Particle analysis system with photochromic filter

A background light filter for use in an optical particle analysis system operates to attenuate light coupled thereto and pass an attenuated intensity light signal. If the light coupled to the filter takes the form of discrete beams or areas forming a pattern, the filter will selectively attenuate in only those portions of the filter struck by the light beams forming the pattern. The optical filter is substantially insensitive to short term variations in light coupled thereto so that changes in the received light pattern which may, for example, be produced by passage of a particle through the light beam producing the pattern, will be passed through the optical filter with substantially no attenuation.

BACKGROUND OF THE INVENTION 
The present invention relates to photoanalysis apparatus and more 
particularly to photoresponsive apparatus for detecting various 
characteristics of small particles such as blood cells. 
There is a great need for accurate analysis of the characteristics of 
groups of small particles. A particularly important field for such 
analysis is in medical research and diagnosis where, for example, blood 
cells and other biological cells must be analyzed. 
Various systems have been developed for analyzing groups of small particles 
such as blood cells. In one system the analysis is accomplished optically 
by entraining the particles such as blood cells in a very thin stream of 
liquid and passing the stream containing the particles through an optical 
scanning station. A photo-optical detecting device is arranged to detect 
the optical reaction of each particle to illumination from a beam of 
light. 
In photoanalysis systems as described above it has been recognized that the 
light scattering effect and the fluorescent effect produced by particles 
in the stream passing through the optical scanning station varies 
according to different characteristics of the particles, including such 
factors as particle size, refractive index, particle staining and 
composition. It has also been found that the beam of light passing through 
the liquid stream forms a light pattern, the light pattern being different 
for the stream with and without particles as well as for different size 
and type particles. Additionally, the thin stream of liquid produces light 
reflections at various interfaces such as the air/water or glass/water 
interface. The reflections, light pattern and light beam all are coupled 
to a photodetector which converts the light signals received to electrical 
signals which are analyzed to determine presence of a particle and 
particle characteristics. 
The light pattern and light reflections developed at the photodetector when 
no particle passes through the light beam may have a rather high 
intensity. This high intensity light will create noise in the electrical 
signals produced by the photodetector. To eliminate the noise the 
photodetector sensitivity may be decreased. 
The variation in the light reflection and light pattern formed at the 
detector in response to passage of a particle through the light beam may 
not be very great so that the resultant variation and electrical output 
signal from the photodetector may be small. Because of the high noise 
level due to the background light and the reduced photodetector 
sensitivity, it may be difficult to detect particles or to accurately 
detect and identify various particle characteristics. It is therefore 
desirable to eliminate or minimize, to the greatest extent possible, the 
light patterns and light reflections received by the photodetector when 
there are no particles in the flow stream passing through the light beam. 
That is, to minimize the background light. 
SUMMARY OF THE INVENTION 
In practicing this invention, a particle analysis system is provided 
wherein a light beam is passed through a fluid stream to a photodetector. 
The light beam passing through the stream produces light reflections and a 
light pattern. Passage of a particle through the fluid stream and through 
the light beam varies the amount and intensity of light passing through 
the fluid stream, the reflected light and the light pattern. In the 
system, a background light filter is positioned between the fluid stream 
and a photodetector. The filter operates in response to the light 
reflections and the light patterns received thereat to slowly increase in 
optical density at the points the light pattern strikes the filter and 
attenuate the pattern passing through the filter to the photodetector. The 
filter is substantially insensitive to rapid changes in the light pattern 
produced, for example, by passage of a particle in the fluid stream 
through the light beam so that the light variations resulting from 
particle passage will pass through the filter to the photodetector 
allowing particle detection and analysis.

