Measuring the albedo of limited-extent targets without the aid of known-albedo masks

•Methods to measure the albedo of limited-extent targets are summarized.•A new mask-free method is proposed.•The mask-free method does not need a known-albedo mask(s) for calibration. Albedo is a non-dimensional parameter used to quantify the proportion of solar radiation reflected by a surface. While a couple of methods are available for measuring the albedo of limited-extent targets, these methods require known-albedo mask(s) for calibration, which interrupt the measurement by manual intervention and make continuous albedo measurements difficult. Here we propose a mask-free method for measuring the albedo of limited-extent diffuse targets without the aid of known-albedo masks for calibration. The albedo of a limited extent target can be measured by leveling an albedometer over the target and assembling a baffle around the lower detector of the albedometer to make the detector receive reflected radiation from the target only. The view factor from the detector through the baffle to the target is found by leveling the albedometer over a large open field with uniform background and then dividing the reflected radiation at the baffle-on case from that at the baffle-off case. Albedos of seven 1 m × 1 m flexible uniform sheets with different colors were measured using the proposed mask-free method and the methods with known-albedo masks for calibration. The results indicated that the mask-free method measured the albedo of limited extent targets as accurately as the methods with known-albedo masks for calibration.