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I am using an LDR (Iduino SE012) to measure light. It returns a 10-bit value of the incoming voltage and I don't know how to convert this into lux, lumen candela or any other light-intensity unit. I was googling for datasheets but nothing useful came up. So is there any way how I can extrapolate a light intensity from the voltage?

I mean one idea that I would have is to measure some things where I know the lux output, but since I don't have such things or a luxmeter I'm pretty stuck. It also returns 1024 pretty quick if you hold it into the sun (Altough the sun is very strong today and it has 36°C). Is there maybe a "minimal value" at wich it returns the full voltage? From that point I could calculate it for all the other values.

Here are some datasheets I found:

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  • I'm in no way known on this subject, but if you have reference lights you could gauge/plot them to find the corresponding values for your sensor?
    – aaa
    Commented Aug 1, 2018 at 13:03
  • The cheapest lux sensor is the bh1750. If you buy a bh1750 you can make a table for the ldr to get the lux value. Maybe you can also add a ds18b20 temperature sensor to compensate the ldr value for the temperature. Then you can use the ldr... wait, did the bh1750 do all of that already? A light sensor may not be used with direct sunlight. Neither the ldr or the bh1750.
    – Jot
    Commented Aug 1, 2018 at 13:05

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You can't.

The LDR has a spectral sensitivity which is very different from the human eye's. The definition of the lux, or any photometric unit for that matter, pretty much requires that your sensor is filtered in such a way as to have the human eye's spectral sensitivity.

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  • Well maybe there's some interference of ultaviolet, radio waves and such things but these should be minor. And if you use it to measure sunlight the amount of visible light and UV-light should be relative to each other. So if I accept interference from other spectres would it then be possible?
    – Cowboy_Patrick
    Commented Aug 1, 2018 at 12:56
  • @The_javascript_King: Most of the spectral response misfit will come from near IR, rather than UV or radio. And it's nowhere near “minor”. If the light you measure has always the same relative spectrum (same relative intensities at various wavelengths), then yes, you could calibrate this in lux. However, the vis/IR ratio of sunlight depends on the height of the sun over the horizon, so you cannot assume a constant ratio.
    – Edgar Bonet
    Commented Aug 1, 2018 at 13:22

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