Gases can be selectively detected by utilization of an infrared (IR) absorption photometer via their specific absorption in the IR range. Known instruments are generally rather expensive, are of considerable size, require careful treatment and as a rule can be operated by skilled personnel only. The microstructured IR absorption photometer of the present invention is developed for (quasi-) continuously controlling a gaseous stream, the photometer being a single-piece shaped part manufactured as a microstructured body. The photometer is compact and robust, suitable for portable instruments and can be manufactured at low cost and in large numbers. The photometer can be made of metal and can be used even at an increased temperature. A flashlight lamp serves as an IR radiation source and a lead selenide receiver as an IR radiation receiver. The pulse repetition frequency of the flashlight lamp is from 0.01 Hz to 10 Hz. The pulse interval preferably amounts to more than a thousand times the pulse duration. By using the inventive photometer the safety of systems in which flammable, toxic, or other gases are contained or may occur can be considerably increased in an economic manner.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a microstructured photometer for the 
infrared (IR) spectral range for detection of absorbing material or 
mixtures thereof, e.g. within fluids. Furthermore, the invention relates 
to a procedure for measuring absorption (extinction) by means of this 
photometer. 
DISCUSSION OF THE BACKGROUND 
Absorption photometry can be carried out using known spectral photometers, 
in which the multi-frequency radiation is dispersed by means of a grating. 
These instruments can be used to address a very wide range of complex 
problems and to obtain accurate results. However, the known instruments 
are of considerable size, are generally used only in a fixed position and 
are comparatively expensive. Tuning through a broader spectral range as a 
rule requires long measuring times. The instruments demand careful 
treatment and handling and as a rule can be operated only by skilled 
personnel. 
In addition to spectral photometers also filter photometers are known, in 
which the spectral range for measuring the absorption is achieved with the 
aid of optical filters, e.g. interference filters. 
Measuring the absorption of a specific material as a rule only requires a 
narrow spectral range. If a plurality of materials are to be analyzed 
absorption has to be measured in several spectral ranges. A reference 
wavelength can be used for eliminating external effects. 
DE -44 43 814.2 describes an IR spectrometric sensor for gases, which 
consists of a microstructured shaped part containing a mirror grating, a 
connection for injecting multi-frequency IR radiation as well as a 
connection for extracting single-frequency IR radiation. 
The known photometers contain a radiation source and at least one radiation 
receiver, which is impinged by a beam being attenuated by the gas to be 
detected. The radiation receiver may be impinged at short intervals by 
dispersed radiation of different wavelengths. This can be achieved 
preferably by rotating the monochromatic illuminator. During the operation 
condition of the photometer the radiation source is permanently switched 
on. The radiation receiver is either permanently exposed to the dispersed 
radiation or the path of rays is interrupted by means of a chopper. The 
chopper may work mechanically. 
In the IR range thermal sources or semiconductor diodes can be used. 
Thermal radiators having a sufficient radiant flux within the IR range 
emit very much heat into the surrounding. In time, this results in heating 
up surrounding components which then emit IR radiation too. Thus the 
output signal is disturbed and masked. This disturbance becomes apparent 
especially in a compactly built microstructured IR photometer. After 
sufficient heating up of the instrument the slit used for injecting IR 
radiation no longer has clear outlines. Thus the resolution of the 
photometer is diminished or lost at all. Furthermore, the known thermal 
radiators having sufficient power are slowly-acting and unsuitable for 
intermittent operation. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide for an IR 
radiation source suitable for a microstructured IR photometer by which the 
instrument is heated up as little as possible. 
This object is achieved according to the invention by a microstructured IR 
absorption photometer which comprises a single-piece shaped part 
manufactured as a microstructured body, which comprises a base plate, a 
mirror grating for dispersing the IR radiation, a connection for injecting 
multi-frequency IR radiation, and at least one connection for extracting 
single-frequency IR radiation; a free space between the mirror grating and 
the connections for the IR radiation; a cover plate above the free space, 
which cover plate is joined to the shaped part; an IR radiation source; 
and at least one radiation receiver characterized in that the 
microstructured absorption photometer comprises a flashlight lamp as an IR 
radiation source and the IR radiation receiver has a response time which 
is shorter than the pulse duration of the flashlight lamp. 
