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Data Acquisition Modes
A considerable part of the power of the PhotoniQ is in its ability to acquire large
amounts of data under very different conditions. Its flexibility and diverse functionality allow it to be
used in applications that include bioaerosol fluorescence detection, particle sizing, flow cytometry, confocal
microscopy, and gamma ray imaging. While the number and types of applications is extensive, the process used
for data acquisition generally falls into one of two categories — stochastic or continuous. For stochastic
systems, events of interest arrive randomly distributed in time often following a Poisson distribution.
The emphasis in a system of this kind is on collecting events uniformly over time at the fastest rate possible
— it is usually not essential that every event be captured. Particle analysis applications such as bioaerosol
detection and flow cytometry are typically in this category. Continuous systems on the other hand, are more
concerned with reliably capturing all events over a predefined interval of time. Although the rate of event
capture can also be critical, it is important that no events are missed during the acquisition interval.
A scanned imaging system like confocal microscopy is an example of a continuous data acquisition system. Here
events are more appropriately referred to as pixels and the acquisition interval as the
scan period.
The PhotoniQ accommodates these two broad classes of data acquisition by employing two distinct data
acquisition modes — particle and image. Particle mode is often used in stochastic
data acquisition applications where the PhotoniQ is configured to optimally acquire random events in the
fastest way possible over an indefinite period of time. Although the trigger source in these applications
can sometimes supply the PhotoniQ with triggers at extremely high rates, the PhotoniQ will only accept
a trigger if it can adequately acquire the event data across all channels, and transfer it to the PC. Any
trigger that would result in an event that cannot be properly acquired and processed is rejected. This
permits the PhotoniQ to operate at a relatively constant acquisition rate for an unlimited period of time.
The Sustained Average Event Rate (SAER) is the figure of merit that describes the data acquisition
performance of a PhotoniQ in particle mode. While a small data buffer in the PhotoniQ allows it to
acquire several consecutive events that briefly exceed the SAER, the maximum average event acquisition speed
is generally equivalent to this specification.
Unlike particle mode where priority is placed on acquiring events uniformly over time, image
mode places priority on acquiring each and every pixel over the scan period (the image frame). Thought of
another way, particle mode assigns priority to transferring all acquired data to the PC whereas
image mode assigns priority to accepting all triggers. In a scanned imaging system each trigger
corresponds to a pixel. Since the pixel rate in image mode can be quite high — sometimes much higher
than the transfer rate to the PC — a large, fast data buffer is used by the PhotoniQ to store a complete frame
of image data. Thus the PhotoniQ operates by accepting all triggers (pixels) up to the point where either the
triggers stop (the end of the scan period) or its buffer becomes full, and transfers the acquired data to the PC.
The Maximum Trigger Rate (MTR) defines the maximum rate of data acquisition in
image mode. Provided that the input trigger rate does not exceed the MTR, the PhotoniQ will fill its
buffer with each and every pixel until it is full. Transfer of the image to the PC takes place at a rate equal
to the SAER. Since in many situations the PC transfer time can exceed the image scan period, it is important
that the PhotoniQ buffer be sized to hold at least one complete image.
For a detailed discussion on PhotoniQ's acquisition modes, see the following application note.
PhotoniQ: Data Acquisition Modes
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