Abstract: A system and method for analyzing smoke or other emissions are provided. An image is analyzed and processed to identify characteristics associated with the emission, such as color, densities, dispersion rates, fuel mixture characteristics, and other suitable analysis factors. If the analysis indicates that abnormal conditions exist or that any user-defined alerts are warranted, a message is sent to an operator terminal. The system and method may continue to capture subsequent images and thus provide real-time data. The data may be stored in memory and collected over time. The data may be associated with a digital signature and used to create reports for company quality control boards, regulatory control agencies, and the public. The system and method thus provide a cost effective, reliable, and repeatable mechanism for real-time analysis of smoke stacks and other environmental changes.

Abstract: A method is provided for making and using measurements in gas filter correlation radiometry. A Gas Filter Correlation Radiometer (GFCR) instrument is moved in a region of space surrounding a heavenly body. An atmosphere of the heavenly body is viewed with the GFCR instrument along a first view direction with the atmosphere and the GFCR instrument experiencing a relative velocity of approximately zero. The atmosphere is also viewed with the GFCR instrument along at least one second view direction that is angularly separated from the first view direction such that atmospheric spectra associated with the second view direction appears Doppler shifted with respect to atmospheric spectra associated with the first view direction. A gas filter correlation radiometry application is performed using the measurement signals obtained from the different view directions.

Abstract: A soot number determining device has an intake line for taking in a soot-containing gas through a filter paper that has been introduced into the intake line. A transport apparatus transports a soot spot, which is produced at an intake position by virtue of the gas being taken in on the filter paper, by transporting the filter paper further to an evaluation position. A device is used to determine the extent of blackening of the filter paper.

Abstract: A diesel particulate matter (DPM) monitor that can be worn, attached to a particular location or piece of equipment, and collects real-time data about DPM levels. In an exemplary embodiment, ambient air is drawn by a flow-controlled diaphragm pump through a particle size selector which only passes submicron particles. These particles are collected on an optically transparent filter. A laser illuminates the filter and the transmittance of the laser beam is measured in real-time by a photo detector. As DPM accumulates on the filter, the laser transmittance decreases. The instrument is calibrated to convert this decrease in transmittance to a real-time concentration of elemental carbon in the air.

Abstract: A projected beam smoke detector includes circuitry and control software to measure a distance a beam travels between the detector's transmitter and receiver. Either a time-based or a phase-based measurement methodology could be used. A sensitivity parameter of the detector could be set in response to the results of the measurement.

Abstract: A focused droplet nebulizer of the invention produces substantially uniform droplets of a predetermined size. Droplets are pushed out through a small outlet orifice by the contraction of a chamber. The droplets can be carried on a substantially non-divergent path in a drift tube. A piezo membrane micro pump acts in response to an electrical control signal to force a droplet out of the outlet orifice. The nebulizer can operate at frequencies permitting a stream of individual droplets of the predetermined size to be sent along the substantially non-divergent path in the drift tube in a preferred embodiment ELSD device.

Abstract: A light scattering type smoke sensor includes a sensor body, light-emitter for emitting light toward an open smoke-sensing space and outputting a light-received signal according to the amount of scattering light received, and a fire judging unit for judging whether fire occurs or not on the basis of the amount of received light determined on the basis of the outputted light-received signal.

Abstract: Described herein is a method and system for providing a countermeasure against laser detection systems using nanocomponent material that is tailored to cloak or obscure a target from detection by transmitted laser radiation. The nanodot material absorbs and/or down-converts the transmitted laser radiation. Similarly, described herein is a method and system for providing a countermeasure against laser systems intended to blind a target through the use of a specifically engineered nanocomponent material for absorbing and/or down-converting the radiation from the laser system.

Abstract: Detecting video phenomena, such as fire in an aircraft cargo bay, includes receiving a plurality of video images from a plurality of sources, compensating the images to provide enhanced images, extracting features from the enhanced images, and combining the features from the plurality of sources to detect the video phenomena. Extracting features may include determining an energy indicator for each of a subset of the plurality of frames. Detecting video phenomena may also include comparing energy indicators for each of the subset of the plurality of frames to a reference frame. The reference frame corresponds to a video frame taken when no fire is present, video frame immediately preceding each of the subset of the plurality of frames, or a video frame immediately preceding a frame that is immediately preceding each of the subset of the plurality of frames. Image-based and non-image based techniques are described herein in connection with fire detection and/or verification and other applications.

