Apparatus for measuring spectrographic images

The present invention discloses an apparatus for measuring spectrum and image with high spatial resolution and spectral resolution. The apparatus comprises an imaging side telecentric lens for collecting light from an object, an optical slit positioned behind the imaging side telecentric lens, an aspheric lens for collimating lights from the optical slits, a dispersing device for separating the lights of different wavelengths into a plurality of sub-rays of different entrance angle, an achromatic lens for focusing the sub-rays, and an optical sensor for detecting the optical intensity of the sub-rays. The dispersing device can be a transmission or reflection diffraction grating, and the optical sensor may consist of a plurality of photo-detectors positioned in a two dimensional array.

BACKGROUND OF THE INVENTION

(A) Field of the Invention

The present invention relates to an apparatus for measuring spectrographic images, and more particularly, to an apparatus for measuring spectrum and image with high spatial resolution and spectral resolution.

(B) Description of the Related Art

Generally speaking, an optical image technique can only measure spatial information of an object, i.e., the image of the object, but cannot acquire spectral information. To acquire both spatial and spectral information, the optical image technique needs extra spectral measuring technique. In other words, the spectral information can only be obtained when an additional light-splitting device is incorporated into an optical imaging system.

Conventional spectrographic images measuring technique can be classified as a point scanning and a global imaging. The point scanning take a relatively long measuring time for moving the probe or the object in two-dimensional manner and the measured data needs to be combined point by point, so the obtained image tends to distort. On the contrary, the wavelength resolution of the global imaging is limited for using light-splitting devices such as the optical filter. Consequently, the above-mentioned two methods are not suitable for measuring an object with a larger area rapidly and at high spatial/spectral resolution. Particularly, to measure an object with large area rapidly at high spatial/spectral resolution can be realized only by line scanning. The line scanning method allows acquiring the spectrographic images of the object just by moving in a single direction. Therefore, it has advantage of rapid measuring speed and the image mapping is simpler without distortion. In addition, the line scanning possesses a higher spectral resolution due to the dispersing device.

Conventional spectral image measuring apparatus possesses a line-shaped field of view (FOV), and the light spot of the object in the field of view after passing lens, reflecting mirror and dispersion device causes great aberration such as spherical aberration, coma aberration, and chromatic aberration. Theses aberration results in severe expansion and distortion of imaging light spots on 2D sensor, and adjacent imaging light spots is not distinguishable due to overlapping. Consequently, neither the spatial nor spectral resolution cannot be improved. Hence, it needs a new design for improving the resolution to develop the spectral machine vision.

FIG. 1is an apparatus10for measuring spectrographic images according to the prior art. The apparatus10uses an optical collector30to guide optical energy14from points on the Y axis in the field of view of an object12to a spherical lens18after penetrating through a optical slit16. The optical energy14is collimated by the spherical lens18, and then enters into a diffraction grating20to disperse into rays22with different wavelengths and take-off angles. The ray22is focused on a charge-coupled device (CCD)26by a focusing lens24to simultaneously pick up the spatial and spectral information of the object12. The opening of the slit16inFIG. 1is parallel to Y-axis by the long side, and to X-axis by the short side.

FIG. 2(a) is a schematic diagram of the collector30according to the prior art. The collector30uses a multi-track fiber head40including several fibers42for close measurement of the object12. The multi-track fiber40is inserted into an F-number matcher43, and the optical energy in the three fibers42can present three light spot46as shown inFIG. 2(b) at the optical slit16by the convergence of the reflecting mirror44and the concave reflecting mirror45. However, the size and spatial resolution of the analyzable field of view on the object12depend on the arrangement, the diameter and quantity of the fiber42of the multi-track fiber40. Consequently, available channels are limited, and the channel of the collector30inFIG. 2is only3. In addition, the optical energy14can be collected from the object12only by closing the multi-track fiber40to the object12, which results in difficulty in measuring. Therefore, such a design is mainly used to measure the spectrographic images in an experiment at a lower resolution requirement.

FIG. 3shows the operation of the optical collector30using an imaging lens50according to another embodiment of the prior art. The imaging lens50collects the optical energy14to the optical slit16, and guides the optical energy14to a grating56via a spherical lens54. The width of the optical slit16and the size of the CCD26determine the size of analyzable FOV of the object12. However, off-axis light beams of the object12enter into the optical slit16via the imaging lens50, and the principle ray and optical axis58form an included angle θ1, i.e., the principle ray is not parallel to the optical axis58. As a result, the off-axis light beam causes a great de-collimation after passing the spherical lens54, which cannot meet the requirement that the light beam enters into the grating56at a collimated manner, and the spectrum resolution present on CCD26is reduces. In addition, such a de-collimation will also cause extra aberration, which further reduces the spectral resolution on the CCD26. Therefore, the channels available to measure cannot be increased due to the limitation of the spectral/spatial resolution. Hence, such a design can only be used in the comparison with low resolution, and cannot generate the spectrographic images with real high spatial/spectral resolution.

