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TMP006 Fresnel lens

Josh Black
Prodigy 40 points
Other Parts Discussed in Thread: TMP006, TMP007

I've been working on a device that measures the depth and temperature of water using the TMP006 and Silicon labs' SI1141. I've been getting relatively stable results on heated water (10 - 50 degrees celcius) at depths between 5cm to 40cm. I'm now working on waterproofing the device and, as expected, a flat sheet of IR transmissive polypropylene is having an adverse effect on the TMP006. Could a polypropylene fresnel lens be used at this range as a water resistant cover, and if so, what kind of lens should I be looking for?

Kind regards,

Josh Black

  • 0
    TI__Expert 3370 points

    Hi Josh,

    You can definitely use a Fresnel lens.For a first order estimate on focal length there are a number of lens calculators on the web, here is one http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html

    Once you have the focal length based on object distance, look for a lens with F/1 aperature or smaller f-number  to gather sufficient  light.

    Polypropylene can defintely be used for a thin lens. In addition there are specialized plastic materials for use in IR lens applications, see for example Fresnel Technologies. The lens material should have good transmission in the 6 to 14um range of IR band to match the TMP006 responsivity. For prototyping purposes, Edmund Optics, ThorLabs and Newport may have something close to your needs in a catalog lens.

    You may also wish to consider the TMP007 which includes a math engine to calculate the temperature and several other features such as ALERT functions. The TMP007 has the same PCB footprint as the TMP006 so it is easy to evaluate both.

    Regards,

    Werner Metz

     

     

  • 0 in reply to Werner Metz
    TI__Intellectual 1811 points

    Josh,

       I've fooled around with an existing Polypropylene fresnel lens..  I purchased from Glolab.  It's a low cost way to do some quick analysis .

    Regards,

    Tommy Santoyo

  • 0 in reply to Werner Metz
    Prodigy 40 points

    Thanks for the quick reply. Would I be able to mount a small fresnel lens to the front of the TMP006/7, then cover the TMP006/7 & my infrared depth sensor with a slightly thicker sheet of acrylic dyed black/grey with an IR transparent dye?

    Regards,

    Josh Black

  • 0 in reply to Josh Black
    Prodigy 40 points

    Apologies, I meant polyethylene, not acrylic.

    Regards,

    Josh Black

  • 0 in reply to Josh Black
    TI__Expert 3370 points

    Hi Josh,

    In principle, yes but depends on the thickness in practics. 1-2mm thickness can work well with approx 50% absorption; more than that and the transmission goes down literally exponentially (See Beer-Lambert Law, http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law )

    Better to have the Fresnal lens be the only item in the TMP006/7 optical path . FOr the IR depth sensor, it depends on teh wavelength used, these are often in the near IR (0.8-1.0 um)

    Regards,

    Werner Metz

     

     

  • 0 in reply to Werner Metz
    Prodigy 40 points

    You are correct, the Si1141 sits between 750nm - 950nm. So, as opposed to covering the TMP006 with a fresnel lens, then covering the entire front of the device with a polyethylene cover, the best course of action would be to cut three holes through the housing - one for the TMP006, one for the depth sensor, and one for the IR led for the depth sensor - then cover the TMP006 hole with the fresnel lens and cover the depth/led holes with polyethylene covers?

    Regards,

    Josh Black

  • 0 in reply to Josh Black
    TI__Expert 3370 points

    Hi Josh,

    Yes, best would be to have three holes in the housing, and use the polyethylene Fresnel lens over the TMP006/7. For the near IR sensor, the polyethylene may cause too much scatter - there are glasses which will transmit with less scatter and might be a better fit

    Regards,

     

    Werner