Patent Application
20120316830
The present invention relates to an apparatus for measuring distance between media and an imaging head for a computer-to-plate (CTP) imaging device.
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.).
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.
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.
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.
Briefly, according to one aspect of the present invention a system for measuring a distance to a substrate includes a first light source, emitting a first wavelength on a region of the substrate though a lens. A second light source emits a second wavelength region of the substrate through the lens. A first and second detector are configured to detect the first and second wavelength light reflected from the substrate. A processor is configured to compute 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 between the imaging device and substrate. A second response function represents reflected light intensity emitted from the second light source as a function of the distance between the imaging device and substrate. A ratio response function represents the ratio of the first and second response function as a function of distance between the imaging device and substrate.
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.
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.
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.
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.
The principle of this disclosure is described herein. The signal measured by the detector, Vd, is proportional and is a function of few parameters:
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.
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.
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.
Processor 340 forms a response function Vd(λ1,z), which is a function of the applied wavelength λ1 and the distance z between the lens 336 and substrate 148. Similarly, processor 340 forms a response function Vd(λ2,z), using a different wavelength λ2. Processor 340 computes along a defined range, a ratio response function which is a division of function Vd(λ1,z) and function Vd(λ2,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.
Furthermore, consider the case where the reflectivity; ρλ1 ρλ2, and the and optical transmittance; T(λ1,z) T(λ,z), are identical or change in the same way. In such a case the ratio signal, Vd(λ1,z)/Vd(λ2,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's core. By adjusting these optical parameters, the ratio Vd(λ1,z)/Vd(λ2,z) may be controlled, achieving for example the right dynamic range and accuracy.
Assuming for simplicity the case where the optical response of the confocal system is the same, both for λ1 and λ2, and described by a Gussian function G(λ,z).
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.
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.
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.
The basic principle of the invention was described via a fiber optic confocal system, described by
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.
Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket No. K000381US01/NAB)., filed herewith, entitled COUPLE MULTI-WAVELENGTH CONFOCAL SYSTEMS FOR DISTANCE MEASUREMENTS, by Eyal; the disclosure of which is incorporated herein.
