Spectral detection of contaminants in containers

To spectrally detect a contaminant in a moving container, a set of reference spectral information related to one or more containers having known contents is stored. Thereafter, radiant energy is directed at liquid near the bottom of the container so that the radiant energy is modified by the contents of the container and travels through the contents of the container in multiple paths of varying length. Spectral information from detected portions of the modified radiant energy is obtained, and is compared to the stored set of reference spectral information using correlation techniques. Based on the relationship between this spectral information and the stored set of reference spectral information, the presence or absence of a contaminant is indicated.

BACKGROUND OF THE INVENTION 
The invention relates to spectrally detecting a contaminant in a container. 
The popularity of refillable containers has increased as the costs, both 
social and financial, associated with disposal of packaging have become 
less acceptable. For example, in many countries, water and other beverages 
are sold in refillable bottles. These bottles are often made from a type 
of plastic known as polyethylene terephthalate. 
After use, refillable containers are returned to a bottling plant where 
they are cleaned and inspected before being refilled. This inspection, in 
addition to checking for physical damage such as cracks, screens the 
containers to eliminate those that include contaminants that might degrade 
the flavor, safety, or other qualities of the product that they contain. 
The risk of contamination is greater when a container is made from 
plastic, as opposed to glass, because some contaminants can be absorbed 
into the plastic walls of the container. Absorbed contaminants can persist 
despite cleaning procedures, and can later leach into the product. 
Though some contaminants, such as detergents and fabric softeners, are 
visibly colored and can be detected by human inspectors, such human visual 
inspection is undesirable when bottles or other containers are moving on 
high speed conveyors and stopping or touching the bottles to perform an 
inspection is undesirable or overly expensive. Moreover, such human visual 
inspection is subject to lapses in attention by the inspectors. 
As an alternative, it has been suggested to use spectrophotometric 
instrumentation to automatically detect colored contaminants. 
Spectrophotometric instrumentation for color detection is well known in 
many fields, including laboratory analysis of chemical solutions, and 
quality control functions in the paint, fabric, and photographic 
industries. In general, spectrophotometric analysis of liquid samples is 
based on Beer's Law, which states that the optical density (i.e., the log 
ratio of transmitted or detected light intensity to incident light 
intensity) is directly proportional to the concentration of the chemical 
compound giving rise to the absorption of light. Beer's law is discussed, 
for example, in H. A. Strobel, Chemical Instrumentation, pp. 148-53 (1960, 
Addison Wesley, Reading, Mass.). Beer's law is limited in that it can only 
be applied if all of the detected light travels the same distance through 
the absorbing medium. In chemical spectrophotometric analysis, this is 
done by placing the liquid sample in a cuvette, or optical cell, having 
parallel windows that are typically spaced apart by ten millimeters. 
When a container is mostly filled with liquid, a narrow optical beam can be 
directed radially through the container so that it intersects the major 
vertical axis of the container in a region where the wall of the container 
is substantially parallel to the major vertical axis. In the case of a 
cylindrical container such as a bottle, if the beam is very narrow 
relative to diameter of the bottle, all of the detected light travels over 
substantially the same path length through the bottle, and the geometrical 
conditions for Beer's law are satisfied. As an alternative, the beam can 
be directed through the bottom of the bottle (i.e., from the bottom of the 
bottle to the top of the bottle, or from the top of the bottle to the 
bottom of the bottle). 
However, when a bottle or other container only contains a few millimeters 
of residual liquid, it becomes much more difficult to satisfy Beer's law. 
For example, in refillable plastic beverage bottles, the walls of the 
bottle curve inward near the bottom of the bottle and are not parallel to 
the major axis of the bottle. Also, many refillable plastic bottles 
include a dome in the bottom of the bottle. In these situations, when 
light is directed through the side of the bottle, refraction and 
reflection result in the detected light travelling over a variety of path 
lengths, and use of Beer's law is not practical. Similarly, when light is 
directed through the bottom of the bottle, the length of the path that the 
light takes through the liquid is likely to be insufficient to allow 
accurate detection. 
One way of dealing with small amounts of residual liquid is to mechanically 
tilt the bottle to pool the residual liquid in a corner of the bottle and 
arrange the incident beam of light perpendicular to the face of the bottle 
to minimize refractive effects. In this case, as long as the beam of light 
is sufficiently thin and the dome avoided, the path length will be well 
defined and free of multiple reflections, and conventional Beer's law 
analysis may be useful. 
