GB2059576A - Forward scatter visibility meter - Google Patents

Abstract

A visibility meter operating on the principle of forward light scatter, comprises a light transmitter 2 and a light receiver 3 for transmitter light scattered forward in the atmosphere, means for measuring the intensity of the received scattered light as an index of the corresponding visibility, and a screen 4 for protecting the receiver from direct non-scattered transmitter light, characterised in that the direction of radiation (17) of the transmitter (2) extends downwardly at an angle and the reception direction (19) of the receiver (3) extends upwardly at an obtuse angle (a) to the direction of radiation (17). The screen preferably has two parallel members each with a triangular cutout on the base and each edge is equipped with a rain gutter, water and insect repellant coating and can be heated to 50-60 DEG C. A translucent plug 40 may provide a test. <IMAGE>

Description

SPECIFICATION Visibility meter (Forward scatter meter) This invention relates to a visibility meter of the kind operating on the principle of forward light scatter, comprising a light transmitter and a light receiver for transmitter light scattered forward in the atmosphere, and comprising means for measuring the intensity of the received scattered light as an index of the corresponding visibility, and comprising a screen for protecting the receiver from direct non-scattered transmitter light.

Meters of this kind (forward scatter meters, FSM) are particularly suitable inter alia for metereological purposes and for road traffic, for switching on and off fog warning signals or appropriate speed restriction signals.

In known art meters, the visibility measurements have frequently been disturbed by climatic and environmental influences, and maintenance (cleaning) is often required because the directions of'radiation of the transmitter and receiver are horizontally aligned to one another on each side of the screen. Consequently, dust particles can lodge on the transmitter and receiver exit windows and result in the measurements being falsified. The risk of the exit windows becoming soiled is particularly high in the case of visibility meters installed at airports.

The invention is based on the finding that a relatively slight change in the directions of radiation can effectively counteract the said soiling.

According to the present invention, there is provided a meter of the above kind wherein the direction of radiation of the transmitter extends downwardly at an angle and the reception direction of the receiver extends upwardly at an obtuse angle to the direction of radiation.

The obtuse angle included by these directions results in a small vertical distance between the apex of the angle and the meter. Consequently, the meter has to be only a slight height above the ground to prevent vegetation projecting into the volume under measurement, e.g. in the summer, and the height of the snow encroaching on the volume under measurement during the winter.

The "volume under measurement" denotes that part of the area beneath the screen in which the radiation cone of the transmitter intersects the cone from whose area the receiver receives. Since the volume under measurement is not above or next to the meter, but below it, the air in the volume under measurement is not heated by the meter due to solar irradiation in the summer or separate heating in the winter to prevent icing up.

If the volume under measurement is situated above the meter, or even if it is situated next to the meter in the case of a lateral air flow, such heating of the volume under measurement is inevitable and results in errors. Finally, noise in the case of a receiver receiving only at an angle from below is less than in the case of a receiver receiving from above or laterally.

One exemplified embodiment of the subject of the invention is described in detail below with reference to the figures of the accompanying diagrammatic drawing wherein: Figure lisa side elevation of the visibility meter in partial section.

Figure 2 is a plan view of the screen of the meter according to Figure 1 and to an enlarged scale.

Figure 3 is a side elevation of figure 2 and Figure 4 is a section on the section station IV--IV in Figure 2 to a larger scale than Figures 2 and 3.

A light transmitter 2 and a light receiver 3 are secured opposite one another to the underside of a support 1. A screen 4 projects down from the support 1 centrally between the transmitter 2 and the receiver 3. The housing 5 of transmitter 2 contains a flashlight 7 powered by a power supply unit 6, and an optical system 8 behind a window 9. Housing 10 of receiver 3 contains an optical system 12 and a photo-diode 13 behind a window 11, said photo-diode being connected to a circuit 14 for measuring the photo-current, said circuit comprising a filter 1 5 and a logic network 1 6.

The axis 1 7 of the radiation cone 1 8 of transmitter 2 extends obliquely downwards and the axis 19 of the cone 20 from whose area the receiver 3 receives light is directed upwards at an angle in the direction of reception. The two axes 17 and 19 include an obtuse angle a. Screen 4 projects upwards to such an extent into the two cones 18 and 20 and is of such width perpendicularly to the drawing plane of Figure 1 that the receiver 3 does not receive direct light from the transmitter 2. In clear visibility (infinite visibility), no transmitter light reaches the receiver.