Commercial gas discharge lamps are suitable flashlight lamps, e.g. 
discharge lamps filled with xenon gas. The smallest models are tubular 
lamps some centimeters long and a few millimeters in diameter. 
Miniaturized models may be even smaller. The pulse duration of the 
flashlight lamp may be some microseconds during which light of 
considerable intensity is emitted. 
Suitable radiation receivers include e.g. such IR radiation receivers the 
response time of which is from 0.5.multidot.10.sup.-6 seconds to 1 second, 
e.g. lead selenide receivers. 
The single-piece shaped part manufactured as microstructure may be made of 
plastic (e.g. poly(methyl methacrylate), polysulphone, polycarbonate) or 
of metal (e.g. nickel, nickel-cobalt, gold, copper). 
By means of the microstructured IR absorption photometer according to the 
invention the absorption of a fluid is measured using a pulse repetition 
frequency from 0.01 Hz to 10 Hz and a pulse duration from 10.sup.-6 
seconds to 1 second. The pulse interval preferably amounts to more than a 
thousand times the pulse duration. 
The microstructured IR absorption photometer may contain more than one exit 
slit and more than one IR radiation receiver mounted behind each slit. 
Thus it is possible to measure the absorption of the fluid to be tested at 
several neighboring wavelengths. A further wavelength at which no 
absorption occurs may be used as a reference wavelength. 
The microstructured IR absorption photometer according to the invention is 
used for quantitative analysis of gases and gas mixtures, e.g. gaseous 
hydrocarbons (methane, ethane, propane, butane, and others) or of carbon 
dioxide, carbon monoxide, nitrogen oxide, water vapor, ammonia, and 
others. 
The material to be tested is located in the preferably open space between 
the flashlight lamp and the entrance slit. Through this space, e.g. the 
gas to be tested may flow by convection, or gaseous components may reach 
this space by diffusion. 
The microstructured IR absorption photometer according to the invention has 
the following advantages: 
(a) Due to the short pulse duration and the low pulse repetition frequency 
the heat dissipation is low. Temperature of the IR photometer is only a 
little bit or not at all higher than the temperature of the environment; 
(b) If fluids are tested the composition of which varies only within long 
periods of time (within minutes or hours) slowly or only insignificantly a 
low pulse repetition frequency is sufficient to detect such variations 
reliably. The microstructured IR absorption photometer is most suitable 
for low pulse repetition frequencies; 
(c) The high intensity of the flashlight within the IR range allows the use 
of inexpensive detectors having moderate sensitivity to detect 
single-frequency IR radiation; 
(d) The flashlight lamp is mechanically less sensitive than a thermal 
radiation source having a spiral-wound glow filament; 
(e) The microstructured IR absorption photometer as a whole is insensitive 
to vibration and is very suitable for portable instruments; 
(f) The service life of the flashlight lamp operated at low pulse 
repetition frequencies amounts to many years and is appreciably greater 
than the service life of thermal radiation sources. Therefore, the 
microstructured IR absorption photometer requires only a very small 
expenditure for maintenance; 
(g) Even at low pulse repetition frequencies the fluid to be tested can be 
monitored quasi-continuously as long as the composition of the fluid 
varies only gradually. 
Accordingly, the present invention provides for a microstructured infrared 
(IR) absorption photometer which comprises a single-piece shaped part 
manufactured as a microstructured body. The shaped part comprises a base 
plate, a mirror grating for dispersing IR radiation, a connection for 
injecting multi-frequency IR radiation, at least one connection for 
extracting single-frequency IR radiation, a free space disposed between 
the mirror grating and the connections for injecting and extracting IR 
radiation respectively, and a cover plate above the free space. 
The microstructured IR absorption photometer further comprises an IR 
radiation source, and at least one IR radiation receiver. 
