  Aerosol particles experience significant photophoretic forces at low
pressure. Previous work assumed the average particle temperature to be very
close to the gas temperature. This might not always be the case. If the
particle temperature or the thermal radiation field differs significantly from
the gas temperature (optically thin gases), given approximations overestimate
the photophoretic force by an order of magnitude on average with maximum errors
up to more than three magnitudes. We therefore developed a new general
approximation which on average only differs by 1 % from the true value.