SUMMARY OF THE INVENTION

An apparatus for measuring spectrographic images comprises an imaging side telecentric lens for collecting optical energy from an object, an optical slit positioned behind the imaging side telecentric lens, an aspheric lens for collimating the light beam from the optical slit, a dispersing device for separating the light beam into a plurality of rays with different wavelengths and take-off angles, an achromatic lens for focusing the rays, and an optical sensor for detecting the optical intensity of the rays. The dispersing device can be a transmissible diffraction grating, and the optical sensor can be a CCD or 2D array sensor consisting of several photo-detectors positioned in a two dimensional array.

Regardless of on-axis or off-axis, the optical energy in the field of view of the object can propagate into the dispersing device in a substantially collimated manner since the apparatus possesses the imaging side telecentric lens and the aspheric collimating mirror. In addition, the apparatus uses the achromatic lens to reduce the position difference caused by the aberration, and rays with different wavelengths in the field of view of the object can fonn a very small imaging spot on the optical sensor regardless of on-axis or off-axis. As a result, the optical energy with different wavelengths from the object can form imaging spots substantially without diffusion effect on the optical sensor. Consequently, the present invention can increase both the spectral resolution and the spatial resolution simultaneously, and therefore can be used to measure spectrographic images with high-density channel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4illustrates an apparatus100for measuring spectrographic images according to the first embodiment of the present invention. The apparatus100comprises an imaging side telecentric lens110for collecting optical energy104from an object102, an optical slit130positioned behind the imaging side telecentric lens110, an aspheric lens120for collimating the light beam from the optical slit130, a dispersing device140for separating the light beam into a plurality of rays with different wavelengths and take-off angles, an achromatic lens160for focusing the rays, and an optical sensor150for detecting the optical intensity of the rays.

The dispersing device140can be a transmissible diffraction grating, and the optical sensor150can be a CCD or 2D array sensor. That is, the optical sensor150can consist of several photo-detectors arranged in 2D array. InFIG. 4, X-axis is dispersion axis, and the dispersing device140separates the optical energy104in the X-axis. The line-shaped FOV101of the object102depends on the size of the opening of the optical slit130and the amplification ratio of the apparatus100.

The optical energy104includes an on-axis light beam180and several off-axis light beams190. The principle ray182of the on-axis light beam180coincides with the optical axis112of the apparatus100. The imaging side telecentric lens110parallels the principle ray192of the off-axis light beam190to the optical axis112, and then the principle ray192enters into the optical slit130. Consequently, the off-axis light beam190can be easily corrected into an approximate collimated light beam (i.e. principle rays being parallel to each other) via the aspheric lens120, and then enters into the dispersing device140, as shown inFIG. 5. Similarly, the aspheric lens120also corrects the on-axis light beam180to an approximate collimated light and then enters into the dispersing device140.

FIG. 6shows the operation of the achromatic lens160according to the present invention. Three rays162,164and166with different wavelengths such as 400 nm, 600 nm and 800 nm, respectively are focused at different positions on the rear. Due to aberration, there is a position difference Δd between the imaging spot of the rays162,164and166. If the surface of the optical sensor150is right on the plane170, the ray162,164and166will form three imaging spots with different size on the optical sensor150, wherein the sequence of the spot size is ray166>ray164>ray162. In order to minimize the size difference of the imaging spots of ray162,164and166on the optical sensor150, the optical sensor150is rotated by a rotating angle θ2to move its surface to the plane172. However, rotating the optical sensor150makes it more different on assembling the optical device of the apparatus100. The achromatic lens160is positioned between the aspheric lens120and the optical sensor150, which will decrease the imaging spot position difference Δd caused by the aberration and allow the rays162,164and166with different wavelengths to form imaging spots with similar size on the optical sensor150.

In short, the present invention parallels the optical energy104from the object102via the imaging side telecentric lens110and collimates the light beam from the imaging side telecentric lens110via the aspheric lens120. Particularly, the imaging side telecentric lens110parallels the principle ray192of the off-axis light190to the principle ray182of the on-axis light beam180. The aspheric lens120parallels all rays of the on-axis light beam180to its principle ray182, and parallels all rays of the offset axis light beams190to its principle ray192.

FIG. 7shows imaging spots of the apparatus100according to the first embodiment of the present invention. Regardless of on-axis or off-axis, the optical energy104in the FOV101of object102can propagate into the diffraction grating140in a substantially collimated manner since the apparatus100uses the imaging side telecentric lens110and the aspheric collimating mirror120. In addition, since the achromatic lens160can reduce the position difference caused by the aberration, rays with different wavelengths in the FOV101of object102can form a very small imaging spot on the optical sensor regardless of on-axis or off-axis. In other words, the optical energy104with different wavelengths from the object102can form imaging spots substantially without diffusion effect on the optical sensor150.

FIG. 8illustrates an apparatus200for measuring spectrographic images according to the second embodiment of the present invention. Compared to the apparatus100inFIG. 4, which uses a transmissible architecture, the apparatus200inFIG. 8uses a reflective architecture. Particularly, the apparatus200uses a reflective aspheric lens220, a reflective diffraction grating240and a reflective mirror260.

Compared to the prior art, the present invention can increase both the spectral resolution and the spatial resolution simultaneously; therefore, it can be used to measure spectrographic images with high-density channel.