Patent Description:
More specifically, the present invention relates to an optical measurement system comprising the features of the preamble portion of claim <NUM>, a method of determining at least one characteristic of particles within a liquid (<NUM>) that is contained in a well comprising the features of the preamble portion of claim <NUM> and an optical measurement system comprising the features of the preamble portion of claim <NUM>.

Such optical measurement systems and methods are known from <CIT>, which, however, is directed to Phase Contrast Imaging based on refracted light.

An optical imaging apparatus is known from <CIT>.

Many applications in the field of analytical research and clinical testing utilize optical methods for analyzing liquid samples. Among those methods are absorbance, turbidity, fluorescence/luminescence, and optical scattering measurements. Optical laser scattering is one of the most sensitive methods, but its implementation can be very challenging, especially when analyzing biological samples in which suspended particles are relatively transparent in the medium. In this case, most of the scattering process occurs in the forward direction near the incident laser beam. To detect this low-angle, forward scattering signal, high extinction of the incident beam is required. But various optical effects (e.g., such as laser beam spatial purity, optical surface scattering, and beam distortions by the free liquid surface) often interfere with the extinction of the incident beam. For this reason, the forward scattering method is rarely applied in spite of its sensitivity.

In the case of fluorescence/luminescence detection, there is a spectral separation between the excitation light and the emitted light, which can help to facilitate the extinction of the excitation light by means of spectroscopic techniques, such as notches, bandpass optical filters, or monochromators. But in many application cases, the fluorescence signal is many orders of magnitude lower compared to the excitation light intensity and the excitation- light extinction by wavelength separation is not sufficient. For this reason, many systems collect the emitted light from a direction that is opposite of or normal to the excitation beam, such that the excitation light does not reach the detector. However, this can result in a rather complex optical layout, sometimes utilizing multiple detectors.

One particularly important application of optical measurements of liquid samples involves a microplate reader used for microbiologic assays. A microplate comprises of multiple open-top wells containing individual samples arranged in a two-dimensional array (e.g., <NUM> x <NUM>). To obtain useful information on the samples content, the microplate reader may utilize one or more types of optical measurements. Because of a two-dimensional arrangement, the optical access is typically available in only the top and bottom directions of the wells. The upper free surface of the liquid sample is normally curved due to the liquid's surface tension. This curvature combined with the relatively small diameter of the wells cause a significant incident beam divergence or distortion, making its extinction very difficult and inefficient before it reaches the detector. This is one reason that forward scatter signal measurement or a fluorescence signal measurement in the input-beam direction is not easily implemented in wells of a microplate.

Accordingly, there is a need for an improved optical measurement system that allows for the detection of the forward scatter signals and/or forward fluorescence signals in the input-beam direction so as to allow for a determination of the size, quantity, and/or concentration of particles (e.g., bacteria) in the liquid.

In order to solved this technical problem, the present invention provides an optical measurement system comprising the features of claim <NUM>, and a a method of determining at least one characteristic of particles within a liquid (<NUM>) that is comprising the features of claim <NUM>.

Advantageous embodiments are indicated in dependent claims.

The present invention involves a method for overcoming the aforementioned problems by generating an input beam with a controlled distortion, and correcting the distortion induced by a free-liquid surface. As such, the method includes a substantially collimated or focused beam at the detector, while separating the desired scattering signals and/or fluorescence signals by efficiently blocking the input beam from detection.

The present invention also involves a system equipped with a single detector array, for high sensitivity scattering measurements at several scattering angles. The detector may receive back scatter signals, low-angle forward scatter signals, fluorescence signals, and absorbance measurement signals, all using a single detector array.

In yet a further preferred embodiment, the present invention is an optical measurement system for measuring at least one characteristic of particles within a liquid that is contained in a well. The system comprises a light source, a tunable optical element, a detector, and an input-beam attenuator. The light source is for transmitting an input beam toward a free surface(s) of the liquid and in a manner that is generally parallel to a central axis of the well. The tunable optical element is located between the light source and the free surface(s) of the liquid. The tunable optical element is for altering the shape of the input beam to compensate for a distortion associated with the free surface(s) of the liquid. The detector is located below the well for receiving a forward scatter signal indicative of at least one characteristic of the particles within the liquid. The input-beam attenuator is for inhibiting a transmitted portion of the input beam from impinging upon the detector. The transmitted portion of the input beam is the portion of the input beam that has transmitted through the liquid.

