thing as meteorological visibility. Similarly, on a hot day, it is not uncommon to
observe objects at a distance, and close to the ground, ‘shimmering’. This is a
form of degradation of quality due to atmospheric turbulence. These two optical
imperfections of the atmosphere are the subject of the next two chapters. In this
chapter we shall deal in depth with the problem of atmospheric attenuation.
Turbulence will then be discussed in depth in Chapter 16.
ln our considerations of atmospheric attenuation we shall be concerned with
the outdoor viewing situation. However, it should always be remembered that
any particles or vapour in an atmosphere will introduce attenuation effects, and
as such it is quite possible to meet atmospheric attenuation indoors. A good
example of a violent indoor effect is the effect of steam from a kettle or bath in
an enclosed space. A less obvious, but significant, one is the effect of tobacco
smoke in a cinema.
15.1 Basic attenuation mechanisms
15.2 Basic attenuation laws for horizontal viewing
15.3 Slant path viewing
15.4 Effect of structured illuminance of the viewing path
15.5 Measurement methods
15.6 Implications on modelling of visual thresholds
16. ATMOSPHERIC TURBULENCE
As briefly mentioned in the introduction to Chapter 15, a second atmospheric
optical effect which can be troublesome in viewing through any significant