How fast is fast? Sampling microsecond dynamics with a high frame rate camera

LAMBDA cameras are capable of taking 2,000 fps continuously without time

gap in between images, making them the fastest large format imaging

cameras currently available. But is this the limit? How fast can LAMBDA go,

what are the quickest dynamics you could image?

To answer these questions, we need to dive into several of the unique

features of the chip. The Medipix3RX chip has two counters in each pixel,

which can either be combined for a high dynamic range of 24 bits or used in

continuous read/write mode, where one counter is being used for counting

while the other one is read out. Switching the counters is easy and happens

instantaneously (i.e., in much less than 1 microsecond). The detector can

also be “gated”, i.e., it counts only when an external signal is provided, for

applications that require special acquisition modes.

Continuous read/write mode

In continuous read/write mode reading the detector out takes 0.5 ms to read

12 bits per pixel. But that's not the limit, we are currently developing faster

readout for lower counter depths. So if 6 bit counter depth is sufficient for

you, you can run the camera at 4,000 fps without time gap. Certain

application may even be possible with 1 bit readout (>= 1 photon or no

photons during counting time), which can run at up to 24 kHz.

24 bit mode

In 24 bit mode, there is one counter of 24 bit, which can be read out in 1 ms

- the dead time between two frames taken in this mode.

However the counting time can be selected by the user to be any value all

the way down to less than a 1 microsecond, at which point the shaping time

of the preamplifier limits further reduction of the counting time. Choosing

such a short shutter time will not give you more than 1 frame every millisecond (since the detector still needs to be read), but will allow

you to have very brief counting periods. These extremely brief counting periods can be advantageous for certain types of applications.

One example would be XPCS, where a long counting time would smear out the coherent properties of the system, resulting in reduced

contrast. Another example are certain pump-probe experiments where a repeatable quick response of the system is externally

triggered. Sweeping the delay between system and camera trigger the entire response dynamics can be sampled down to the

microsecond level.

Gating mode

Gating is a feature that allows multiple (usually very short) exposures that are added up within one frame. The advantage of this

approach is that starting and stopping of the counter is much faster than the 1 ms needed to read out the detector, this feature is also

sometimes called an “electronic shutter”. This mode allows you to take data exclusively during defined time windows, which can be

synchronized to your source or an experimental trigger, thereby suppressing background photons that are arriving outside of your time

window.

In summary

LAMBDA cameras are sensitive to dynamics on the microsecond time scale and offer multiple acquisition modes enabling up to 24 kHz

sampling frequency, making LAMBDA an excellent choice for a large class of XPCS and pump-probe type experiments.

© Copyright X-Spectrum GmbH 2014-2017 Impressum
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