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Forum Index : Microcontroller and PC projects : picomite mmbasic howto
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| Frank N. Furter Guru  Joined: 28/05/2012 Location: GermanyPosts: 959 |
Hi Volhout, our intern would like to use the MLX90614 for a measurement. However, with your code (MLX90614-DCI_V02.zip), we are getting values that deviate by approx. 3°C (IR values too low, T ambient too high). Do you have any idea why this might be? Thanks!!! Frank |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5367 |
Hi Frank, The inside of the sensor (with it's DSP) could be few degrees warmer than actual ambient. I see 1-2 degrees difference. The IR value being different is (I expect) caused by the emission coeficient of the surface you are probing. I have several sensors here at my desk that are within 1 degree C when pointed at a rough black surface at controlled temperature. Shiny surfaces should be avoided. On unknown surfaces I add a black sticker before measuring. You can make a suitable surface with a piece of kitchen alu foil that you blacken with smoke or charcoal. Another reason for the lower value could be that the emissive surface is not lighting the pixel sensor completely. Calculate the (6 degrees opening angle) surface you are viewing, and the size of the hot surface/component. If you are observing a small spot, the camera lens should be right above it, and not at 20mm distance. I think the program you are using has some compensation value hidden somewhere that I took out in later versions. My latest version is here (for Game*Mite) MLX90614 Volhout P.S. if you know the emission coeficient, you can program it's value in the sensor (register 4). Edited 2025-10-09 00:01 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Frank N. Furter Guru Joined: 28/05/2012 Location: GermanyPosts: 959 |
Hi Volhout, thank you very much for your link! The only difference I can see is the averaging function you have implemented in the new version. We have now compared the values with an MLX90640 and a commercial IR thermometer – there are actually no significant differences... Frank |
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| Frank N. Furter Guru Joined: 28/05/2012 Location: GermanyPosts: 959 |
Hi Volhout, have you evver changed the emissivity factor? I can't figure out from the data sheet what I need to enter where...  Frank |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5367 |
Hi Frank, For the MLX90614 I did never change the emissivity factor. I saw it was register #4, but I have not got into it to find the format (fixed point/floating point/divider ratio). If you look at the datasheet for the MLX90641, you see that they pack data in these registers in the strangest ways. I had to decode the formats for my thermal camera. Maybe the format is the same as in the MLX90641 (or 90621 for that matter). Volhout PicomiteVGA PETSCII ROBOTS |
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| Frank N. Furter Guru Joined: 28/05/2012 Location: GermanyPosts: 959 |
Yes, it seems very unclear how this factor is created and transferred.  Thank you for your reply!!! Frank |
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