Method and apparatus for fluorescent sensing

A sensing system for the detection of trace gases in the atmosphere or contaminates on a surface. Particular chemical substances are detected at remote points by placing a sensor unit in the vicinity of the area to be examined and illuminating the area with a laser beam generated by the sensor unit. An optical detector, also carried by the sensor unit, registers the fluorescence produced by the substance illuminated and relays this information by a telemetry link back to the base site. The utility of the system is broadened by providing a chemical reactant selected to react with the substance to be detected to produce a reaction product that fluoresces strongly at the wavelength of the laser light. The chemical reactant is carried by the sensor unit and is sprayed into the area to be examined. The sensor unit carrying the chemical reactant can also be used on location, for example, with a hand-held unit that sprays the chemical reactant on a surface suspected of being contaminated and detecting the resulting fluorescence of the reaction product.

BACKGROUND OF THE INVENION 
1. Field of the Invention 
This invention relates to the sensing of surface contaminates and trace 
gases in the atmosphere and more particularly to the sensing of such 
materials at remote locations. In one embodiment, the detection results 
from fluorescence initiated by illumination with laser light, and in 
another arrangement, differential absorption of the substance serves as 
the basis for detection. Local or remote sensing may be accomplished by 
examination for fluorescence produced by a reaction product of a chemical 
reaction between the substance to be detected and a selected reagent. 
2. Description of the Prior Art 
Laser systems have been explored for the remote detection of chemical 
substances since early in the 1960's. One method has made use of 
fluoresence by the substance of interest initiated by laser light. In this 
method, the laser wavelength is selected to be near or at the peak of an 
absorption resonance in the trace gas or chemical element to be detected. 
This absorption resonance is selected to be one that causes strong 
fluorescence. In one such arrangement, a pulse of laser light is 
transmitted in the atmosphere in the area where the presence or absence of 
the particular substance is to be determined. A receiver, directed toward 
the radiant laser light, is arrranged to respond to the fluorescent 
radiation. For discriminating against spurious background fluorescence, a 
differential method is used. In this case, two successive pulses of 
different wavelengths are transmitted: one lying at a wavelength near or 
at the peak of the absorption line of the substance to be detected and the 
other at a wavelength appreciably removed from the absorption line. The 
fluorescent signals from the two pulses would appear predictably at 
different intensities making it possible to discriminate against spurious 
background fluorescence. In general, however, because the fluoresence from 
the substance at a remote location occurs over a solid angle of 4 pi 
radians, only a relatively weak signal is available to the receiver. This 
is equally true whether the fluorescence is from a reaction product or 
when the presence of biological agents is to be detected by the natural UV 
fluorescence. Such arrangements require a high energy laser beam and a 
sensitive receiver: the greater the distance between the substance to be 
detected and the location of the sensor unit, the greater must be the 
intensity of the laser light and the sensitivity of the receiver. The 
receiver usually requires narrow-band filters and a narrow field of view 
to discriminate against incident daylight. With the best of equipment, 
such arrangements are effective only over limited distances. 
Contaminates in the air or on a surface have been detected by spraying a 
reactant chemical, selected to produce a reaction product having strong 
fluorescence, into the air or onto the surface and illuminating the area 
with laser light to cause fluorescence of the reaction product. Such 
arrangements have been limited to laboratory use and generally have been 
unsatisfactory for field use. 
The detection of nerve gas and other chemical and biological agents is of 
great importance in military and other situations, but remote sensing of 
such agents has not been particularly sucessful. One problem, in addition 
to the problems of long-distance laser illumination and fluorescence 
detection, is caused by the fact that some substances do not have a strong 
fluorescence intensity that is readily detected. Another factor is that 
objectionable chemical agents may be camouflaged by releasing a chemical 
with similar fluorescence properties. In some instances, it is possible to 
provide a chemical reactant capable of reacting with the suspected 
contaminate to produce a reaction product selected to be one having a 
strong fluorescence. 
A specific example is a remote sensor for agent specific detection of nerve 
gas soman (GB) with indole and sodium perborate in a mixture of acetone 
and water. From an early investigation in 1957 [See B. Gehauf and J. 
Goldenson, Anal. Chem., 29, 276 (1957)], it is known that in a two step 
process, the reaction produces indoxbyle, which fluoresces strongly when 
subjected to laser radiation in the 350 nm wavelength region. A XeF laser 
at 350 nm, or a nitrogen laser at 337 nm, can be used. The fluorescence 
spectrum peaks at 450 nm and has a width of about 50 nm. See Alan 
Hartford, Quarterly Progress Report AP-4-82:109, July 18, 1982, Page 1, 
Los Alamos National Laboratory, University of California, Los Alamos, N.M. 
