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I'm currently working a project involving the VEML6040 color sensor (Link to application note). I am reading through the application note and stumbled on the graph depicted below. As far as i understand, this graph is about the responsivity of the sensor vs the wavelength of the light that it recieves.

However i don't understand the unit on the y-axis. It says relative responsivity (uW/cm^2). And then it says 2000,4000 etc.. How should interpret this? Does this mean that for example the blue channel will output a value of 6000 when light with a wavelength of 450nm and an intensity of 1 uW/cm^2 hits it?

enter image description here

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  • \$\begingroup\$ Why was this question downvoted? \$\endgroup\$ – Bort May 4 '18 at 14:30
  • \$\begingroup\$ I first linked to a wrong datasheet (fixed it later), i think that has been the reason. \$\endgroup\$ – Mauricio Paulusma May 4 '18 at 14:35
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That graph doesn't make much sense. The Y axis reads Relative Responsivity and gives a unit of µW/cm², which is not a relative unit.

It is probably the count value they obtained with some fixed irradiance and varying wavelength for the different sensors. This doesn't help much without knowing their test setup, which isn't given in the application note.

So I would just ignore this and work with the datasheet, which is still a bit of a mess, but gives better values to work with.

First we have the irradiance response given in counts per µW/cm² for different wavelengths: irradiance responsivity

Which is closer to what you need. But then they chose the wavelengths to measure this not at the peak responsivity. Which are:

peak responsivity

Which does not correspond to the plot they give later on: Normalized Response

So you can now scale the curves according to the given points in the table. Which would make the peak of red read something like 96/0.7 = 137 counts/(µW/cm²), the peak of green 74/0.5 = 148 counts/(µW/cm²) and the peak of blue 56/0.9 = 62 counts/(µW/cm²). For accurate numbers use Engauge Digitizer.

I have no idea what is going on with white. It's almost non existent in the datasheet and the normalized response shows it being very sensitive in IR, which at least I cannot see and I usually don't count as white...

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  • \$\begingroup\$ Aah alright it's clear to me now. Many thanks for the torough answer! And also thank you for the tip of Engauge Digitizer, didn't knew that piece of software. \$\endgroup\$ – Mauricio Paulusma May 4 '18 at 14:14
  • \$\begingroup\$ @MauricioPaulusma you are welcome. I discovered Engauge Digitizer a few years ago and find it immensely helpful with getting values out of datasheet graphs. \$\endgroup\$ – Arsenal May 4 '18 at 14:16

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