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Optical measurements of pollution dispersion at
commercial airports
M Bennett1, S Christie1 and A Graham2
1
School of Chemical Engineering and Analytical Science, University of
Manchester, Sackville Street, PO Box 88, Manchester. M60 1QD. UK.
2 Centre for Air Transport and the Environment, Manchester Metropolitan
University, Chester Street, Manchester. M1 5GD. UK.
Corresponding author: [email protected]
Abstract. We have been using a range of optical techniques to measure the
dispersion of pollution at commercial airports. In such an application, it is of
course essential that any such techniques be eye-safe. For this reason, we have
converted an existing rapid-scanning Lidar based on a frequency-doubled YAG
system (532 nm) to frequency-tripled YAG (355 nm). At 33 mJ/pulse and a beam
divergence of 1 mrad, this can be shown to be eye-safe in almost all practical
situations. The system has been used to obtain cross-sections of the aerosol
emitted by aircraft on take-off and landing at Heathrow and Manchester airports.
We similarly deployed a Doppler Lidar system based on 1550 nm technology to
obtain wind and turbulence profiles up to 200 m range. Using a collimated rather
than a focused beam guaranteed eye and skin safety at all ranges. Range
resolution was obtained through an optical coherence technique. This wavelength
gives a better SNR than visible or NIR radiation and is orders of magnitude safer.
A focused UV system could give range resolution out to 800 m but would not be
inherently safe.
1. Introduction
A range of optical techniques has been employed to measure the dispersion of pollution
from commercial aircraft at London Heathrow and Manchester airports. Measurements
in the wake of several hundred flights have been obtained. Data were acquired in early,
intermediate and advanced stages of the takeoff ground run; from airborne aircraft in
both departure and arrival; and on touchdown and over the landing ground run.
The optical techniques employed included: a backscatter Lidar operating at a
wavelength of λ=355 nm to obtain cross sections through the aerosols emitted by
aircraft; a coherent Doppler Lidar system based on 1550 nm technology to obtain wind
profile measurements at up to 200 m range; a number of passive miniature
spectrometers [1]; and a 50 m open path UV spectrometer (uv-Falcon) measuring from
200 – 270 nm. This latter instrument is discussed elsewhere [2]. In addition to the optical
systems, supporting measurements came from a meteorological station (temperature,
humidity, wind strength, wind direction and short wave insolation), a phased array sodar
for wind profile measurements, and the airport’s own meteorological data.
For the application of optical instrumentation at busy commercial airports, it is of
course essential that they be eye-safe: while it was the intention to target the emissions
from the aircraft rather than the aircraft themselves, it was inevitable that UV laser
pulses would strike aircraft from time to time. Consequently, the hazard calculations and
assessments were necessarily meticulous. This paper will discuss them in detail.
The safety arrangements for the University of Manchester’s backscatter RapidScanning Lidar (RASCAL, cf. figure 1) system were first developed for the original and
much more hazardous 532 nm wavelength system. At this visible wavelength the eyesafe distance for a single shot was calculated to be as much as 3.1 km and for multiple
shots 6.4 km. The arrangements put in place to manage this hazard included:



(a)
A dead man’s handle arrangement with a lookout. If the lookout saw anything
potentially sensitive in the field of view, he would take his thumb off the button
and laser output would be instantly suppressed.
CCTV monitoring of the field of fire. The operator could see where the beam was
pointing at all times.
Computer control. All scanning directions had to be tabulated within the
computer. Dummy runs had to be made before the laser would fire.
Standard operating procedures. For example, the Lidar could only be operated
above the horizon except in controlled terrain.
(b)
↓
Figure 1. (a) The mobile RASCAL backscatter lidar facility at Heathrow; (b) Aircraft on final
approach at Manchester airport: RASCAL may just be discerned in the background.