In poor visibility, some light is scattered by the fog particles in the atmosphere. Scattered light from the measured volume reaches the photo-diode 1 8 of receiver 3, and the resulting photo-current is amplified and is an index of the turbidity and hence of the visibility, which decreases as turbidity increases.

The logic network 1 6 is an AND gate controlled by the transmitter to suppress the photo-current in the intervals between the flashes (on the principle known from the specification of Swiss Patent 566013).

As will be seen in detail in Figures 2, 3 and 4, screen 4 consists of two plates 21 disposed parallel to one another at a distance b. Light scattered by a water droplet (rainwater, water from melting snow) hanging from the bottom edge of the left-hand plate 21 in Figure 1 is prevented from reaching the receiver 3 by the right-hand plate 21, since it would simulate a shorter visibility than the actual visibility. A water droplet hanging from the bottom edge of the right-hand plate 21 cannot result in scattered light because it would be in the shadow of the left-hand plate. The bottom edges of the plates 21 are at the same level.

Each plate 21 of screen 4 has a triangular cutout 22 at the bottom (see Figure 2) so that its bottom edge consists of two edge parts 24 extending downwardly and outwardly. The bottom screening edge of each plate 21 is provided with a rain gutter 25 on the outside, the cross-section of the gutter being acute-angled. The result of the cut-out 22 is that water droplets run to the bottom outer corners of the screen which, given appropriate dimensions of the width of the screen, are situated outside the cone 18 in which the transmitter 2 radiates and outside the cone 20 from which the receiver receives. A water droplet hanging cannot cause any light scatter which would influence the measurement. The gutters 25 prevent any droplet formation at the bottom edge of the screen even in heavy downpours.They are in the form of acute-angled bends at the free ends of the limbs 26 of a U-section sheet-metal strip 27, which spaces the plates 21 at the bottom by an amount b, and can be, for example, screwed or riveted to the plates 21.

Screen 4 is advantageously coated with a water-repellent coating, e.g. Teflon (Registered Trade Mark). An insectisidal or insect-repellent coating may also be used to keep insects away. It has, however, hitherto been impossible to keep spiders away, although this is very desirable and in some cases necessary, because 2 cobweb hanging from the screen 4 simulates a visibility of about 1 km when the actual visibility is greater.

Experiments aimed at solving the problem of keeping spiders away from the screen have shown that spiders keep away and cannot spin webs if the screen is kept at a temperature of from 50 to 600 C. To this, end, screen 4 is provided with an appropriate heater element 28 which, in the exemplified embodiment, is disposed on the Usection metal strip 27 secured to the bottom edges cf the plates 21, the edges of said strip being bent at an acute angle to form the gutters 25.

With this relatively high screen temperature it is important that the screen should be disposed above the measured volume, as already stated, i.e., so that it does not heat the measured volume, since this would change the light scatter and hence falsify the measurements. A lower temperature is sufficient in winter to prevent snow and icing. The heating is therefore advantageously adjustable to different temperatures.

Particularly in the tropics, bacteria and fungal spores penetrating the housings 5 and 10 of the transmitter 2 and receiver 3 may result in disturbing coatings on the lenses 8 and 12, the flashlight 7 and the photo-diode 13. To prevent this, the housings 5 and 10 are air-tight and they contain a drying medium (silica gel bags or cartridges 29) to prevent any condensation which might occur in the air-tight housings on cooling.

To avoid the disturbing coatings caused by bacteria or fungal spores on the windows 9 and 11 of the transmitter 2 and receiver 3, the windows may be made from baryta or quartz, since it has been found that such coatings do not form in that case.

A difficult problem is that of vandalism, particularly in the case of meters installed in lonely areas. The best protection has proved to be unattractive paintwork, the use of camouflage paint. It has been found that meters thus painted are less subject to vandalism.

In late autumn (Indian summers), spiders' webs carried on the wind may stick to the meter and impair its function as already stated. To prevent this, the meter is surrounded by a mesh fence which prevents insects, more particularly butterflies, from flying through the volume under measurement. The mesh should be as close as possible but should not obstruct the airflow. A mesh width of 1 cm has proved satisfactory. Such steps are unnecessary with meters mounted on buoys for navigational use.

In the case of meters erected on the ground, vegetation must be prevented from projecting into the volume under measurement, and at least the zone enclosed by the mesh fence is provided with a surface to prevent vegetation, e.g. concrete, asphalt, a plastic or sheet-metal plate, which is made in one piece to prevent plant growth in cracks.