Abstract: The invention relates to a photoacoustic detector for measuring the concentration of fine dust particles, especially carbon particles in gas, comprising a pulsed light source for providing excitation light, wherein a size distribution of the fine dust particles can be determined by changing the pulse length and/or the pulse repetition rate.

Abstract: A method of obtaining a fugitive emission flux measurement of airborne matter is provided. The method involves measuring the airborne matter along one or more than one measurement plane that spans the fugitive emission using two or more than two measurement beam paths where each of the two or more than two measurement beam paths are parallel to each other, or substantially parallel to each other, and determining a mass per unit length measurement for the measurement plane, determining a representative wind velocity at or near the one or more than one measurement plane, and calculating the fugitive emission flux of the airborne matter in mass per unit time using the mass per unit length determination and representative wind velocity.

Abstract: A photoelectric smoke detector is provided which is suitable for being downsized without detection capability deterioration. The present invention relates to a photoelectric smoke detector for detecting presence of smoke by utilizing a light emitting element and a light receiving element whose optical axes intersect with each other.

Abstract: An apparatus for monitoring the exhaust of an engine includes a flow-through chamber for receiving exhaust, a source of electromagnetic radiation and a detector. The source provides electromagnetic radiation in a range comprising the infrared, visible and ultraviolet wavelengths. The source and a detector are arranged so that radiation passing through the chamber is incident on the detector. An electronic circuit is connected to the detector to provide a signal indicative of the attenuation of the radiation by particles in the exhaust in the chamber. The detector provides respective measures of radiation which it receives for at least two different wavelengths of the radiation, and the electronic circuit provides corresponding electrical signals indicating the attenuation of the two different wavelengths by particles within the exhaust in the chamber. The wavelengths are selected to be those for which the attenuation caused by nitrogen dioxide in the exhaust is substantially the same.

Abstract: An apparatus is provided for detecting presence of transient particulate in gas within a duct (1) comprises (i) at least one emitter (3) of illumination (4) capable of being swept over essentially the entire cross-section of the duct from outside and the duct, and (ii) at least one detector (6) for detecting presence and position of any sparkle of the illumination from any particulate within the illumination as the beam is being swept over the cross-section of the duct, wherein the detector is configured to be mounted externally of a duct as the beam is being swept over the cross-section of the duct and outside the zone projecting the swept area of the duct.

Abstract: An absorption spectroscopy gauge to measure chemical concentrations in a post-detonation combustion cloud of energetic materials. A broadband light source coupled to an optical fiber guides light into a gauge via a first leg where a plano-convex lens collimates the light source internally. The light reflects off a mirror and passes through an absorption region before entering a second leg of the gauge where it is refocused into a different fiber and sent to a time-resolved spectroscopy system for analysis. The time-resolved spectroscopy system can include a spectrometer and a steak camera. The two legs of the gauge are arranged as separate halves connected by a plurality of rods that can be adjusted to change the length of the absorption region. The gauge is arranged to include stainless steel cone shaped tips to minimize added turbulence brought upon by its use.

Abstract: In a smoke sensor, a first light-receiving element which receives light from a light-emitting element and is disposed in a position where the quantity of received light changes according to a density of smoke and a second light-receiving element which monitors the quantity of light of the light-emitting element are arranged symmetrically with respect to the light-emitting element. Furthermore, a signal from the first light-receiving element and a signal from the second light-receiving element are amplified by an identical amplifier circuit. In a microcomputer, the density of smoke is computed based on a difference between an output obtained by amplifying an output of the first light-receiving element with the amplifier circuit and an output obtained by amplifying an output of the second light-receiving element with the amplifier circuit.

Abstract: The present invention relates to a system for detection of forest fires, based on the chemical analysis of the atmosphere through optic spectroscopy. The smoke originated from a fire has a chemical composition different from that of a normal atmosphere. This chemical composition is determined by the analysis of light absorption, which passes through the smoke, in its different wavelengths, carried out by a spectrometer. In this case, the spectrometer is associated to a telescope and solar light is used as the light source allowing the detection of smoke originated from a fire in a specific area of the horizon. The maximum distance from which the smoke can be detected depends only on the potency of the telescope and may be of many kilometers. The installation of the system on a rotating support and the use of computational logarithms makes the detection in any point of the horizon possible, a completely autonomous way.

Abstract: Provided is an electric device which is free from causing troubles in opening/closing the cover even if the housing is distorted.

