, but it may have any other proportion between the transmitted and reflected light. A mirror, or a pellicle, or any other beam-splitting device supporting the required functionality may substitute beam splitter  140 . The optical setup can be changed also, so that the beam splitter  140  transmits instead of reflecting the target image to the camera. 
         [0029]      FIG. 9  is a schematic illustration of a further embodiment of an optical set-up for measurement of the lateral color aberration. This embodiment is characterized in that it uses two illumination systems  160  and  162 , for example located on the opposite sides of the optical path. Each illumination system may include a source of illumination (not shown), a field stop  166  and  165 , and illumination beam forming optics  168  and  167 . Illumination systems  160  and  162  have their field stops  166  and  165  optically conjugate with the plane where measurement target  176  is located. One of the illuminations systems provides the first or reference illumination and the other one provides the second or the measurement illumination. Illumination systems  160  and  162  direct illumination to optical system  172  that is tested. Optical system  172  collects the illuminations reflected and transmitted by target  176  and directs it with the help of beam splitter  178  and tube lens  180  or other auxiliary optics to a camera  184  where image  182  of target  176  is formed. Target  176  may be similar in its structure to the above-described targets. The principles of measurement of image  182  have been explained above. Beam splitter  178  could be a 50%-50% beam splitter or may have any other proportion between the transmitted and reflected light. A mirror, or a pellicle, or any other beam-splitting device supporting the required functionality may substitute beam splitter  178 . The optical setup can be changed also, so that the beamsplitter  178  transmits instead of reflects the target image to the camera. 
         [0030]      FIGS. 10A and 10B  are schematic illustrations of additional exemplary embodiments of illumination systems for measurement of the lateral color aberration.  FIG. 10A  shows an illumination system that may use a beam combiner  204  to combine into one illumination beam the illumination that illumination systems  190  and  192  located on the respective sides of beam combiner  204  provide. Each illumination system may include a source of illumination  196  and  197 , a field stop  198  and  199 , illumination-forming optics (not shown) and a set of exchangeable color filters  200  and  202 . Field stops  198  and  199  may cover complementary sections of the field view of the imaging systems. The color filters  200  and  202  may be positioned anywhere between the light sources  196  and  197  and the beam combiner  204 . For instance, if the illumination is brought to the optical system through fiber optics, the color filters may be positioned at the distant end of the fiber optics. Each of systems  190  and  192  may provide the first or reference and the second or measurement illumination and their filters are selected accordingly. Polychromatic (“white”) illumination is usually provided by incandescent lamps, white LEDs or other illumination sources and does not require use of filter, unless certain predefined spectrum is needed. If illumination systems  190  and  192  instead of “white” light source use LEDs there might be no need in filters.  FIG. 10B  illustrates an alternative illumination system  210  that includes light sources  208  and proper spliced light guides  212  and  214 , such as fiber optics guides. Plane  216  may be conjugate to the target plane in the optical system. 
         [0031]    Tube lenses  144  ( FIG. 8) and 180  ( FIG. 9 ) may affect the results of the measurements. In order to reduce or eliminate the influence of color aberrations of tube lenses a reflective optical path, as illustrated in  FIG. 11 , may be used. In this embodiment, target  220  may be illuminated by any one of illumination sources described above. Optical system to be tested  224  collects the reflected or transmitted illumination and directs it to a reflective optical set-up that may further include an optional folding mirror  226  and a reflector  228 . Folding mirror  226  and reflector  228  may have spherical, parabolic or any other type of aspherical surfaces that may be tilted. The reflective optical set-up forms image  230  of target  220  in a predefined plane where a camera (not shown) may be located. It is clear that in case of finite imaging system tube lens, reflector or any other additional optical element for providing the image on the camera is not needed. 
         [0032]      FIG. 12  is a schematic illustration of an additional embodiment of a set up for measurement of the lateral color aberration of optical systems. Here, instead of taking the two or more images at different times (temporally separated), the separation is done spatially. The light beams  310  and  311  are splitted by a beamsplitter  300  before falling on the imager and fall at the end onto two separated imagers  301  and  302  or on different parts of the same imager (optical scheme for this version not shown here). Beam  310  comes from first illumination and  311  from the second illumination. Filters  304  and  305  are designed so that they do not affect the “color” of the first illumination, but do affect the second illumination. Filter  304  transmits from the second illumination only a chosen wavelengths band (called here “E”) and filter  305  transmits only wavelengths band “F” of the second illumination. The LCA can be evaluated by comparing images from the two imagers  301  and  302  (or from images of different parts of the same imager; optical scheme for this version not shown here).  301 - 1  and  302 - 1  represent the areas, where edges and/or features from the first illumination are imaged, which can be used for the reference and  301 - 2  and  302 - 2  are the areas, where the aberrations are measured. As mentioned, the filters  304  and  305  do not affect the “color” of the first illumination. This can be done for instance by leaving the relevant part of the filters uncoated, by cutting the filters accordingly or by use of an appropriate coating on the filters. Instead of the filters  304  and  305 , the beam splitter  300  could be designed so that color “E” is transmitted through it, whereas color “F” is reflected. 
         [0033]    In another embodiment for measuring the lateral color aberration a special configuration of filter assembly is used as exemplified in  FIG. 13A . The filter assembly is actually an array of filters with different spectral transmittance. In the example in  FIG. 13A  two kinds of sub-filters, named “E” and “F” are shown, but the filter may be built from more kinds of sub-filters and the array might be also two-dimensional. These filters can be used for example in an optical set up, as shown in  FIG. 1 . The filters, which may cover the whole field of view, may be positioned instead of filter  58  in  FIG. 1 . The image  350  of an edge illuminated in a set up using such kind of filter may look as shown in  FIG. 13B  and from this image it may be possible to calculate the lateral color aberration, assuming that edge used is straight enough and that the lateral color aberration does not change rapidly in the field of view. It will be understood that various configurations of filter assembly may be used. 
         [0034]    Another possible placement for the filter is near to the image plane; the filter may even be attached to the imager. Actually many color imager make use of such kind of filters; often filters termed “Bayer filters” are used in these imagers. The same idea can be used also with a slightly different type of imager, as described for instance in the U.S. Pat. No. 6,727,521. In this case the color separation is done vertically instead of horizontally. 
         [0035]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.
Disclosed is a method of measuring an optical system artifact that includes introduction into an optical path of a measurement target ( 64) having at least one edge. Illuminating a section of the edge by a first illumination and illuminating another section of the edge by a second illumination. The difference of the edge images generated by the optical system ( 60) when illuminated by the first illumination and the second illumination measured in a predefined plane represents the optical artifact.