The present invention relates to a method and apparatus for detecting foreign matters that might be present in liquids. More particularly, the present invention relates to a method and apparatus for detecting undesirable small foreign matters that might be present in medical fluids filled in transparent containers such as injection ampoules and vials, which often contain foreign matters like glass chips, small particles, and fibers. The presence of such foreign matters is not desirable for quality, and it is necessary to select defective containers containing foreign matters by testing all the containers filled with medical fluids.
In a conventional method the detection of foreign matters is accomplished as follows: An ampoule to be tested is turned at a high speed and then brought to a standstill quickly. The suspended foreign matters that swirl together with the liquid in the ampoule are illuminated. The beam of light which has passed through the liquid is received by a light detector, and a decrease in light received is regarded as an indication of the presence of foreign matters. In such a method it was difficult to detect foreign matters sensitively with a single photoelectric element as a light detector, because foreign matters to be detected are extremely small as compared with the detection visual field and the difference of photocurrent caused by the presence of foreign matters is also extremely small. Another conventional method in which is used a light detector consisting of many small light sensitive elements equivalent in size to foreign matters and outputs of respective light sensitive elements are scanned, has a fatal drawback that complex electric circuits are required for signal processing and the result of detection is affected by the shape of foreign matters.
In the case of the small light sensitive elements having a light sensitive area equal to or smaller than the projected area of the minimum size of foreign matter particles, sensitive detection can be accomplished because the light sensitive surface is shaded by a foreign matter particle and an extremely great difference occurs in quantity of light received between the shaded elements and the unshaded elements. For instance, a light sensitive area of 100.mu..times.100.mu. square will be completely shaded by a particle of about 100.mu..times.100.mu. size, and sensitive detection will be accomplished. However, a long and narrow foreign matter, say 50.mu..times.200.mu. in size, will not cover the square light sensitive surface completely, although the projected area is the same. Therefore, such a foreign matter may not be detected.
Another conventional method that detects diffused reflection from foreign matters cannot discriminate foreign matters of different sizes because the ratio of reflection varies depending on the kinds of foreign matters.
In the present invention which has been made to overcome the above-mentioned drawbacks, the light sensitive surface of the light detector is divided into a multiplicity of small sections measuring 0.01 mm.sup.2 to 1 mm.sup.2 so that each light sensitive element generates an output signal proportional in magnitude to the projected area of foreign matters, and detection is accomplished by comparing the output signal with the reference value.
Foreign matters vary in shape, and typical shapes are particle and fiber. Foreign matters of fibrous shape generally measure 20.mu. in diameter and more than ten times the diameter in length. Thus, a particle measuring 100.mu..times.100.mu. has the same projected area as a fiber measuring 20.mu..times.500.mu.. If the limit of detection is to be set up for particles measuring 100.mu..times.100.mu. and fibers measuring 20.mu. in diameter (or 500.mu. in length), the light sensitive surface of the detector should be divided into sections measuring 500.mu..times.500.mu. (0.25 mm.sup.2). By measuring the quantity of received light with each section of the divided light sensitive surface, it is possible to obtain an output signal proportional to the projected area of foreign matter regardless of its shape--particle, fiber, and others, and to obtain output signals having a sufficient S/N ratio, without mistaken detection due to small particles inherent to medical fluids.
According to a preferred embodiment of this invention, the light sensitive surface of the light detector is provided with a bundle of optical fibers connected to photoelectric elements in such a manner that each divided section of the light sensitive surface corresponds to each photoelectric element. Thus, the individual sections of the light sensitive surface are substantially continuous and there is no dead zone which might result in failure of detection.
In addition, photoelectric converting elements such as phototransistors, photodiodes, and photocells may be staggered directly on the light sensitive surface without using optical fibers, so that foreign matters suspended in the swirling liquid are detected by either row of the detecting elements. Such arrangement eliminates any dead zone.
In continuous and automatic detection the visual field of detection should be changed according to the size of ampoules to be tested, and this is accomplished by replacing the light receiver or by covering optically or mechanically a part of the light receiver. Such operation, however, needs skill, and a simple and certain method has been searched for.
According to the present invention the visual field for detection is changed as follows: The output signals from the group of small light sensitive elements are compared with the preset reference values to produce an output. Thus obtained output is then introduced into the detection visual field selector circuit that controls the number of small light sensitive elements to be used according to the size of objects to be detected.
The detection visual field selector circuit is a circuit to select a proper number of small light sensitive elements to be used according to the change of visual field relative to the size of objects to be detected. More particularly, it is so designed as to make presetting by means of a selector circuit according to the size of objects to be detected so that the number of small light sensitive elements to be used corresponds to the size of detection visual field. This is accomplished by the built-in matrix circuit such as diode matrix circuit and wired OR-circuit. Thus, it is possible to obtain by simple operation necessary and sufficient outputs from the output signals generated by the small light sensitive elements through the comparators. And the visual field thus changed is extremely accurate.