Optical methods for measuring local heat transfer coefficients using thermochromic liquid crystals are discussed. Two techniques using radiative steady state and transient heating have been used to measure local heat transfer on the fin of a plate fin-tube heat exchanger. It is estimated that the errors in the steady state technique should be no more than +/- 10% and the results are given. The mean value of the heat transfer coefficient over the whole surface is 35 W m(-2) K-1, rising to 73 W m-2 K-1 at the leading edge. The transient method does not give results consistent with the steady state method when heat transfer coefficients are based on the fluid inlet temperature. It is suggested that the steady state method is preferable since it yields numerical values that are closer to those based on local temperature and hence easier to interpret when observing the effects of changing the local surface geometry.