SUMMARY OF THE INVENTION 
The invention features detection of contaminants in a moving container by 
extracting and examining spectra of the container and contents thereof, 
without requiring uniform optical path lengths or relying on an analysis 
under Beer's law. According to the invention, contaminants in a container 
moving at high speed are detected without requiring the mechanical 
complexity associated with tilting the container, even when the container 
includes only a minimal amount of liquid. 
In one aspect, generally, the invention features automatic spectral 
detection of contaminants in a moving container. Preferably, liquid is 
added to the container to ensure the presence of a minimal amount needed 
for proper detection. Spectral characteristics of the liquid are 
determined by subjecting the container and the liquid to radiant energy 
and obtaining a resulting spectrum. This spectrum, which is derived from 
radiant energy that travels through the liquid in multiple paths of 
varying length, is then compared against a library of stored reference 
spectra to determine whether contaminants are present. If contaminants are 
determined to be present, the container is rejected. 
For improved performance, two spectra are obtained for each container, and 
each is compared to corresponding reference spectra. The spectra are 
obtained, for example, by measuring a first spectrum when a container is 
located in a first position, moving the container to a second position, 
and measuring a second spectrum. In this approach, the two spectra can be 
obtained using only a single source of radiant energy and a single 
detector. 
Typically, the source of radiant energy and the detector are angled 
relative to each other (i.e., the source of radiant energy and the 
detector are not coaxial), and are positioned so that the radiant energy 
that passes from the source to the detector passes through a region 
substantially near the bottom of the container. The preferred range for 
this angle is from 100.degree. to 140.degree. with the most preferred 
angle being at about 120.degree.. 
The amount of liquid and the concentration of contaminants in a container 
can vary to a large degree. Thus, to avoid falsely rejecting 
uncontaminated containers, features of the spectra that are relatively 
unaffected by variations in liquid level or concentration are employed. 
In a preferred embodiment of the invention, the spectra employed are an 
"absorption" spectrum, which provides a measurement of the radiation 
absorbed by the liquid and any contaminants therein, and a "reflection" 
spectrum, which provides a measurement of the radiation reflected by the 
liquid and any contaminants therein. While the absorption spectrum and the 
reflection spectrum each provide accurate contaminant detection, improved 
accuracy of detection is achieved through use of both spectra. 
The library of reference spectra against which the measured spectra are 
compared can include spectra associated with only uncontaminated 
containers, only contaminated containers, or both contaminated and 
uncontaminated containers. When the library includes spectra for only 
uncontaminated containers, the presence of a contaminant is indicated when 
the spectra of the container differs from all of the spectra in the 
library by a predetermined threshold. When the library includes spectra 
for only contaminated containers, the presence of a contaminant is 
indicated when the spectra of the container matches, within a 
predetermined threshold, a spectrum from the library. Finally, when the 
library includes spectra for both contaminated and uncontaminated 
containers, the presence of a contaminant is indicated when the spectra of 
the container differs from each of the reference spectra associated with 
an uncontaminated container by more than a predetermined threshold or 
matches, within a predetermined threshold, a reference spectrum associated 
with a contaminated container. 
Generally, only a small amount of liquid is added to a container prior to 
directing radiant energy into the container. In some applications, this 
amount is less than ten milliliters. Often, only four and a half 
milliliters are used. Typically, water or a dilute aqueous solution is the 
liquid added. 
Various forms of radiant energy can be used in generating the spectra. In 
certain applications, either visible light, infrared energy, or a 
combination of the two are preferred. 
The invention is particularly useful for detecting contaminants in clear 
plastic bottles, such as those made from polyethylene terephthalate. 
However, the invention is also useful in detecting contaminants within 
other types of containers, containers made from other materials, and 
tinted containers. Generally, the only limitation on suitable containers 
is that they be made from materials that are translucent to the radiant 
energy being employed. 
In another aspect, generally, the invention features a spectral contaminant 
detection system that includes a radiant energy source or illuminator that 
directs radiant energy at a container, a detector that detects radiant 
energy from the illuminator that has been modified by the contents of the 
container and produces spectral information related to the detected 
radiant energy, and a processor that compares spectral information from 
the detector with a library of reference spectra and indicates the 
presence or absence of a contaminant based on the relationship between the 
spectral information and the reference spectra. This system, which is 
preferably entirely automated, works effectively even when containers are 
moving past the system at rates on the order of 400 containers per minute 
or greater. 
