Visibility-range meter

O. A. Volkov, S. A. Denisenko, and K. V. Konstantinov

OAO LOMO, St. Petersburg

R. A. Kruglov

A. I. Voe�kov State Hydrometeorology Observatory, St. Petersburg�Submitted July 22, 2009�Opticheski� Zhurnal 76, 71–74 �October 2009�

This paper describes the construction principles of transmissometers and nephelometers and dis-cusses their advantages and disadvantages. A measurement of the visibility range of the NIF-1,which uses the advantages of transmissometric and nephelometric measurement principles, ispresented. The features of the construction of the optical and electric systems of the device arediscussed. Practical results are presented. © 2009 Optical Society of America.

INTRODUCTION

A number of meteorological phenomena �haze, fog, rain,snowfall, mist, dust storms, etc.� can significantly degradevisibility, and this presents danger to moving transport andhampers the activity of a number of branches of the nationaleconomy. Therefore, visibility observations are among thebasic meteorological observations on the network of hy-drometeorological stations of all the countries of the world.

Visual observations of the visibility do not allow data ondangerous phenomena to be obtained continuously. More-over, they are characterized by subjective errors. Therefore,equipping an observation station with devices for measuringvisibility is an important governmental task. Visibility-measuring devices evaluate the meteorological visibilityrange �MVR�, which is defined as the path length in theatmosphere needed to attenuate the light flux of a collimatedlight beam to 0.05 of the initial value. Two methods are usedin current practice for instrumental observations of thevisibility—the transmissometer �TM� method and thenephelometric method. The TM method is based on the fol-lowing principle: the transmittance � of the atmosphere, cal-culated per kilometer as unit length, is determined in a layerof specified length �the so-called measurement base�. This isthen converted to the MVR, using the Koschmiederformula.1 The TM method is the most accurate but does notprovide measurements of MVR values higher than 8000 m,which are limited by the measurement base in accordancewith the formula

�S = 0.29S

B�� , �1�

where �S is the measurement error of the MVR, S is thevalue of the MVR, B is the measurement base, ��=�� /�,�� is the photometry error, and � is the part of the light fluxattenuated by the layer of atmosphere �recorded by the de-tector�.

This method is used on aviation-meteorological stationsfor meteorological assurance of the takeoff and landing ofaircraft, where data are needed only concerning limited vis-ibility.

Transmissometers are divided into single-base and two-base.

647 J. Opt. Technol. 76 �10�, October 2009 1070-9762/2009/1

A single-base TM consists of two units, located at theends of the measurement base: a radiation unit at one endand a photoelectric detector at the other.

In a two-base TM, a transmitter-radiator unit is located atone end of the measurement base, and a reflector and a lightdetector are at the other end.2

Two-base TMs are currently predominant world-wide.This class of equipment includes the FI-2 pulsed photom-eters and the FI-3 modern visibility-range meter, developedand marketed by OAO LOMO.3 The measurement rangeMVR of the FI-3 extends from 60 to 8000 m.

The required visibility-measurement range extends from30 m to 50 km on the network of hydrometeorological sta-tions in all countries of the world. Such a wide range can beprovided by the nephelometric �NM� method of measure-ment. The NM method is based on the phenomenon, well-known in physical optics, according to which a light beamtransmitted through a turbid medium undergoes scattering atthe particles suspended in it and consequently acquires ap-preciable brightness. This brightness varies as a function ofthe degree of turbidity of the medium; the measured bright-ness value makes it possible to judge how transparent thismedium is.

The NM method of measuring the transparency of theatmosphere does not require basis sections, is suitable in thelight and dark parts of the day, and can be used on openterrain, in woods, at sea, etc.

A disadvantage of the NM method is that the data areless reliable when precipitation in the form of rain or snow ispresent. In order to compensate the methodological errors inthe presence of precipitation, the design of a nephelometerincludes additional sensors and converters to detect the type�solid, liquid� and intensity of the precipitation. However, theinfluence of methodological errors cannot be completelycompensated.

