Principles Of Operation - CODEL TunnelTech 601 Operation And Maintenance Manual

CODEL
Page 2 Operation and Maintenance

2 Principles of Operation

The TunnelTech 601 is built around a metal/glass encased reverse biased silicon diode photocell that
determines the luminance levels of a given area, together with an eye response optical filter with a
20° viewing angle. Optical filters are used to provide measurements of luminance to the required
standard human visible spectral response. The receptor is perfectly linear within the measuring range
and its response to light is virtually instantaneous, sending near constant updated information via a
4-20mA output.
The unit is placed at the safe stopping distance from the tunnel entrance, aimed and centred into the
portal.
The output of the photocell is fed into a low noise chopper stabilised amplifier with further stages
provide current limiting and gain adjustment before being fed into the 4-20mA output current
generator.
Protection against transients is provided for the PCB power input and signal output.
A thermostatically controlled heater is fitted to prevent fogging and correct operation.
Depending on primary power, either an isolation transformer or a DC to DC convertor is fitted to
supply power to the PCB. The cell is practically temperature independent, the correction being less
than +/-1º within 50ºC from the 25ºC calibration temperature.
The TunnelTech 601 complies with CIE recommendations that luminance should be monitored as the
luminance contained within a conical field of view, subtending an angle of 20° and mounted on the
tunnel approach road approximately 120 – 200 metres from the portal, depending on the approach
speed of the vehicles and their stopping distance.
Issue: A Revision: Date: 10/10/18 Ref: 100533