Holographic gas analyzer utilizing holographic optics

The present invention relates to an analyzer for determining the concentration of substances, such as chemicals, present in a fluid medium, such as gas, vapor or liquid, using holographic optics. The analyzer of the present invention is based on the infra red absorbance of the gas, vapor or liquid to be measured, where the optical functions of an infra red absorbance gas analyzer are performed by holographic functional representations of the required optical components.

RELATED APPLICATION 
This application claims priority of co-pending United States provisional 
application Ser. No. 60/012,870 filed on Mar. 5, 1996. 
BACKGROUND OF INVENTION 
1. Field of the Invention 
This invention relates to analyzers for determining the concentration of 
substances, such as chemicals, present in a gas or vapor, and more 
particularly to an analyzer utilizing light absorbance of a gas, vapor, or 
solute in liquid to be measured in association with holographic optics. 
2. Description of the Related Art 
Chemical compounds absorb light, such as infra red light, with different 
effectiveness at different wavelengths. Infra red absorbance analyzers use 
this property to determine the concentration of chemicals in an air and 
gas or vapor mixture. A beam of infra red light of a discrete wavelength 
or band known to be absorbed by a chemical is passed through a chamber 
containing an air and gas or vapor mixture filtered to remove dust. At the 
end of the infra red light beam path is a photo detector. 
When volatile contaminants are introduced into the chamber of a light 
absorbance gas analyzer, the substances present absorb some of the infra 
red light, reducing the intensity of the light signal at the photo 
detector. The amount of attenuation of the light signal is proportional to 
the concentration of the contaminant and the path length of the beam of 
light. The light detection circuit is calibrated based on the 
proportionality between airborne contaminant concentration and the signal 
absorbance, and outputs a signal or meter value proportional to the 
contaminant concentration. A reference cell may be used which contains the 
same optics without the introduction of the gas/vapor (or with a reference 
gas) to provide a zero reference point. 
The prior systems for analyzing a gas or vapor used physical mirrors, 
reflectors, diffusers, lenses, or light guides, and required either a 
diffraction grating or a prism to provide monochromatic light. 
Alternatively, light was limited to a range of wavelengths, or bands by 
using filters. Narrow slits, focusing and/or directing lenses were used to 
provide a narrow beam of light. Depending on the inherent absorbance of 
the compound and the concentration range of interest, a rather long light 
path length may be required for an adequate minimum detection of the 
compound. Increasing device sensitivity requires increasing path lengths 
and therefore a more complex reflective array. In typical instruments, 
this path length may be on the order of twenty meters. In some specialized 
devices it is much longer. In order to provide sufficient light path 
length in a practical sized and often portable device, mirrors and mirror 
arrays were used to fold the light path within a chamber for the gas or 
vapor mixture. As instrument response time is a direct function of the 
volume of the measuring chamber and the rate of air flow through it, 
reduced chamber size is important to obtain a quick response time. 
The characteristics of the optics of the prior art devices such as 
sensitivity, fragility, and expense, made the prior devices impractical 
for rugged use and were unworkable in situations where a rapid response 
was required as the devices could not be made small enough in a practical 
sufficiently maintainable and affordable manner. 
SUMMARY OF THE INVENTION 
The present invention relates to analyzers for determining the 
concentration of light absorbing matter, such as chemicals, present in a 
gas, vapor or liquid, collectively referred to herein as a "fluid medium." 
The analyzer of the present invention is based on the absorbance of light, 
such as infra red light, of the fluid medium to be measured, where the 
optical functions of the gas analyzer are performed by holographic 
functional representations of the required optical components. The fluid 
medium is transparent to the analytical wavelength of light used to 
quantify light absorbing matter present in the fluid medium. 
A basic analyzer in accordance with the present invention comprises a 
measuring chamber or collecting area for the fluid medium; a monochromatic 
or polychromatic light source; holographic optics emulating one or more of 
the following functions: optical slit(s), diffraction grating(s) or 
equivalent focusing lenses or surfaces, mirror(s); and a light 
detector/amplifier. In the preferred embodiment of the present invention, 
holograms are onlaid as single or multiple pads onto the interior surfaces 
of the measuring chamber. Each holographic pad could have single or 
multiple functions and may be either a reflective or transmission type as 
appropriate. 
