In a communications system, the signal level is the signal power or intensity at a specified point and with respect to a specified reference level, e.g., 1 mW.
A further mechanism by which the level of noise relative to the signal influences imaging capabilities is in the number of gray levels that can be distinguished in an individual image pixel, which in effect establishes a limit on the dynamic range of the image.
Since noise, because of its Poisson characteristics in the fluorescence microscope, is equivalent to the square root of the mean signal, the dynamic range is the signal divided by the square root of the signal, and is therefore equivalent to the square root of the signal.
As stated previously, the overall signallevel in biological fluorescence microscopy applications is typically low for several reasons, one of which is that fluorophore photobleaching upon illumination is always a factor, and must be minimized by limiting the total exposure to excitation light.
Instantaneous values of the input signal that are low, relative to the reference level, are increased, and those that are high are decreased.
Signal compression is usually accomplished by separate devices called "compressors." It is used for many purposes, such as (a) improving signal-to-noise ratios prior to digitizing an analog signal for transmission over a digital carrier system, (b) preventing overload of succeeding elements of a system, or (c) matching the dynamic ranges of two devices.
Signal compression (in dB) may be a linear or nonlinear function of the signallevel across the frequency band of interest and may be essentially instantaneous or have fixed or variable delay times.
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