Page 288 - Fundamentals of Light Microscopy and Electronic Imaging

IMAGING PERFORMANCE OF A CCD DETECTOR 271

DR S/N

0 2 4 6 8 10
(4×) (2×) (1×)
Gain (e/ADU)

Figure 14-10
The effect of gain on dynamic range and signal-to-noise ratio. As gain is increased, the
number of electrons/ADU decreases. For example, for a gain factor of 1 , 2 , and 4 , the
number of electrons/ADU is typically 8, 4, and 2 electrons/ADU, respectively. By convention,
1 gain is usually defined as the saturating number of electrons per pixel divided by the
read noise of the camera. For images exposed to give a constant accumulated electrons,
increasing the gain causes the dynamic range (number of gray level steps) to increase
exponentially. While having a large number of gray levels can be beneficial, notice that as the
gain increases, the S/N ratio, the measure of signal clarity, decreases. For images exposed
to give a constant number of ADUs, increasing the gain decreases the S/N.

where the signal voltage is amplified by multiplication by a constant factor. Gain is usu-
ally applied when there are a limited number of photons and it is desirable to utilize a
large number of gray levels. The disadvantage of increasing the gain is a corresponding
decrease in the accuracy of digitization; at high gain, the noise from inaccurate digitiza-
tion can cause images to look grainy. There is a limit to how much gain can be applied,
because at very high gain, image quality deteriorates. Nevertheless, by increasing the
gain, the exposure time can be reduced, while retaining a large number of gray levels.

IMAGING PERFORMANCE OF A CCD DETECTOR

Image quality can be described in terms of four quantifiable criteria: resolution of time
(sampling rate), resolution of space (ability to capture fine details without seeing pix-
els), resolution of light intensity (number of gray-level steps or dynamic range), and
signal-to-noise ratio (clarity and visibility of object signals in the image). As we will
see, it frequently occurs that not all four criteria can be optimized simultaneously in a
single image or image sequence. For example, to obtain a timed sequence of a live flu-
orescent specimen, it may be necessary to reduce the total exposure time to avoid photo-
bleaching and phototoxicity. This can be accomplished by exposing the specimen less
often (loss of temporal resolution), binning the image (loss of spatial resolution), and/or
by applying a high gain (reduction in dynamic range and S/N ratio). Alternatively, to
maximize dynamic range in a single image requiring a short exposure time, you could