Additional aspects of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments, which is made with reference to the drawings, a brief description of which is provided below.

While the invention is susceptible to various modifications and alternative forms, specific embodiments will be shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

The drawings will herein be described in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. For purposes of the present detailed description, the singular includes the plural and vice versa (unless specifically disclaimed); the words "and" and "or" shall be both conjunctive and disjunctive; the word "all" means "any and all"; the word "any" means "any and all"; and the word "including" means "including without limitation.

<FIG> schematically illustrates the various optical measurements that may be taken on a vial <NUM> containing a liquid <NUM> having suspended particles (e.g., bacteria). The incident input beam <NUM> (also called an "excitation" beam in the case of fluorescence / luminescence) passes through the liquid <NUM> and interacts with the suspended particles. As one example, the input beam <NUM> may be derived from an LED.

The various types of optical measurements in <FIG> are individually known in the prior art. A first detector <NUM> is located on the opposite side of the input beam <NUM>. The input beam <NUM> is partially absorbed within the liquid <NUM> such that an output beam <NUM> is received by the first detector <NUM> and is indicative of the absorbance of the particle-filled liquid <NUM>. Additionally, the receiver <NUM> can detect a scatter signal <NUM> resulting from the input beam <NUM> scattering from the various particles within the liquid <NUM>. Additionally, a second receiver <NUM> measures a high-angle side scatter signal <NUM> in a direction that is generally perpendicular to the input beam <NUM>. And a third receiver <NUM> measures the back scatter signal <NUM> caused by energy reflecting from the particles in a direction generally opposite to the input beam <NUM>. Finally, a portion of the input beam <NUM> is absorbed by particles, which is then re-emitted at a wavelength that is characteristic to the particle molecular structure. The process is known as fluorescence, which is a type of photo-luminescence. This fluorescence signal <NUM> is generally emitted in all directions, and can be picked up by all the detectors sensitive in the spectral range of the emitted radiation. It should be noted that <FIG> is for the purpose of illustrating the individual types of signals that are detectable when an input beam is passed through a liquid. While these types of signal detectors are available individually within the prior art, all of their combinations are not, however, necessarily a part of the state of the prior art.

In summary, there are several types of optical measurements available when the input beam <NUM> is transmitted into the liquid <NUM> containing particles. The first type of optical measurement detects the fluorescence signals <NUM>, which is generally emitted in all directions and can be emitted at multiple wavelengths. The second type of optical measurement detects absorbance, which is performed by evaluating the intensity of the output light beam <NUM> that has passed through the liquid <NUM>. The third type of optical measurement involves light that is elastically scattered by the particles in the liquid <NUM> & in various directions. As such, the input beam can be both absorbed and scattered by the suspended particles, both of which prevent the incident input beam from reaching the first detector <NUM>. The amount of absorbed and scattered light is related to the characteristics of the particles, such as particle concentration and can be determined through known special calibration techniques. When the particles are bacteria, they can be detected and counted by various techniques, which are generally described in <CIT> and <CIT>, both of which are commonly owned.

Regarding this third type of optical measurement involving scattering, there are three major types of scattering signals - back scatter signals <NUM>, forward scatter signals <NUM>, and side scatter signals <NUM>. Detection of side scatter signals <NUM> is also known as nephelometry, which evaluates a parameter (sometimes called "turbidity") that, in certain cases, can be also be linked directly by calibration to particle concentration.

<FIG> illustrates a common microplate <NUM> having a two-dimensional array of wells <NUM> extending downwardly from a top surface <NUM>. Each of the wells <NUM> typically has an elongated shape and contains a liquid sample for measurement. However, optical access for all of the wells <NUM> is possible only from the top and the bottom of the microplate <NUM>, while optical access from the side is very limited for the interior wells <NUM>.

<FIG> illustrates a typical layout for optical measurements detected from a liquid <NUM> within one of the wells <NUM> of the microplate <NUM>. The liquid <NUM> has a free surface <NUM> that is characterized by a meniscus due to the surface tension of the liquid <NUM>. A laser <NUM> provides an input beam <NUM> that enters the liquid <NUM> from above the top surface <NUM> of the microplate and is detected from the bottom. In particular, the input beam <NUM> results in an output beam <NUM>, a forward scatter signal <NUM>, and a fluorescence signal <NUM> that are transmitted towards a detector <NUM>.