To ensure that spectral information is obtained from the detector at proper 
times, the system can include a first position sensor and a second 
position sensor that signal the processor when a container is in, 
respectively, a first position or a second position. The processor 
responds to the signal from the first position sensor by obtaining first 
position spectral information from the detector, and to the signal from 
the second position sensor by obtaining second position spectral 
information from the detector. The processor then compares the first and 
second position spectral information against a library of reference 
spectra to determine whether a contaminant is present. 
The system preferably is positioned downstream of a liquid supplier that 
adds a quantity of liquid to the container before the container arrives at 
the illuminator. To minimize the amount of liquid required, the 
illuminator and detector are positioned so that radiant energy from the 
illuminator reaches the detector after passing through a region 
substantially near the bottom of the container. Typically this region is 
within one inch, and often within one quarter of an inch, of the bottom of 
the container. 
Other features and advantages of the invention will be apparent from the 
following description of preferred embodiments, and from the claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference to FIG. 1, a spectral contaminant detection system 10 is 
positioned to detect contaminants in containers, such as bottles 12, by 
analyzing spectral characteristics of liquids contained in the bottles 12, 
as the bottles 12 move along a conveyor 14 in the direction indicated by 
arrow 15. Because contaminants may be present as liquids in the bottles 12 
or may leach or desorb from walls of the bottles 12 into liquids contained 
therein, the spectral characteristics of the liquids indicate the presence 
of such contaminants. Thus, by comparing the spectral characteristics of a 
bottle 12 and the liquid contained therein to characteristics of bottles 
containing contaminated or uncontaminated liquids, system 10 determines 
whether contaminants are present in the bottle 12. 
As used herein, "contaminant" means any substance that can be detected in a 
container by the detection system of the invention and whose presence is 
incompatible with the product with which the container is to be filled. 
For example, detergents are contaminants with respect to beverage 
containers, and flavored beverages may be contaminants with respect to 
bottled water. 
System 10 includes a radiant energy source or illuminator 16 and a detector 
18. Illuminator 16 is positioned to direct radiant energy at a bottle 12 
so that the radiant energy encounters liquid contained in the bottle 12. 
Detector 18 is positioned to detect radiant energy from illuminator 16 
after that radiant energy has encountered the liquid contained in bottle 
12. 
Illuminator 16 includes a fiber optic bundle 20 coupled to a lens 22. Fiber 
optic bundle 20 transmits radiant energy from a lamp 24 located in a 
control unit 26 to lens 22, which focusses the radiant energy and directs 
the focussed radiant energy toward a bottle 12. The lamp 24 is typically a 
halogen lamp, but other sources of radiant energy such as, for example, a 
xenon flashtube that is controlled to strobe at appropriate times, could 
be used. 
Detector 18 includes an optic fiber bundle 28 that receives some of the 
radiant energy from lens 22 after it has encountered liquid in the bottle 
12. Fiber optic bundle 28 transmits the radiant energy to an optical 
spectrometer 30 in control unit 26. Within optical spectrometer 30, a 
series of mirrors focusses the transmitted radiant energy on a diffraction 
grating that separates the transmitted radiant energy into wavelength 
components and directs each wavelength component to a different pixel of a 
linear detection array 32. Typically, linear detection array 32 is 
implemented as a diode array or a charge coupled device ("CCD") having 
about one thousand pixels. 
Use of fiber optic bundles 20 and 28, which may be two meters or greater in 
length, allows control unit 26 to be positioned a substantial distance 
away from the conveyor 14 and bottles 12, and thereby minimizes the 
exposure of control unit 26 to the potentially wet or otherwise hostile 
environment at conveyor 14. 
With reference to FIGS. 2 and 3, illuminator 16 and detector 18 are 
positioned so that the radiant energy from lens 22 is directed at a region 
33 near the bottom of each bottle 12, with lens 22 being centered about 
one half inch above the bottom of bottle 12 and fiber 28 being centered 
about one quarter inch above the bottom of bottle 12. Illuminator 16 and 
detector 18 are aimed such that their axes of emission and reception are 
not aligned (i.e., illuminator 16 and detector 18, respectively, emit and 
receive radiation in directions that are not parallel to each other) and 
are not normal to the direction of movement of bottles 12 (see FIGS. 1, 4, 
and 5). This positioning requires the presence of only minimal amounts of 
liquid 34 in bottle 12. In the preferred embodiment, each bottle 12 needs 
to contain as little as about four and one half milliliters of liquid 34. 
