A new liquid-crystal thermometry method is described to determine an isothermal map on a heat transfer surface coated with a cholesteric liquid-crystal layer that changes color according to temperature. This method is based on the use of a set of sharp band-pass optical filters, one of which is attached to a black-and-white video camera to take a monochromatic image having a specified color. From the image, an isothermal line was drawn with the aid of a digital image processing technique that excludes human color sensation. We obtain as many isothermal lines as band-pass filters and can determine an isothermal map. An experiment is presented as an application of the present method to measure temperature distributions on a heated surface cooled by air flow and disturbed by a short attached cylinder.