Page 266 - Fundamentals of Light Microscopy and Electronic Imaging

DIGITAL IMAGE PROCESSORS 249

called the Nyquist criterion. (See Castleman in Shotton (1993) and Inoué and Spring
(1997) for theory and details.)

DIGITAL IMAGE PROCESSORS

A digital image processor (digital signal processor) is a valuable tool for increasing the
contrast, smoothness, and signal-to-noise ratio of the raw video image and for perform-
ing complex operations such as background subtraction and enhancement of image con-
trast. This unit (a dedicated computer) is positioned between the camera and the TV
monitor and other recording devices. The processor converts the raw image into digital
form by an A/D converter, performs image processing operations digitally, and recon-
verts the processed signal back to analogue form through a D/A converter. The final
processed image can be displayed on a TV monitor or sent to other devices such as a
thermal printer, VCR, or computer, which print, record, or receive the processed ana-
logue signal. Available image processing operations are displayed in a menu on the TV
screen and are activated by clicking on them with a computer mouse. The current
processors are quite sophisticated and perform many of the functions contained in com-
puter software programs used for processing and analyzing images from digital cam-
eras. Although they are expensive, image processors are indispensable for many video
applications. The operations performed by a high-end digital image processor include
the following:

Live: Displays the live raw image.
Real-time background subtraction: A background image containing image artifacts
such as dust, scratches, and uneven illumination is captured and stored in mem-
ory. The background frame is subtracted from each incoming raw image frame,
and the corrected background-subtracted frame is displayed in real time (30
frames/s) on the TV monitor.
Frame averaging: Improves the S/N ratio and smoothness of the image. The desig-
nated number of frames to be averaged is held in a frame buffer, and the average
is computed and displayed at video rate. At each calculation/display cycle, a new
frame enters the buffer and the oldest frame is discarded.
Frame accumulation: Accumulates the signals of the designated number of frames
into a single image frame to produce a clearer image (higher S/N ratio).
Subtraction: Does a simple subtraction of the input image from a memorized
image.
CCD on-chip integration: Integrates the image for the designated number of frame
equivalents instead of accumulating or averaging individual 1/30 second frames.
This procedure gives a clearer picture of low-light specimens.
Background subtraction plus accumulation: Combines the functions of background
subtraction and frame accumulation.
Sequential subtraction: Removes those features that remain stationary in a stack
with a designated number of frames. Only moving objects are displayed.
Rapid transition isolation: Removes image features from the processed image that
remain stationary or do not change in intensity, leaving an image representing
features that exhibit rapid change.