Lx values calculated incorrectly in Z2M

[MikaFromTheRoof]

I recently wondered why all the devices in Z2M always show at least 1 lx illuminance and never 0, even if they are in pitch black surroundings.

Because of that I did some research and found out, that the Zigbee standard defines the following formular for calculating the lux value from the raw illuminance value, that the Zigbee devices report:

lx = 10^(value/10000)-1
This has also been mentioned in this old issue: Koenkk/zigbee2mqtt#2837 (comment)

Back then the formular got corrected in the fromZigbee converter.

Now I found, that the formular used in the modernExtend.ts is wrong again:

zigbee-herdsman-converters/src/lib/modernExtend.ts

Line 811 in 5f2f223

result = 10 ** ((result - 1) / 10000);

The -1 is located in the wrong position, which would mathematically result in the lx value never being 0.

Is it possible to fix that globally in Z2M?

Thanks for great work!

[Koenkk]

Fixed, thanks!

Changes will be available in the dev branch in a few hours from now and in the next release which is every 1st of the month.

[andrei-lazarov]

Sorry, your "research" is just wrong. The formula was correct.

In Zigbee spec we get:
value = 10000 * log10(lx) + 1

Quick math:

value - 1 = 10000 * log10(lx)
(value - 1) / 10000 = log10(lx)
10 ^ ((value - 1) / 10000) = lx

Value 0 and 0xffff are special cases.

• see Illuminance Measurement Attributes (Value 0 and 0xffff) zigbee2mqtt#31431

It is expected that your sensor shows 1 lx. You can even see minMeasuredValue is at least 1

I'll revert this and see how to treat the special cases..

[MikaFromTheRoof]

Okay, good to know but also weird that according to the Zigbee spec you mentioned it is just impossible for Zigbee devices to ever report 0 lx.
I just wrote an external converter for one of my DIY Zigbee devices, that uses the formula I mentioned above and it works just fine. It is displayed as 0 lx and I don't have any problems or errors

[jamsoft]

I have some el cheapo Zigbee Tuya Lux sensors that report 0 lx. I stumbled here as I'm looking at your Zigbee Illuminance demo at the moment as I'm building one around a VEML7700.

[MikaFromTheRoof]

How are your Tuya sensors labeled inside of Z2M? I could imagine, that the converter for this specific device might be calculating the illuminance differently and therefore the raw illuminance values reported by that device might be converted to values down to 0 lx. Maybe we could look, how conversion is done there and use the same method for the modernExtend.ts, if it is done well.