Apparatus for detecting an aqueous liquid in bottles and containers
The present invention relates to apparatus for detecting an aqueous liquid in translucent bottles and containers (hereinafter generally called &#34;bottles&#34;), and particularly to the detection of liquid residues in bottles which have been washed in a caustic solution prior to filling with beer, soft drinks or other products. 
     BACKGROUND OF THE INVENTION 
     The specification of U.S. Pat. No. 4121103 describes an inspection system based on the absorption of infra-red radiation by water, which is the main constituent of liquid residues, in which light rays including energy in the infra-red region are directed upwards through the bottom of a bottle to be inspected and on to a detection means for detecting the energy of particular infra-red wavelengths which is absorbed if there is any liquid residue in the bottle, thereby to produce a signal to reject any bottle which contains an aqueous residue. 
     Such a system can operate satisfactorily when inspecting clear bottles, but it cannot satisfactorily inspect coloured bottles which also absorb radiation in or near the infra-red region. Different bottles of the same colour can vary considerably in their absorption of infra-red radiation. The ranges of observed transmission coefficients for different coloured bottles, at the wavelength used for infra-red measurement, are: 
     for clear bottles--0.5-0.99 
     for amber bottles--0.09-0.35 
     for green bottles--0.008-0.61 
     For these reasons a simple measurement of transmitted intensity will not give a reliable indication of water content. The present invention has for an object to alleviate these difficulties and provide an inspection system for coloured and clear bottles and which is capable of use where differently coloured bottles, as well as clear bottles, are inspected on the same line. 
     SUMMARY OF THE INVENTION 
     In the detection system according to the invention the intensities of the energy transmitted through a bottle in two parts of the spectrum in and near the infra-red region, in one of which water is relatively transparent and in the other of which water absorbs more strongly, are compared to provide a signal indicative of an aqueous residue in the bottle. 
     The inspection apparatus conveniently uses two detectors, preferably of different types, to measure the transmitted intensity in the two parts of the spectrum. 
     Thus while first detector which is sensitive in a first part of the spectrum, e.g. about 0.8 to about 0.9 microns, in which water is fairly transparent, and which therefore essentially measures the absorption by the glass, and the second detector which is sensitive in a second part of the spectrum, e.g. about 1.2 to about 1.6 microns, in which water is more strongly absorbent and which measures the absorption by both the glass and any water present, the ratio 
     
         intensity of energy received by second detector/intensity of energy received by first detector 
    
     is sensitive to water content but insensitive to variations in thickness or colour of the glass. 
     Although the compensation is not perfect since the compensation measurement is taken at a different wavelength to the water measurement, and the infra-red absorption characteristics of differently coloured glasses have different shapes, experimental results have shown that the degree of compensation is adequate for practical purposes. 
     In one embodiment, one detector, conveniently a silicon detector with a filter to limit its short wavelength response, measures the intensity at 0.85 microns where water is relatively transparent. This detector therefore measures the absorption by the glass of the bottle and provides a calibration signal. The other detector, conveniently a germanium detector with a silicon filter to limit its short wavelength response, measures the intensity in the range of 1.2 to 1.6 microns where water absorbs more strongly. 
     A pass/fail decision is based on the ratio of the outputs of the two detectors, i.e. 
     
         fail if I.sub.Ge &lt;nI.sub.Si 
    
     where I Ge  and I Si  are respectively the intensities measured by the germanium and silicon detectors and n is a constant, the value of which is determined by the relative intrinsic sensitivities of the detectors. The value of n can be chosen to give the desired sensitivity of the system since an increase in n will decrease the amount of water required for a bottle to fail, and vice versa. 
     The use of the second wavelength to calibrate the bottles, in effect, reduces the variation which, as indicated by the above observed transmission coefficients, exceeds 100:1 between the most absorbing green bottle and the least absorbing clear bottle, to the range of 2.5:1, and ensures that all bottles, whatever their colour, fail if they contain a minimal depth of water, for example, between 1 mm and 4 mm of water. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a schematic diagram of the detector apparatus, 
     FIG. 2 is a block diagram of one embodiment of electronic circuit for use with the detector apparatus, 
     FIG. 3 is a diagram showing the pass/fail criterion, 
     FIG. 4 shows the transmission characteristics of the two detector/filter combinations used in the inspection system, and 
     FIG. 5 is a graph showing the transmission of radiation of water as a function of wavelength in the infra-red region of the spectrum. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 the radiation from a filament lamp 1 located beneath a diffuser 2 passes axially upwards through the bottle 3 to be inspected. Above the bottle the silicon detector 4 and the germanium detector 5 are mounted towards the upper end of simple collimator tubes 6,7, their respective filters 8,9 being mounted at the lower ends of the collimator tubes. This ensures that only radiation which has passed through the base of the bottle is detected. Filter 8 may, for example, be a Kodak Wratten No. 87 filter, and filter 9 is silicon filter and may comprise a 0.75 mm thick slice from a single crystal of silicon. The silicon detector may be a silicon photodiode whilst the germanium detector may be a germanium phototransistor. 
     FIG. 4 shows the spectral response of the two detector/filter combinations used in the detector system. Curve Si is the relative response of the silicon detector with Kodak Wratten No. 87 filter and curve Ge is the relative response of the germanium detector with a silicon filter. 
     FIG. 5 shows a graph of the transmission of radiation by water as a function of wavelength. The calibration and measuring wavelengths are indicated by the arrows Si and Ge respectively. 
     Referring to FIG. 2, the output from the silicon detector 4, after amplification in the amplifier 10, is fed to a variable gain amplifier 11, the output from which is compared in a comparator 12 with the output from the germanium detector 5, after amplification in an amplifier 13. 
     The output from the comparator is fed to a responder 17 which responds when the comparator output departs from a predetermined value. 
     If the germanium detector response is reduced, due to water-based liquid residue in the bottle, below the pass-fail line 14 in FIG. 3, e.g. to the point 15, the bottle would fail. The response from a typical empty bottle is at point 16 and the bottle would pass. The slope of the line 14 can be adjusted by the sensitivity control amplifier 11. The responder 17 can control a bottle inspection reject mechanism. 
     The liquid detector may be mounted next to the main station of an inspection apparatus for detecting foreign bodies in bottles, such as described in U.S. Pat. No. 3,727,068, or built into such apparatus, and the output from the responder 17 may be used to control a common reject mechanism.