Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-08-23
2004-09-14
Gagliardi, Albert (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S351000, C250S353000, C356S305000, C356S328000
Reexamination Certificate
active
06791086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for efficiently and robustly measuring gas concentrations/partial pressure of respiratory and anesthetic gases.
2. Description of the Related Art
It is well known by those of ordinary skill in the art that gas analyzers of the nondispersive infrared (NDIR) type operate on the principle that the concentration of specific gases can be determined by (a) directing infrared radiation (IR) through a sample of a gaseous mixture, (b) separately filtering this infrared radiation to minimize the energy outside the band absorbed by each specific gas (c) measuring the filtered radiation impinging upon one or more detecting devices and (d) relating a measure of the infrared absorption of each gas to its concentration. Gases that may be measured exhibit increased absorption (and reduced transmittance) at specific wavelengths in the infrared spectrum such that, the greater the gas concentration, the proportionally greater absorption and lower transmittance. An extension of this NDIR technique uses a continuous, linear bandpass filter, followed by a linear array of detectors.
Gas analyzers are widely used in medical applications and may be characterized as being located either in the main path of the patient's respiratory gases (mainstream analyzers) or in an ancillary path usually paralleling the main path (sidestream analyzers). A mainstream analyzer is situated such that the subject's inspired and expired respiratory gases pass through an airway adapter onto which the analyzer is placed. Mainstream designs require the optical and electronic components to be interfaced to a patient's airway or to a respiratory circuit in communication with a patient in a location in relatively close proximity to the patient. As a result, to be accepted in clinical use, the mainstream gas analyzer must be designed as a compact, lightweight yet robust structure unaffected by typical mechanical abuse and temperature variations associated with prolonged use in health care facilities.
While conventional mainstream gas analyzers work well for a small number of specific, non-overlapping spectrum wavelengths, it is difficult to change wavelengths of interest. The system becomes increasingly inefficient if there are more than 2 or 3 wavelengths of interest, and it is very difficult and expensive to provide resolutions significantly better than 0.1 micron, FWHM (full-width at half maximum) in the IR region.
It is known to use grating spectrometers for gas analysis. There are two general configurations of grating spectrometers: the spectrograph, which originally spreads the spectrum out over a strip of photographic film or a linear array detector, and the spectrometer, which uses a single detector that is set at an appropriate location or angle to register a particular spectral element.
For IR gas measurements, an IR source provides broadband energy that is collimated and passed through a gas sample cell. The collimated broadband energy, now attenuated at certain wavelengths, is directed to a diffraction grating where it is diffracted from the grating, spread out into a continuous spectrum, and focused with a mirror onto a small detector. The diffraction grating is rotated about an axis parallel to the grating lines, and substantially coaxial with the face of the diffraction grating. As the diffraction grating is rotated, the spectrum is scanned past the single detector. Since the diffraction grating rotation is synchronized with the detector readout electronics, specific, but arbitrary, spectrum features can be isolated and registered.
One major drawback of many conventional spectrometers is that the rotation of the diffraction grating requires a motor of some sort, oscillating linkages to drive the diffraction grating from the motor, and a bearing assembly. While such an arrangement can deliver good results, such a structure is relatively large, heavy and expensive. Other conventional spectrometers use an oscillating motor, sometimes called a galvanometer drive, in place of the motor and linkage. Such arrangements are less expensive, but still large, heavy and relatively expensive.
U.S. Pat. No. 6,249,346 (2001) to Chen, et al., U.S. Pat. No. 6,039,697 (2000) to Wilke, et al., and U.S. Pat. No. 5,931,161 (1999) to Keilbach, et al. all disclose relatively smaller sized spectrometers, but of designs that are of undue bulk and, in some instances, complexity.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a spectrometer that overcomes the shortcomings of conventional gas analyzing devices. This object is achieved according to one embodiment of the present invention by providing a robust spectrometer apparatus for determining respective concentrations or partial pressures of multiple gases in a gas sample with single, as well as multiple and even overlapping, absorption or emission spectra that span a wide spectral range.
The present invention adapts a grating spectrometer for use in a compact respiratory gas analysis instrument. Specifically, the present invention employs a scanning spectrometer, which scans, or sweeps, the spectrum across a fixed detector. From an optical point of view, this apparatus may be characterized as a modified Ebert scanning monochrometer.
A very small, inexpensive oscillating mirror may be made using a MEMS (MicroElectroMechanical System) fabrication process. With a diffraction grating added to the mirror surface, this structure provides a very low cost, small, lightweight but rugged scanner for an in-line IR gas analysis instrument.
Spectrum resolution is primarily a function of the grating size, aperture, line pitch, diffraction order, and collimation. In the present invention, the required grating width is in the 1 to 2 mm range, which is well suited to existing MEMS technology. The other parameters are easily obtained or controlled, at least well enough for necessary accuracy.
The diffraction grating may be formed separately and glued on to the “mirror” surface or, preferentially, the diffraction grating may be formed in the surface of the mirror as part of the MEMS fabrication processing. The drive to make the mirror oscillate may be magnetic, wherein the mirror either has a planar coil formed on the back or the mirror itself is made magnetic or, alternatively, the mirror may be driven electrostatically. Because the required angular amplitude is relatively small, an electrostatic drive is currently preferred.
The apparatus of the present invention may also be configured in several additional ways. In one instance, the oscillating grating may be removed and replaced by a scanning (oscillating) mirror. In an embodiment of this approach, the mirror scans the input light over a fixed grating, which disperses the spectrum. As before, the spectrum is focused by a mirror onto the detector plane. While this alternative method requires one additional component, the manufacturing cost may be less because the MEMS oscillating element does not need to have a grating fabricated on its surface.
In yet another alternative embodiment, the oscillating mirror may be positioned to direct the attenuated broadband energy beam back through the gas sample cell, with the grating and detector on the same side of the gas sample cell as the IR source. The advantage of this arrangement is higher sensitivity (due to the double pass through the gas in the cell), and a somewhat narrower package. Alternatively, in the double pass configuration, the mirror on the side opposite to the source may be fixed, and an oscillating mirror/fixed grating (or oscillating grating) and detector system located on the source side. These various embodiments may be configured in a single plane or the oscillating mirror, scanning grating or a focusing mirror may be rotated in orientation to direct the beam in a different plane, so that different package configurations may be easily accommodated.
A diffraction grating can provide diffracted beams in several orders. Ordinarily, the
Gagliardi Albert
Haas Michael W.
Respironics Inc.
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