Moreover, since an insignificant volume of the air me-dium is used, measurements in an inhomogeneous atmo-sphere for a takeoff-landing strip up to 5 km long are non-representative. The reliability of the MVR measurementscannot be increased by using a large number of implementa-tions, since the time per measurement is limited. Accordingto the requirements of the International Civil Aviation Orga-

64700647-04$15.00 © 2009 Optical Society of America

nization, instrumental measurements of the visibility must beaveraged on an interval of 60 sec.4

Since a nephelometer is not a self-calibrating device, itneeds to be calibrated by comparing it with a TM undervarious weather conditions. The drawbacks of the two mea-surement methods can be compensated only by comprehen-sive use of a nephelometer and a TM, and this is imple-mented, for example, in the TL 31 device made by Vaisala�Finland�. However, it is inexpedient from a technical andeconomic viewpoint to use two devices, since this makes themeasurements appreciably more expensive and more com-plex. This is not acceptable for the network of hydrometeo-rological stations.

The goal of this article is to show that it is possible inprinciple to create a device that combines the advantages ofthe TM and NM methods of measuring visibility.

COMPOSITION AND FUNCTIONAL LAYOUT OF THE NIF-1VISIBILITY-RANGE METER

The NIF-1 visibility-range meter developed at LOMOcombines two measurement channels, united in such a waythat a common source of light pulses is used. This ensuresacceptable cost without degrading the measurement quality.The composition and design of the NIF-1 is shown in Fig. 1and the functional layout is shown in Fig. 2.

The radiation unit �RU� forms pulsed optical signals at awavelength of �=0.85 �m and a light flux of �0 at the out-put. Light flux �, attenuated by the layer of the atmosphereat a distance of 50 m, arrives at the TM receiver unit�TMRU�. Transmittance � of the layer of atmosphere at adistance of 50 m is calculated in the electronic unit �EU� asthe ratio �=� /�0, which is independent of the luminescencebrightness of the radiating LED. The calculated value of �from the EU travels along the communication line to themeasurement-and-display unit �MDU�. The coefficient � inthe MVR is converted in the MDU by means of an algorithmbased on the well-known Koschmieder formula, and then itis displayed in digital form on the panel of the MDU.

The light scattered when it interacts with suspended par-ticles in a local volume of air arrives at the nephelometer

1

2

3

4

5

FIG. 1. Composition and structure of the NIF-1. 1—Measurement-and-display unit, 2—electronics unit, 3—radiator unit, 4—nephelometer detectorunit, 5—transmissometer receiver unit.

648 J. Opt. Technol. 76 �10�, October 2009

detector unit �NDU� at an angle of 45°. An estimate of theMVR from the NM method is then obtained from the expres-sion

S = 3/� , �2�

where � is the scattering coefficient.Measurements are carried out by the NM method when

the MVR becomes more than 4000 m. Thus, the MVR read-ing is not affected by precipitation in the form of rain orsnow.

THE OPTICAL SYSTEM OF THE NIF-1 VISIBILITY-RANGEMETER

The optical system of the NIF-1 is shown in Fig. 3.It contains the nephelometer detector channel �9�, the

TM detector channel �14�, and the radiating channel of theRU, common to them �5�. The detector channel of the

Us

RU

Urad 220 V

MDU

Weather observer'sroom

EU50 Hz

Communicationline

up to 8 km

SupplyHeater

Φ0TMRU

50 mUs

NDU

Φ

FIG. 2. Functional layout of the NIF-1 �see text for explanation�.

12

3

45 6

78

9

10 11

12 13 14

15

16

135° 45°

FIG. 3. Optical layout of the NIF-1 �see text for explanation�.

648Volkov et al.

nephelometer is placed at an angle of 135° �forward scatter-ing at a 45° angle� to the transmitting channel. This angleprovides a linear connection between the attenuation index�G� and the scattering coefficient ��� and, for the size distri-bution of the particles of interest to us in the probe volume,is determined as

��45 ° �/G = 1.45. �3�

The maximum deviation from a value of 1.45 does not ex-ceed �10%.

The receiver channel of the TM is located opposite thetransmitting channel at a distance of 50 m and serves to mea-sure the MVR from 30 to 4000 m, to obtain MVR data dur-ing precipitation, and to check the NM channel.

The transmitting channel consists of the EP505L-350IR4emitting laser diode �3� with a wavelength at the radiationmaximum of �=0.85 �m, a five-lens objective �4� �with fo-cus f�=24.53 mm and input aperture A=0.6, correspondingto included angle 73°30��, which forms a parallel beam ofradiation 40 mm in diameter, the reference-channel photodi-ode �1�, a narrow-band interference filter �2�, and an externalprotective glass �6�.

