Page 265 - Fundamentals of Light Microscopy and Electronic Imaging

248 VIDEO MICROSCOPY

and an S/N ratio 10 might be considered barely acceptable. The S/N ratio is also a sta-
tistical parameter that expresses the confidence level at which fluctuations of a certain
percent can be detected. The meaning of this statistic is discussed in greater detail in
Chapter 14.

ALIASING

Aliasing is the phenomenon whereby periodic structures in an object are not faithfully
represented in the object image, but instead by a false period that can be mistaken for
being real. Aliasing (from the Latin alius, meaning another, other, or different) is an
imaging artifact and is a property inherent to all detectors with periodic structures such
as raster lines or pixels. We commonly observe aliasing on television. This occurs when
there is insufficient magnification produced by the lens of a TV camera focused on peri-
odic structures such as the pattern of pinstripes in an announcer’s shirt, bricks in the wall
of a house, or seats in an empty stadium. Under these conditions the images of these
objects exhibit false patterns of wide bands. An example of aliasing in a diatom imaged
by video microscopy is shown in Figure 13-12.
Aliasing has its origins in signal sampling theory and can be described mathemati-
cally. If the image of a specimen with periodic structure (a sine wave target or the peri-
odic pattern of pores in a diatom) is reconstructed from sample points that occur at a
much higher frequency than detail in the specimen, it is possible to reconstruct and dis-
play an accurate image of the specimen. However, at the point where the sampling fre-
quency is less than twice the specimen frequency, it is no longer possible to represent
the signal accurately, and a false period (aliasing) is observed. Below this sampling
limit, the display of periodic elements in the image actually increases as the sampling
frequency becomes further reduced. The factor of 2 requirement for oversampling is

Figure 13-12
Aliasing in a video image of a diatom. The spacing period between rows of pores in the image
becomes false (aliased) when the magnification is too low to allow proper sampling of the
image on the faceplate of the camera. Pleurosigma, DIC microscopy.