The invention of lasers in the early 1960s enhanced the rapid
development of optoelectronics which had introduced various optical
measurement methods. A typical example of the methods is found in
measurements of velocity. It is well recognized that optical velocity
measuring methods have important advantages, such as noncontacting and
nondisturbing operations, over c- ventional methods employed previously.
These fundamental advantages are indicated by the enormous research
e?ort which has gone into their devel- ment for many years. One of the
optical methods proposed and studied to measure the velocity is laser
Doppler velocimetry which was proposed in the early 1960s and
extensively studied by many investigators and is at present applied to
practical uses. Another is spatial ?ltering velocimetry which was also
proposed in the early 1960s and studied by a number of investigators. In
comparison with laser Doppler velocimetry, spatial ?ltering velocimetry
had not received much attention from investigators but was studied
steadily by several research groups mainly in Japan and is now
practically used in various ?elds of engineering. Several important
books on laser Doppler velocimetry have already been published, but
there has been no book on spatial ?ltering velocimetry. This book is the
?rst contribution to spatial ?ltering velocimetry. Therefore, the
Introduction of Chapter 1 provides in detail a historical review of
spatial ?ltering velocimetry, relating it to other optical methods and
discussing its practical relevance. In the book following Chap.
