Albedo measurement

Albedo, also known as solar reflectivity, is the ability of a surface to reflect sunlight. The albedo of a surface also determines the amount of heat reflected by that surface. Albedo is an important aspect in the study of climate change. Albedo acts as an important climate regulator, as surfaces with a higher albedo reflect more solar energy back into space than they absorb, helping to cool the planet.

Human activities affect global albedo, and therefore climate. Air pollution, greenhouse gas emissions and changes in land use can alter the albedo of land surfaces, making them darker and contributing to increased heat absorption on the planet, creating hot spots in cities known as "urban heat islands".

One of the most striking examples of this human influence on albedo is the melting of ice in the Arctic. Ice reflects much of the sun's radiation, but the areas of ice that have melted are replaced by open water, which absorbs much of the sun's radiation.

What's more, cities, with their asphalt surfaces, have a lower albedo than the surrounding natural areas, making them less reflective and warmer. This urban heat island is caused by changes in land use around cities, the creation of roads and buildings, and other human activities. Cities with darker surfaces (bituminous concrete, slate roofs, etc.) tend to be warmer and absorb more energy.

In addition to climate and urban planning, there's one sector that needs quantitative albedo data: space. A launch vehicle or satellite in space is subject to a vacuum. This implies that thermal exchanges are exclusively radiative: heat dissipation by radiation from a body according to Planck's law weighted by emissivity, and solar gain weighted by albedo. If the latter is not controlled, it represents, for a surface above the atmosphere, a value of around 1,300 W/m². We therefore need to protect against it with an appropriate reflective coating.

For this, scientists need reliable tools to estimate albedo. Climatologists prefer remote sensing, i.e. satellite-based methods. In the urban planning sector, the preferred method is an in situ field method, and on certain structural elements a laboratory measurement. For the aeronautics and space sectors, laboratory methods are used predominantly.

Albedo is an extremely important parameter in the fight against global warming and urban heat islands. It is essential information for the thermal design of spacecraft. Its measurement meets strict normative requirements.

The article describes the concept of albedo and how it can be measured in the laboratory using a spectrometer that scans the entire solar spectrum. The albedo is then calculated by weighting the sun's...