In use of the analyzer of the present invention, a beam of light of a 
discrete wavelength or band known to be absorbed by a chemical is 
transmitted through the chamber or area for collecting a fluid medium, 
such as an air and gas/vapor mixture, to the holographic optics and to the 
light detector. Any contaminants present in the fluid medium absorb some 
of the transmitted light, reducing the intensity of the light signal at 
the light detector. The amount of attenuation of the light signal is 
proportional to the concentration of contaminants present in the fluid 
medium and the path length of the beam of light. The path length can be 
varied by the positioning and number of the holographic optics encountered 
by the light path. In this manner, the concentration of contaminants 
present in the fluid medium can be determined. 
Some examples of applications of the holographic analyzer of the present 
invention include chemical process control systems, environmental 
monitoring systems, automotive emission control systems, indoor air 
quality monitoring, and industrial hygiene monitoring. 
The holographic analyzer of the present invention provides the following 
advantages over prior systems: 
Complex holographic arrays can be easily reproduced, such that functional 
complexity can be increased without significant increases in costs as the 
production of reference cells, or multiple wavelength systems would be 
inexpensive; 
High sensitivity in flow through systems can be achieved without 
sacrificing response time; 
A holographic system will not require a larger volume to accommodate more 
complex light paths; 
Holographic optics can be more easily modified for use in harsh 
environments by application of coatings, or placement of holographic pads 
behind the surface substrate, such that with proper coatings the present 
invention could be applied to measurements in static or flowing liquids; 
Design flexibility can be further increased using either transmissive or 
reflective holograms; 
Simplicity, and therefore inherent reliability, would be greater than the 
standard units due to reduced parts count; 
A holographic system is simpler, easier to manufacture and therefore would 
be of lower cost to produce and sell than the devices of the prior art; 
and 
The lower cost and reliability of the present invention would facilitate 
use in applications not currently considered feasible. 
These and other advantages of the present invention will become apparent 
from a review of the accompanying drawings and the detailed description of 
the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, a basic embodiment of the holographic gas analyzer 
system of the present invention is shown and generally referred to by the 
numeral 10. The holographic gas analyzer 10 comprises a chamber 14, a 
light source 20, one or more holographic reflective pads 22a and 22b, and 
a detector 24, such as a photodetector which may include an amplifier. 
Infra red detectors are well known in the art and detectors appropriate 
for this application are commercially available by a number of companies 
including Electo-Optical Systems, Inc. of Phoenixville, Pa. 
A fluid medium, such as a mixture of air and a gas (or vapor), to be 
analyzed is introduced into chamber 14 through an inlet 12 using a pump or 
other air moving device (not shown). The mixture of air and gas is 
distributed inside the chamber 14 as necessary and drawn out of the 
chamber 14 through an exhaust outlet 16. 
A light beam 18 is emitted from the light source 20, which can be 
monochromatic or multi-chromatic. The light beam 18 passes through the 
chamber 14 and the air and gas mixture therein. The light beam 18 is 
reflected off of one or more holographic reflective pads 22a and 22b on 
surfaces inside the chamber 14, until ultimately the light beam 18 strikes 
the detector 24 positioned at an appropriate angle with respect to the 
holographic reflective pads 22a and 22b. Any contaminants present in the 
air and gas/vapor mixture will reduce the intensity of the light signal at 
the detector 24. The light intensity at the detector 24 is used to 
determine the concentration of the contaminants in the gas. It is 
appreciated that the inner configuration of the chamber 14 is not limited 
to a cube shape and that the chamber 14 may be sealed or may be a ported 
chamber through which a gas or fluid flows. It is also appreciated that 
contaminant entry and exit into and out of the chamber can be through 
diffusion membranes. The light path 18 is similarly not restricted to one 
plane and may span multiple planes as discussed in detail below. 
Because different wavelengths will reflect at different angles from the 
holographic reflective pads 22a and 22b, to function as a prism or not 
reflect at all depending on the wavelength of the light, proper placement 
of the detector 24 with respect to the holographic reflective pads 22a and 
22b limits the wavelength that reaches the detector 24. 
