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In this thesis a proof-of-principle demonstration is developed that uses the heat flux between a probe and a sample as a proxy for their separation. The proposed architecture uses a probe that consists of a bilayer cantilever with an attached sphere at its free end. The deflection of the cantilever that is caused by the heat input is measured using the optical beam deflection method. To eliminate temperature dependent effects the temperatures of the probe and the sample are kept constant. Moreover, a total internal reflection microscopy is included to provide an independent measurement of the separation between the probe and the sample. This architecture allows the measurement of the heat flux as a function of only the separation. An equation is derived that relates the output signal of the instrument directly to the heat flux that is absorbed by the probe. It couples the top-level design parameters to the system output and is used to study and design the separate elements. In addition to the design of the instrument, the research contributes a detailed study of the influences of the microsphere and the microcantilever on the heat flux measurement.