Background Luminance Sensor LM21

Fully featured ambient light sensor for Runway Visual Range (RVR) applications

Model: LM21
Manufacturer: Vaisala – Finland


  • Intelligent, standalone sensor
  • Verified accuracy and photopic response
  • Extensive self-diagnostics
  • Window contamination measurement and compensation
  • Optical path blockage detection
  • High power heaters to prevent snow accumulation
  • Calibration traceable to measurement standards
  • Field calibration device available

Vaisala Background Luminance Sensor LM21 is an intelligent, standalone precision photometer.

Vaisala Background Luminance Sensor LM21 is a state-of-the-art luminance sensor for runway visual range (RVR) assessment. The background luminance has an effect on the distance from which the pilot can see the runway lights.

Resembles Human Eye

LM21 is a precision photometer with a verified photopic spectral response. It measures the total amount of light coming in from an angle of 6 degrees, and converts the measured data to cd/m2. The sensor sends the measurement data to the interface unit of a Vaisala transmissometer or forward scatter visibility sensor. The interface unit combines both the visibility and background luminance data into the same message and sends it to the main RVR computer.

LM21 can be connected to Vaisala Transmitter LT31 and Vaisala Forward Scatter Sensor FS11. LT31 and FS11 are connected using a RS-485 serial line

Comprehensive SelfDiagnostics

LM21 is an intelligent, standalone sensor. To ensure reliable and uninterrupted operation, it has extensive selfmonitoring functions with high power heating features. The sensor measures and compensates for the attenuation effect of window contamination. It ensures that measurement accuracy is maintained between window cleanings, and also extends the cleaning interval. The optical path clearance monitoring circuitry verifies that measurement is not affected by obstructions.

Easy to Calibrate

The calibration of LM21 is traceable to international measurement standards for luminous intensity. Vaisala Field Calibrator LMA21 is a unique field calibration device that is available for quick field calibration. It provides a stabilized and diffused white light beam for LM21 calibration.

The calibration coefficients for light intensity have been defined during factory calibration and stored in the LMA21 memory. Calibration starts automatically when LMA21 is connected to LM21. LM21 reads the calibration coefficients from the LMA21 memory and performs the calibration. The status and result of the calibration is displayed with 5 LED indicators on the cover of LMA21.

Measurement Performance


Measurement range 2 … 40 000 cd/m² 
Accuracy ±10 %


Optical Specifications 


Spectral response 400 … 700 nm, photopic according to CIE standards
Peak wavelength 553 nm ± 5 nm
Field of view 6° (94 % of measured luminance)
Effective diameter of reception lens 24 mm (0.94 in)


Operating Environment


Operating temperature -55 … +65 °C (-67 … +149 °F)
Operating humidity 0 … 100 %RH


Inputs and Outputs


Connection Power/signal cable provided
Power supply 10 … 38 VDC, or alternatively 

8 … 28 VAC, 6 W

Heater power supply 28 VAC, 50 W
Output signal RS-485 (RS-232), frequency output
Overvoltage protection Power supply lines and output lines are protected by current limiting series resistors and transient suppressors 


Mechanical Specifications


Weight  1230 g (2.71 lb)
Cable length 2.3 m (7 ft 7 in)
Color White
Housing Aluminum, weatherproof
Mounting With a mounting clamp onto LM21 Support Arm of LT31 or Optional Support Arm of FS11




LM21 sensor is CE compliant. The compliance has been verified according to the following EMC product family standards:
Electrical equipment for measurement, control and laboratory use – EMC Requirements IEC 61326-1: 2012-07 (Edition 2.0), Environment class: Industrial
Electrical equipment for measurement, control and laboratory use – EMC Requirements EN 61326-1: 2013-01 (Edition 2.0), Environment class: Industrial

Background Luminance Sensor LM21


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