Sensor Timing, CW Sources and Pulsed Sources
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In the above discussion of sensor timing and triggering, we implicitly assumed that the LightLike source modules were of the continuous-wave (CW) type. Of course as a numerical code LightLike only computes wavefronts at discrete instants. The meaning of "CW" here is that a CW source can emit a wavefront wheneverrequired by the sensor timing and triggering parameters. Key CW sources in LightLike are UniformWave, GaussianCwLaser and PointSource. In previous discussion of sensors and timing, we noted two key facts:
(1) setting sensor timing parameters causes the so-called CW sources to emit a wavefront at corresponding discrete times (accounting for propagation delay);
(2) sensor output is always a temporally-integrated quantity (such as J/m2), over a sensor exposureLength.
From these two facts, it follows that a pulsed source can be easily represented by LightLike's so-called "CW" sources. We need only specify the correct combination of source strength and sensor exposureLength, so that the total energy emitted by the source during exposureLength is the desired pulse energy. This technique is an important alternative in LightLike to the use of an explicitly pulsed source (it may be, for example, that a source module with desired characteristics does not exist in pulsed form).
Nevertheless, LightLike does provide a number of source modules that, in certain respects, explicitly represent physically pulsed sources. An example is the module PulsedPointSource. The module names can unfortunately be a bit confusing, because (1) the names of the pulsed-source modules do not always parallel the CW modules, and (2) the names sometimes but not always contain the word "Pulsed" (just as CW sources only sometimes contain the word "CW") . The user should scan LightLike.Sources for the current list of available modules.
When a pulsed source module is used, the user must still set up sensor timing and triggering parameters exactly as explained in connection with the CW sources. The only really new feature is that timing parameters may be specified so that a source pulse may not be entirely contained within an exposure window. The pulsed-source modules are not designed to accurately model various temporal envelopes of pulses. In all pulsed-source modules, the intensity profile of each pulse is modeled as triangular in time, and symmetric. For example, the intensity of a one-microsecond pulse rises linearly from zero to its peak over the first half microsecond, then falls linearly to zero over the second half microsecond. As long as the entire pulse falls within the exposure window, all of its energy will be deposited on the detector. If a pulse arrival should overlap the beginning or end of an exposure window, only the corresponding portion of its energy will be detected (based on area under the triangle envelope). If no part of a pulse overlaps an exposure window for a given sensor, then the sensor will report zero energy for that exposure window.
Pulsed sources all contain a pair of parameters named "pulseInterval" and "pulseLength", and an input named "on". (For an example, see PulsedPointSource). The meaning of "pulseInterval" and "pulseLength" is self-explanatory. The input "on" has the same triggering functionality as the "on" input in sensor modules. That is, "on" can either:
(1) be assigned the value "true", which turns the source on at t=0, or
(2) be connected to a SquareWave module, which can then be set to trigger the first source pulse at an arbitrary start time. The section on sensor timing and triggering illustrates more fully the use of SquareWave for triggering. The triggering principles are the same whether applied to sensors or pulsed sources.
Although the extra functionality provided by the pulsed-source modeling is fairly restricted, the user may find it a convenient way to explicitly model the time multiplexing in physical source systems. The user should be aware, though, that this can always be done with LightLike's CW sources. Pulse energy can be represented as discussed in the introductory paragraph, and some adjunct procedures can be used to control which sensors see which sources. These adjunct methods include spectral filters and "polarization" tags. The use of "polarization" tags in LightLike is discussed in using Polarizers to separate light from different sources. We put "polarization" in quotes here because in LightLike the specified polarization state is just a numerical tag that allows one to differentiate two beams of the same wavelength. The main point we want to emphasize is that the choice between using a CW or a pulsed source is generally a matter of user experience or procedural preference, rather than strict LightLike necessity.