Abstract: An exhaust gas analyzer and an exhaust gas analyzing method capable of analyzing the concentration of particulate matter contained in exhaust gas from an internal combustion in real time are provided.

Abstract: A gas analyzer system includes an optical source, an optical filter assembly, a controller, and an analyzer. The optical source generates an optical signal. The optical filter assembly includes different optical filters in which to filter the optical signal. During operation, the controller selects sequential application of each of the different optical filters in a path of the optical signal to modulate the optical signal using different frequency bands of optical energy. The modulated optical signal passes through an unknown sample. The optical analyzer analyzes the modulated optical signal after passing through the sample to detect which types of multiple different gases are present in the sample.

Abstract: An exhaust gas analysis method comprises applying a laser beam to the exhaust gas discharged from an internal combustion engine, receiving the laser beam that has passed through the exhaust gas, and measuring the concentration of any of the components contained in the exhaust gas based on the received laser beam. The exhaust gas analysis method further comprises detecting the absorption spectrum of the laser beam absorbed in the exhaust gas by using the received laser beam (S1), calculating the concentration of the specific gas component by using the absorption spectrum (S2), calculating the temperature of the exhaust gas by using the absorption spectrum (S3), calculating the pressure of the exhaust gas by using the absorption spectrum (S4 to S6), correcting the calculated concentration of the component contained in the exhaust gas by using the calculated temperature (S7), correcting the concentration by using the calculated pressure (S8), and outputting the true concentration value (S9).

Abstract: A projected beam smoke detector includes circuitry and control software to measure a distance a beam travels between the detector's transmitter and receiver. Either a time-based or a phase-based measurement methodology could be used. A sensitivity parameter of the detector could be set in response to the results of the measurement.

Abstract: A spray data acquisition system includes a spray pump actuator that is capable of providing a pumping force to a pumping device to generate an aerosol spray plume along a spray axis. The system also includes an illumination device that illuminates the aerosol spray plume along at least one first geometric plane that intersects the aerosol spray plume. The system further includes an imaging device that acquires data representative of an interaction between the illumination and the aerosol spray plume along at least one geometric plane.

Abstract: An apparatus and method allowing an optimized illumination in a light profile microscope by excitation of a sample with an elliptically collimated beam. The beam, which is typically supplied by a laser is collimated with unequal beam waist radii (and Rayleigh ranges) along major and minor axes orthogonal to a propagation direction, and approximates a plane sheet of illumination. The plane sheet of illumination is aligned with a thinnest width dimension thereof along the optic axis of the microscope objective, and with a center thereof at the object plane of the objective. Excitation light in a test sample is thereby confined to within a narrow thickness of the object plane of the objective lens, which minimizes out-of focus light in the image. The major axis width of the plane illumination sheet is typically a factor of ten or more greater than the minimum width, allowing a large area of the test sample to be illuminated and imaged.

Abstract: In a smoke sensor, a first light-receiving element which receives light from a light-emitting element and is disposed in a position where the quantity of received light changes according to a density of smoke and a second light-receiving element which monitors the quantity of light of the light-emitting element are arranged symmetrically with respect to the light-emitting element. Furthermore, a signal from the first light-receiving element and a signal from the second light-receiving element are amplified by an identical amplifier circuit. In a microcomputer, the density of smoke is computed based on a difference between an output obtained by amplifying an output of the first light-receiving element with the amplifier circuit and an output obtained by amplifying an output of the second light-receiving element with the amplifier circuit.

Abstract: Photography is employed to objectively quantify opacity of fluids such as smoke plumes and dust via a method termed the Digital Optical Method (DOM™). The DOM™ quantifies the ratio of radiance values by means of a camera response curve obtained using objective measures. The radiance ratios are then used to calculate opacity of target fluids such as smoke plumes. The DOM™ quantifies opacity during both daytime and nighttime conditions with a much broader range of subject types, e.g., white, gray and black smoke plumes, and environmental conditions, e.g., non-blue-sky, building, and mountain backgrounds, than existing systems while not requiring human interpretation for any application. In one embodiment, the DOM™ quantifies opacity from digital photos using a pre-designed algorithm and an inexpensive digital camera. Very little training is needed to implement the DOM™ and it yields consistent objective quantitative results, while providing a permanent photographic record easily digitally archived.

