Light scattering method and apparatus for detecting particles in liquid sample

A method and apparatus for detecting particles contained in a liquid sample includes using a nozzle to form the liquid sample into an unconfined stream having substantially flat sides and directing the stream into intersection with a laser beam in the external cavity of a laser, wherein the sides of the stream are oriented at the Brewster angle with respect to the laser beam. Light scattered from the laser beam by particles in the liquid stream as they pass through the laser beam is collected and directed by a lens to a photodetector which, in response to the impinging scattered light, emits signals for counting and measuring the particles contained in the liquid stream.

BACKGROUND OF THE INVENTION 
The present invention relates to a particle size measuring cell and, more 
particularly, to a measuring cell in which the sizes of particles 
entrained in a liquid stream are measured by passing the liquid stream 
through a laser beam and detecting the light scattered from the laser 
beam. 
In known particle size measuring cells of the above type, individual 
particles in the liquid stream passing through the laser beam scatter the 
light toward a photodetector, which transmits a signal in response. The 
amount of scattered light impinging on the photodetector is proportional 
to the size of the particle causing the scattering, and the amplitude of 
the signal transmitted by the photodetector is determined by the amount of 
impinging light, so that the amplitude of the signal transmitted by the 
photodetector is a measure of the size of the particle passing through the 
laser beam. 
The scattered light is collected by a lens and focused on the 
photodetector, which generates its pulses in response to each particle 
passing through the laser beam. The liquid stream is typically carried 
through the laser beam by a conduit of transparent material. As a result, 
there are substantial losses in the energy of the beam as it passes from 
air through one wall of the conduit, the fluid stream, and then the other 
wall of the conduit, thus, passing through numerous interfaces between 
various media having different indexes of refraction. Furthermore, since 
the conduits are typically circular in cross-section, the fluid stream is 
circular in cross-section and, therefore, fairly thick at the center where 
the laser beam intersects, thereby increasing the losses. For these 
reasons, prior devices for detecting and measuring particles in liquid 
streams have been limited to use with a laser beam emitted from a 
conventional laser. An external cavity laser has not been used in 
connection with measuring particles in liquid streams since losses in the 
light beam as it passed through the liquid stream have rendered it 
difficult, if not impossible, to maintain lasing. 
SUMMARY OF THE INVENTION 
By the present invention, a liquid sample containing particles to be 
detected is formed by a nozzle into a thin unconfined stream having flat 
sides and is passed at the Brewster angle through a laser beam in an 
external cavity of the laser. Particles in the liquid stream scatter light 
from the beam, and the scattered light is focused by a lens and directed 
to a photodetector. The use of a flat-sided free-falling stream flowing at 
the Brewster angle significantly reduces losses in the laser beam as it 
passes through the stream, thereby allowing lasing of the beam in the 
external cavity to be maintained. The laser beam within an external cavity 
of a laser is far more intense than a laser beam which has been emitted 
from a conventional laser. The greater intensity of the light in the beam 
of an external cavity laser results in stronger signals from the 
photodetector, which allows smaller particles to be detected and measured. 
Since the unconfined stream at the Brewster angle reduces losses in a 
laser beam enough so that lasing in an external cavity can be maintained, 
an external cavity laser can be employed and greater sensitivity can be 
achieved for detecting and measuring particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As can be seen from the FIGS. 1 and 2, the liquid stream particle size 
measuring cell according to the present invention includes a laser 10 
which projects a collimated beam of light 11 through a Brewster window 12. 
The light beam 11 is reflected by a concave mirror 14 back through the 
Brewster window 12 and into the laser 10 to define an external cavity of 
the laser 10. 
A sample of liquid containing particles to be detected and measured is 
formed by a nozzle 15 into an unconfined thin stream 16 having opposed 
flat sides. A nozzle suitable for forming the liquid into a thin stream 
having substantially planar and parallel smooth surfaces is disclosed in 
U.S. Pat. No. 3,766,489 to Rosenberg et al. The liquid stream 16 is 
directed through the laser beam 11 in the external cavity of the laser 10 
at the Brewster angle, so that the energy losses in the laser beam as it 
passes through the fluid stream are minimized. The losses are also 
minimized by the fact that the stream is unconfined, that is, it is not 
contained within a tube and does not flow along any plates, either of 
which would also cause losses to the laser beam, since the laser beam 
would be required to pass through them. 
When the light of the laser beam 11 strikes a particle in the fluid stream 
16, some of the light is scattered, and some of the scattered light falls 
on a lens 18, which collects the light and focuses it on a photodetector 
20. The photodetector 20 emits a signal in response to the reception of 
light from the lens 18. The number of pulses of light is an indication of 
the number of particles in the fluid stream, and the amplitude of the 
signals is proportional to the amount of light scattered and received by 
the photodetector, which is a measure of the size of each particle. 
Apparatus for processing the signals from the photodetector 20 in order to 
count and determine the sizes of the particles in the stream are well 
known in the art. 
It can be appreciated from the foregoing that a particle detecting 
apparatus of greater sensitivity is provided by the present invention, 
since a more intense beam of light can be employed, which allows smaller 
particles to be detected than could be detected by previously known 
apparatus. It can further be appreciated that various modifications can be 
made to the above-described apparatus without departing from the spirit 
and scope of the present invention, which is defined in the appended 
claims.