The holographic reflective pads 22a and 22b are holographic images 
manufactured in the conventional manner known to those skilled in the art 
and may reflect and/or transmit light. The techniques for generating 
holographic optical elements and the equipment and materials required are 
described in detail in reference books, including Optical Holography, 
Collier, Burkhardt & Lynn Academic Press (1971), incorporated herein by 
reference. The holographic reflective pads 22a and 22b are created to a 
known reflective angle for a selected wavelength of light. Light at 
different wavelengths, if reflected at all, would reflect at different 
angles from the design wavelength. Proper placement of one or more 
detector 24, and possibly with baffles (see FIG. 2a), would therefore 
provide a means for wavelength selection, or bandwidth narrowing. 
A basic example of an application of the holographic gas analyzer 10 would 
be a carbon dioxide detector using the methods described herein to 
generate a reflective surface of sixty (60) degrees and thirty (30) 
degrees for infra red light with a wavelength of 4.3 microns. It is 
appreciated that other angles are also possible. 
The holographic gas analyzer of the present invention may have a plurality 
of optional configurations. For example, the number of holographic 
reflectors encountered by the light path may be increased. By increasing 
the number of holographic reflectors inside the chamber 14, it is possible 
to increase the light path folding and therefore increase the ultimate 
light path length within a fixed chamber volume. The number of holographic 
reflectors encountered by the light path may be increased in a number of 
ways. 
Referring to FIG. 2a and 2b, one method for increasing the number of 
holographic reflectors is to increase the number of surfaces of the 
gas/vapor containing chamber having holographic pads. FIG. 2a shows an 
example of a folded light path on a plane using four reflective inner 
surfaces of the chamber 14 with at least one holographic reflector pad 
22a-22d on each surface of the chamber 14. It is appreciated that it is 
possible to utilize all or part of the inner surfaces of the chamber 14 to 
reflect the light path 18. 
FIG. 2b is a top view of FIG. 2a showing a complex light path configuration 
and the placement of optional baffles 26 for providing a means for 
wavelength selection. There is a sixty (60) degree angle of reflection 
from the incident light beam to the surface of two holographic reflector 
pads and a thirty (30) degree angle of reflection from the incident light 
to the surface of the other two reflector pads. It is appreciated that 
these angles can vary with the use of different media and wavelengths of 
light. 
Referring to FIG. 3, multiple reflector and/or transmissal holographic 
reflective pads could be placed on each inner surface of the chamber 14. 
Each holographic reflective pad could have a different angle of 
reflection. As shown in FIG. 3, additional holographic reflector pads 
22a-f are used to shift a light beam 18 from one plane to another plane. 
The use of different planes for the reflection of the light path increases 
the length of the light path within the chamber 14. 
Referring to FIG. 4, for circumstances where a more narrow light band width 
is required, or the wavelength selectivity achieved will be inadequate 
from reflective selection alone, a holographic diffraction grating pad 28 
is incorporated into one of the inner surfaces of the chamber 14. The 
grating pad 28 could be either a reflective grating or as a transmissive 
grating. Discrete holographic gratings are known in the art and 
commercially available. 
It is appreciated that other holographic functions could be added to pad 
functions on multipad surfaces such as light focusing and beam splitting. 
Multi-chromatic devices can be built for measuring absorbance at multiple 
peaks for a single compound, or to simultaneously measure exclusive 
absorbent wavelengths for multiple compounds. These devices can be built 
using one or more of the following methods. 
Referring to FIG. 5, multiple holographic reflector pads 122a-122d with 
different design wavelengths could be placed on the inner surfaces of the 
chamber 14, or on multiple surfaces attached to each other. As necessary, 
different photographic emulsions can be screened into different areas of 
the inner surfaces of the chamber 14. Holographic images for different 
wavelengths can be layered on top of each other. Advantage can be taken of 
different order reflection surf holograms to produce reflection surfaces 
which reflect multiple discrete wavelengths at different angles. 
Depending on the application, the light beam(s) could be directed to the 
same or different detectors 24. Alternatively, a multi-chromatic system 
could use a single detector 24 with beam selection controlled by a stepper 
motor or vibrating arm. 
As holographic optics can be more easily modified for use in harsh 
environments by application of coatings, or placement of holographic pads 
behind the surface substrate, such that with proper coatings the present 
invention could be applied to measurements in static or flowing liquids. 
While the present invention has been described in detail with regards to 
the preferred embodiments, it is appreciated that other variations of the 
present invention may be devised which do not depart from the inventive 
concept of the present invention.