The albedo of a surface is the ratio between the overall intensity of light reflected from it and the incident intensity. In diffuse reflection the incident light is reflected at many angles, rather than at one angle as in specular (mirror-like) reflection. Surfaces for which the reflection is completely diffuse are called Lambertian, after the Swiss scientist Johann Heinrich Lambert (1728 - 1777) who first studied such effects. Fresh snow is one example of a nearly Lambertian surface; others include fine powders, foams, and many natural materials and surfaces such as grass, soil and concrete when viewed from an appropriately large distance. The closest approximations to an ideal Lambertian surface are provided by synthetic materials optimized for this purpose, such as "Spectralon." The albedo of snow and other terrestrial surfaces is a key factor in global climate change. For example, if artic ice melts, then the resulting open water surfaces or forests absorb far more incident solar radiation than the ice did, accelerating the melting process in a dangerous positive feedback effect.

In this project, we measured the albedo of snow and some other diffusely-reflecting extended surfaces (grass, concrete, dirt, a paved parking lot) using a simple but effective technique. Our technique consisted of alternately taking readings of the incident sunlight and light reflected upwards on to a silicon photodetector (Thorlabs DET-110) suspended face-down from a 2 meter high tripod. The photodetector is most sensitive in the red and near-infrared, with a peak sensitivity at about 900 nm. We found that the ratios of these readings were nearly independent of height from h = 10 - 100 cm and later explained this result by creating a mathematical model of the reflection process. Our measured albedos are in good agreement with literature values except in the case of snow, where we found R = 30%, significantly lower than the 40 - 90% generally reported. This unexpected result could have several possible explanations, but unfortunately we had no opportunity to repeat our one observation. In a separate series of experiments, still underway, we are studying the angular dependence of diffuse reflections from a sample of Spectralon material illuminated with red light (λ = 633 nm) from a 30 mW HeNe laser. We also hope to study other diffusely-reflecting surfaces such as fine white powder and paper towels.