SUMMARY OF THE INVENTION 
There are two major difficulties in the remote sensing of chemical 
substances by the detection of fluorescence. The first results when 
attempting to detect substances in which the fluorescence occurs over a 
broad and featureless unresolved line. This has been a major difficulty in 
the remote sensing of chemical and biological agents such as nerve gas 
where the presence of such agents can be camouflaged by introducing other 
chemical substances with similar fluorescence emission. One method of 
overcoming this difficulty, as pointed out above, is to spray into the 
area of the substance to be detected, a second chemical agent capable of 
selective reaction with the substance to be detected and whose reaction 
product is capable of strong fluorescence. The presence of the reaction 
product is determined by laser illumination of the reaction product at a 
strong absorption frequency. In accordance with this invention, such a 
chemical reactant is carried by the sensor unit that also includes a laser 
and an optical sensor and is arranged so that the reactant is sprayed into 
the area or onto the surface to be examined by laser illumination and 
optical scanning. 
Another difficulty arises, as discussed above, because of the distance 
between the sensor unit and the area to be examined for the substance of 
interest. The difficulty of long distance laser illumination and equally 
long distance detection of fluorescence severly limits the capability for 
remote sensing. 
The present invention overcomes these difficulties by placing the source of 
the laser illumination and the fluorescence detection at the site of the 
substance to be detected and transmitting the results to the measuring 
site by a telemetry signal. 
To illuminate a substance from a distance with laser light of sufficient 
intensity to cause substantial fluorescence, requires a large and bulky 
laser system: one that is costly and not ideally suited for field 
operations. However, it is possible, by locating the laser adjacent the 
area to be examined for the presence of the substance, to make use of a 
low power laser; one that may be small, self-contained, have a short 
operating life, and be low in cost. Similarly, the detection problem can 
be solved by placing the fluorescence detector adjacent the area to be 
examined. 
One example of an operating system may thus comprise a telemetry receiver 
at the base site and a remote sensor unit located in the vicinity of the 
area to be examined for the presence of a particular substance. The sensor 
unit includes a laser that radiates a relatively low-power beam, an 
optical detector that responds to the presence of the fluorescence, and a 
telemetry transmitter that radiates a signal responsive to the optical 
detector. Fluorescence may thus be detected by the telemetry receiver at 
the base site without the use of high-power lasers or ultra-sensitive 
receivers. This method is particularly suitable for the detection of 
agents by means of UV fluorescence. The differential method described 
above can be used for discriminating against spurious background 
fluorescence. Moreover, a time delayed observation can be used for further 
discrimination. In this case, the receiver will be gated to respond at a 
delayed time with respect to the incident UV pulse. The delay is adjusted 
to take advantage of the fluorescence decay-time of the agent to be 
detected. This time delayed observation cannot be used to advantage in the 
previous systems for the remote sensing of fluorescence from an extended 
volume. 
For biological agents, generally existing in the form of aerosol droplets, 
the natural UV fluorescence from protein molecules can provide the 
signatures needed for detection and identification. It is known that 
tryptophan residues in a protein molecule have two strong absorption 
peaks, one of which is centered at about 295 nm and the other at 235 nm. 
The fluorescence caused by an incident UV pulse within these absorption 
peaks occurs in the 350 nm range. The 350 nm fluorescence appears in two 
broad and overlapping emission bands with different decay times. These 
decay times are of the order of several nanoseconds. It is possible to 
generate UV laser pulses in the 200 nm to 300 nm region by harmonic 
generation of a tunable dye laser in the 400 nm to 600 nm range. The 
pumping laser to drive the dye laser may be a pulse laser of the kind 
described in my co-pending United States patent application Ser. No. 
496,069 filed May 19, 1983, abandoned. With this method, UV pulses within 
the 295 or 235 tryptophan absorption peaks can be generated with a pulse 
duration of about two nanoseconds or less. A gated electronic detector 
with a gate-width of one to two nanoseconds at several nanoseconds delayed 
time can be used to observe the time-delayed 350 nm fluorescence. This 
makes it possible to obtain a time resolved spectrum of the tryptophan 
residue in the protein molecules. For detailed spectral signatures, a 
small grating with an array of detectors can be used to observe the 
fluorescence at a number of discrete wavelengths within the 350 nm 
fluorescence spectrum. The overlapping bands are also polarization 
sensitive. In the sensor, the behavior of observed time-delayed 
fluorescence versus wavelength, together with the polarization behavior of 
the fluorescence signals, can be used in diagnostics for discrimination 
against spurious background fluorescence. Moreover, in the presence of 
bacteria and viruses, the similarly obtained fluorescence signals will 
depend upon the specific bacterial or virus. These dependencies can be 
used in the diagnostics to specify the type of biological agent detected 
by the sensor. 
