Optical coupling arrangement for particulate detector

Apparatus for detecting particulates (46) within a medium in a chamber (10) comprises a photo-detector (14) which is maintained at a stable low temperature by a Peltier type cooling device (42). Scattered light from the particulate (46) is focused by a spherical lens (34) onto the input face (30) of a rod lens (22). The latter has an optical pitch of 0.5 and transfers the image to its output face (25) whence it passes via a light pipe (18) to the sensitive area (16) of the photo-electric device (14). The rod lens (22) provides an inexpensive means for transferring the light and which provides a thermal barrier. Thus, although the photo-electric device (14) is held at a low temperature, the input face (30) of the rod lens (22) can be held at the temperature of the medium within the chamber (10) and is not subjected to the formation of mist or ice. The lens (34) is mounted by means of a collar ( 28) which is slidable into a position in which the lens (34) focuses the input light into the face (30) of the lens (22), and then secured in this position by ultra-violet-cured adhesive fillets (32, 34).

BACKGROUND OF THE INVENTION 
The invention relates to particulate detecting and optical coupling 
arrangements. One such arrangement may be used within a system intended 
for sensing the presence of particulates within a gaseous or liquid 
medium; the medium is illuminated and the light scattered by any 
particulates present in the medium is collected by the optical coupling 
arrangement and transferred to a suitable photo-detecting device. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a particulate detecting 
arrangement, comprising light directing means for directing a beam of 
light into an area in which particulates may be present whereby the light 
is scattered by any such particulates, and light detecting means 
positioned to receive such scattered light and to respond to it and to 
detect it, characterised in that the light directing means is a laser and 
the light detecting means is an avalanching photo-diode. 
According to the invention, there is also provided an optical coupling 
arrangement for collecting light present within a predetermined area as a 
result of the presence of particulates therein and for coupling the said 
light to light detecting means, characterised by an elongate optical 
member having a longitudinal dimension substantially greater that its 
transverse direction and extending longitudinally into the enclosure from 
wall means thereof and adapted to receive the said light and to transmit 
it to and to focus it onto the said light detecting means situated outside 
the enclosure. 
According to the invention, there is further provided an optical coupling 
arrangement for focussing light onto light detecting means, characterised 
by lens means providing a thermal barrier.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The arrangement of FIG. 1 incorporates a chamber 10, part of whose wall is 
shown at 12, in which flows the medium in which the presence of 
particulates is to be sensed. The medium may be gaseous or liquid. A light 
source (not shown), preferably a laser beam, illuminates the medium within 
the chamber 10. Any particulates present within the medium will scatter 
the impinging light and the purpose of the optical coupling arrangement to 
be described is to collect the scattered light and to detect it by means 
of a photo-detector, thereby indicating the presence of the particulates. 
The photo-detector is shown generally at 14 and comprises, in this example, 
an avalanche-type photo-detector. The sensitive area of the detector 14 is 
shown at 16 and it incorporates an integral light pipe 18 which conducts 
light from its face 20 to the sensitive area 16. 
The optical coupling arrangement includes a graded index rod lens 22 which 
extends through a bore 24 in the wall 12 of the chamber 10 and has a face 
25 which is optically coupled to the light pipe 18 and the face 20 of the 
photo-detector by means of a UV-cured adhesive 26. The rod lens 22 extends 
well into the chamber 10 and has a metal collar 28 attached adjacent its 
inner face 30 by means of UV-cured adhesive 32. The metal collar 28 
supports a lens 34 which is in the form of a complete sphere and which is 
attached to the collar 28 by means of further UV-cured adhesive 36. 
The photo-detector 14 has to be maintained at a stable temperature for 
effective operation. For this purpose, the detector is mounted within a 
metal block 40 and a Peltier-type solid state cooling device 42 is 
sandwiched between this block 40 and the wall 12 of the chamber. The 
Peltier device acts as a heat pump, collecting heat from the block 40 and 
the photo-detector 14 and transferring it to the wall 12 of the chamber 
which is designed to act as a heat sink. The temperature of the 
Photo-detector 14 is maintained stably at a relatively low value, minus 5C 
in this example. In order to assist in this, the photo-detector 14, the 
metal block 40 and the Peltier device 42 are enclosed within thermally 
insulating foam 44. 
Instead, the temperature of the photo-detector could be stably maintained 
at a temperature corresponding to normal ambient temperature by some other 
suitable temperature control device. 