The IR radiation source is a flashlight lamp, and the IR radiation receiver 
has a response time which is shorter than a pulse duration of the 
flashlight lamp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, FIG. 1 
illustrates a view of the microstructured IR absorption photometer as seen 
from an open side of a free space. Arranged on the base plate (1) there 
are a planar mirror grating (2) also seen in FIG. 1a, a slit (3) for 
injecting multifrequency IR radiation, and a slit (4) for extracting 
single-frequency IR radiation. Concave mirrors (5) and (6) are situated 
opposite the mirror grating (2). Between the mirror grating (2) and the 
concave mirrors (5) and (6) there is the free space. The longitudinal 
sides of the free space may be open or closed. A flashlight lamp (7) as an 
IR radiation source is arranged on the outside of the injection slit (3) 
and an IR radiation receiver (8) is arranged on the outside of the 
extraction slit (4). The radiation of the flashlight lamp (7) is focused 
on the injection slit (3) by means of a concave mirror (9). The parts 
shown hatched in FIG. 1 project above the base plate (1). The base plate 
(1) together with the elements (2) and (6) fixedly arranged thereon form 
the microstructured single-piece shaped part. 
By means of the two concave mirrors (5) and (6), the IR beam is reflected 
within the free space in order to extend its path. 
The material to be tested is located in the space between the flashlight 
lamp (7), concave mirror (9) and entrance slit (3). This space is open and 
a gas, e.g., can flow through this space. 
FIG. 2 illustrates a longitudinal section through the inventive 
microstructured IR absorption photometer. The free space is covered by a 
cover plate (10). The radiation coming from the IR radiation source 
undergoes multiple reflection on the walls of the free space. 
Example 
Microstructured IR absorption photometer for flammable gases 
Flammable hydrocarbons such as methane, ethane, propane and butane, absorb 
IR radiation in the range of 3.38 .mu.m (2960 cm.sup.-1). The detection of 
propane in the air makes use of a microstructured IR absorption photometer 
according to the invention. 
The microstructured IR absorption photometer according to FIG. 1 is 
approximately 25 mm long and approximately 20 mm wide. The free space has 
a height of approximately 500 .mu.m. The inside of the microstructured 
shaped part fabricated by means of the LIGA technique and made of 
poly(methyl methacrylate) and the inside of the cover plate made of 
poly(methyl methacrylate) are gold-plated, as is the mirror grating. The 
mirror grating has 200 lines/mm. The blaze angle is matched to the maximum 
reflection of the mirror grating in the range 3.4 .mu.m (2940 cm.sup.-1) 
in the first order of diffraction. 
The IR photometer entrance slit (3), which has a width of approximately 0.4 
mm is illuminated with a flashlight lamp, e.g. the xenon gas filled flash 
tube BGA 1020 TAR 3 (manufactured by Heimann). This flash tube is 34 mm 
long and has a diameter of 3.15 mm. The tube is made of hard glass. The 
bias voltage amounts to 320 V and the ignition voltage to 11 kV. The mean 
energy amounts to about 2 Ws per pulse, and the pulse duration is about 10 
.mu.s. This flashlight lamp has a high radiant intensity in the range 
around 3.4 .mu.m (2940 cm.sup.-1). The light of the flashlight lamp (7) is 
focused on the entrance slit (3) by means of the concave mirror (9) which 
is mounted about 2 cm in front of the entrance slit. 
The radiation reflected by mirror grating (2) is directed onto the exit 
slit (4) having a width of approximately 0.4 mm. The slit (4) is situated 
at the position at which the wavelength 3.4 .mu.m (2940 cm.sup.-1) 
appears. Behind the exit slit (4) a lead selenide radiation receiver is 
arranged, the response time of which is about 1.5 .mu.s; this is less than 
the pulse duration of the flashlight. 
The space between the concave mirror (9) and the entrance slit (3) serves 
as a gas cuvette without side walls. The gas is flowing by convection 
through this space with a throughput of about 100 cm.sup.3 per minute. The 
gas essentially consists of air and may at times contain propane. 
As soon as propane is present in the gas to be tested, the intensity 
received by the radiation receiver at 3.38 .mu.m (2960 cm.sup.-1) 
diminishes in accordance with the extinction law as the concentration of 
propane increases. 
This IR absorption photometer can be used to detect propane percentages in 
air, which amount to approximately 10% of the propane concentration of the 
ignitable mixture of propane and air, i.e. approximately 0.2% propane in 
air. 
This makes it possible to detect e.g. leaks in gas-operated systems. The 
lower explosion limit of a propane-air mixture is about 1.8% propane. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.