K8064 nonlinearity

Application description
Two dimmers are used to control two different incandecent lightbulbs of 240V/100W. While one (white bulb) is fading off, the other (green bulb) is getting more bright.

Input signal to dimmer
dimmer 1 (white) linear dc-voltage ramp from 1 to 9 V between 2 to 20 s
dimmer 2 (green) linear dc-volgage ramp from 9 to 1 V between 2 to 20 s
and vice versa

Problem
Brigthness of bulbs are changing by bounds (fits and starts) even dc-voltage ramp to ANOLOG IN of dimmer and internal ANALOG signal to dimmers PIC16F676 (IC1, pin9) do not show such unlinearities at all.

Hello,

Yes, this is true. The K8064 can not be linair because of the nonlinearity of the input section. This input section with an optocoupler is nesessary for the galvanic shielding (optical isolation) between your contol voltage and the mains circuit.
A second reason is that the ouptut power curve is not linair, even your load is not linair.