Friday, May 07, 2021

Chronoptics on iToF Camera Design Challenges

Chronoptics CTO Refael Whyte publishes an nice article "Indirect Time-of-Flight Depth Camera Systems Design" about different trade-offs and challenges in ToF cameras. Few quotes:

"The table below compares two image sensors the [Melexis] MLX75027 and [Espros] EPC635, both of which have publicly available datasheets.


The MLX75027 has 32 times more pixels than the EPC635, but that comes at a higher price. The application of the depth data dictates the image sensor resolution required.

The pixel size, demodulation contrast and quantum efficiency are all metrics relating to the efficiency of capture of reflected photons. The bigger the pixel active area the bigger the surface area that incoming photons can be collected over. The pixel’s active area is the fill factor multiplied by its size. Both the MLX7502 and EPC635 are back side illuminated (BSI), meaning 100% fill factor. The quantum efficiency is the ratio of electrons generated over the number of arriving photons. The higher the quantum efficiency the more photons are captured. The demodulation contrast is a measure of the number of captured photons that are used in the depth measurement.

Illumination sources should be designed for IEC 60825–1:2014, specification for eye safety. The other aspect of eye safety design is having no single point of failure that makes the illumination source non-eye safe. For example, if the diffuser cracks and exposes the laser elements, is it still eye safe? It not the crack needs to be detected and the laser turned off, or two barriers used incase one fails. Indium tin oxide (ITO) can be used as a coating, as it is electrically conductive and optically transparent, the impedance will change if the surface is damaged. Or a photodiode in the laser can be used to detect changes in the back reflection indicating damage. The same considerations around power supplies shorting and other failure modes need to be considered."

6 comments:

  1. Federico Montagni , R&D ESPROSMay 11, 2021 at 2:18 PM

    Just to mention, that the epc635 has a quantum efficiency of >50% @ 940nm and a full well depth of 8’000 ke- (yes: 8 Me-) including an automatic common mode rejection in the charge domain. Perfect for any long range outdoor application.

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    Replies
    1. Any reason those numbers are not on the datasheet?

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    2. In epc635 datasheet, table 5 and 6 (see dowload area on espros.com), the data needed to design a 3D TOF camera is listed.
      QE and full well capacity are not really the direct parameters to define the detection sensitivity and the ambient light suppression capability.
      However, you are right, QE and FW capacity are values to compare different vendor technologies.

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  2. Could you elaborate on that conclusion a bit more? an increased quantum efficiency will also lead to increased sensitivity to background light I believe

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    Replies
    1. Yes, the quantum efficiency will improve the capture of all photons. ToF cameras are active illumination sensors, and power is required to generate the signal photons, by improving the quantum efficiency of the sensor it reduces the power of the light source required, as it makes the entire system more efficient.

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  3. Federico Montagni, ESPROS Photonics AGMay 12, 2021 at 12:49 PM

    The tables 5 and 6 in the datasheet of the epc635 show the data needed to design a 3D TOF camera. QE and full well capacity are not really the direct parameters to define the detection sensitivity and the ambient light suppression capability. However, you are right, QE and FW capacity are values to compare different vendor technologies.

    ReplyDelete

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