If the fluorescence signal <NUM> is to be measured, an optical filter or a monochromator <NUM> is utilized to remove the incident output beam <NUM> before it reaches the detector <NUM>. But for either an optical filter or a monochromator <NUM>, the ability to discriminate the wavelength of the incident output beam <NUM> from the fluorescence signal <NUM> is limited by the filter's rejection ability or the monochromator's quality, which is typically on the order <NUM>-<NUM> to <NUM>-<NUM>, respectively. But in many cases, the fluorescence signal <NUM> is weak, or the particle concentration in the liquid <NUM> is low, thereby requiring extremely low fluorescence signals <NUM> to be measured. And for those cases, a higher rejection ratio should be implemented, such that physical blocking of the incident output beam <NUM> is required, which is problematic because it limits the ability to also detect the fluorescence signal <NUM> from the bottom. As will be described in more detail, the present invention resolves this problem with the fluorescence signal <NUM> because the incident beam is blocked, leaving the fluorescence signal <NUM> (perhaps filtered through a filter) as the primary signal received at the detector.

The liquid <NUM> contained in the microplate well <NUM> typically has a highly curved free surface <NUM> due to surface tension. This free surface <NUM> has an optical power (sometimes with a significant optical aberration), substantially distorting the input beam <NUM> and causing it to diverge as shown in <FIG>. Because of this effect on the input beam <NUM>, blocking the output beam <NUM> or detecting the front-scatter signal <NUM> at low angles is impossible. Accordingly, the only practical optical measurements that are left in place are detection of the absorbance, the back scatter signal, and backward-emitted fluorescence. If absorbance is measured, there is no need for the optical filter or a monochromator <NUM> in <FIG>. In summary, the divergence of the input beam <NUM> caused by the liquid's free surface <NUM> negates the ability to effectively shield or filter the detector <NUM> from the output beam <NUM>, thereby inhibiting the detection of the forward-emitted fluorescence <NUM> or the forward scatter signal <NUM>, leaving only absorbance to be detected from the bottom of the microplate <NUM>. The present invention, which is generally described in <FIG>, helps to resolve this problem by correcting the distortions caused by the free surface <NUM>.

<FIG> illustrates one embodiment of the present invention that permits the detection of the forward scatter signal as well as the forward emitted fluorescence. Specifically, the system includes a laser <NUM> as an optical source that produces an input beam <NUM> generally parallel to the central axis of the well <NUM>. In one preferred embodiment, the input beam <NUM> of the laser <NUM> is in a wavelength in the visible to near infrared (e.g., <NUM> to <NUM>) and has a power in the range from about <NUM> milliwatts to <NUM> milliwatts. The input beam <NUM> is shaped by a tunable output element <NUM> located above the top surface <NUM> of the microplate <NUM>. The tunable output element <NUM> creates a specifically shaped converging input beam <NUM> that enters the liquid <NUM> from the free surface <NUM>. Notably, the tunable output element <NUM> achieves a certain convergence that negates the distorting effect caused by the meniscus at the free surface <NUM>, thereby resulting in a substantially collimated beam <NUM> (or a focused beam) that travels through the liquid <NUM> within the well <NUM> of the microplate <NUM> toward a detector <NUM>. In one particular embodiment, the tunable output element <NUM> can be adjusted in the range from -<NUM> and +<NUM> dpt for a well <NUM> with a <NUM> diameter. The collimated beam <NUM> is acted upon by a beam dump <NUM> (or other type of attenuator) after leaving the well <NUM>, but before reaching the detector <NUM>. In this case, the collimated beam <NUM> can be physically blocked by the beam dump <NUM>, enabling measurement of the low-angle forward scatter signal <NUM> and/or the fluorescence signal <NUM> with a high extinction ratio relative to the collimated beam <NUM>. As such, the limited geometric area of the collimated beam <NUM> can be controlled, such that the detector <NUM> receives substantially only the forward scatter signal <NUM> and/or the fluorescence signal <NUM>.