Abstract: A smoke detector (1) has a spherical chamber (2) including a plurality of holes (15,17) for allowing smoke and other particles to enter the chamber. The majority of the internal surface (3) of the chamber (2) is covered with a high reflectivity Lambertian surface, that is a material that scatters incident light equally in all directions and at all wavelengths. The remaining portion of the internal surface (3) is coated with a light absorbing material (13) such as a matt black coating. A scatter sensor (9) is directed towards the absorbing coating (3), and an integrating detector (5) is configured to detect radiation directly from the entire Lambertian surface. A first LED (19) emits blue light into the chamber (2), and a second LED (21) emits infrared light into the chamber. Processing means (23, 25 and 27) are provided to analyse the signals from the detectors (5,9), including means for discriminating between signals from the sensors indicative of different frequencies of received radiation.

Abstract: A method of detecting clouds in a digital image comprising, for an area of the digital image, determining a reflectance value in at least three discrete electromagnetic spectrum bands, computing a first ratio of one reflectance value minus another reflectance value and the same two values added together, computing a second ratio of one reflectance value and another reflectance value, choosing one of the reflectance values, and concluding that an opaque cloud exists in the area if the results of each of the two computing steps and the choosing step fall within three corresponding predetermined ranges.

Abstract: The invention relates to a smoke detector designed to be placed in a room subjected to ambient light, the detector comprising: emitter means suitable for emitting a light beam; at least one reflector suitable for reflecting said light beam towards receiver means; receiver means suitable for receiving the reflected light beam; and a processor circuit suitable for receiving a signal that is characteristic of the emitted light beam and a signal that is characteristic of the received light beam, the processor circuit being suitable for generating a smoke detection signal as a function of said received signals. The detector further comprises at least one mask disposed adjacent to the reflector, the mask being suitable for selecting those light beams that, at the inlet of said mask, have emission directions lying in a predetermined angular range.

Abstract: Method and assembly for determining the presence of soot particles in a gas stream such as in the air. To prevent the disturbing effect of salts and the like that may be deposited on soot particles, it is proposed to introduce the soot particles into water as a suspension. The salts concerned dissolve in the water and have no effect when determining the change in optical properties resulting from the presence of the soot particles. This determination comprises in particular a light transmittance measurement. The presence of the soot particles can be determined after the gas stream is first passed through a filter, as a result of which a particle size distribution can be determined. Suspension of the soot particles can be achieved by subjecting these to a stream of steam, whereby the steam condenses on the soot particles and then causes these to precipitate out of the gas. The various processes can be expedited by using a cyclone.

Abstract: A method for etalon suppression in a gas detection device by determining an etalon fringe period during a calibration step without gas in dependency of the DC drive current. A measuring signal which is a function of the gas absorption and substantially independent of an intensity modulation of an initial light signal at an initial frequency (f) is generated by determining a first pre-measuring signal when the laser source is operated at the center of the gas absorption peak, a second pre-measuring signal when the laser source is operated with a DC drive current below the gas absorption peak of the gas to be detected, and a third pre-measuring signal when the laser source is operated with a DC drive current above said gas absorption peak, with a difference between said DC drive currents which corresponds to the etalon fringe period determined in a calibration step before.

Abstract: A system and method for correcting for “wavelength drift” in an RES system. One advantage provided by the invention includes the ability to correct for offset amounts of “wavelength drift” that are not divisible by an integer number of pixels (e.g., fraction pixel offset amounts) in an optical detector used in the RES system. Correction of fractional pixel offset amounts may enhance spectral matching, enable substantially continuous calibration of an RES system, and/or other benefits.

Abstract: An improved elemental mercury analyzer utilizes a fluorescence assembly in combination with a fluorescence quenching reduction mechanism to detect the concentration of elemental mercury within an emission gas sample, via fluorescence of the mercury within the gas sample, while minimizing fluorescence quenching of the gas sample. In one arrangement, the analyzer contains the emission gas sample under a vacuum or negative pressure while detecting fluorescence of the elemental mercury within the emission gas sample. By performing fluorescence detection of the emission gas sample at reduced pressure relative to the pressure of the as-sampled emission gas, the analyzer reduces the number of particle collisions within the emission gas sample over a certain period of time. Such collisional deactivation, and/or the addition of oxygen depleted gas such as nitrogen to the gas sample, reduces fluorescence quenching of the emission gas sample, improving accuracy of detection of mercury.

