Optical probe

A compact optical probe is disclosed particularly useful for analysis of emissions in industrial environments. The instant invention provides a geometry for optically-based measurements that allows all optical components (source, detector, rely optics, etc.) to be located in proximity to one another. The geometry of the probe disclosed herein provides a means for making optical measurements in environments where it is difficult and/or expensive to gain access to the vicinity of a flow stream to be measured. Significantly, the lens geometry of the optical probe allows the analysis location within a flow stream being monitored to be moved while maintaining optical alignment of all components even when the optical probe is focused on a plurality of different analysis points within the flow stream.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for transmitting light to and 
receiving light from a remote sample for analysis and more particularly to 
an optical probe for use in measuring the optical response of a remote 
sample. 
The ability to monitor particulate matter in process streams and emissions 
to the air from industrial operations, and particularly the ability to do 
so in-situ and in real-time, is becoming increasingly important in many 
industrial processes. This is the case not only because of the desire to 
control and modify various processes in real-time to improve their 
efficiency but also to comply with various environmental regulations 
governing the composition, quantity and quality of industrial emissions. 
Air emissions of toxic, hazardous and regulated materials are coming under 
increasing scrutiny by the regulatory community. Not only are industrial 
operations being required to monitor their air emissions more closely but 
also they are being required to do so on a continuous basis. Measurement 
of hazardous metal concentrations in stack emissions is a difficult task. 
Currently, air emissions of these metals from industrial operations are 
measured using extractive sampling followed by off-line chemical analysis, 
a procedure that is costly and typically has long turnaround times. 
Furthermore, because of the many manual operations involved in extractive 
sampling there is a significant potential for introducing sampling errors. 
Complete analyses of stack measurements typically are not available for 
two to four weeks from the time that samples are collected. Furthermore, 
certification tests require that more than one sample be taken for a given 
operating condition and that at least one sample be taken for each 
operating condition. The long turnaround times inherent in extractive 
sampling prevent the use of air emissions measurements as a method of 
controlling operating parameters in real-time. Continuous measurements of 
industrial air emissions could ultimately provide real-time information 
that could be used by facility operators to modify operating parameters to 
improve efficiency or reduce air emissions. Although most of the metal air 
emissions are in the particulate phase, vapors may also be significant and 
must be measured simultaneously. Furthermore, the particles that contain 
metals may be quite inhomogeneous and particulate metals may be present in 
any of a large number of compounds. 
Optical methods, because they can provide instantaneous data readouts, can 
generally be located within or in proximity to a flow stream (process 
stream or source of air emissions) and can be placed in remote locations, 
are particularly useful as a means of monitoring particulate air emissions 
and controlling operating parameters. Optical methods can be classified 
into two principle classes: one, wherein an output optical signal, 
produced in response to an input optical signal, is measured; two, wherein 
a change in the input optical signal, produced in response to the medium 
through which the input optical signal has traveled, is measured (i.e., a 
transmittance measurement). 
A wide variety of instruments are currently available for on-line analysis 
of flow streams. However, the optical probes that these instruments use 
are typically designed for analysis of the concentration of constituents 
in fluid streams. These optical probes generally contain bundles of 
optical fibers and specialized lenses and mirrors that are not suitable 
for use in the harsh environments encountered in monitoring particulate 
emissions from industrial boilers, incinerators and furnaces. Furthermore, 
many of these instruments employ beam dividers or splitters, an 
arrangement which causes more than 75% of the available light to be lost. 
Because of the requirement for a second probe that receives light 
transmitted through the sample, instruments that operate in the 
transmittance mode are generally unsuited for use in the harsh 
environments of stack emissions from boilers, incinerators, furnaces and 
the like. 
A method for circumventing many of the problems discussed above was 
disclosed in U.S. Pat. No. 4,637,716. Here, an anemometer measures light 
scattered from particles in a fluid, wherein an entrant light beam from a 
laser passes through a hole in a mirror inclined 45.degree. to the beam 
axis and is focused by a focusing lens onto the end of an optical fiber. 
Scattered light collected by the optical fiber emerges from the fiber at a 
larger angle than the entrant light and is converted to a relatively large 
diameter beam by the focusing lens. The parallel beam of returning light 
is subsequently reflected by the inclined mirror into a detection system 
via a second lens system. While the method of getting light into and out 
of an analysis area described in the '716 patent overcomes many of the 
deficiencies noted earlier, this method does not permit focusing the 
entrant light beam on any particular area or particle selected for 
analysis within a flow stream. Furthermore, while optical fibers are 
useful for transmitting an incident light beam from a low power 
(.about.1-2 Watt) laser they are completely unsatisfactory for 
transmitting the high intensity incident light beam from a high power 
(.about.kilowatts) laser such as would be used for laser spark 
spectroscopy, for example, because of severe degradation of the optical 
fiber by the high intensity laser light. 
