Minduck wrote:I testet it also whit my Aputure 120D (6000K).
The nearest I get was 5800K but most of the time it sets to 5100-5400K.
You're assuming that a cheap LED light is going to be producing light at the exact kelvin at which it is rated. That's rarely ever the case. Plus, LED lights have spectral spikes that also affect the read. And, even tungsten and HMI bulbs aren't exact and will drift over time as they age. So, no light is going to be exactly a specific temp or tint. For example, before a professional film shoot with multiple HMI lights, one of the prep tasks is to use an accurate color meter and determine which gels will need to be added to each light to bring them all as close as possible, and even then it's ballpark, not dead on. This is the case even for the most expensive ARRI HMIs of all the same model as the bulbs are never the same exact temp and tint.
Also to note is that unless you're in a completely neutral grey environment, the color of any light bouncing off the environment will definitely affect the color of the light reflecting off the grey card.
All this stuff is somewhat relative rather than absolute. That said, I do find that BMD's auto white balance sets the temp and tint quite well if I point it at a grey card.
If you are using different light sources or needing to make sure a white background is really reading as white, I recommend checking out this article by Art Adams on using RGB parade for matching and balancing lighting temps + tints on set:
https://www.provideocoalition.com/art-manual-white-balance/