Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 39
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the functionality of the Vaisala Transmissometer
LT31.
Operating Principles
This section describes the basic principles behind the operation of the
Vaisala Transmissometer LT31. The most direct and accurate method for
assessing visibility is the measurement of transmittance. Vaisala
Transmissometer LT31 directly measures the atmospheric transmission
between the light transmitter and the light receiver.
Concept of Visibility
Visibility is a complex psycho-physical phenomenon, closely tied to the
factors involving human sight. The estimation of visibility is subject to
variations in individual perceptive and interpretative abilities, as well as
the light source characteristics and transmission factors. Any visual
estimation of visibility is therefore subjective.
Human observations of visibility are more difficult to define and control
in daylight, as human observation is strongly dependent upon the
selection of lighted visibility markers. It is possible, however, to define
and estimate night-time visibility in terms of equivalent day-time
visibility to ensure that no artificial changes occur in the estimate at dawn
and at twilight. Although this approach has the advantage of permitting
the use of instrumental means to measure visibility, it does not always
meet the requirements of some users.
The concept of visibility is mostly used in meteorology in the following
ways:
- As one of the parameters identifying air-mass characteristics,
especially for the needs of synoptic meteorology and climatology.
For this purpose, visibility must be representative of the optical state
of the atmosphere.