For the reasons set forth above, it is highly desirable to have an optical 
probe that permits measurements to be made at a plurality of locations 
within a flow stream, is rugged enough to be used for monitoring emissions 
from industrial boilers, incinerators and furnaces and can introduce the 
optical input to the measurement location and extract the optical response 
through limited access ports in a chamber or duct enclosing the emissions. 
It is further desired that the probe should be reliable, suitable for 
remote sampling and easy to align and operate. 
The instant invention provides an optical probe whereby all of its optical 
components (source, detector, relay optics, etc.) can be located in 
proximity to one another and generally exterior to the flow stream being 
monitored thereby permitting a compact and rugged system. The geometry of 
the optical probe disclosed herein provides a means for making optical 
measurements in environments where it is difficult and/or expensive to 
gain access to the vicinity of a measurement point from more than one 
direction, making it particularly useful for remote sampling operations in 
industrial environments. Most important, this optical probe geometry 
allows the measurement location to be moved within the flow stream being 
monitored while maintaining optical alignment of all optical components 
thus simplifying alignment and operation of the optical probe. 
SUMMARY OF THE INVENTION 
The optical probe of the present invention comprises a lens system for 
focusing an incident light beam onto a plurality of analysis locations 
within a flow stream being monitored and for collecting a return light 
beam from each of the plurality of analysis locations. The optical probe 
also includes means for separating the incident light beam from the return 
light beam whereby the return light beam can be transformed to provide a 
real-time analysis of material present at each of the analysis locations. 
In one embodiment, the optical response to the incident light beam at an 
analysis location can be collected by a first lens that focused the 
incident light. The optical response, collimated by the first lens, is 
coaxial with the incident light beam but propagating in a direction 
opposite to that of the incident light. In a second embodiment an incident 
light beam is input to a focusing lens by means of inclined mirror. This 
embodiment is particularly advantageous, because it permits the light 
source to be mounted at an angle relative to the beam axis, e.g., 
perpendicular, rather than in-line thereby permitting a more compact 
monitoring instrument. By coaxially separating the collimated input light 
beam from the collimated response, the optical arrangement disclosed 
herein not only permits the incident light beam and the optical response 
beam to enter and exit the analysis point through the same aperture but 
also provides for moving the analysis location within the flow stream 
being monitored, while maintaining optical alignment of all components, 
simply by translating the first lens so that a new analysis point, located 
at the focal point of the first lens can be acquired; no further changes 
or adjustments are required. Measurements can be made at any of a 
plurality of locations in a flow stream comprising a gas, liquid, or 
aerosol or combinations thereof, or on a solid/gas, solid/liquid or 
liquid/gas interface, or internal to a transparent or partially 
transparent solid.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates generally to optical probes and particularly 
to a novel compact optical probe useful for measuring the optical response 
at an analysis point or within a small volume of a flow stream being 
monitored and is most particularly useful for monitoring particulate air 
emissions in various industrial operations and in the field. 
To better understand the present invention, the following discussion is 
provided. If diverging light rays originate at the focal point of a lens 
system, they can be collimated by that lens system. Furthermore, if 
parallel light rays are incident on a lens system, the rays exiting from 
the lens system will converge to the focal point of the lens system. Thus, 
a system composed of two lenses that are positioned coaxially such that 
light that originates at the focal point of the first lens will be 
collimated by that first lens and will be incident on the second lens 
which, in turn, focuses that light onto the focal point of the second 
lens. Thus a plurality of analysis points can be acquired by simply 
translating the first lens and its associated focal point so that the new 
analysis point is located at the focal point of the first lens; and as a 
consequence, no further changes or adjustments are required in the second 
lens that still focuses the collimated return light, originating at the 
focal point of the first lens system, onto, for example, a 
transmitting/detecting means. On the other hand, if the return light from 
the first lens were diverging, any changes made in the position of the 
first lens would require compensating translational changes in the second 
lens that focuses the return light onto the transmitting/detecting means. 
Furthermore, the optical response from any point other than the focal 
point of the first lens will not be collimated by this lens and therefore 
will not be focused onto the transmitting/detecting means located at the 
focal point of the second lens and thus will not be efficiently measured. 
