Copyright 2013 Optical Sensors
Optical Fog Sensor - OFS
OFSMk2 is a low price sensor for visibility. The power consumption is low. The main application is for traffic purposes. Using so called backscatter technique it measures the amount of water particles i e fog in the air that limit the visibility.
The sensor is used at different places for different applications. The validity and the stability has been good in all cases resulting in satisfied customers.

The sensor is microprocessor controlled making possible some features like RS232 output giving:
- the calculated visibility in meters
- measured ambient light etc

How it works optically - backscatter technique:
A narrow beam of red laser light comes out of an opening on the front. A detector behind a lens in another opening is sensitive for incoming laser light in a narrow lobe that overlaps the transmitter beam.
The design is very compact
If there are fog particles in the overlap zone light will be scattered back and reach the detector causing a signal on the sensor raw signal output. The sensitive zone is located about 30 cm ahead of the sensor and its volume is less than 1 cubic centimeter.
More information about different methods to measure visibility can be found on Measuring visibility.

Electrical signals:
The raw signal V is analog and it is a measure of the amount of backscattered light from the overlap zone. So the more fog in the overlap zone the more signal.
Signal processing:
The following expression can be derived for the raw signal V:
................(1)
where:
V
is the measured raw signal level
konst
is a calibration constant
VIS
is the visibility in meters. The principal relationship between the raw signal output and the visibility, expression (1), above is shown in the following graph, konst is in this case set to 10:
The output voltage V as funcion of the visibility.
Expression (1) can also be written:
Expression (2) can thus be used to convert the measured signal V to visibility. If the measured raw signal is too low, lower than noise and temperature drift we can't be sure that there is fog present. Therefore we get an upper limit on the measurable visibility. In the specifications we have set the limit to 5 000 meters. This is somewhat conservative. If the sensor is checked, cleaned and calibrated frequently the limit can be set higher. This also depends on the required accuracy in percent of measured visibility at the higher part of the visibility range. The processed digital output saturate at 10 000 meters visibility but the accuracy is limited above about 5000 meters.
In order to get a value of the visibility as experienced by the eyes, mean values of samples from about one minute from the output are taken and processed. If that is not done very large fluctuations in the signal will occur during fog. The reason is probably that there are spatial variations in the particle density that our eyes can't see.
Microprocessor analysis
The analysis described above is done in an integrated microprocessor. The primary signal is first sampled during one minute and then the calculation according to formula 2 is done.

Digital output
The calculated visibility is presented in digital form as an ASCII string on the RS232 output, 2400 baud 8N1, that is transmitted "streaming" every 60 seconds. Polling action can also be delivered as an option. At good visibility the string is typically:

"+0.00000,10000,05200,00005 ".

The first figure is the extinction,
the second is the calculated visibility in meters,
the third figure is a measure of the laser power
and the fourth figure is an uncalibrated measure of the ambient light

The relations between visibility, fog density and extinction are the following:
 
visibility= 3/extinction.

This string can be received by many loggers with RS232 inputs but also by a PC with a terminal program like Hyper Terminal (part of WIN-98) We recommend a freeware called Br@y terminal.

The microprocessor can as an option control a potential free semiconductor switch that is opened when the calculated visibility is below a certain value (default is 1000 meters). And the switch is closed when the visibility is higher than that value.

Microprocessor controlled analog output
The microprocessor also controls the analog output giving the visibility directly ( VIS =1 km gives 1 Volt, and VIS = 500 meters gives 0.5 Volt etc ). If the visibility is larger than 5000 meters the analog output will saturate at 5 Volts .This output is also updated every 60 seconds. During the first minute of operation after switch-on the signal on the analog output will therefore be zero. (This may be a bit confusing at setup)

Mounting the unit:
The unit should be mounted so that the laser beam is directed approximately north and horizontal, i e sunlight must not reach the detector. The beam should not hit anything within a distance of about 10 meters. Some simple shield, protecting the sensor from direct sunlight and the lenses from rain, reaching not more than 100 mm in the laser beam direction helps to keep away precipitation from the optics but in most cases the small shield on the sensor front is sufficient.

Electrical supply connections:
A floating DC 11-15 Volt, min 200 mA power supply is connected on the "Power"-terminal marked plus and minus on the screen print (red and black wire on the standard cable).
The outputs are connected to the "Analog" - yellow wire - and "RS232" terminals - green and white -. Texts on the screenprint on the PCB will define the signal found on the respective outputs. Note that separate ground wires are used for signal and power.


Some short data of the OFS sensor:
................(2)
Dimensions:
120*120*90 mm, for details see drawing
Weight:
about 1kg
Warm up time:
about 1 minute
Current consumption:
200 mA from a 12 Volt ( 11-15) supply (without heating about 60 mA)
Outputs:
RS232 2400 baud 8N1, analog 0-5 Volt
optional switch that changes state at 1000 meters visibility
Temp. range:
-20 to +50 deg C
Laser output power:
less than 5 mW. Laser safety class: 3R
Laser wavelength:
650nm
Housing:
IP 65 aluminium box, openings sealed with O-rings
Visibility range:
Visibility shorter than 10 km - down to 20 meters