Given such careful arrangements, RASCAL had operated without significant incident
since 1987. Clearly, however, it would not be practicable to manage a laser hazard of
this magnitude at a busy commercial airport. For this reason, and following the lead of
Eberhard et al. [3], we have recently converted the system to operate at 355 nm in the
UV-A. At this wavelength, the radiation is no longer focused by the lens of the eye to a
high intensity spot on the retina. The most sensitive location now becomes the cornea,
at permissible energy density 3-4 orders of magnitude greater than at 532 nm [4].
The second Class 4 instrument deployed at the airport, the Doppler Lidar system, uses
optical coherence tomography to obtain range resolution [5]. It uses IR radiation at λ = 1550
nm, with a DFB laser diode as the primary source. This beam is split between a reference
path and a signal path along single-mode fibres. The signal path then passes through an
erbium fibre amplifier (EDFA) that raises the power to up to 1 W before it is emitted into the
atmosphere as a collimated beam from the optical head. Some proportion of this beam is
scattered by atmospheric aerosol back towards the instrument. The motion of this
aerosol towards or away from the source gives rise to a Doppler shift in frequency, which
may be detected by the instrument and interpreted as the radial component of wind
speed. In this system, the beam is not scanned but is directed into the sky at a fixed
azimuth and elevation. The beam must of necessity be directed above the horizon since
the return from a hard target would swamp the desired signal from clear air. Our normal
practice is to set up the beam at a fixed elevation of 30o from the zenith. On occasion,
however, we have directed it at a lower elevation between buildings in order to study
flows within wakes (figure 2). It is found that with an output power of 1 W, the system
can now usually provide a signal from clear air at visibilities of up to ~40 km.
Radiation at 1550 nm is strongly absorbed by water and so cannot penetrate the eye.
It is thus the radiation intensity at the cornea or on the skin which presents the limiting
safety factor.
(a)
(b)
Figure 2. (a) Optical head of Doppler Lidar system; (b) Doppler Lidar system in operation
at a chemical plant in Italy [ref 6].
2. Calculations of laser safety
2.1 RASCAL system (355 nm)
Maximum permissible exposure (MPE) at the cornea may be taken from the relevant
British Standard [4]. We find that, for exposure times between 1 ns and 10 s, the MPE
for the cornea is given by 5600 t 0.25 J m-2, where t is in s. Thus for the 8 ns pulse from
our system, the permissible energy density from a single pulse is 53.0 J m -2. The pulse
energy obtainable with the UV system has been measured as 33 mJ/pulse, when the
system is optimized. The output from the laser is taken through a beam diverger and
reflected by a series of mirrors to give an output diameter of about 30 × 37 mm
(elliptical). At the output from the system, the laser energy for a single pulse will thus be
38 J m-2, i.e. eye-safe. The laser operates at 30 pulse/s. If a bystander were so foolish
as to continue to peer into the beam as it emerged from the system, a simple calculation
shows that he would have accumulated the MPE after 8.2 s (i.e. 247 shots).
At some distance from the system, the radiation intensity is substantially reduced
since the beam diverger has been adjusted so that the beam diverges at >1 mrad. Thus
at a range of 500 m the beam diameter will have grown to >533 mm, implying a pulse
intensity of <148 mJ m-2 and a time-averaged intensity of <4.44 W m-2. For long
averaging times (>1000 s), the requisite limit is 10 W m-2, i.e. the beam would be safe for
continuous viewing.
It should be noted that the passengers and crew of passing aircraft benefit from an
additional large safety margin in that aircraft windows are specifically designed not to be
transparent in the UV. It is in any case only the single pulse intensity which is of
relevance to aircraft, since they are in general moving, there being no intention to fix the
laser beam on a moving aircraft.
For ground-based targets, there remains the possibility that the beam might be
viewed through an optical viewing aid. A spectator on a nearby building, for example,
might stare at the Lidar through binoculars out of a very reasonable curiosity. At 355 nm,
standard optical glass (BK7) is poorly transparent to UV: the loss is about 13% per cm.
