Albedo
Posté le 21 November 2018 dans News
Albedo is the solar reflectance coefficient of a surface.
This dimensionless coefficient represents the ability of a surface to reflect sunlight.
Principle of albedo measurement
A surface is considered as white if albedo is greather than 0.8.
On the contrary, a surface is considered
principle of laboratory albedo measurement methodas black if albedo is less than 0.03.
Albedo measurement is important to determine the surface temperature under sunlight. aerospace engineers need this value because in the space there is only radiative heat transfers.
The ability of Earth to reflect sunlight is very important to predict temperature of the planet. Meteorologists need albedo for their calculation.
To measure albedo two approach are possible: a field condition method describe in ASTM E1918-06 and a laboratory approach.
Field method :
The folowing figure from ASTM E1918-06 shows the experimental dispositive. the principle consist to measure the incident flux with a pyranometer and to rotate the pyranometer for 180° to measure the reflected flux.
Albedo measurement according to ASTM E1918-06
This method is very usefull for measuring albedo of heterogeneous surface like soil and vegetation.Pyranometer used for albedo measurement: pyranometer
Laboratory method :
principle of laboratory albedo measurement method
the spectral reflectance is measured with a spectrometer and a summation is done weighted by the solar spectral irradiance. The solar spectral irradiance at sea level has been measured and the values are in the ASTM G173-03 standard :
solar spectral irradiance according to ASTM G177-03
albedo values for several surfaces :
albedo value