The problem was to measure the optical energy scatteredin the specified limited volume and was solved by means ofthe proposed optical layout. The optical layout of thenephelometer’s detector channel made it possible to collectthe maximum optical energy on the sensitive area of thephotodetector, and this in turn increased the information con-tent of the detector channel.

The optical system of the nephelometer’s detector chan-nel �9� consists of the FDUK-2 silicon photodiode �11� withreceiver area 77 mm, a narrow-band interference filter�10�, and a four-lens objective �8�, which collects theforward-scattered radiation in a narrow angular range.

The detector channel of the TM consists of detector �15�with light filter �16�, a field stop, and a single-lens objective�13�.

Protective glasses �6, 7, 12� are installed in the detectorand the radiator modules in order to seal them off.

To minimize the background illumination, narrow-bandinterference filters with a working wavelength of �p

=0.86 �m and a pass band at the 0.5 level of no greater than0.05 �m are mounted in front of the detectors. The transmit-tance of the optical layout at the working wavelength is atleast 0.6 of the background transmission in the sensitivityinterval of the photodetector—no more than 0.01. All theradiation detectors are protected with these filters.

All the optical surfaces are coated with antireflectioncoatings that have a reflectance at the working wavelengthno greater than 0.5%.

STRUCTURAL FEATURES OF THE ELECTRIC PROCESSINGSYSTEM

An energy calculation showed that, when MVR5000 m, the signal level lies at the noise level of the pho-todetector, and the photocurrent is I�=4 nA. Synchronous-detection methods are therefore used to separate out the use-ful signal on the noise level.

649 J. Opt. Technol. 76 �10�, October 2009

A reference channel that is identical for the measurementsetup in both the NM and TM measurement channels is usedto stabilize the measurements.

The use of the same type of photodiodes in both chan-nels reduces the influence of climate changes of the sur-roundings. Thermal stabilization of the inner volumes of theunits RU, NDU, and TMRU is used.

Since the forward scattering is measured in open spacein the presence of illumination by the sun, amplitude modu-lation of the illumination by the EU is used.

Because the forward-scattered energy from the radiatoris extremely small and is measured in units of nanowatts, thesecondary sources of radiation and electromagnetic noisefrom the power grid affect the measurement error even at amodulation frequency of 3000 Hz. Therefore, rather thanchoosing a multiple of 50 Hz as the modulation frequency, itis chosen as 3010 Hz, at which the influence of the noisementioned above is strongly attenuated.

CONSTRUCTION OF THE DETECTOR UNIT

The device involves technical solutions that ensure thatthe device will operate under large backgroundillumination—for example, installation of a narrow-band in-terference filter, synchronous detection, differential switch-ing of a photodiode, etc.

All the signal processing is carried out by the processors�which are located in the EU and in the MDU and are col-lected in the EPF8636AL184�, using specially developedprograms.

CONCLUSION

As a result of this work, it was shown that it is possiblein principle to create a new class of devices that combine theadvantages of transmissometric and nephelometric methods.A prototype of the NIF-1 visibility-range meter has beendeveloped and fabricated at LOMO. Comparative tests of theNIF-1 and the FI-3 showed acceptable agreement of the re-sults.

The limits of error when the NIF-1 was used to deter-mine the MVR in the ranges h are as follows:

30�h�200 m 20%200�h�3000 m 10%3000�h�30 000 m 20%

Development of software is being carried out in parallel,which will make it possible to eliminate the transmissometerchannel.

a�Email: const_cv@mail.ru

1V. A. Kovalev, Visibility in the Atmosphere and Its Determination�Gidrometioizdat, Leningrad, 1988�.

2Yu. M. Goncharov, O. A. Volkov, R. A. Kruglov, and S. M. Persin, “Abaseline photometer,” Russian Patent 2070717 �1996�.

3O. A. Volkov, R. A. Kruglov, S. A. Denisenko, and V. A. Protsenko, “Abaseline photometer,” Russian Patent 83136 �2009�.

4V. A. Gavrilov, Visibility in the Atmosphere �Gidrometioizdat, Leningrad,

649Volkov et al.

1966�.

650 650J. Opt. Technol. 76 �10�, October 2009 Volkov et al.