Abstract:
A distance measurement method includes imaging a first light source emitting a first wavelength, on a region of a substrate with a dispersive confocal lens; imaging a second light source emitting a second wavelength with the dispersive confocal lens on the region of the substrate; measuring intensity of light reflection emitted from the first light source; measuring intensity of light reflection emitted from the second light source; and generating a first response function wherein the first response function represents reflected light intensity emitted from the first light source as a function of the distance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket No. K000338US01/NAB), filed herewith, entitled COUPLE MULTI-WAVELENGTH CONFOCAL SYSTEMS FOR DISTANCE MEASUREMENTS, by Eyal; the disclosure of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates to a method for measuring distance between media and an imaging head for a computer-to-plate (CTP) imaging device. 
       BACKGROUND OF THE INVENTION  
       [0003]    The basic confocal technique was invented by Marvin Minsky and is since well known in the literature in different forms. The fundamental principles and advantages of confocal microscopy are described in U.S. Pat. No. 3,013,467 (Minsky et al.). 
         [0004]    Shafir et al. in the article, “Expanding the realm of fiber optic confocal sensing for probing position, displacement, and velocity,” Applied Optics Vol. 45, No. 30, 20 Oct. 2006, uses different wavelengths and adjusts the fiber tips at different focal planes of the imaging lens. Shafir et al., however, does not use the ratio of signal for distance measurements. 
         [0005]    U.S. Pat. No. 6,353,216 (Oren et al.) also uses a confocal system and different wavelengths. The different signals in this patent are used in order to determine the direction of the movement. The idea of using the ratio of two signals for distance measurements is not mentioned. 
         [0006]    The confocal signal obtained in the referenced prior art is dependent on the reflectivity of the sample. Furthermore the confocal signal is also dependent on the optical transmittance of the medium in front of the sample. There is, therefore, a need for a confocal signal that will be immune or at least less dependent on the reflectivity and optical transmittance of the medium. 
       SUMMARY OF THE INVENTION  
       [0007]    Briefly, according to one aspect of the present invention a distance measurement method includes imaging a first light source emitting a first wavelength, on a region of a substrate with a dispersive confocal lens; imaging a second light source emitting a second wavelength with the dispersive confocal lens on the region of the substrate; measuring intensity of light reflection emitted from the first light source; measuring intensity of light reflection emitted from the second light source; and generating a first response function wherein the first response function represents reflected light intensity emitted from the first light source as a function of the distance. 
         [0008]    The present invention suggests a confocal system in which the sample is illuminated simultaneously by two different wavelengths. The ratio of the back reflected signals from the sample is immune or less sensitive to parameters such as the reflectivity and the optical transmittance of the medium in front of the sample. 
         [0009]    These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  a prior art illustration of confocal sensor used to measure the reflection from an imaged substrate; 
           [0011]      FIG. 2  a prior art schematic showing a response function of reflected light intensity from an imaged substrate—maximal value represents focus; 
           [0012]      FIG. 3  an illustration of a confocal system using two light sources with different wavelength each; 
           [0013]      FIG. 4A  illustrates the shift between two response functions; and 
           [0014]      FIG. 4B  illustrates the ratio of two response functions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
         [0016]    While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as covered by the appended claims. 
         [0017]      FIG. 1  illustrates a common and well known structure of fiber optic confocal sensor  100 . The confocal sensor  100  is comprised of a light source  104  coupled to optical fiber  124  and to fiber optic coupler  116 . Rays  136  emitted from optical fiber  128  via imaging lens  144  are imaged on the surface of substrate  148 . The back reflected light  140  is coupled to the emitting optical fiber  128  and reaches light detector  112  via coupler  116  and optical fiber  132 . The intensity measured by light detector  112  is a function of the distance, z,  160  to substrate  148 . 
         [0018]    The principle of this disclosure is described herein. The signal measured by the detector, Vd, is proportional and is a function of few parameters:
       Vd(λ, z) α Io×G(λ, z)×ρ(λ)×T(λ, z). Where, α represents a proportional sign.   Io is the intensity of the light that impinges on the sample.   ρ(λ) is the reflectivity of the sample.   T(λ, z) is the optical transmittance of the medium between the sample and the imaging lens.   Z is the distance to the sample.   G(λ, z) is a function describing the overall optical response of the confocal system. It is a function of the distance, z, and of the wavelength λ, and defined also by optical parameters of the confocal system such as the numerical aperture of the lens and of the diameter of the fiber&#39;s core.       
 
         [0025]      FIG. 2  is graph describing typical and well known confocal signal where a symmetrical curve describes Vd(λ, z) as a function of the distance Z. Such a curve is measured by simultaneously reading Vd(λ, z) and while scanning with the confocal system along the z axis and at known positions. The best focus is defined at the maximum  204  of the symmetrical function. The graph describes the ambiguity of a typical confocal system. A single value of Vd(λ, z) corresponds to two different values of the position z. 
         [0026]    The scan along the z axis can be done in several techniques, for example by using an autofocus system embedded within a compound lens  336 , constructed from several optical elements, where some of them can be moved and controlled in order to change and adjust the lens focal distance. 