In those instances where a chemical reaction is to be produced before 
examination for fluorescence, the sensor unit is provided with a supply of 
chemical reactant and arranged to spray the reactant into the area or onto 
the surface to be examined. The laser, chemical spray, and optical 
detection functions can be combined into a small unitary structure, the 
sensor unit, that can be projected or otherwise propelled into the area to 
be examined. 
The sensor unit with spray chemical ability can also be used as a hand-held 
unit without need for the telemetry system where examination is to be made 
for a surface contaminate or a specific trace gas in the atmosphere. 
In a preferred embodiment, the unit carries a reactant gas that is specific 
for a particular substance and produces a strongly fluorescing reaction 
product. The unit optically monitors for the fluorescence and provides a 
suitable visual or audible signal. The laser may be a pulsed unit of the 
kind described in my co-pending United States application Ser. No. 
496,069, mentioned above, titled: "Laser System with Interchangeable 
Modules and Method for Changing such Modules". Such a unit may be 
battery-powered, generate the necessary laser light of such frequency and 
intensity as to be eye-safe in operation and, of major importance, low 
enough in cost to permit quantity usage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the various figures, similar elements are indicated by the same numbers 
or by the same numbers followed by a differentiating suffix. 
As illustrated by FIG. 1, a typical system for the detection of trace gases 
or suspended chemical substances includes a base site, generally indicated 
at 1, having a telemetry receiver 2 coupled to a receiving antenna 3, and 
an output indicator, shown diagrammatically at 6. At a position remote 
from the base site, in the vicinity of the area to be examined for the 
substance of interest, is a sensor unit, generally indicated at 12. The 
sensor unit 12 includes a low-power pulsed laser 14 that emits a laser 
beam having a wavelength suitable for producing fluorescence in the 
particular substance whose presence is to be determined or in a particular 
reaction product of that substance. 
This laser 14 may be similar to the one mentioned above as described in my 
co-pending application. If the beam emitted by the laser illuminates the 
substance to be detected, the substance is caused to fluoresce and this 
fluorescence is detected by an optical-detection device 18 that forms part 
of the sensor unit 12. Optical detection devices capable of responding to 
fluorescence are well known along with means for adjusting the device for 
optimum response to the particular fluoresence that is expected and 
minimizing the effect of ambient illumination. When fluorescence is 
detected, a telemetry transmitter 22, operating in conjunction with a 
signal processor 46, that forms part of the sensor unit 12 is activated 
and caused to transmit, through an antenna 23, a telemetry signal to the 
receiving antenna 3 at the base site 1 where it produces a signal to 
activate the indicator 6. 
In order to increase the sensitivity of the test, the substance to be 
detected may be concentrated prior to the examination. Nerve gas, for 
example, may be dispersed in tiny droplets with a density so low that 
direct detection is difficult. In this example, a duct 24 is provided with 
an exhaust fan 26 that forces a continuous flow of air from an intake 
screen 27 on the bottom of the unit through a filter 28 that collects the 
contaminate to be examined. A beam from the laser 14 illuminates the 
surface of the filter 28 which is examined for fluorescence by the optical 
detector 18. Information from the optical detector is transmitted to the 
base site by the transmitter 22. 
In many instances, a more reliable and more sensitive result may be 
achieved by providing a chemical reactant capable of reacting with the 
substance to be detected to produce a reaction product that fluoresces 
strongly at the selected frequency of the laser light. For example, the 
filter 28 may comprise a pad of cotton or other porous material saturated 
with an indole sodium perborate solution or other reactant. 
The specific components to be selected will depend upon the particular 
circumstances of use. For example, if the distance between the base site 1 
and the remote location is not great, the telemetry antennas 3 and 23 may 
be non-directional and no particular orientation of the antennas will be 
necessary. For greater distances, known methods for achieving directivity 
in transmission and reception may be employed. In general, the sensor unit 
12 will be located in a position where power is unavailable or, as 
described later, may actually be in flight while measurements are being 
made. For this reason, the operation of the sensor units is battery 
powered by a common battery power source 30 carried within the sensor unit 
package. The sensor unit assembly may be positioned at the remote site by 
any desired means: it may be hand-carried, dropped by parachute, carried 
by a drone plane or projected from a gun or other launching device. 
As shown in FIG. 2, in an actual field operation, the sensor unit 12 may be 
dropped by a parachute 32 into a cloud, generally indicated at 34, which 
is suspected of containing a specific contaminant. The unit 12 telemeters 
the presence or absence of the contaminate as it passes earthward through 
the cloud. This information is received at a base site, which in this 
instance may be a tank 36 or other military vehicle. After the unit lands 
on the earth, it may continue to examine the atmosphere and telemeter 
information to the base site. At the moment illustrated by FIG. 2, similar 
sensor units 12d and 12e are already on the ground and are continuing to 
examine the atmosphere for contaminates. 