In addition to being positioned near the bottom of bottle 12, illuminator 
16 and detector 18 are positioned close to conveyor 14, typically within 
one eighth of an inch. As best illustrated in FIG. 3, a beam of light 35 
from illuminator 16 is directed at and above the horizontal plane occupied 
by detector 18. As also illustrated in FIG. 3, the mean light path from 
lens 22 of illuminator 16 to fiber 28 of detector 18 is along path AE. 
Although bottles and containers of various shapes may be inspected by 
system 10, the bottle 12 shown in FIGS. 2-5 has a base 36 with a convex 
bulge 37 in its bottom that causes liquid 34 near the bottom to form a 
concentric annular ring around convex bulge 37. Because base 36 has a 
smaller diameter than a main portion 38 of bottle 12, and illuminator 16 
and detector 18 are positioned near the bottom of bottle 12, bottles 12 
can be moved along conveyor 14 with no spacing--i.e., in contact with 
other bottles, with no interference by a bottle 12 with measurement taken 
by system 10 on an adjacent bottle 12. 
A liquid supplier 39, positioned upstream of illuminator 16, adds a 
sufficient amount of liquid 34 to each bottle 12 to ensure that radiation 
emitted from illuminator 16 will encounter liquid in the bottom of each 
bottle 12. Generally, because extra liquid 34 does not affect the 
performance of system 10, liquid supplier 39 adds liquid 34 to each bottle 
12 without regard to whether a bottle 12 already contains liquid. Addition 
of liquid 34 by supplier 39, which may be an injector timed to inject a 
pulse of liquid into the open top of each bottle 12 as it passes 
underneath the supplier 39, may assist in leaching contaminants from the 
bottle walls as well as ensuring the presence of a sufficient amount of 
liquid for detection. Typically, the liquid 34 supplied by liquid supplier 
39 is water or a dilute aqueous solution. However, in some applications, 
other liquids could be used. For example, a liquid that changes color in 
the presence of an otherwise difficult to detect contaminant could be used 
to ease detection of that contaminant. 
With reference to FIG. 4, in which, for simplicity, only a single bottle 12 
is shown, in operation of spectral contaminant detection system 10, a 
first position sensor 40 signals a processor 41 (FIG. 1) in control unit 
26 when bottle 12 is positioned suitably to produce a first spectrum for 
liquid 34 and any contaminants contained therein. First position sensor 40 
signals processor 41 when bottle 12 is positioned so that a portion of the 
radiant energy that reaches detector 18 from illuminator 16 travels along 
a path ABC, reflects from the inside surface of a wall 42 of bottle 12, 
and continues along a path CDE to detector 18. Because this position 
maximizes the length of the path that radiant energy takes through liquid 
34, and thereby maximizes the absorption of radiant energy by liquid 34 
and any contaminants contained therein, the measured spectrum is referred 
to as an absorption spectrum. Typically, illuminator 16 and detector 18 
are positioned so that the angle ACE is within a range from 
100.degree.-140.degree. with about 120.degree. being most typical. 
In actual operation, the portion of the radiant energy produced by 
illuminator 16 that actually reaches detector 18 travels by multiple paths 
that are significantly more complicated than the path described above. For 
example, the actual path is affected by reflection from wall 42 of bottle 
12 and the interface between liquid 34 and air above liquid 34 in bottle 
12. In addition, due to the presence of liquid 34, the radiant energy is 
refracted at points B and D, so that some of it travels approximately 
along a path BD before reaching detector 18. The radiant energy is 
affected also by scattering at wall 42 of bottle 12 and at convex bulge 
37, and can travel along complicated paths that include several internal 
reflections within bottle 12. 
First position sensor 40 signals processor 41 when the leading edge of a 
bottle 12 crosses a line FG between first position sensor 40 and a first 
light source 44. When bottle 12 crosses line FG, bottle 12 interrupts or 
otherwise causes a change in the level of light (radiation) from first 
light source 44 that reaches first position sensor 40. First position 
sensor 40 generates the signal to processor 41 in response to this change 
in the level of light. 
Upon receiving the signal from first position sensor 40, processor 41 
causes linear detection array 32 to record the spectrum produced by 
spectrometer 30 of the radiant energy detected by detector 18. Processor 
41 then sequentially reads linear detection array 32 to generate a vector 
that represents the intensity of radiant energy received at each pixel of 
linear detection array 32, and stores the vector as an absorption spectrum 
associated with the bottle 12 being examined. Typically, each pixel of the 
absorption spectrum is represented by twelve bits. In the preferred 
embodiment, processor 41 is implemented using an Intel 486 processor 
running at sixty six megahertz. 
