Transmissometer LT31

The world’s best, most accurate, and most frequently used visibility measurement instrument for runway visual range

Model: LT31
Manufacturer: Vaisala – Finland


  • Single baseline for the MOR measurement range 10 … 10 000 m (33 … 32 800 ft)
  • State-of-the-art white light source
  • Automatic alignment with quality control
  • Automatic calibration
  • Internal back-up battery
  • Compatible with Vaisala

   Transmissometer MITRAS and

    SKOPOGRAPH II Flamingo

  • Window contamination compensation
  • Meets the ICAO and WMO requirements for RVR and visibility
  • Based on decades of field experience
  • Sophisticated self-diagnostics

Vaisala Transmissometer LT31 enables accurate and reliable single baseline measurement for CATIIIb category airports.

Accurate and reliable determination of runway visual range (RVR) reduces airport downtime and improves safety of operations. LT31 is the qualified observer of RVR providing correct visibility data without interruptions and with minimal maintenance

Measurement Range from 10 to 10 000 m (33 to 32 800 ft)

LT31 provides the most accurate and reliable solution for the automatic measurement of RVR.

The meteorological optical range (MOR) measurement range is 10 … 10 000 m (33 … 32 808 ft), which covers the full required RVR range (CATI … CATIIIb), as well as the range required for aeronautical visibility (the ICAO-defined visibility). LT31 accuracy meets the ICAO and WMO requirements. This full measurement range is achieved with a single baseline system, which makes measuring easier and more economical.

White LED Provides Wide Spectrum Light Source

LT31 incorporates a white LED as a ligh source. White light is needed for the best accuracy in transmittance measurement. The WMO recommends the use of a wide spectrum (white) light sources for transmissometers as narrow spectrum light source (for example, lasers or colored LEDs) will cause measurement error with some weather phenomena.

Automatic Systems

Automatic Calibration with Integrated Forward Scatter Sensor

Calibration of a transmissometer is traditionally based on human observations. A reliable and accurate calibration has required very high visibility, stable conditions, and skilled and well trained personnel.

The Vaisala-patented automatic calibration method for transmissometers is based on an integrated forward scatter sensor/present weather sensor. The system automatically detects drift and adjusts the sensor settings accordingly. Weather conditions do not need to be as good as for manual calibration; LT31 automatically recognizes suitable conditions.

Automatic Fine Alignment

One of the major error sources in transmittance measurement is alignment drift. Checking and adjusting the alignment has also required skilled and trained personnel.

To maintain the measurement accuracy, LT31 performs an automatic optimization of the alignment. The alignment quality is also continuously evaluated without any human intervention.

The automatic fine alignment also provides easy handling of LT31 during installation. The alignment quality during harsh weather conditions is ensured by a double mast construction. The outer tube works as wind and solar radiation shield. The inner support structure is separated from thermal and mechanical stress caused by solar radiation and wind load.

Contamination Reduction

In general, precipitation leads to an increased amount of window contamination. LT31 has long and narrow weather protection hoods that reduce the amount of window contamination caused by precipitation.

For wind-driven precipitation or dust, the instrument is equipped with a powerful blower. The blower creates an air curtain in front of the instrument window. The air curtain is specifically designed not to disturb the measurement path and cause the measurement errors that have been a problem with conventional blower designs.

Up-to-date Data

Automatic Window Contamination Compensation

Window contamination is a significant source of error in transmissometers. Maintaining high accuracy has required frequent cleaning. However, the effects of contamination can be automatically compensated if the window transmittance can be measured accurately.

In LT31, window contamination is also compensated for by the most accurate method: by measuring the transmittance directly through the window glass

Extensive Self-diagnostics LT31 has a sophisticated self-diagnostics that provides detailed status information for all functional units. In addition, this feature helps to locate possible technical failures. It records a history of significant operational situations, warnings, and alarms from the instrument.

Internal Backup Battery

LT31 can be equipped with an internal backup battery. It provides steady data availability during short power breaks, for example while backup generators are started.

Present Weather Reporting

The optional present weather reporting from RVR sites gives a full picture of the prevailing weather conditions within the entire airport area. The arrival of weather fronts and the presence of local showers can easily be monitored due to the multiple present weather observation points. The integrated present weather sensor provides the type and intensity of precipitation for METAR and local weather reporting purposes.

Background Luminance Sensor LM21

The optional LM21 sensor offers the means for measuring the ambient light level or background luminance in RVR applications. The background luminance sensor is used for measuring the background against which the runway lights or runway markings are seen.

Measurement Performance

Transmittance measurement resolution 20 bit
Recommended Baseline Lengths 
30 m (98 ft) baseline length (optimal) MOR: 10 … 10 000 m (33 … 32 808 ft) Transmittance range: < 0.01 % … 100 %
75 m (246 ft) baseline length MOR: 37.5 … 10 000 m (123 … 32 808 ft) Transmittance range: < 0.02 % … 100 %
Purchased Option
25 m (82 ft) baseline length 1/3 × baseline length to 10 000 m (32 808 ft)
Accuracy (RVR range) Exceeds the ICAO (Annex 3) recommendations
Output data Automatic or polled data messages Standard message including MOR, LT31 status and BGL data (option) Message including present weather data (option) 

MITRAS transmissometer compatible message 

SKOPOGRAPH II Flamingo transmissometer compatible message FD12 Visibility Meter compatible message 

Present weather reporting option Identifies 7 different types of precipitation (rain, freezing rain, drizzle, freezing drizzle, mixed rain/ snow, snow, ice pellets) 
Reports 49 different codes from WMO 4680 code table


Transmitter Optical Specifications


Light source White light-emitting diode (LED) – Certified class 1 product according to EN 60 825-1
Optical monitoring Light source stability control 

Window contamination measurement and compensation circuitry


Operating Environment


Operating temperature  -40 … +60 °C (-40 … +140 °F) standard 

-55 … +60 °C (-67 … +140 °F) hood heater option

Operating humidity 0 … 100 %RH
Wind speed Up to 60 m/s (134 mph)
Electrical safety IEC 60950-22 / EN 60950-22/A11 / 

IEC 60950-1 / EN 60950-1 / UL 60950-1


Inputs and Outputs


AC supply 100/115/230 VAC + 10 %, 50 … 60 Hz
Power consumption Max. 800 VA (for complete LT31 system) with all options
Battery backup option  Battery 2 Ah, backup time 60 min at +20 °C (+68 °F) when a proper and fully charged battery is used
Obstruction light Optional
Outputs Serial data line RS-232 or optoisolated RS-485 (2-wire) or optional data modem Separate maintenance line RS-232


Mechanical Specifications


Dimensions (H × W × D) 2685 × 420 × 1022 mm (105.71 × 16.54 × 40.24 in)
Transmitter Unit LTT111 85 kg (187.39 lb)
Receiver Unit LTR111 82 kg (180.78 lb)
IP rating IP66




Radiated emissions CISPR 22 / EN 55022
Conducted emissions AC CISPR 22 / EN 55022
Harmonics current emissions IEC 61000-3-2 / EN 61000-3-2
Electrostatic discharge IEC 61000-4-2 / EN 61000-4-2
RF field immunity IEC 61000-4-3 / EN 61000-4-311 V/m (80 MHz–1 GHz) 4 V/m (1 GHz–4 GHz)
Electric fast transient IEC 61000-4-4 / EN 61000-4-4
Surge IEC 61000-4-5 / EN 61000-4-5
Conducted RF immunity IEC 61000-4-6 / EN 61000-4-6

Transmissometer LT31 Datasheet


Vaisala Transmissometer LT31


A clearer approach to RVR


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