Optical detection of particles in a liquid medium
Particles in a liquid medium are detected by directing a source of laser light through a container containing a liquid sample with a concentration of microscopic particles in suspension. An optical detector located off the optical axis receives scattered light from the particles and provides a signal with a dc component and a varying component. The DC component is removed to provide a filtered signal representing the varying component. Movement is produced in the sample liquid, e.g., by convective stirring, to extend the band of frequencies of the varying component towards higher frequencies. The apparatus can be used for growth curve detection of biological samples and provides increased sensitivity.
1. Optical apparatus for detecting particles in a liquid medium, comprising
a container for a liquid sample with a concentration of microscopic particles in suspension;
a source of coherent light arranged to direct coherent light along a predetermined optical axis through a sample in said container to provide an illuminated volume of said sample;
a detector located off said optical axis and arranged to receive light from said source which has been scattered by particles in said illuminated volume of said sample, said detector providing a signal representing the intensity of said received light and comprising a dc component dependent on the concentration of said particles in said illuminated volume and a time varying component with frequencies in a band, a filter removing said dc component to provide a filtered output signal and a sample stirring device to produce movement of the sample liquid in said illuminated volume so as to extend said band of frequencies towards higher frequencies.
2. Apparatus as claimed in claim 1 , wherein said sample storing device comprises a heater arranged to heat the sample in the container to produce convective stirring.
3. Apparatus as claimed in claim 2 , wherein said heater is arranged to heat the sample differentially in a horizontal plane.
4. Apparatus as claimed in claim 3 , wherein said heater comprises a heater block for receiving said container, said heater block being asymmetric about a vertical plane containing said optical axis.
5. Apparatus as claimed in claim 1 , including at least a first matched pair of said detectors equally spaced at a common radial distance on opposite sides of a plane containing said optical axis, and difference means receiving input signals respectively from said pair of detectors and providing a difference signal representing the difference between said input signals.
6. Apparatus as claimed in claim 5 , including at least one further said pair of detectors on opposite sides of said plane and at a respective different common radial distance from said optical axis, said difference means receiving input signals from said further pair of detectors and providing a further difference signal representing the difference between the input signals from said further pair, and a summing means receiving said difference signals and providing a sum signal representing the sum of said difference signals.
7. Apparatus as claimed in claim 5 , including at least one further said pair of detectors on opposite sides of said plane and diametrically opposed to said first pair with respect to said optical axis, said difference means receiving input signals from said further pair of detectors and providing a further difference signal representing the difference between the input signals from said further pair, and a summing means receiving said difference signals and providing a sum signal representing the sum of said difference signals.
8. Apparatus as claimed in claim 5 , including at least one further said pair of detectors at a common radial distance on opposite sides of a respective different plane containing said optical axis, said difference means receiving input signals from said further pair of detectors and providing a further difference signal representing the difference between the input signals from said further pair, and a summing means receiving said difference signals and providing a sum signal representing the sum of said difference signals.
9. Apparatus as claimed in claim 1 , wherein the or each detector has a sensitive aperture which is greater than ten times the coherence area for interfering scattered light at the detection plane normal to the optical axis.
10. Apparatus as claimed in claim 9 , wherein the illuminated volume of said sample has an area normal to the optical axis which is greater than 1 mm 2 .
11. Apparatus as claimed in claim 9 , wherein said illuminated volume has a dimension along said optical axis which is greater than 3 mm.
12. Apparatus as claimed in claim 11 , wherein said dimension is greater than 7 mm.
13. Apparatus as claimed in claim 9 , wherein said illuminated volume is greater than 3 mm 3 .
14. Apparatus as claimed in claim 13 , wherein said illuminated volume is greater than 30 mm 3 .
15. Apparatus as claimed in claim 9 , wherein said sensitive detector aperture is between 0.5 and 2 mm 2 .
16. Apparatus as claimed in claim 9 , wherein the distance between the mid point of the illuminated volume along the optical axis and the detection plane is between 1 and 5 cm.