Vaisala LM21 Technical data

P.O. Box 26, FIN-00421 Helsinki, FINLAND

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Vaisala Background Luminance Sensor LM21

Features/Benefits

• Intelligent, stand-alone sensor

• Extensive self-diagnostics

• Window contamination
measurement and compensation

• Optical path blockage detection

• High power heaters to prevent
snow accumulation

• Calibration traceable to
measurement standards

Va is a l a
Background
Luminance Sensor
LM21 is an
intelligent, stand­alone precision
photometer.

• Field calibration device available

www.vaisala.com

The Vaisala Background Luminance
Sensor LM21 is a part of Vaisala's airport
RVR assessment equipment. The
background luminance has an effect on
the distinctiveness of the runway lights
as seen by a pilot.

Resembles human eye

The LM21 is a precision photometer
with a spectral response similar the
human eye. The LM21 measures the
total amount of light coming in from an
angle of 6 degrees, and converts the
measured data to cd/m2. The LM21
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.

The LM21 can be connected to the
Vaisala Transmissometers LT31 and
MITRAS, as well as to the Vaisala
Forward Scatter Sensors FS11 and FD12.
The LT31 and FS11 are connected
using a RS-485 serial line. For the
MITRAS and FD12 connection, the
measurement result is converted to a
frequency that corresponds linearly to
cd/m2 divided by 10.

Comprehensive self­diagnostics

The LM21 is an intelligent, stand-alone
sensor. To ensure reliable and
uninterrupted operation, the LM21 has
extensive self monitoring functions with
high power heating features. The LM21
measures and compensates for the
attenuation effect of window
contamination. It ensures that
measurement accuracy is maintained
between window cleaning, and also
extends the cleaning interval. The
optical path clearance monitoring
circuitry verifies that measurement is
not affected by obstructions.
Tradi tiona lly, back gro und lumin anc e
sensors have either incorporated
rudimentary self-monitoring features or
none at all.

Easy to calibrate

The calibration of the LM21 is traceable
to the measurement standards. The
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 the LM21 calibration.
The calibration coefficients for light
intensity have been defined during
factory calibration, and stored in the
LMA21 memory. The calibration is done

automatically when the LMA21 is
connected to the Vaisala Background
Luminance Sensor LM21. The 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 the LMA21.

The Vaisala
Background
Luminance Sensor
LM21 measures
the ambient light
level or background
luminance in RVR
applications. In the
picture, the LM21
is installed to the
Va is a l a
Transmissometer LT31.

LM21

Technical Data

Operational

Measuring range 2 ... 40 000 cd/m
Measurement accuracy 10 %

Optical

Spectra l response 400 ... 700 nm,

resembles the spectral response of the human eye
Peak wavelen gth 550 nm
Field of view 6°
Effective diameter of reception lens 24 mm

Electrical

Connection power/signal cable provided
Power supply 10 VDC to 38 VDC,

or alternatively 8 VAC to 28 VAC, 6 W
Heate r powe r suppl y 28 VAC, 50 W
Output si gnal RS-485 (RS-232),

frequency output
Output signal frequency
(LM21FD and LM21MITRAS) 1.4 ... 3000 Hz

(corresponds to 4 ... 30 000 cd/m
Frequency signal amplitude 12 V
Frequency signal max.current 10 mA
Overvoltage protection power supply lines and output lines

are protected by current limiters

and transient suppressors

Environmental

Operating temperature -40 ... +65 °C
Operat ing humidity 0 ... 100 %RH

Mechanical

2

Weight 1.1 kg
Cable length 2.3 m
Cable length of LM21FD and LM21MITRAS 4 m
Color white
Housing aluminum, weatherproof
Mounting with a mounting clamp onto

the LM21 Support Arm of LT31 or

Option Support Arm of FS11

wi th S ens or F ast ene r As sem bly on t op o f

MITRAS Light Transmitter or FD12P

Electromagnetic compatibility

The LM21 sensor is CE-compliant. This compliance has been verified
according to the following EMC directives:
Radiated emissions EN55022
Radiat ed su sceptibili ty IEC 1000-4-3, 10 V/m
Conduct ed emissions EN55022
Conducted susceptibility IEC 1000-4-6
EFT immu nity IEC 1000-4-4

2

)

ESD immunity IEC 1000-4-2

Dimensions

Dimensions in mm.

142

31

154

100

Specifications subject to change without prior notice.
© Vaisala Oyj

Ref. B210425en 2004-03