Tuesday, February 02, 2016

ESPROS Talks about ToF Imaging Challenges

ESPROS Photonics' mailed me the Jan. 2016 Newsletter updating on the company's ToF business success and technical challenges:

"The ESPROS TOF technology is gaining huge momentum. The delivery start of the 3D-TOF QVGA imager was excellent. Customers immediately recognized the performance of our OHC15L technology. A QE of more than 80% at 850nm wavelength and pixels with 100% fill factor provide unmatched sensitivity with very low illumination power. A reference design of a camera engine with a horizontal FOV of 94° achieves a 10m range in the full field. Ok, this is on a white target. But the exposure time is a few milliseconds only."

Then, the newsletter shares the company experience dealing with temperature drift challenges:

"Several components of a TOF system are temperature sensitive: On the 3D TOF chip e.g. pixel-field, LED driver and the phase detector (DLL), etc.; on camera level e.g. the illumination (e.g. LEDs, VCSELs), illumination driver, connectors, IR filters, lenses, etc. Figure 1 shows the signal path with the contributors to temperature drift.

The main contribution comes from the pixel-field which is approx. 85% of the total distance drift by temperature (Figure 2).

The temperature drift in the pixel field is related on the one hand to the drift speed of electrons in solid which changes by temperature. It is a physical constant so it can be compensated by simple temperature measurement of the chip temperature. On the second hand, the drift speed is also related to the field strength in the photodetector. This field is
generated by the the backside voltage VBS. A simple rule says the higher the voltage the faster the electrons (Figure 3). However, there is a voltage limit where other effects start to become dominant. It is to note that a variation of VBS varies the measured distance. Thus, an accurate and constant VBS voltage is mandatory for accurate measurement.

Figure 4 shows a 2nd order fit for the compensation of the temperature drift in the pixel field."

1 comment:

  1. It looks like there are three tempco's in the system emitter,detector and the circuitry. Few questions

    1. Is the drift(thermal) behavior signal dependent.
    2. Is this behavior repeatable from system to system (can we use a golden set of coefficients on all the systems).
    3. How does the system behave in the presence of ambient lighting and temperature varying simultaneously.


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