In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. (…)
In SI derived units these are measured in lux (lx), or equivalently in lumens per square metre (lm·m−2).
Here are the values I am measuring over a day (there were some tests today, so please look at the general shape)
Since Wikipedia gives values of the order of 1000 to 10000 lux for a typical day, I believe the top chart is the illuminance it has in mind (in my case its values max out at 600, which is reasonable for today (a covered day, the sensor is not pointing directly outside)).
I am trying to guess what the other measure could be. It is not simply the scaled version of the top one and since [illuminance is] wavelength-weighted by the luminosity function to correlate with human brightness perception I wonder whether this could be the raw measure.
I found a discussion of the luminosity function but it does not seem to match the scaling: it rather seems that the bottom chart gives a preference for lower values of the illumination.
So your values would rather match with my lower graph (including the “logarithmic behaviour” = lower values being raised, higher lowered) but that graph is explicitly unit-less (while the other has units) and the scale is linear.
Bright sunlight is about 110,000 lx. My sensor reads a little low because of it’s position. Unless you were measuring indoor levels that top graph looks way too low.
I measure indoors “ambiant light” - the sensor points to a wall opposite of the one it is on, to avoid direct light from the window.
This is just the second day I have it so this is just a test - I will see what is best in terms of such a measure (and I will switch on the lights depending on a moving average anyway)
Just got this sensor running with zigbee2mqtt. The relationship is (for me, at least)
illumination_lux = e^(illumination/4342.79)). I was hoping for better resolution around 0 to 1 but not with this conversion anyway.
I just calculated mine: e^(15683/4342.79) = 37, this is also what is provided by the device itself.
You do not have the direct calculation in lux directly available?
This is what I see in the entity details:
Out of curiosity: how did you get that exponential relationship?
I just took a few (~20) values in range from 3 to 22000 lux with the “illumination” value paired and did a curve adaptation for A+Be^(Cx). Since it matched perfectly for every single value (bar rounding) I presume that’s what’s used internally.
My next plan is to verify the lux values against other sensors (I have one handheld and another sensor connected to a raspi that goes down to <1 lux).
