Phototransistor output voltage (spec(ular)/mirror voltage)
This parameter represents the intensity of the light reflected
on the mirror, that is, the thickness of the dew or frost layer
on the mirror.
Thus, this voltage data should be constant while the
humidity content is within the limit of the Peltier cooler.
In practice, we need the following empirical temperature
compensation to the measured voltage.
ptv2 = ptv * ( 1.46 + tref * ( -0.0133 + tref * 81.7 * 10^-6))
ptv2: compensated phototransistor voltage
ptv : measured phototransistor voltage
tref: Snow White reference temperature
When the mirror temperature is at the dew or frost point temperature,
the compensated voltage should be around 1.5 V.
If the mirror becomes completely clear up due to insufficient
cooling ability, the voltage will become around 2 V,
which is the dry mirror voltage.