Apparatus and method of analyzing particles contained in a fluent medium. An optical system receives light from a laser, and focuses the light at a focal spot in the fluent medium. Optical pulses resulting from the backscattering of light by particles in the focal spot are detected and used to produce an electrical transit time signal comprising a series of electrical pulses. The length of each electrical pulse corresponds to the time required for a particle to pass through the focal spot. The apparatus also includes a laser Doppler system that receives light from the focal spot region, and produces an electrical velocity signal corresponding to the velocity of particles within such region. A processor receives the transit time and velocity signals, and combines the signals to produce data representing the sizes of particles passing through the focal spot.

Apparatus and method for particle analysis

Wind turbines are known to produce shadow flicker by interruption of sunlight by the turbine blades. Known parameters of the seizure provoking effect of flicker, i.e., contrast, frequency, mark-space ratio, retinal area stimulated and percentage of visual cortex involved were applied to wind turbine features. The proportion of patients affected by viewing wind turbines expressed as distance in multiples of the hub height of the turbine showed that seizure risk does not decrease significantly until the distance exceeds 100 times the hub height. Since risk does not diminish with viewing distance, flash frequency is therefore the critical factor and should be kept to a maximum of three per second, i.e., sixty revolutions per minute for a three-bladed turbine. On wind farms the shadows cast by one turbine on another should not be viewable by the public if the cumulative flash rate exceeds three per second. Turbine blades should not be reflective.

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Wind turbines, flicker, and photosensitive epilepsy: characterizing the flashing that may precipitate seizures and optimizing guidelines to prevent them.

A description is given of the design and construction of two instruments for studying atmospheric optics One device, the Recording Polar Nephelometer, measures the volume scattering index of light passing through a sample of natural atmosphere, with scattering angle, polarization, and wavelength as variables A new calibration procedure has been developed which employs a diffusing screen of known reflectance and transmittance as the standard The second device, the Portable Transmissometer, measures the extinction coefficient with an accuracy of 5% under all conditions These instruments are transported by a specially-equipped station wagon to form a mobile research unit Samples of results obtained in fog and clear air are included

Two Instruments for Atmospheric Optics Measurements

A technique is described for computing the aureole radiance field about a point source in a medium that absorbs and scatters according to an arbitrary phase function. When applied to an isotropic source in a homogenous medium, the method uses a double-integral transform which is evaluated recursively to obtain the aureole radiances contributed by successive scattering orders, as in the Neumann solution of the radiative transfer equation. The normalized total radiance field distribution and the variation of flux with field of view and range are given for three wavelengths in the uv and one in the visible, for a sea-level model atmosphere assumed to scatter according to a composite of the Rayleigh and modified Henyey-Greenstein phase functions. These results have application to the detection and measurement of uncollimated uv and visible sources at short ranges in the lower atmosphere.

Aureole radiance field about a source in a scattering-absorbing medium.

A system for detecting a reflected light beam includes a source of coherent light such as a laser beam, an optical fiber spaced away from the source for receiving an incident beam of coherent light from the source at one end of the fiber, means for reflecting a portion of the incident beam back through at least a portion of the fiber to radiate a reflected beam which includes narrow angle signals having a beam width corresponding to that of the incident beam and wide angle signals having a beam width substantially wider than that of the incident beam, receiving means between the source and the optical fiber for receiving the reflected beam and having an optical aperture therein for transmitting the incident beam and the narrow angle signals in the reflected beam, and a projector lens between the receiving means and the optical fiber for directing the incident beam into the optical fiber and for directing the narrow angle signals in the reflected beam into the optical aperture of the receiving means. This system may be used as a velocimeter for determining the velocity of moving particles adjacent the opposite end of the optical fiber from the end that receives the incident light beam. The receiving means typically comprises a mirror for reflecting the wide angle portion of the reflected beam into a photoelectric transducer, or the receiving means may itself be a photoelectric transducer.