With reference also to FIG. 5, in which, for simplicity, a single bottle is 
shown, a second position sensor 46 signals processor 41 when bottle 12 is 
positioned suitably to produce a second spectrum for liquid 34 and any 
contaminants contained therein. Second position sensor 46 signals 
processor 41 when bottle 12 is positioned so that radiant energy that 
reaches detector 18 from illuminator 16 travels approximately along a path 
AH, reflects from liquid 34 near the inner surface of wall 42, and 
continues approximately along a path HE to detector 18. Because most of 
the radiant energy reaching detector 18 does so by reflection rather than 
transmission, the measured spectrum for this second position of the bottle 
is referred to herein as a reflection spectrum. 
It should be understood that the path of the radiant energy for the 
measurement at a second position of the bottle 12 is, like that of the 
first position, more complicated than illustrated in FIG. 5. For example, 
light could also travel along a path AI, refract at the interface between 
wall 42 and liquid 34, travel along a path IJ, refract at the interface 
between liquid 34 and wall 42, and travel along path JE to detector 18. 
Moreover, the intensity of the energy received by a detector 18 for the 
second position is typically lower than that received for the first 
position since, when bottle 12 is in the second position, most of the 
energy is transmitted beyond point H. 
Second position sensor 46 signals processor 41 when a leading edge of 
bottle 12 crosses a line KL between second position sensor 46 and a second 
light source 48. Second position sensor 46 and second light source 48 are 
typically located downstream of first position sensor 40 and first light 
source 44 by slightly less than the diameter of bottle 12, and operate 
identically to first position sensor 40 and first light source 44. To 
prevent cross talk caused by first position sensor 40 responding to light 
produced by second light source 48, or by second position sensor 46 
responding to light produced by first light source 44, the sensors and 
light sources are positioned with first position sensor 40 and second 
light source 48 on one side of conveyor 14, and second position sensor 46 
and first light source 44 on the other side of conveyor 14. In an 
alternative approach to preventing cross talk, the sensor/light source 
pairs could be configured to respond to different frequencies of light. 
Upon receiving the signal from second position sensor 46, processor 41 
causes linear detection array 32 to record the spectrum produced by 
spectrometer 30 of the radiant energy detected by detector 18. Processor 
41 then stores the recorded spectrum as a reflection spectrum associated 
with the bottle 12 being examined. 
Processor 41 determines whether a bottle 12 contains contaminants by 
comparing the absorption and reflection spectra associated with the bottle 
12 to a library of reference spectra associated with bottles containing 
acceptable substances. For example, for bottles to be filled with a 
beverage, acceptable substances would include water, beverage residue, and 
the aqueous solution supplied by liquid supplier 39. Processor 41 compares 
the spectra by computing either the Pearson's correlation or the 
Spearman's correlation for the vectors representing each spectrum. 
Pearson's correlation, which is described, for example, in Pfaffenberger & 
Patterson, Statistical Methods For Business and Economics, p. 429 (1977, 
Richard D. Irwin, Inc., Homewood, Ill.), determines whether two vectors 
are related by a linear mapping, and r, the Pearson's correlation 
coefficient, is determined as: 
##EQU1## 
where x.sub.i equals the ith component of the vector X minus the average 
value of the components of the vector X, y.sub.i equals the ith component 
of the vector Y minus the average value of the components of the vector Y, 
and n equals the number of components in vector X or vector Y. 
Spearman's correlation, which is described in Pfaffenberger & Patterson at 
p. 679, arranges the elements of each vector in rank order and determines 
whether two vectors have similar rank orders, and .rho., the Spearman's 
correlation coefficient, is determined as: 
##EQU2## 
where R(X.sub.i) equals the rank of the ith component of the vector X 
relative to the other components of the vector X, R(Y.sub.i) equals the 
rank of the ith component of the vector Y relative to the other components 
of the vector Y, and each tied rank is assigned the average of the ranks 
that would have been assigned had there been no ties (e.g., if the fifth 
and sixth ranked components have equal values, they are each assigned a 
rank of 5.5). 