Abstract: An advanced gas purged optical densitometer that significantly decreases the likelihood of optical component contamination and unstable optical path reduction includes an optical emitter, an optical receiver, and an optical path purge device. The optical emitter emits a light beam along an optical axis, through the optical purge path device and a test fluid. The optical receiver receives the light beam after it traverses the optical path purge device and the test fluid, and supplies a signal representative of intensity of the received light beam. The optical path purge device is configured to prevent contamination of the optical emitter and optical receiver, and to maintain a substantially optical path length of the test fluid.

Abstract: Embodiments of photoacoustic devices, systems, and methods are disclosed. One device embodiment, among others, comprises a flow-through configuration for a test sample gas and a modular set of two or more solid-state or gaseous based lasers.

Abstract: A passive sensor housing has an air gap formed therein between a collimating lens and a lens focused mirror reflecting optical light signals transmitted through the lens into the air gap within which smoke is received through screened openings in the housing. The optical light signals which are generated within an opto-electronics unit are returned thereto by reflection through the air gap into a fiber cable connected to the collimating lens of the sensor and to an optical coupler within the opto-electronic unit to which generated light signals are delivered from a light emitting diode and returned from the sensor for delivery to a receiver within which such signals are processed into an output signal reflecting smoke density within the air gap.

Abstract: A device for measuring aerosol size distribution within a sample containing aerosol particles. The device generally includes a spectrometer housing defining an interior chamber and a camera for recording aerosol size streams exiting the chamber. The housing includes an inlet for introducing a flow medium into the chamber in a flow direction, an aerosol injection port adjacent the inlet for introducing a charged aerosol sample into the chamber, a separation section for applying an electric field to the aerosol sample across the flow direction and an outlet opposite the inlet. In the separation section, the aerosol sample becomes entrained in the flow medium and the aerosol particles within the aerosol sample are separated by size into a plurality of aerosol flow streams under the influence of the electric field.

Abstract: A gas sensor arrangement comprises a first half-shell mateable with a second half-shell. The first half-shell has a detector receiving opening and radiation source receiving apertures. A gas measuring chamber is formed between the first half-shell and the second half-shell. The gas measuring chamber extends between the detector receiving opening and the radiation source receiving apertures. Concave mirrors are arranged around the radiation source receiving apertures. The concave mirrors are formed by inner surfaces of the first and second half-shells. Tubes extend from the concave mirrors to the detector receiving opening. The tubes are formed by the inner surfaces of the first and second half-shells. The inner surfaces are coated with a reflective material.

Abstract: A gaseous target analyte present as a minor constituent in an admixture with at least one other gaseous species can be detected using a cavity enhanced optical spectrometer by a process comprising the steps of: i) identifying a plurality of strong spectral absorption peaks of the target analyte which are present within the scanning range of the spectrometer, ii) determining for the identified peaks the pressure region above which the peak width increases substantially with increasing pressure and below which the peak width is substantially independent of pressure, iii) determining which of the peaks identified in step i) are, within the pressure region determined in step ii), free from spectral interference by any of the other components of the admixture. iv) measuring the spectrum of the admixture at the pressure region identified in step ii).

Abstract: This invention relates to a method and apparatus for improved process control in combustion applications, and particularly those relating to the steelmaking industry. An apparatus is provided for process control in a combustion application comprising a laser to transmit a near-infrared laser beam through off-gas produced by the combustion application, a detector to detect the transmitted laser beam and convert the detected laser beam to an electrical signal, and a control system for providing adjustment of select inputs to the combustion application in response to the electrical signal from the detector. The method of this invention comprises transmitting a near-infrared laser beam through off-gas produced by the combustion application, detecting the transmitted laser beam, and adjusting select inputs of the combustion application in response to the detected transmitted laser beam.

Abstract: A method for the detection and signaling of smoke by means of an optical assembly in a detector casing, the optical assembly including at least one optical transmitter element and at least one optical receiver element, and which emits a reception signal which is representative of the incident amount of light. An electronic evaluation device compares the reception signal to a setpoint and an alarm signal is generated when the reception signal reaches the predetermined threshold value. The temperature is measured on or in the detector casing and the temporal characteristic of the temperature is correlated with the temporal characteristic of the reception signal of the optical receiver element and a dew film signal is generated when the rise in the reception signal is correlated to a rise in temperature.

