| | Optical properties of ice and snow
Optical properties of a material influence the way optical radiation reacts when hitting its surface. Each material has its own specific spectral signature due to the degree of reflection, absorption and transmission at different wavelengths of the received radiation.
Ice and snow generally show a high degree of reflection at visible wavelengths (VIS; ca 0,4 – 0,75 µm), lower reflection in the NIR (Near InfraRed, wavelength ca 0,78 - 0,90 µm) and very low reflection in the SWIR (ShortWave InfraRed, wavelength ca 1,57 - 1,78 µm). The low reflection of ice and snow in the SWIR is related to their microscopic liquid water content. (VIS and NIR together are often abbreviated as VNIR). However, the characteristic reflection varies with the actual composition of the material, and therefore differs for snow, firn, glacier ice, and dirty glacier ice.
Reflectance curves of snow, ice, vegetation, and water
Reflectance curves of snow, vegetation, water, and rock
The coloured area in the above graphic indicates the degree to which the Earth's atmosphere enables electromagnetic radiation of a certain wavelength to pass through. Zones with high atmospheric transmission are suitable to observe the Earth from space. The numbered rectangles indicate the spectral bands at which sensors, here ASTER and Landsat Thematic Mapper, record radiation.
The reflectance curves for firn and glacier ice are interrupted for the larger wavelengths because they would overlap with the reflectance curve of snow. The reflectance curve for dirty glacier ice is also interrupted at the larger wavelengths because reflectance would vary a lot depending on the type and amount of debris. It is thus not possible to draw a general reflectance curve for dirty glacier ice for larger wavelengths.
See an example (right).
| |