In operation, light scattered by a particulate shown diagrammatically at 46 
is imaged by the lens 34 onto the face 30 of the rod lens 22. The rod lens 
has an optical pitch equal to 0.5, so that the image received at its inner 
face 30 is transferred to its outer face 25 where the light exits and 
passes into the light pipe 18 and thus onto the sensitive area 16 of the 
detector 14 which thus detects it. 
The UV-cured adhesive fillets 26,32 and 36 provide optical gating in 
addition to structural attachment. The position of the metal collar 28 on 
the lens 22 is adjusted, before curing the adhesive, so as to provide 
correct focussing of the light onto the face 30 of the rod lens. 
The rod lens 22 has low thermal conductivity and provides a thermal barrier 
within the optical path. Therefore, although its outer face 25 is cooled, 
by being in close proximity to the cooled detector 14, its inner face 30, 
being immersed in the medium within the sensing chamber 10, will be at the 
temperature of the medium. Assuming, as will normally be the case, that 
this medium is at a reasonably elevated temperature, the face 30 will 
therefore not attract moisture or ice formations. Furthermore, 
fluctuations in temperature at the face 30, caused by changes in 
temperature in the medium, are not, or are only slightly, transmitted to 
the face 25, thus minimising the demands on the temperature control 
device. 
If the photo detector used does not have its own in-built light pipe 
(corresponding to the light Pipe 18), the rod lens 22 will be arranged to 
have an optical pitch less than 0.5 so as to transfer the image form its 
inner face 30 directly to the sensitive area of the detector. 
The lens 34 can be a standard type of lens used for coupling optical 
fibers, such lenses being widely available and relatively inexpensive. 
In the arrangement shown in FIG. 2, items corresponding to those in FIG. 1 
are correspondingly referenced. 
The arrangement in FIG. 2 differs from that of FIG. 1 in that the 
arrangement of FIG. 2 does not incorporate the spherical lens 34 or its 
supporting collar 28. In this arrangement, the light scattered by the 
particulate 46 is directly collected by the face 30 at the end of the 
graded index rod lens 22. The optical pitch of the graded index rod lens 
is selected so that it focuses light received from the particulate onto 
the face 20 of the detector 14. In this example, the optical pitch of the 
rod lens is approximately 0.85. However, it could have any other value, 
for example less than 0.5, which is suited to its length and such as to 
ensure that the light is focused onto the detector 14. 
In the arrangement shown in FIG. 3, items corresponding to those in FIGS. 1 
and 2 are again similarly referenced. 
In the arrangement of FIG. 3, a graded index rod lens is not used. Instead, 
a support tube 50 extends through the bore 24 in the wall 12 and 
terminates adjacent to the photo detector 14. Collection of light 
scattered from the particulate 46, and focussing of this light onto the 
photo detector 14, is carried out by means of micro lenses 54 and 56. Each 
of these lenses is a converging lens such as a plano-convex lens. Lens 54 
is attached to the outer end of the tube 50 by ultra-violet cured adhesive 
58 and collects the scattered light and passes it into the tube 50. Lens 
56 is also converging, such as plano-convex and receives the light 
transmitted and focuses it onto the photo detector 14. 
Although the arrangement of FIG. 3 does not use a graded index rod lens, 
the tube 50 which it uses instead also acts as a thermal barrier so that 
fluctuations in temperature in the vicinity of the lens 54 are not, or are 
only slightly, transmitted to the area of the photo detector, thus 
minimising the demands on the temperature control device of the 
photo-detector. 
The arrangements described provide simple (and particularly in the case of 
the FIG. 1 and 2 arrangements) inexpensive methods of collecting very weak 
light levels scattered from particulates in gaseous or liquid media, and 
have the advantage of maximising the solid angle of collection. This is at 
least partially achieved by the types of construction illustrated which 
use an elongate optical member (the graded index rod lens 22 or the tube 
50) which extends substantially into the enclosure from the wall thereof. 
The arrangements avoid the use of large lenses. They also provide a 
thermal barrier within the optical path, thus blocking temperature 
fluctuation in the medium from being transmitted to and adversely 
affecting the operation of the photo-detector. 
The systems described may advantageously be designed to operate in a 
sampling mode. The enclosure 10 becomes a Pipe into which a sample of the 
medium to be tested for particulates is drawn.