         [0027]    The signal, Vd(z), as can be seen from the equation, is dependent also on the reflectivity, ρ(λ), of the sample and the optical transmittance, T(λ,z), of the medium. This means that at best focus, different intensities will be measured for samples having different reflectivity. 
         [0028]    Furthermore, for a specific sample and although positioned at best focus, the intensity measured by the detector, will change if the sample reflectivity or the optical transmittance of the medium change during the measurement procedure. In such cases, therefore, one has to repeatedly scan the peak in order verify the position of the best focus. 
         [0029]      FIG. 3  describes the basic principle of the present invention using a fiber optic confocal system where at least two coupled light source and detector units  344  and  348  are used. Light sources  304  (from unit  344 ) and  308  (from unit  348 ) each emitting different wavelengths. Light source  304  is coupled via fiber optic coupler  320  to detector  312 . First detector  312  is constructed to be sensitive just to wavelength λ 1 , emitted by first light source  304 . Second light source  308  is coupled via fiber optic coupler  324  to second detector  316 . Second detector  316  is constructed to be sensitive just to wavelength λ 2 , emitted by second light source  308 . Units  344  and  348  are further coupled by fiber optic coupler  328  to emit combined light via a single output port  332 . Output optical port  332  is imaged via a dispersive optical element  336  on substrate  148 . Due to the dispersion of  336  the wavelengths are focused on two different planes, shifted relative to each other by Δz. 
         [0030]    Processor  340  forms a response function Vd(λ 1 , z), which is a function of the applied wavelength λ 1  and the distance z between the lens  336  and substrate  148 . Similarly, processor  340  forms a response function Vd(λ 2 , z), using a different wavelength λ 2 . Processor  340  computes along a defined range, a ratio response function which is a division of function Vd(λ 1 , z) and function Vd(λ 2 , z). The computed ratio response function is an absolute and monotonic function of the distance z. Hence the ambiguity (related to common confocal systems) of the function Vd((λ, z) where one value fits two different z positions is omitted. 
         [0031]    Furthermore, consider the case where the reflectivity; ρλ 1  ρλ 2 , and the and optical transmittance; T(λ 1 , z) T(λ, z), are identical or change in the same way. In such a case the ratio signal, Vd(λ 1 , z)/Vd(λ 2 , z), will be independent or less sensitive to the reflectivity, ρ, and to the transmittance T. G(λ, z), describing the optical response of the confocal system is a function of optical parameters such as the numerical aperture of the lens and of the diameter of the fiber&#39;s core. By adjusting these optical parameters, the ratio Vd(λ 1 , z)/Vd(λ 2 , z) may be controlled, achieving for example the right dynamic range and accuracy. 
         [0032]    Assuming for simplicity the case where the optical response of the confocal system is the same, both for λ 1  and λ 2 , and described by a Gussian function G(λ, z).  FIG. 4A  describes a lateral shift along the z axis between normalized function G(λ 1 , z) and normalized function G(λ 2 , z). This lateral shift is due to the dispersion of the imaging lens.  FIG. 4B  describes the ratio between G(λ 1 , z) and G(λ 2 , z). 
         [0033]    Practically, optical detectors such as  312  and  316  can be made to be sensitive just to a single wavelength by using different types of detectors. One can also use identical detectors where adequate band pass filters are inserted in front of the detectors. Different bandpass filters can be used, for example, filters based on thin film technology or filters made from fiber Bragg gratings. 
         [0034]    Different optical fibers and fiber optic couplers can be used in order to implement the invention. For example, multi and single mode optical fibers and couplers, wavelength and polarization dependent fiber optic couplers and fiber optic elements can be used. 
         [0035]    Measurement can be done simultaneously by activating the light sources and measuring detected signals at the same time. Measurements can also be done by sequentially activating the different light sources and performing measurement with their related detectors. When operating in simultaneously sequential mode, there is no need to spectrally isolate the light detectors, since measurements are done at different times. 
         [0036]    The basic principle of the invention was described via a fiber optic confocal system, described by  FIG. 3 . However, the principle can be implemented by using free space optics or by using a hybrid system where both fiber optic elements and free space optics are used. In the case of free space optics the output port  332  maybe for example a pin hole aperture. 
         [0037]    While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. 
       PARTS LIST  
       [0038]      100  confocal sensor 
         [0039]      104  light source 
         [0040]      112  light detector 
         [0041]      116  fiber optic coupler 
         [0042]      124  optical fiber connecting light source to coupler 
         [0043]      128  optical fiber emitting light on substrate 
         [0044]      132  optical fiber connecting coupler to detector 
         [0045]      136  emitted rays to substrate 
         [0046]      140  back reflected rays from substrate 
         [0047]      144  imaging lens 
         [0048]      148  substrate 
         [0049]      160  distance, z, from lens to printing plate 
         [0050]      204  maximal focus 
         [0051]      304  first light source 
         [0052]      308  second light source 
         [0053]      312  first detector 
         [0054]      316  second detector 
         [0055]      320  coupler 
         [0056]      324  coupler 
         [0057]      328  coupler between first and second light sources 
         [0058]      332  output optical port 
         [0059]      336  dispersive lens 
         [0060]      340  processor 
         [0061]      344  coupled light source and detector unit 
         [0062]      348  coupled light source and detector unit