Optics: measuring and testing – By dispersed light spectroscopy
Reexamination Certificate
2001-09-13
2003-10-21
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
C356S326000, C356S328000
Reexamination Certificate
active
06636305
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to the field of spectroscopy. More particularly, the new and useful invention claimed in this document pertains to an apparatus and method for producing a substantially straight image produced by a spectrograph, monochromator, or a similar optical measurement device (collectively in this document, “instrument”). The apparatus and method for producing a substantially straight instrument image in a spectrally dispersed focal plane is particularly, but not exclusively, useful for inducing image curvature adjustments to spectral data associated with a light or radiation signal (collectively, “light beam”) by providing entrance slit configurations to overcome curvature, coma, astigmatism and other optical spectroscopy measurement and instrumentation aberrations to the signal shape (collectively, “aberrations”).
BACKGROUND OF THE INVENTION
Spectroscopy is a general term for the process of measuring energy or intensity as a function of wavelength in a beam of light or radiation. Many conventional instruments, including spectroscopes, include basic features and components such as an entrance slit, a collimator for producing a parallel beam of radiation; one or more prisms, mirrors, gratings, reflectors and similar components for receiving and dispersing radiation through differing angles of deviation based on wavelength; an exit slit or an apparatus for collecting spectral data; and perhaps apparatus for displaying and adjusting the image of the dispersed radiation. At least one application of spectroscopy is to use absorption, emission, or scattering of electromagnetic radiation by atoms, molecules or ions to qualitatively and quantitatively study physical properties of matter.
For purposes of both qualitative and quantitative analyses of physical matter, a light beam may be admitted at varying angles into an instrument through an entrance slit, directed through, past, or at one or more prisms, mirrors, reflectors, gratings and other electronic and optical devices (collectively, “instrument components”) producing one or more beams of light or radiation that may be directed at a sample of the physical matter (collectively, “incident radiation”) to in turn produce one or more beams from which to measure property characteristics of a sample (collectively, “resultant beam”). A resultant beam may provide one or more frequencies associated with the sample, as well as the intensities of those frequencies. The frequencies and intensities may be used to identify chemical characteristics of a sample, and resultant spectral data my be collected by any of a variety of detectors, such as a charge coupled device. Spectral data also may be adjusted by algorithms, mathematical formulae, or other means to present the shape of the resultant beam spectral data on an electronic display such as a computer screen.
Efforts to display and present images of a resultant beam have not always resulted in uniform, predictable results, or in acceptable levels of precision and accuracy of spectral measurements. At least one problem is that spectroscopic measurements may be affected by the instrument itself. One or more of the instrument components may contribute to undesirable instrumentation variabilities that affect spectral data measured by the instrument. Problems associated with instruments and methods used to employ phenomena associated with the Raman shift have been addressed and resolved in exemplary fashion by the apparatus and methods shown in U.S. Pat. No. 6,141,095 issued on Oct. 31, 2000 to Allen, et al., and in U.S. Pat. No. 6,281,971 B1, issued on Aug. 28, 2001 to Allen, et al. Until the present invention, however, at least one problem persisted, namely presenting one or more useful and desirable shapes of images from light beams passing through an instrument. Providing substantially straight images from light beams admitted to an instrument through a curved entrance slit in a spectrally dispersed focal plane in an on-axis instrument, without affecting spectral resolution, has presented a number of perplexing problems.
Converting spectral data that has passed through a number of chemical, electronic, electrical and optical devices, and doing so rapidly and accurately, while providing consistently reliable human-readable images with high resolution, may be affected by the instrument itself, by the shape of the entrance slit of an instrument, by angles of incidence of a light beam entering an instrument, by the wavelength of the light beam, by diffraction orders, by diffraction angles, by the focal length of mirrors and lenses, and by related parameters (collectively, in this document, “instrument spectral parameters”).
Depending on the optical elements selected to configure an instrument, a dispersive instrument may be either on-axis or off-axis. An on-axis dispersive spectrometer, however, generally is one based primarily on lenses, although such an instrument also may include one or more mirrors. As used in this document, the term “on-axis” generally refers to an instrument in which the optical elements are oriented so that the axes of the optical elements are substantially co-linear with the center of a light beam passing through or past the optical elements. Use of a dispersive on-axis instrument having a straight or substantially straight entrance slit tends to yield a curved image. In off-axis spectrometers, image curvature may be caused at least by the arrangement and orientation of the optical elements which are oriented so that the axes of the optical elements are not substantially co-linear with the center of a light beam passing through or past the optical elements. The term “off-axis” as used in this instrument generally refers to an instrument that is primarily mirror based, and in which the optical elements are oriented so that the axes of the optical elements are not substantially co-linear with a light beam passing through or past the optical elements. In an off-axis instrument system, the axis of symmetry with respect to reflective surfaces generally forms an angle with regard to each other and with regard to light reflected from the mirrors. As used in this document, the terms “on-axis” and “off-axis” are used not to limit the scope of the present invention, but simply to clarify industry differences in the way instrument components, particularly the orientations and arrangement of optical elements, may be assembled.
FIGS. 1A and 1B
are useful in appreciating the difference between an off-axis mirror based instrument, and on-axis lens-based instrument. As shown in
FIG. 1A
, an off-axis mirror-based instrument is shown with a light beam entering through an entrance slit. The light beam is collimated by an off-axis Mirror A, diffracted by a Grating, then focused on a detector by a second off-axis Mirror B. A conventional on-axis lens-based instrument is shown in
FIG. 1B
, which shows a light beam entering an entrance slit of the instrument. The light beam is collimated by an on-axis Lens A, dispersed by a Dispersive Element, in this case a prism, and focused on a detector by a second on-axis Lens B.
Mirrors are reflective and therefore difficult to use in an on-axis environment. Accordingly, mirrors are usually used in an off-axis environment, but are known to introduce one or more image aberrations. Lenses, on the other hand, are transmissive, and therefore may be used in an on-axis environment to substantially eliminate off-axis aberrations. Lenses, however, are not typically used in spectrographs or monochromators where the intent is to cover a wide range of wavelengths because lenses have chromatic aberrations which cause light beams of different wavelengths to have different focal lengths. On the other hand, if an intended application involves a substantially narrow spectral window, so that chromatic aberration is not significant, as in Raman spectroscopy, lenses may be used in an on-axis environment to yield superb imaging quality.
An exit slit in a monochromator, as opposed to an entrance slit, maybe located a
Allen Fritz S.
Zhao Jun
New Chromex, Inc.
Regan Ray R.
Smith Zandra V.
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