System for detecting reflected laser beams

When transmittance of the atmosphere is determined by comparing the irradiancy of a distant light source to the calculated value for the irradiancy in vacuum, the result obtained is dependent on the field of view of the receiver. Measurements of this dependence over water have been made at sea level. For an uncollimated source, the results for any wavelength in the visible and near ultraviolet regions of the spectrum are represented approximately by the relation Tθ=T+0.5(1−T)(1−e−θ),where Tθ is the transmittance at a particular wavelength measured with an instrument having a field of view θ radians in diameter, and T is the transmittance which would be obtained if unscattered light alone were accepted by the measuring equipment.Observations have been made in several wavelength regions between 3600A and 6235A for atmospheres in which the transmission for visible light was between 0.35 and 0.85 per sea mile. Two-mile and nine-mile optical paths were used. Observations of Tθ through the nine-mile path were made with a fixed field of view 13.5 degrees in diameter, and those through the two-mile path were made with several values of θ between 5° and 25°.

The Influence of Field of View on Measurements of Atmospheric Transmission

An instrument for responding to backscattered or backreflected radiation from a path through a medium in which a beam of electromagnetic radiation is transmitted along the path, and a receiver system is arranged to receive backscattered radiation returning along the path substantially coaxially with the beam, the receiver system including detection means for the backscattered radiation and equalizing means for producing a substantially equal response regardless of the point of backscatter along the path. The equalizing means comprises a passive radiation transmission structure interposed in the path of return prior to the detection means, the passive structure having a substantially constant acceptance solid-angle for the returning radiation from points over the range of the path whereby radiation reaching the detection means from various points along the path does not vary in intensity due to the different distances of the points from the detection means. The passive structure is shown in the form of a full circular hole and a doughnut-shaped hole in an opaque plate, with the return beam focused to fill the hole out to a range of interest. Other passive structures are illustrated by a Fabry-Perot filter combined with a telescope and a bundle of opaque walled tubes. Transmissivity of a medium out to a point r is determined by the ratio of the integral of power from r to an outer limit range L in which the passive structure is still functional, as numerator, over the integral from the instrument to limit L as denominator. The coaxial array is shown achieved by use of a beam splitter, the beam from the source proceeding via the beam-splitter outwardly, the beamsplitter sized so that all return signal reaching the detector is treated equally by the beam-splitter. A multiple beam-splitter is shown by which multiple beams from a single source are produced, with the respective returns concentrated on a single detector.

Instrument responsive to back-scattered or back-reflected radiation having passive system for range correction

An experimental investigation was made to determine the general characteristics of the spectral transmission of the atmosphere in the vicinity of Washington, D. C, on the Chesapeake Bay, in the Gulf of Mexico, and in the Central Pacific. Transmission measurements were made at the wavelengths of approximately 15 of the Hg discharge lines in the interval 2500A to 6000A. Values of the spectral atmospheric attenuation coefficients (km−1) have been computed. For all observations made, these values increase toward shorter wavelengths and this is most pronounced for wavelengths below 3200A, where true absorption as well as scattering contributes to the attenuation.

An experimental study of atmospheric transmission

A new method for flame temperature measurement has been developed involving measurement of radiation, emitted by flames, in a narrow wavelength interval within the ultraviolet OH emission band. The mean value of flame emissivity within the wavelength interval is evaluated by measurement of mean flame transmission for a continuous spectrum source. The method requires that the wavelength interval studied be so narrow that the emission of the source used for transmission measurement and that of a blackbody at flame temperature both be approximately constant over the interval.The method has been tested on steady flames over temperature ranges from 2000°K to 3000°K and excellent agreement obtained with sodium line reversal measurements. Temperatures of pulsed flames have been determined from photographic traces obtained with a monochromator-photo-multiplier-oscilloscope-camera combination. Two high speed light choppers were used, one to interrupt the radiations entering the monochromator and the other to interrupt, at a different frequency, the radiations from a hydrogen discharge lamp transmitted by the flame to the monochromator. The time resolution was 0.10 millisecond and the accuracy in temperature measurement about 40°K in the range 2000°K to 3000°K. The method was calibrated by means of sodium line reversal although it is easily adaptable to absolute calibration.

Harold S. Stewart

Papers

The Influence of Field of View on Measurements of Atmospheric Transmission

Instrument responsive to back-scattered or back-reflected radiation having passive system for range correction

An experimental study of atmospheric transmission

A Method for the Determination of Flame Temperature from Emission in the Ultraviolet OH Band

Fixed-angle three-wavelength nephelometer for balloon use (A)