Thus the use of a pair of 16x binoculars might lead to an intensification of the flux at the
cornea by a factor of (say) 0.75 × 162 = 192x. For a building of more than 500 m from the
Lidar, the pulse intensity at <192 × 0.148 = 28.4 J m-2 would remain below the MPE. For
intermediate viewing times (10 – 1000 s), however, the MPE is only 104 J m-2. Integrated
over many pulses, this limit would be reached after >12 s. The implication of these
calculations is that scanning should always be above the horizon where practicable, and
that the laser should never be fired steadily at any potentially inhabited building or
stationary aircraft.
The UV beam itself is invisible and so should not present a distraction hazard. There
remains a trace of the secondary, 532 nm output, but this diverges rapidly and does not
remain perceptible at any distance from the Lidar. The beam makes white objects
(paper, clothing) fluoresce, but it is hard to see how this might present a significant
distraction at hundreds of metres from the source.
2.2 Doppler Lidar system (1550 nm)
Again the maximum permissible exposure at the cornea may be taken from the British
Standard [4]. We find that, for exposure times of less than 10 s, the MPE for the cornea
is given as 10 kJ m-2, while for continuous viewing it is 1 kW m-2 (i.e. equivalent to the
intensity of bright sunlight). The beam is output from a 50 mm objective lens and has a
width at this point of about 30 mm. Its energy density is thus of order 1.4 kW m -2. If a
bystander were foolish enough to ignore the hazard notices (figure 2) and to continue to
peer into the beam as it emerged from the system a simple calculation shows that he
would have accumulated the MPE after about 7 s. In practice, the power density is
sufficient to give a perceptible warming on the back of the hand.
There is a theoretical risk here to optical equipment. Standard optical glass has an
excellent transparency at 1550 nm: a bystander attempting to take a photograph of the
optical output of the Lidar could well seriously damage his camera!
At some distance from the system, the radiation intensity is gradually reduced as the
beam diverges diffractively. Thus at a range of 500 m the beam diameter will have
grown to about 41 mm, implying an intensity of 0.76 kW m -2, i.e. the beam would be safe
for continuous viewing.
An additional safety margin arises from the power lost within the system between the
optical amplifier and the output beam (> 5%), and also through absorption and scattering
in the atmosphere (> 5%). Since the beam should be directed above the horizon,
sensitive targets should in general be moving.
In the case of ground-based targets, there remains the possibility that the beam might
be viewed through an optical viewing aid. The infra-red beam is invisible and so will not
present a distraction hazard. As with the RASCAL, however, a spectator on a terminal
building might stare at the system through binoculars. The use of a pair of 16x
binoculars would lead to an intensification of the flux at the cornea by a factor of 256x. At
both airports, the terminals were more than 500 m from the Lidar sites, so the intensity
on the cornea would become <256 × 0.76 = 194 kW m-2, an extremely hazardous level.
It is thus clear that the Doppler Lidar beam must never be directed at any potentially
inhabited building or stationary aircraft. As noted above, for operational reasons one
would not want to do this anyway.
Moving targets (e.g. taxiing aircraft), however, would be relatively safe in these
circumstances. The time to accumulate the MPE at this intensity is 57 ms. Since the
beam is 41 mm in diameter at 500 m, a lateral velocity of only 0.7 m s -1 would be
sufficient to keep the instantaneous dose below the permitted 10 kJ m-2.
2.3 Choice of Doppler Lidar system parameters in relation to eye safety.
Some of the technical aspects involved in the choice of wavelength for a Doppler Lidar
system have been discussed in ref [5]. There is a range of issues that affect the
signal/noise ratio (SNR) which may be obtained. The proportionalities involved are:





Atmospheric backscatter (Mie scattering)
Irradiance of scattered light (for a focused system)
Efficiency of capture of this irradiance to a single mode fibre
Photon energy
Bandwidth of beats signal
Averaging of beats signal
λ-1.3
λ-1
λ2
λ
λ
λ-0.5
Adding powers, we see that the overall SNR of the beats spectra averaged over a
given short period is proportional to λ1.2, i.e. long wavelengths are better. Indeed, the
traditional wavelength for such systems is 10.6 μm from a CO2 laser. Practically,
however, the cheapness and convenience of modern communications technology
outweighs the 10 dB loss implicit in the move to 1.55 μm; at least for near-field
measurements. Clearly, it would be futile to go to shorter wavelength NIR (λ < 1.4 μm)
or to visible wavelengths since the SNR would be worse, while the eye safety limits
would be several orders of magnitude lower.