A first embodiment of the optical probe of the instant invention is shown 
in FIG. 1. In the embodiment illustrated in FIG. 1, an optical probe can 
be used, for example, to analyze particles (not shown) present in the flow 
stream of an industrial stack (i.e. emissions) 100 by means of laser spark 
spectroscopy. Here, a light beam 110, issues from a light source 115, 
preferably a laser, and is collimated by collimating lens 105. The 
collimated beam 110 then passes through an aperture 120 in a mirror 125, 
the mirror being inclined at an angle to input beam 110. The collimated 
beam 110 can then be focused by focusing lens 130, onto a selected point 
A.sub.1 within the stack, for analysis of the particles within a volume 
about analysis point A.sub.1 (the analysis location). The optical response 
of the particles within the volume being analyzed is in the form of a 
return light beam 140 from analysis point A.sub.1. The return light beam 
140 is collected and collimated by focusing lens 130 and is reflected by 
mirror 125 and mirror 127 onto lens 145, which act in cooperation to 
provide means for focusing return light beam 140 onto light transmitting 
sensor 150, which can be the face of a optical fiber cable or bundle that 
transmits light to a detection means (not shown). For the purpose of 
simplifying further discussion, this coaxial, counterflow lens system will 
be hereafter referred to as a single-ended lens system to distinguish it 
from double-ended lens systems that require separate lens configurations; 
one for introducing the light beam into the flow stream (incident beam) 
and another for collecting the return light beam (optical response). The 
detection means can be preceded by a spectrometer or other wavelength 
selecting device. New analysis points A.sub.i (where i=1, 2, . . . , n) 
within the stack can be acquired by translating lens 130 such that the 
focal point of lens 130 is focused on a different analysis point A.sub.i 
(not shown). No other changes in the optical probe are required. 
A simple variation on this geometry can further reduce the number of 
optical components in the optical system. By curving the surface of mirror 
125 (curved surface not shown), mirror 125 can also act as a focusing 
element. By positioning transmitting means 150 at the focal point of 
curved mirror 125 the collimated output beam 140 of light that originates 
at analysis point A.sub.i can be focused directly on the transmitting 
sensor 150 thereby eliminating the need for intermediate means to focus 
the return light beam (i.e., mirror 127 and lens 145) onto transmitting 
means 150. 
A second embodiment of the instant invention is shown in FIG. 2. Here an 
input light beam 215 issues from light source 205 and is collimated by 
collimating lens 212. Collimated laser beam 215 is input to focusing lens 
230 by means of inclined mirror 210. Input beam 215 can then be brought to 
a focus by focusing lens 230 onto point A.sub.i for analysis. The optical 
response of particles (not shown) at point A.sub.i can be collected by 
lens 230 in the form of a return beam 220 which is brought to a focus, by 
focusing lens 240, onto a light detector (not shown), such as a photodiode 
or charge coupled device, or onto a optical fiber cable or bundle 245 for 
transmitting light to a light detector. A beam expanding telescope 255 
consisting of lenses 212 and 250 can be inserted into the input light beam 
215 if desired. As set forth above, new analysis points A.sub.i (where 
i=2, . . . , n) can be acquired by simply translating focusing lens 230 so 
that new analysis point A.sub.i is located at the focal point of the lens; 
no further changes or adjustments are required. This embodiment is 
particularly advantageous, because it permits the light source to be 
mounted at an angle relative to the beam axis, e.g., perpendicular, rather 
than in-line thereby permitting a more compact monitoring instrument. 
Lens 130 or 230 can be translated by mechanical means such that new 
analysis locations can be acquired. These mechanical means can be, but are 
not limited to, a rack and pinion arrangement 160, which can be motor 
driven, or a screw arrangement wherein the lens retaining assembly 270 can 
be threaded and can be fitted into a receiving member 275, having spiral 
grooves, and acting in cooperation with lens retaining assembly 270 to 
translate lens 230 as lens 230 is rotated about its axis, or equivalents 
thereof. 
Thus, the first and second embodiments of the present invention disclose 
means for coaxially separating counterpropagating light beams employing 
differential beam size or diameter and also maintain the alignment of the 
optical probe as it focuses on different analysis points within the flow 
stream. 
From the foregoing description and examples, one skilled in the art can 
readily ascertain the essential characteristics of the present invention. 
In particular, it will be appreciated by those skilled in the art that the 
optical probe of the instant invention can be also be used with any of a 
variety of other optical techniques that rely on measuring the optical 
response at a point (or in a small volume or on a surface or interface) to 
an input optical signal. Optical techniques for which the present optical 
probe provides an advantage comprise; laser spark spectroscopy, Raman 
scattering, laser-induced fluorescence, Rayleigh scattering, Mie 
scattering and laser-induced incandescence. The description and examples 
are intended to be illustrative of the present invention and are not to be 
construed as limitations or restrictions thereon, the invention being 
delineated in the following claims.