As shown in FIGS. 3 and 4, a sensor unit 12a, similar to the sensor unit 
12, is arrranged to be incorporated into an artillery shell, generally 
indicated at 38. The sensor unit 12a has a nose opening 40 through which 
the atmosphere to be tested enters the sensor. While the shell 38 is in 
transit, the incoming air passes through a filter 28a and is exhausted 
through a port 42. The filter 28a, which may be saturated with a reactant 
chemical such as an indole sodium perborate solution, collects and 
concentrates the contaminate from the air stream. The filter 28a is 
illuminated by a laser beam from a source 14a that is reflected from a 
mirror 44. Fluorescence at the filter surface is detected by an optical 
detector 18a that together with a signal processor 46a produces a signal 
indicating the presence or absence of fluorescence at the filter 28a. This 
signal is transmitted to the base sight, in this instance a tank 36a, by a 
telemetry transmitter 22a. All components are powered by a battery pack 
32a. 
In operation, the tank 36a may fire a series of shells 38 in a forward 
direction to sample the air for contaminates as the tank moves forward. 
The telemetry system may include 2-way controls so that the sensor unit 
12a is activated for operation by a signal transmitted from the tank 36a. 
The sensor 12a then collects a sample along a path length indicated at 48 
and then examines the collected substance and transmits a telemetry signal 
to indicate the presence or absence of the suspected con- taminate. 
Where the distance between the base site and the area to be examined for 
the presence of a chemical substance is greater, or for other reasons, it 
may be desirable to provide continuing flight and orientation control for 
the sensor unit as illustrated by FIGS. 5 and 6. In this instance, a 
sensor unit 12b is carried by a drone aircraft 52 illustrated entering a 
cloud 34b to be examined for the presence of a particular contaminating 
substance. The base site 1, in this instance, may comprise a tank 36b or 
it may be a more remote location. The sensor may be carried by any 
remotely controlled projectile or other vehicle. The use of laser beams 
and other methods for controlling the path and orientation of such a 
projectile are well known and are not described in detail here. 
The sensor unit 12b is mounted on the drone plane 52 in such manner that an 
intake screen 27b is exposed to the surrounding atmosphere which is drawn 
through a duct 24b by a fan 26b. A filter 28b, mounted in the duct 24b, 
collects the contaminating chemical which is then illuminated, through a 
window 54 in the duct 24b, by a laser 14b. As in the other examples, the 
filter may be saturated with a reactant chemical to increase the accuracy 
and sensitivity of the test. The filter surface is examined for 
fluorescence by an optical detector 18b which in conjunction with a signal 
processor 46b and a telemetry transmitter 22b transmits an appropriate 
signal to the base site at tank 36b. 
In operation, the drone 52 is directed to the area to be examined. The 
laser 14b and the optical sensor 18b may operate continuously or may be 
turned on at the appropriate time by a signal from the base site 36b. The 
laser-detection assembly as used in the examples of FIGS. 1-6 becomes a 
disposable item returning data only while in flight through the area to be 
examined and, for that reason, the use of low cost components is a 
necessity. The system described here, making use of the low-cost laser 
previously referred to, meets that requirement. 
FIG. 7 illustrates a hand-held sensor unit 12c which is generally similar 
to the sensor unit 12, but contains no transmitter or antenna. The release 
of the reactant spray through a nozzle 56 from a container 58 and 
activation of the laser 14c are initiated by a manually operated 
push-button 62. The spray is applied to a surface suspected of 
contamination, which is illuminated by the laser light. Any resulting 
fluorescence may in some instances be detected by visual examination and 
in other instances by the optical sensor 18c that may include a spike 
filter 64 and a photodetector 66 arranged to actuate a visual indicator, 
shown diagrammatically at 6c. The entire sensor unit is contained in a 
housing 68 provided with a handle 72 by which the sensor is supported 
while the surface suspected of contamination is being scanned. Although 
the hand-held unit of FIG. 7 does not require any communication channel 
with a base site, a telemetry channel may be included to permit auxiliary 
monitoring at the base site. 
As used herein the term "remote location", and similar terms, mean a 
location removed from the reference point by a distance greater than the 
maximum distance over which fluorescence of the substance to be sensed can 
be produced by the particular laser being used and detected by the 
particular optical detection means being used, both at a common location. 
It is apparent that various modifications of the system may be made to best 
fit the arrangement to each particular application without departing from 
the intended context of the invention. The foregoing examples are not 
intended to delineate the scope of the invention, which is to be limited 
only in accordance with the following claims.