If the spectra associated with the bottle 12 correlate within a predefined 
threshold--e.g., by greater than 90%--to a pair of reference spectra 
representing acceptable bottle content, processor 41 allows the bottle 12 
to continue along conveyor 14 for filling or other testing. If not, 
processor 41 sends a signal to a suitable rejector 50, and rejector 50 
responds by removing the bottle 12 from conveyor 14. 
An advantage of comparing the spectra associated with a bottle 12 to 
reference spectra associated with bottles containing acceptable materials, 
rather than comparing with spectra associated with bottles containing 
unacceptable contaminants is that the former imposes less computational 
burden on processor 41. Moreover, detection accuracy of system 10 may be 
higher due to a reduced likelihood of failing to detect contaminants. 
The computational burden is reduced because the number of acceptable 
reference spectra is typically quite limited, while, considering the 
number of potential contaminants and the various ways in which the 
contaminants can be combined, the number of unacceptable reference spectra 
may be virtually unlimited. For example, to detect contaminants in 
refillable polyethylene terephthalate cola bottles, it has been found that 
a library consisting of ten reference spectra--the absorption and 
reflection spectra associated with two liquid levels of water and three 
liquid levels of cola--is adequate. 
The detection accuracy is higher because, unlike a system in which only 
bottles having spectra similar to reference spectra associated with known 
contaminants are rejected, system 10 is able to reject a bottle 12 that 
contains a previously unknown contaminant or a previously unknown 
combination of known contaminants. 
Though comparison to acceptable reference spectra offers considerable 
advantages, the spectral contaminant detection system 10 can be configured 
to compare the spectra associated with unacceptable spectra or a 
combination of acceptable and unacceptable spectra. For example, to screen 
out, for testing or other purposes, only bottles containing particular 
contaminants such as a blue fabric softener or a green disinfectant, the 
spectra associated with the bottles could be compared to reference spectra 
associated with the particular contaminants. Similarly, if the spectra 
associated with a bottle containing a particular contaminant were close to 
the spectra of an uncontaminated bottle, it would be useful to accept the 
bottle only when its spectra are sufficiently similar to the spectra of 
the uncontaminated bottle and sufficiently different from the spectra of 
the contaminated bottle. 
Lamp 24 may be a broadband source that produces radiant energy in a 
wavelength range from about 250 nanometers to about 2000 nanometers, which 
corresponds to visible light and infrared energy. Linear detection array 
32 may produce spectra for the wavelength range from 320 to 1200 
nanometers, which corresponds primarily to visible light. Within this 
range, for correlation purposes, the range from about 485 to about 600 
nanometers has been found to be most useful for absorption spectra, and 
the range from about 350 to about 750 nanometers has been found to be most 
useful for reflection spectra. 
In another embodiment, production of spectra for infrared wavelengths is 
emphasized, for example, to detect the presence of sugars in bottles to be 
filled with water (sugar absorbs radiation of wavelengths between 1300 and 
1600 nanometers). In this embodiment, the spectrometer is modified by 
replacing the diffraction grating with one that operates in the desired 
wavelength range. Because infrared spectra can be used to identify almost 
all organic compounds, a system emphasizing the infrared spectra could be 
used as a general chemical detector. 
The invention may be in the form of other embodiments. For example, though 
conveyor 14 is illustrated as a straight conveyor, system 10 could be 
applied effectively to a system in which bottles 12 are held in the 
periphery of a rotating wheel as they pass by illuminator 16 and detector 
18. In this case, though bottles 12 would travel in an arc as they moved 
from the first position to the second position, their spectra would still 
be determined as illustrated in FIGS. 3 and 4. 
In another variation, the contaminant detection system may generate and 
utilize only a single spectrum, such as an absorption spectrum or a 
reflection spectrum, for each bottle. Limited tests with a system 
utilizing an absorption spectrum alone or a reflection spectrum alone have 
generally shown lower overall accuracy in detection of contaminants, and 
have tended to produce more false positives, than a system generating and 
using both absorption and reflection spectra. However, a single spectrum 
may be adequate in certain applications. For example, an absorption 
spectrum may be sufficient for detection of contaminants in 
liquid/contaminant mixtures of high transmissivity. 
Also, instead of varying the position of the bottle 12 relative to 
illuminator 16 and detector 18 to obtain different spectral 
characteristics, two or more sets of illuminators and detectors, having 
similar or different characteristics, and being operable simultaneously or 
sequentially, could be employed. For example, a first illuminator and 
detector pair could be configured and oriented to obtain a visible 
absorption spectrum while a second illuminator and detector pair is 
employed to obtain an infrared reflection spectrum.