When implementing an optical measurement system according to the schematic illustration of <FIG>, it may be useful to include additional optical components. For example, a pinhole aperture (e.g., <NUM> pinhole) can be placed adjacent to the laser <NUM> to limit the output beam <NUM> to a specific size and shape. Similarly, the pinhole aperture (e.g., <NUM> to <NUM> pinhole) can also be placed below the tunable output element <NUM> and above the upper surface <NUM> the microplate <NUM> to help limit the light energy passed into the free surface <NUM> of the liquid <NUM>.

<FIG> illustrate feedback loops for the tunable output element <NUM><NUM> illustrated in <FIG>. In <FIG>, the detector <NUM> acquires the signal from the collimated beam <NUM> after being acted upon (likely partially) by the beam dump <NUM>. The characteristics of the optical signal from the collimated beam <NUM> are received by a controller <NUM> that executes an image acquisition algorithm at step <NUM> to extract data corresponding to the position, the dimension, and the intensity of the collimated beam <NUM> that are received on the image plane of the detector <NUM>. These optical signals received by the detector <NUM> can be considered as stray light. Based on the known characteristics of the focused convergent beam <NUM> before entering the free surface <NUM> of the liquid <NUM>, and the data corresponding to the position, dimension, and intensity distribution on the detector <NUM>, a physical profile of the free surface <NUM> is calculated or approximated. In other words, the optical characteristics of the free surface <NUM> are now known, as if it were a lens. The controller <NUM> executes a stray-light minimization algorithm at step <NUM> that calculates the wavefront compensation to be introduced into the input beam <NUM> by the tunable optical element <NUM> to minimize the beam distortion caused by the free surface <NUM> of the liquid <NUM>, thereby achieving the collimated beam <NUM>. The controller <NUM> then adjusts the tunable optical element <NUM> at step <NUM>, thereby altering the optical settings of the tunable optical element <NUM>. The process may continue periodically, or iteratively, until the desired collimated beam <NUM> is produced, thereby creating a minimum amount of stray light on the detector <NUM>. When achieved, the system can operate at a more optimum state for receiving the forward scatter signal <NUM> and/or the fluorescence signal <NUM>, as shown in <FIG>.

In the feedback loop of <FIG>, the detector <NUM> is not used as part of the feedback loop. Rather, the free surface <NUM> of the liquid is measured directly or indirectly in other ways, such as a mechanical measurement, optical power measurement, or calculations based on knowing surface tension parameters. The determined profile of the free surface <NUM> is used by the controller <NUM> to determine the optimal settings for the tunable optical element <NUM>. Accordingly, the controller <NUM> receives data at step <NUM> to determine the surface profile of the free surface <NUM> (e.g., measures reflections from the free surface <NUM>). The controller <NUM> executes a stray-light minimization algorithm at step <NUM> that calculates the wavefront compensation to be introduced into the input beam <NUM> by the tunable optical element <NUM> to minimize the beam distortion caused by the free surface <NUM> of the liquid <NUM> and achieve the collimated beam <NUM>. The controller <NUM> then adjusts the tunable optical element <NUM> at step <NUM> to alter the optical settings of the tunable optical element <NUM>. The process continues periodically, or iteratively, until identifying the desired convergent beam <NUM> that exits the tunable optical element <NUM> will result in the desired collimated beam <NUM>.

<FIG> illustrate exemplary implementations of the tunable optical element <NUM>. In <FIG>, the tunable optical element 12a is a mechanically adjustable assembly comprising a first structure <NUM> attached to first optical element <NUM> (such as a lens) and a second structure <NUM> attached to a second optical element <NUM>. In response to control signals received from the controller <NUM>, an actuator <NUM> (such as a motor) imparts a force on the first structure <NUM> and/or the second structure <NUM> to adjust the distances between the first optical element <NUM> and the second optical element <NUM>. The adjustment of the distances between the first optical element <NUM> and the second optical element <NUM> affects the shape of the convergent beam <NUM> (<FIG>) that exits the tunable optical element 12a.

In <FIG>, the tunable optical element 12b includes a mounting structure <NUM> that holds a mechanically or electronically tunable lens <NUM>. The controller <NUM> adjusts the electrical output of a circuit <NUM> that directly affects the shape of the electronically tunable lens <NUM>. Alternatively, the controller <NUM> adjusts the output of the circuit <NUM> to alter the shape of the mechanical mounting structure <NUM>, thereby indirectly affecting the shape of the tunable lens <NUM>. The adjustment of the shape of the tunable lens <NUM> affects the shape of the convergent beam <NUM> (<FIG>) that exits the tunable optical element 12b.

