Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 41
In terms of transmittance, this can be written as follows:
ln (
t
)
V
N
=
ln(T)
x
B
where:
V
N
= So-called standard visibility.
t
= The contrast threshold, still just perceivable by the observer.
T = Transmittance, meaning the section of the incoming light
which remains within a focused beam after having passed the
distance B.
B = Distance.
The WMO recommends the value 0.05 for
t
. The standard visibility V
N
is called meteorological optical range (MOR) for this case:
ln (0.05)
MOR
=
B
x
ln (
T
)
Visibility Measurement
The LT31 directly measures the atmospheric transmission between the
light transmitter and the light receiver. It makes an assessment of the
mean extinction coefficient, including both the scattering and absorption
contributions to the measurement. The LT31 provides a reliable method
for assessing visibility or MOR.
MOR values can be further used to assess the runway visual range
(RVR).
The transmittance is measured using an effective baseline which is the
distance between the light transmitter protective window surface and
light receiver protective window. (For information about the
measurement unit's windows, see sections Introduction to LT31 on page
25 and Measurement Unit on page 32.)
Auto-calibration
The calibration of a transmissometer is traditionally based on human
observation.
Vaisala Transmissometer LT31 does no longer need to be calibrated
manually. LT31 uses an integrated forward scatter sensor of the Vaisala
PWD series to fully calibrate the system automatically.