We should note that for the incoherent system represented by RASCAL, only the first
of the above terms is relevant. For simple backscatter, shorter wavelengths are better.
Our Doppler Lidar system is continuous wave (CW). Within the same eye safety
constraints, we would do better with a quasi-pulsed system, i.e. instead of emitting a
continuous 1 W, we could emit (say) 250 ms bursts of 4 W. The time-mean power
dumped on a sensitive target would be the same, but the SNR would be 6 dB greater.
(About this length of burst is probably optimal since it matches the period over which the
target volume might be considered to be frozen). Such an output, however, would be
difficult to deliver with our existing optical amplifier.
Our system uses a collimated beam with, as mentioned, an optical coherence
technique to obtain a nominal 35 m range resolution. Given the system parameters, this
implies a loss of 13 dB relative to an equivalent focused Doppler Lidar system. Note,
however, that some care must be taken with the focused system, particularly in an
application such as is shown in figure 2(b). With output optics of 50 mm diameter and a
range of 200 m, an output power of 1 W would give a power density of 27 kW m-2 at the
focal point. If the beam is moved, this is not a problem – a scan rate of as little as 6
mrad min-1 would be enough to ensure that the time integrated power received by a
target would never exceed the permitted 10 kJ m-2. But in fixed mode, great care would
have to be taken in aligning the beam. This also implies that we must also take care in
setting up our own system to ensure that the output beam is indeed collimated and not
focusing itself unseen at some greater distance.
Our reason for choosing the optical coherence technique, despite the loss of signal
power, was to obtain some useful range resolution beyond 200 m. Because of the
length of the Rayleigh waist, this is the greatest useful range for a focused system with
50 mm optics at a wavelength of 1.5 μm. The length of focus, of course, is proportional
to the wavelength, so one might reasonably ask whether it would be feasible to
implement a focused system in the UV. The optimal wavelength here would be a shade
below the lower limit of the visible at 400 nm. With the same optics, this could then
provide useful range resolution out to 800 m. From the above analysis, however, it is
clear that for equivalent performance an output power of about 5 W would be required.
The permitted integrated energy received at the cornea is 10 kJ m -2, so for a 50 mm
output beam this would be accumulated in rather less than 4 s. This is clearly
undesirable, but more serious is the power density at the focus: at a range of 200 m, the
focal spot would have a diameter of less than 2 mm and an implicit power density of 2
MW m-2. Again, one could in principle scan the beam to ensure that the integrated
energy dumped on any target remains below 10 kJ m-2, but the rate of scan now
increases to 110 mrad min-1. Clearly, such a Lidar would be a very specialized item of
equipment, and probably unsuitable for airports.
3. Conclusion
We have shown that, with appropriate choice of system parameters, high power laser
systems may be deployed at airports to obtain useful measurements of the dispersion of
effluent from commercial aircraft.
For measurements of aerosol distribution, a
backscatter Lidar operating at 355 nm is highly effective. For measurements of wind
speed and turbulence, Doppler Lidar operating at 1550 nm can be made safe and
practical.
Acknowledgements
We should like to thank the EPSRC and the Department for Transport for funding the work at
Manchester and Heathrow airports respectively and for the conversion of RASCAL to 355 nm.
We are grateful to BAA for permission to operate at Heathrow and to Manchester Airport PLC for
permission to operate at Manchester. The Doppler Lidar was constructed with funding from
National Power and the EPSRC. We are grateful to Dr Michael Harris of Qinetiq for helpful
discussions and to Prof David Raper of MMU for advice and encouragement.
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