DETAILED DISCUSSION OF THE PREFERRED EMBODIMENTS: 
Referring to FIG. 1, there is shown a photoanalysis system including a 
container 10 for storing a fluid containing particulate matter to be 
analyzed. It is to be understood that container 10 may include all 
necessary pressurizing and pressure regulating apparatus for causing the 
fluid to exit container 10 continuously at a prescribed flow rate. 
In operation, the fluid in container 10 exits via orifice 12 and forms a 
fluid column which flows downward. A light source 18, which in the 
preferred embodiment is a laser, produces a laser beam 20 which intersects 
the fluid column 14 at a first location 22. 
Referring now to FIG. 2, there is shown a somewhat different apparatus 
configuration wherein particle laden fluid from a container 10 is coupled 
to and out of an inner nozzle 24 and a particle free sheath liquid is 
coupled to and through an outer nozzle 26. Details of such a structure may 
be found in U.S. Pat. No. 3,710,933. In the aforementioned structure, the 
particle containing fluid is entrained in the center of the flowing stream 
of sheath fluid, and in FIG. 2, the following stream is passed through an 
optical chamber 28. Optical chamber 28 is formed from a highly light 
transmissive medium such as glass or plastic. In the embodiment shown in 
FIG. 2, light beam 20 from laser 18 is passed to and through optical 
chamber 28 and the fluid contained therein. Light beam 20 also strikes and 
passes through fluid column 14 shown in FIG. 1. In the embodiment of FIG. 
2, the glass air interface at the outer surface of optical chamber 28 will 
produce light reflections represented by a light beam 30. The interface 
between the fluid stream and the wall of optical chamber 28 also creates a 
reflected light beam represented by beam 32. Turbulence in the fluid 
stream also causes reflections and one such beam 34 is identified. In 
addition to the noted reflected light beams, the passage of the beam 20 
through the fluid stream itself can produce a distinct light pattern. The 
reflected light beams identified, the light pattern produced by passage of 
light beam 20 through optical chamber 28 and light beam 20 itself all pass 
to a background light filter 40. In FIG. 1 the air water interface and 
fluid turbulence produce reflections, which along with the light pattern, 
pass to filter 40. 
Filter 40 is a structure which has a variable light attenuation 
characteristic. More specifically, the light attenuation of optical filter 
40 will increase slowly or the light transmission will decrease slowly in 
response to a light beam or light pattern striking the filter. The 
attenuation increase and transmission decrease will only occur at the 
specific locations on filter 40 where the beam or pattern strikes. For 
example, in FIG. 2, the light attentuation of filter 40 will increase only 
at the points where the light beams 30, 32 and 34 strike the filter. 
Although it is not shown in FIG. 2, it should be understood that the light 
pattern formed by the intersection of flow stream and light beam 20 will 
also cause attenuation of filter 40, only at the location on filter 40 
struck by the pattern. The amount of attenuation provided by filter 40, at 
the noted locations, is dependent upon the length and intensity of the 
light striking the filter and the material used for the filter itself. At 
least during the time that no particle passes through the light beam then, 
photodetector 42 will receive substantially no light. Because very little 
if any light is received at photodetector 42 the gain of photodetector 42 
may be set at a maximum level without concern for a high ambient noise 
level normally produced by a high ambient light level reaching detector 
42. 
In the preferred embodiment, filter 40 takes the form of a photochromic 
mechanism. The reaction in filter 40 is similar to that which takes place 
when light strikes the emulsions of many photographic films containing 
silver halides. However, when the film is developed, the darkened image is 
permanently fixed whereas with the photochromic filter of the instant 
invention, the darkening and attenuating process is completely reversible 
depending upon the amount and intensity of light present. 
The preferred form of photochromic mechanism used for background light 
filter 40 is a photochromic glass such as is presently manufactured and 
sold by the Corning Glass Works of Corning, N.Y. under the trademarks 
"PHOTOGRAY" or "SUNGRAY". 
When a particle such as particle 46, shown in FIG. 2, passes through light 
beam 20, it will create a light pattern different from the light pattern 
present when no particle is present. This pattern will be created for only 
a very short time period, specifically the time period that particle 46 
passes through beam 20. The light pattern created is coupled to filter 40. 
As previously mentioned, filter 40 slowly increases in light attenuation 
and decreases in light transmissivity in response to received light so 
that any variation in received light which occurs quickly and for a short 
time period, will not affect the existing attenuation characteristics of 
filter 40. Consequently, the light pattern produced by the passage of 
particle 46 will pass through filter 46 substantially unattenuated. In 
FIG. 2 the light pattern passes to a converging lens 48 which operates to 
focus the beams forming the light pattern to photodetector 42. In FIG. 1, 
the light beams passed by filter 40 proceed directly to photodetector 42. 
Photodetector 42 receives the light pattern and develops electrical 
signals in response to the light pattern which are coupled to a detector 
50. Because of the high sensitivity of photodetector 42, a substantially 
greater amount of the light pattern received at detector 42 can be 
converted to electrical signals and coupled to detector 50. The additional 
information provided by the additionally detected signals may be employed 
for identifying additional characteristics of the particle, which 
characteristics were not previously identifiable because of the high 
ambient light level. 
While the present invention has been described by reference to specific 
examples, it is to be understood that modifications may be made by those 
skilled in the art without actually departing from the invention shown and 
described herein. It is therefore intended that the appended claims cover 
all variations that fall within the scope and spirit of this invention.