Page 286 - Fundamentals of Light Microscopy and Electronic Imaging

CAMERA ACQUISITION PARAMETERS AFFECTING CCD READOUT 269

ANALOGUE AND DIGITAL CCD CAMERAS

As discussed in the previous chapter, CCDs can be used as photodetectors in analogue
equipment such as camcorders and video cameras. Digital and video CCD cameras dif-
fer in the way the photoelectron signal is processed and displayed. In digital systems,
quanta of photoelectrons stored in the pixels are sent as an analogue voltage signal over
a short distance to an analogue-to-digital converter (ADC) that changes the signal into
the binary code of the computer. The converter is contained in the camera head, or in the
camera control unit, or on a card in the computer. In video cameras there is no digitiza-
tion step, and the signal remains in analogue format as a variable voltage signal. Video
cameras also add synchronization pulses to the signal to generate the composite video
signal required by the raster scanning mechanism in the TV monitor. Because a video
CCD imager is read at a faster rate (30 frames/s or greater), read noise is higher, caus-
ing the dynamic range and S/N of a video image to be lower than in images produced by
a slow-scan digital CCD camera. Finally, video signals are commonly recorded on a
VCR tape, while digital images are usually stored as image files on the hard drive of a
computer. Sometimes the distinction between digital and video cameras becomes
blurred, since some CCD cameras contain dual video and digital output, and because
video and digital signals can be easily interconverted.

CAMERA ACQUISITION PARAMETERS
AFFECTING CCD READOUT AND IMAGE QUALITY

Readout Rate

Fast readout rates are needed for applications requiring high temporal resolution. At
very high readout rates such as those approaching a video rate of 30 frames/s, the image
appears live and does not flicker on the computer monitor, and with the addition of sub-
array readout and binning, acquisition at rates of hundreds of frames/s is possible. Most
scientific CCD cameras can be adjusted to operate at different readout rates ranging
from 0.1 to 10 MHz, the processing speed of the ADC and camera electronics (1 MHz
6
10 byte processing operations/s). However, high readout speeds increase the level of
noise in the image. Various noise components are always present in the pixels compris-
ing an electronic image, among which readout noise is one of the major sources.
Accordingly, low-intensity images with low pixel values should be read out at slower
rates to reduce noise and maintain an acceptable S/N ratio and image quality.

Subarray Readout

It is possible to define a small subset of pixels on the CCD (a subarray) corresponding
to only a portion of the full image area for acquisition and display on the monitor. The
subarray is defined by entering the boundaries of the region in the acquisition software
or by defining the region of interest (ROI) with a mouse on the computer screen. Subar-
ray readout is fast because the unwanted pixels are not processed by the ADC and are
discarded. Image files, particularly image sequences of time-lapse acquisitions, are also
correspondingly smaller and more manageable.