By making this surface with low heat absorption, i.e. making it white or at least light, the soil is prevented from being heated by solar radiation, since this would result in heating of the volume under measurement and hence thermals or fog dispersion in the volume under measurement.

In areas where snowfalls must be expected, the meter is mounted above the ground to a height such that the point of intersection 30 of the bottom generatrices 31 and 32 of the cones 1 8 and 20 is above the maximum snow height.

Since the visibility criterion is the sight of the human eye, which does not respond to infrared, although conventional light sources and photoelectric transducers radiate and respond to infrared as well, and unlike visible light infrared is practically unaffected by fog, the transmitter 2 and/or the receiver 3, but preferably the latter, is provided with an infrared filter or else the windows 9 and 11 or the optical systems 8 and 1 2 consist of a material which absorbs infrared radiation in order to prevent the measured visibility from being affected by the infrared radiation.

To reduce the power consumption, particularly in meters with their own power supply, and in order to increase the life, particularly of the flashlight, and since visibility always changes only slowly, an embodiment of the meter intended for automatic continuous measurement is provided with a control device which periodically switches the meter on for a short period but does not trigger any change of display until a change in the measurements extends over a number of periods.

In this way, measurements falsified by accidental events, e.g. a butterfly flying through the volume under measurement, are disregarded. For example, measurements can be taken for one second every four to five minutes, the measurement can be stored and compared with the next measurement, or can be integrated over several periods.

In the case of meters for road traffic, which switch on a signalling device (a warning or maximum speed signal) when the visibility falls below a specific critical value, the critical value is advantageously the metereological visibility of 140 metres for example, during the day, but a shorter metereological visibility of 90 metres, for example, during the night, at which the tail lights of a vehicle are distinguishable at a distance of 140 m. The same applies to seagoing traffic, the ship's position lights corresponding to the tail lights and the signalling device being a foghorn.

To this end, the meter has a threshold circuit which switches on the signalling device when the visibility falls below a specific threshold which, under the control of a time switch or a brightness meter, corresponds to a greater visibility during daylight or lower visibility during the night or in inadequate daylight. In road traffic, one of various maximum speeds can be signalled again according to the measured visibility.

To test the meter, a light-permeable plate may be slidable into the volume under measurement either manually or by means of a solenoid for remote control, the plate providing a scatter corresponding to a predetermined visibility. In this way it is possible to check not only whether the meter is functioning at all, but also whether the indication is correct, because the measured visibility is reduced by the plate in accordance with its predetermined scatter.

If it is desired to maintain uninterrupted testing of the functioning of a meter of this kind, this can be done easily as follows: The hole 40 shown in Figures 1, 2 and 3 may accommodate a lightpermeable but matt plug 41, e.g. of Teflon (Figure 3). In the simplest case, just one hole 40 of an appropriately smaller diameter may be drilled (Figure 1). Alternatively, to keep water droplets away from the edges of the hole 40, a ring 41' (Figure 2) may be used instead of a plug 41. The object of this system is for some of the light of the transmitter to reach the receiver in the form of stray light through said hole 40 or the lightpermeable plug 41. This light simulates a specific visibility.Depending upon the range to be covered by the meter in practical operation, the amount of stray light will be determined empirically so that the resulting artificial visibility value is greater than the maximum value to be determined by measurement.

EXAMPLE In a road fog warning system, the fog warning signals are generally switched on when the visibility falls below 1 20 metres. Let it be assumed that this corresponds to an output current of 0.5 mA. If the self-testing system stray light is now so dimensioned through the agency of the size of the hole 40 or the size and the length of the plug 41 as to always simulate 800 metres visibility, it will be assumed that this value is equal to a basic current of 0.1 mA. If the meter output current falls below that value, there is an error or else the hole or surface of the plug or the diffuser is dirty. Maintenance is thus necessary. The calibration of the range of operation of the meter shifts slightly as a result of this self-testing system, there usually being an additive effect.

In that case, 0.5 mA in the above example would become 0.6mA to correspond to the setvalue of 120 m.

The simple self-testing system described may, as already stated, be in the form of a diffuser disposed in one of the two holes 40 e.g. if the length of a plug, e.g. of Teflon, absorbs excessive amounts of light.

