To determine the surface absorptance of a material without knowing its radiant emittance, one can illuminate the surface using a square wave modulated or pulsed light source. The reflected energy should be detected as an AC signal, allowing for analysis of the surface's response. The DC output measured will correspond to the emittance of the material. This method enables the calculation of absorptance by isolating the effects of emittance. Accurate measurements can lead to a better understanding of material properties in thermal applications.