Abstract: Excess oxygen in the combustion process of a facility that bums carbon-based fuels may cause a visible plume in the atmosphere at the stack of the facility. Traditional optical based opacity monitors may be unable to detect this plume or the plume may form at a location downstream from the opacity monitor toward the stack. The present invention discloses methods to utilize common combustion control variables to detect and signal the presence of a visible plume of exhaust gasses. Also disclosed are systems that detect the visible plume and provide a signal so that the combustion process may be manually or automatically adjusted to reduce or eliminate the visible plume.

Abstract: A lightweight and portable analyzer is provided. At least one component of the analyzer is made from a lightweight material, such as ABS. A manifold can have a plate and gas passages ultrasonically welded together. By having at least one component made from a lightweight material, the analyzer is lighter.

Abstract: An infrared absorption measurement method includes: (a) measuring infrared absorption of a measurement target component in a state in which a sample gas including the measurement target component is decompressed; (b) calculating an absorption area in a peak region of the measurement target component in the infrared absorption shown in a graph which shows the relationship between the wave number and absorbance of the sample gas; and (c) calculating a concentration of the measurement target component in the sample gas based on the absorption area and pressure of the sample gas during decompression.

Abstract: A pathogen detector has a sample area for containing environmental air, a light source on one side of the sample area for directing a collimated beam of light through the sample air so that part of the light beam will be scattered by any particles present in the air while the remainder remains unscattered, and a beam blocking device on the opposite side of the sample area for blocking at least the unscattered portion of the beam of light and directing at least part of the scattered light onto a detector. The detector produces output pulses in which each pulse has a height proportional to particle size and a pulse height discriminator obtains the size distribution of airborne particles detected in the air sample at a given time from the detector output. An alarm signal is activated if the number of particles within a predetermined pathogen size range of around 1 to 7 ?m exceeds a predetermined normal level.

Abstract: An optical smoke detector for detecting particulates in an air sample. The detector includes a detection chamber defining an enclosure for the air sample. At least one opening is in fluid communication with the detection chamber whereby the air sample can be introduced and discharged from the detection chamber. A cleaning port defines a passageway to the detection chamber from a position external to the smoke detector and a resilient valve seals the cleaning port with a substantially air-tight seal. The valve member is accessible from a position external to the smoke detector. The valve member is openable, permitting passage of air through the cleaning port into the discharge chamber, by engaging the valve member with a nozzle, such as an air nozzle mounted on a canister of pressurized air.

Abstract: A filter structure of a vehicle air conditioner comprises a filter cartridge provided with a plurality of filter papers within a case and a light source. The light source is provided at a position within the filter papers for radiating light toward one side of the case. The light is blocked when pollutants deposited on the filter papers are enough to cause a contaminated state of the filer to reach a preset state. A display section is provided on the case for transmitting the light, radiated from the light source, to be visually checked on the external side. Therefore, the filter structure allows a driver to check a proper exchange time of the filter visually through a display section in which a display state changes according to a contaminated state of the filter.

Abstract: Methods for analyzing particle systems using polarized scattered light are provided. An exemplary method comprises the steps of: providing models of multiple arbitrary particle systems; performing ray-trace analysis with respect to the models over a range of scatter angles, the ray-trace analysis involving only use of second-order rays; receiving information corresponding to a particle system of interest; and predicting at least one characteristic of the particle system of interest using information generated during the ray-trace analysis. Systems and other methods are provided.

Abstract: Methods for analyzing particle systems of surface facets using polarized scattered light are provided. An exemplary method comprises the steps of: providing models of multiple arbitrary particle systems, the particle system comprising surface facets; performing ray-trace analysis with respect to the models over a range of back-scatter angles, the ray-trace analysis involving only use of second-order rays; receiving information corresponding to a particle system of interest; and predicting at least one characteristic of the particle system of interest using information generated during the ray-trace analysis. Systems and other methods are provided.

Abstract: A weather identification system is provided which employs a first photosensitive receiver positioned directly in the path of a beam of light a predetermined distance from a partially coherent light beam source. The first photosensitive receiver is in direct optical communication with the light beam source to produce electronic signals in response to scintillations caused by particle movement between the source and the first receiver. A second photosensitive receiver is positioned out of the path of the beam of light and is oriented at an oblique angle relative thereto to provide an output indicative of forward scattering of light from scintillations that occur in the beam of light. The signals from the second photosensitive receiver are AC coupled to provide an output indicative of rate of fluctuation and having a signal strength characteristic that even distinguishes between light drizzle and fog.