In <FIG>, the tunable optical element 12c includes a spatial light modulator (SLM) <NUM>. The controller <NUM> adjusts the SLM <NUM> to provide a great amount of control of the beam <NUM> that transmits through the free surface <NUM>. The SLM <NUM> may compensate for virtually any surface distortion free surface <NUM>, but at a limited optical power.

The tunable optical element <NUM> may also be a combination of any of the above elements in <FIG> such as, for example, a tunable lens with an SLM. Mechanically controlled telescopes, deformable adaptive mirrors, or other arrays of tunable optical elements can also be utilized for the functionality required by the tunable optical element <NUM>.

<FIG> illustrates an alternative system that can receive multiple optical measurements when an input beam is transmitted along the primary axis of a vial or well. Further, the system receives the multiple optical measurements on a single detector <NUM>. The exemplary system in <FIG> is illustrative for implementation of various components used for multiple optical measurements, which can be combined in any other arrangements.

In <FIG>, a back scatter signal <NUM> is collected by an optical fiber <NUM> near the free surface <NUM> of the liquid <NUM>. The collection of the back scatter signal <NUM> may be facilitated by the addition of optical elements in front of the optical fiber <NUM> or by shaping the end facet of the optical fiber <NUM>. The back scatter output signal <NUM> from the end of the fiber <NUM> is located at a physical position on the detector <NUM> that is unused for the measurement of the forward scatter signal <NUM> or the fluorescence signal <NUM>.

In cases when optical access generally normal to the input beam direction is possible, a side scatter signal <NUM> and/or a side fluorescence signal <NUM> may be transmitted by a second optical fiber <NUM> to the detector <NUM>. The optical fiber <NUM> is located near the vial's side surface and may collect the side scatter signal <NUM> and/or the side fluorescence signal <NUM> with additional coupling optics (as shown in <FIG>), although additional coupling optics are not necessary. The output light <NUM> exiting the output end of the second optical fiber <NUM> is coupled to the detector <NUM> using a prism <NUM>. Because the detector <NUM> must be located near the bottom surface of the well, the prism <NUM> allows the second optical fiber <NUM> to enter the detector region from the horizontal direction, thereby minimizing the height requirement for mechanical access to the detector <NUM>. One or both sides of the prism <NUM> may contain a dielectric coating for rejection of the incident light wavelength, which can be useful in fluorescence measurements. The output signal(s) <NUM> from the side scatter signal <NUM> and the side fluorescence signal <NUM> can also be spatially separated via optics (like the prism <NUM>) before being transmitted to the detector <NUM>. The output signal(s) <NUM> from the side scatter signal <NUM> and/or a side fluorescence signal <NUM> are then sent to a second physical position(s) on the detector <NUM> not used for the forward scatter signal <NUM> or the fluorescence signal <NUM>, which are transmitted past the beam dump <NUM>. Accordingly, when side access is available to a vial, several types of optical measurements of the liquid <NUM> can be detected by the single detector <NUM> and the information can then be used to determine the types, amounts, and/or concentrations of the particles located within the liquid <NUM>.

<FIG> illustrates an exemplary layout of the detector <NUM> for multiple measurements from a liquid sample. The dashed circular line region <NUM> of the detector <NUM> represents a physical position of the blocked beam due to the beam dump <NUM>. The forward scatter signal <NUM> is detected in a region <NUM> of the detector <NUM> that is located concentrically around the dashed circular line region <NUM>. The various optical fibers (e.g., the optical fibers <NUM>, <NUM> in <FIG>) from the top and sides of the vial carry several complementary signals that are also detected by the same detector <NUM>. For example, the back scatter signal <NUM> from the first optical fiber <NUM> can be detected at the top middle region <NUM> of the detector <NUM>. The side scatter signal <NUM> can be measured at the left middle region <NUM>. A first fluorescence signal can be detected at the lower middle region 351a (e.g., within a first wavelength band or at a different angle), while a second fluorescence signal can be detected at the lower left region 351b (e.g., within a second wavelength band or at a different angle) of the detector <NUM>. An additional side scatter signal at a different angle can also be detected at the top left region <NUM> of the detector <NUM>. Of course, more or less output signals can be detected by the detector <NUM>. Furthermore, while <FIG> is provided for the purpose of illustrating multiple measurements on a single detector, only a single measurement (forward scatter signal <NUM> in <FIG>) can be measured, as would be the case for the detector <NUM> when no forward fluorescence is to be detected.

