Background Luminance Sensor LM21


    Vaisala Background Luminance Sensor LM21


    The Background Luminance Sensor 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 to assess the visibility range of runway lights or warning lights.

    The LM21 features:

    • 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
    • Easy-to-use field calibration device available

    It integrates with the Vaisala Transmissometer LT31 and Vaisala Forward Scatter FD70, FS11, or FS11P to create an expanded runway visual range (RVR) system.


    To ensure reliable and uninterrupted operation, the LM21 has extensive self-monitoring functions and high-power heating features built into its design.

    Optical path clearance monitoring circuitry verifies measurements are not affected by obstructions. Attenuation compensation for window contamination ensures measurement accuracy between window cleanings, and also extends the cleaning interval.


    The calibration of LM21 is traceable to international measurement standards for luminous intensity. The calibration coefficients for light intensity have been defined during factory calibration and stored in the LM21 memory.

    Quick field calibration can be done using a unique field calibration device (LMA21) that provides a stabilized and diffused white light beam for LM21 calibration. Calibration starts automatically when the calibration unit is connected.

    Vaisala team collaborating in office

    Get Started with Background Luminance Sensor LM21 Today

    Contact Vaisala to learn how this stand-alone ambient light sensor can deliver crucial data while helping simplify your Runway Visual Range (RVR) system.