Transmission of electromagnetic radiation in fogs in the 0.53-10.1-μm wavelength range M. R. Clay and A. P. Lenham

Royal Military College of Science, Physics Department, Shrivenham, Swindon, Wiltshire SN6 8LA, U.K. Received 26 March 1981. 0003-6935/81/223831-02$00.50/0. A problem exists in predicting the effects of fogs (and par­

ticulates) on the performance of detection systems which rely on the transmission of electromagnetic waves in the visible and IR. Most reported experimental contemporary programs on atmospheric propagation1-3 are largely concerned with relatively long ranges and time-averaged absorption and will yield little information for fogs in which the visual range is <1000 m. We have concentrated on conditions in which the visual range is less than this. We show the wide range of spectral attenuation curves that can occur both from fog to fog and at various stages during a single fog.

Few measurements have been published showing the wavelength dependence of attenuation in fogs (referred to later as the attenuation curve). The main source of data is found in Ref. 4 which has formed the basis of several discus­sions5-6 which show that the attenuation curves can also yield important information on the size distribution for droplets in a fog.

Arnulf et al.4 used relatively long sampling paths (50-1200 m). They found many of the fogs they studied appeared spatially and temporally inhomogeneous. Their sampling period was not stated but appeared to be several minutes. In this work we have adopted a shorter measuring period to re­duce the effect of inhomogeneity.

Our transmissometer was designed to be operated con­tinuously at the Field Station of the Royal Military College of Science. The site has already been described.7

The attenuation coefficient λ (km-1) at each wavelength in the atmospheric windows is

where Ifλ is the transmission in fog, and I0λ the transmission in clear air, and l the length of the path (in kilometers).

Two transmissometers were used in our measurements operating over 17-m optical paths whose axes were never more than 0.25 m apart. One transmissometer had a quartz halo­gen lamp as source, collimating and focusing optics, and a lead sulfide detector. Six filters, mounted on a rotating disk (5 Hz), sample spectral regions in the wavelength range from 0.53 to 2.69 μm.

The second transmissometer had a Nernst filament as source, two interference filters (4.01 and 10.1 (μm), and py-roelectric detectors. Details of the filters are given in Table I.

Each wavelength was sampled every 200 msec, the full spectral range being covered sequentially during this period to minimize errors in the spectral attenuation curves caused by fluctuations in the fogs. The measurements were fed to a microcomputer which was programmed to record them at preset intervals (from 1 to 5 min).

The presence of the Field Station at one end of the path must modify the fog close to it. The measurements reported were made with wind directions that minimized this effect.

Fogs are spatially inhomogeneous4 so the values of derived represent an averaged value over the path length. Attenua­tion curves were obtained during 20 fogs occurring in the last quarter of 1980 and January 1981.

A considerable variety was found in the form of the atten­uation curves both from fog to fog and during an individual fog. A selection of attenuation curves for six fogs is presented in Table II. No fixed rules were applied to the selection of these curves other than to avoid the repetition of similar data and to illustrate the changes that can occur during the evo­lution of a fog. Each of the first four fogs lasted for <10 h while the last two represent stages in a period of fog and mist that lasted for nearly three days.

In the early stages of development of all the fogs studied (or redevelopment of a fog dispersed by a temporary increase in wind) the attenuation is smallest at longer wavelengths— indicating the predominance of small (<l-μm radius) drop­lets. As they deepened some of the fogs developed attenua­tion curves that showed a maximum attenuation in the 1-4-μm wavelength range (e.g., 11 Dec. 1827 to 2058). Sometimes a minimum attenuation occurred in this range (e.g., 31 Oct. after 0540) indicative of a multimodal droplet size distribu­tion.

Table I. Details of the filters in the Transmissometer

a These filters correspond to atmospheric absorption bands.

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Table II. A Selection of Values of Attenuation λ ( k m - 1 ) for Different Wavelengths Measured in Six fogs

In dense stages in long persisting fog the attenuation at longer wavelengths often becomes as large, or even larger, than that in the visible (29/30 Jan. after 2106 and lasting for over 11 h) indicating the presence of a significant number of large droplets.

The attenuation curves of fogs show more variety than previously reported4 especially in the existence of long per­sisting stages where the attenuation is at least as large at 10 μm as in the visible. It is not safe to assume that the perfor­mance of detection systems in the IR is always better in fogs than that at visual wavelengths. In general, the transmission at 4 and 10 μm was better than at visual wavelengths in thin fogs but not in dense, long persisting, fogs.

3832 APPLIED OPTICS / Vol. 20, No. 22 / 15 November 1981

References

1. T. Bakker, AGARD Conf. Proc. No. 183, Sec. 14 (1976). 2. P. J. Wright, AGARD Conf. Proc. No. 183, Sec. 3 (1976). 3. W. Jessen, A. Kohnle, H. Hipp, and d. H. Höhn, Infrared Phys. 20,

175 (1980), Figs. 4(a) and (b). The denser stages of fog show zero transmission reading.

4. A. Arnulf, J. Bricard, E. Cure, and C. Véret, J. Opt. Soc. Am. 47, 491 (1957).

5. C. Tomasi and F. Tampieri, Appl. Opt. 15, 2906 (1976). 6. R. G. Eldridge, J. Atmos. Sci. 23, 605 (1966). 7. R. K. Hauser, A. P. Lenham, J. Griffin, and M. Hawkins, Bound­

ary-Layer Meteorol. 11, 397 (1977).

We wish to thank R. H. West for his interest in this work and for valuable comments on the first draft of this note.