<FIG> illustrates a beam attenuation device <NUM> that can be used instead of the beam dump <NUM> (<FIG>, <FIG>, and <FIG>). In the beam attenuation device <NUM>, a central portion <NUM> of the incident beam that has traveled though the liquid <NUM> is attenuated, and the peripheral portion <NUM> is blocked, preferably by an opaque structure.

In <FIG>, the beam attenuation device <NUM> allows for an absorbance measurement in the right middle region <NUM> of the detector <NUM>. The peripheral region <NUM> on the detector <NUM>. around the right middle region <NUM> has substantially no signal due to the preferably opaque peripheral portion <NUM> of the beam attenuation device <NUM>. Because the forward scatter signal <NUM> (<FIG>) is particularly useful at low particle concentrations or with weak scattering signals, simultaneous absorbance and scattering measurements are challenging because a wide dynamic range of the detector <NUM> is required. According to this aspect of the present invention, a part of the input beam is partially transmitted to the central region <NUM> of the detector <NUM> via the beam attenuation device <NUM>, bringing it to intensities low enough that the same detector <NUM>$ can measure it simultaneously with the forward scatter signal <NUM>. Using this alternative arrangement of <FIG>, an absorbance measurement can also be carried out, enabling measurement of high particle concentrations. Furthermore, the present invention contemplates a system that uses both the beam dump <NUM> and the beam attenuation device <NUM> that are separately moved into and out of the path of the incident beam to allow the same detector <NUM> to provide the output detection patterns illustrated in both <FIG>.

Because it is advantageous to limit the number of back reflections that are detected by the detector <NUM>, <NUM>, the present invention also contemplates the use of an angled surface at the bottom of each well (as opposed to a horizontal surface) within the microplate, or the bottom surface of an individual vial. The angled surface helps to eliminate some of the back reflections that are incident upon the detector <NUM>, <NUM>. Furthermore, it is also advantageous to have the sidewalls of each of the wells be made of absorbing material (such as a black plastic) to absorb some of the retro-reflections. The bottom surface is preferably very thin, transmissive, and has a surface roughness below <NUM>.

It should be noted that the present invention has been described relative to a free surface of a liquid located within a liquid-containing well. However, the present invention is useful on one or more free surfaces, such as the free surface of a drop of a liquid sample that has a curved free surface or surfaces. Furthermore, while the sample-containing well has been described as being elongated, the well could also have a much more shallow shape. For example, the length of the well may have a dimension that is similar to its diameter.

Claim 1:
An optical measurement system for measuring at least one characteristic of particles within a liquid (<NUM>) that is contained in a well (<NUM>) having a bottom surface, said optical measurement system comprising;
a light source (<NUM>) for transmitting an input beam (<NUM>) toward a free surface (<NUM>) of the liquid (<NUM>) and generally parallel to a central axis of the well (<NUM>);
a tunable optical element (<NUM>) located between the light source (<NUM>) and the free surface (<NUM>) of the liquid (<NUM>);
a detector (<NUM>; <NUM>) located below the bottom surface of the well (<NUM>) for receiving a forward scatter signal indicative of at least one parameter of the particles within the liquid; and
an input-beam attenuator (<NUM>);
wherein
the tunable output element (<NUM>) is controllable to alter the shape of the input beam (<NUM>) to compensate for a distortion associated with the free surface (<NUM>) of the liquid (<NUM>) by creating a converging input beam (<NUM>) for entering the liquid (<NUM>) from the free surface (<NUM>),
characterized in that
the input-beam attenuator (<NUM>) is configured to inhibit the transmitted portion of the converging input beam (<NUM>) from impinging upon the detector (<NUM>; <NUM>),
the transmitted portion of the converging input beam being the portion of the input beam that has transmitted through the liquid (<NUM>), and
the geometric area of the transmitted portion of the converging input beam (<NUM>) is controlled such that the detector (<NUM>; <NUM>) receives substantially only a forward scatter signals (<NUM>) and/or a fluorescence signals (<NUM>) indicative of at least one characteristic of the particles within the liquid (<NUM>).