Claims (23)

1. A visibility meter operating on the principle of forward light scatter, comprising a light transmitter and a light receiver for transmitter light scattered forward in the atmosphere, and comprising means for measuring the intensity of the received scattered light as an index of the corresponding visibility, and comprising a screen for protecting the receiver from direct nonscattered transmitter light, wherein the direction of radiation of the transmitter extends downwardly at an angle and the reception direction of the receiver extends upwardly at an obtuse angle to the direction of radiation.

2. The visibility meter according to Claim 1, wherein the screen has two parallel edges at the bottom extending at a distance from one another and at the same height.

3. The visibility meter according to Claim 1 or 2, wherein the screen has a triangular cut-out at the bottom such that its bottom edges each consist of two edge portions extending downwardly and outwardly.

4. The visibility meter according to Claim 3, wherein the bottom screen edges are provided with a rain gutter of acute-angled cross-section.

5. The visibility meter according to any one of Claims 1 to 4, wherein the screen is coated with a water-repellent insect-repellent coating.

6. The visibility meter according to any one of Claims 1 to 5, wherein the screen is provided with a heating deyice adjustable to a screen temperature of 50 to 60 C and a lower screen temperature above freezing point, or is adjusted to one of these temperatures.

7. The visibility meter according to any one of Claims 1 to 6, wherein the transmitter and receiver are disposed in air-tight housings containing a drying medium.

8. The visibility meter according to any one of Claims 1 to 7, wherein the meter is painted with an unattractive camoufiage paint.

9. The visibility meter according to any one of Claims 1 to 8, wherein the transmitter and the receiver are each disposed behind a baryta or quartz glass window.

1 0. The visibility meter according to any one of Claims 1 to 9, erected on the ground, wherein the meter is surrounded by a mesh fence extending down to the ground and at least the ground area enclosed by the mesh fence has a low heat absorption surface, which prevents any vegetation.

1 The visibility meter according to any one of Claims 1 to 10, erected on the ground, wherein the height from the ground of the area in which the radiation cone of the transmitter intersects the cone from whose area the receiver receives is greater than the maximum snow height.

12. The visibility meter according to any one of Claims 1 to 1 wherein at least the receiver is provided with an infrared filter or an optical system which absorbs infrared radiation.

13. The visibility meter according to any one of Claims 1 to 12, wherein the light source of the transmitter is a flashlight and the receiver has a filter for the electrical pulses corresponding to the received flashes, the pass range of said filter being adapted to the most intensive part of the frequency spectrum of said pulses.

14. The visibility meter according to any one of Claims 1 to 13, wherein the transmitter is a light pulse transmitter and the receiver contains a logic circuit controlled by the transmitter and suppressing the output signal of the photoelectric transducer of the receiver during the pauses between the transmission pulses.

1 5. The visibility meter according to any one of Claims 1 to 14, for automatic continuous measurements, wherein by control means which periodically switch the meter on for a short period and respond to any change in the measured visibility only when a change in the measurement is maintained for at least two periods.

1 6. The visibility meter according to Claim 15, wherein a threshold circuit switches on a signalling device when the visibility falls below a specific threshold which, under the control of a time switch or a brightness meter, is equivalent to greater visibility during daylight or lower visibility during the night-time or inadequate daylight.

17. The visibility meter according to any one of Claims 1 to 16, wherein by test means comprising a light-permeable plate slidable -- manually or by means of a solenoid for remote control - into that part of the area beneath the screen where the radiation cone of the transmitter intersects the cone from whose area the receiver receives in order to produce a scatter there corresponding to a predetermined visibility.

18. The visibility meter according to any one of Claims 1 to 1 6, wherein the screen system is formed with a hole through which a small amount of stray light always passes directly from the transmitter to the receiver and thus produces at basic test current.

1 9. The visibility meter according to Claim 18, wherein a ground glass is provided in one of the holes, the diffuse light passage of which passes sufficient light from the transmitter to the receiver to ensure a basic test current.

20. The visibility meter according to Claims 18 and 19, wherein the joint holes in the screens contain a light-permeable but matt plug, the surface of which has water-repellent properties.

21 ..The visibility meter according to Claim 18, wherein the holes of the self-testing means are provided with an inner ring of a water-repellent material.

22. The visibility meter according to Claims 18 to 21, wherein the value of the self-testing current is added to the meter output measurement and circuitry is provided which, in the event of the value falling below the test current, actuates a remote-control system to indicate the defective state of the meter.

23. A visibility meter constructed and arranged substantially as herein described and as shown in the figures of the accompanying drawing.