Different surface features reflect and absorb the sun's electromagnetic radiation in different ways. The reflectance properties of an object depend on the material and its physical and chemical state, the surface roughness as well as the angle of the sunlight. The reflectance of a material also varies with the wavelength of the electromagnetic energy. The amount of reflectance from a surface can be measured as a function of wavelength, this is referred to as Spectral Reflectance. Spectral Reflectance is a measure of how much energy (as a percent) a surface reflects at a specific wavelength. Many surfaces reflect different amount of energy in different portions of the spectrum. These differences in reflectance make it possible to identify different earth surface features or materials by analyzing their spectral reflectance signatures. Spectral reflectance curves graph the reflectance (in percent) of objects as a function of wavelengths.
Spectral Reflectance of Earth Surface Features
In general, healthy vegetation is a very good absorber of electromagnetic energy in the visible region. Chlorophyll strongly absorbs light at wavelengths around 0.45 (blue) and 0.67 µm (red) and reflects strongly in green light, therefore our eyes perceive healthy vegetation as green. Healthy plants have a high reflectance in the near-infrared between 0.7 and 1.3 µm. This is primarily due to healthy internal structure of plant leaves. As this internal structure varies amongst different plant species, the near infrared wavelengths can be used to discriminate between different plant species.
In its liquid state, water has relatively low reflectance, with clear water having the greatest reflectance in the blue portion of the visible part of the spectrum. Water has high absorption and virtually no reflectance in near infrared wavelengths range and beyond. Turbid water has a higher reflectance in the visible region than clear water. This is also true for waters containing high chlorophyll concentrations.
Ice and Snow
Ice and snow generally have high reflectance across all visible wavelengths, hence their bright white appearance. Reflectance decreases in the near infrared portion and there is very low reflectance in the SWIR (shortwave infrared). The low reflection of ice and snow in the SWIR is related to their microscopic liquid water content. Reflectance differs for snow and ice depending on the actual composition of the material including impurities and grain size.
Bare soil generally has an increasing reflectance, with greater reflectance in near-infrared and shortwave infrared. Some of the factors affecting soil reflectance are:
- Moisture content
- Soil texture (proportion of sand, silt, and clay)
- Surface roughness
- Presence of iron oxide
- Organic matter content
Measuring Spectral Reflectance
There are many different ways that the spectral reflectance pattern of an object or surface can be recorded. Reflectance measurements can be made in the laboratory, in the field using a field spectrometer or captured by other remote sensors including those mounted on aircraft and satellite.
The level of detail of the reflectance curve (number of data points collected) depends on the spectral resolution of the sensor. Laboratory and field spectrometers typically collect hundreds of data points, measuring the percent reflectance of a material at hundreds of wavelengths. The Landsat 8 OLI (Operational Land Imager) only measures reflectance at nine different bands (or specific wavelengths) between 400 to 2500 nanometers, while advanced hyperspectral sensors like Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) measure 224 bands in the same wavelegnth range.
In the above image, the Landsat Thematic Mapper (TM) has only six bands in the visible to shortwave-infrared spectrum which isn't sufficient resolution to detect any absorptions typical in minerals. AVIRIS, however, has sufficient spectral range and resolution to resolve many common absorption bands found in a wide variety of minerals and other compounds.
U.S. Geological Survey Digital Spectral Library
The objective of the U.S. Geological Survey (USGS) spectral library is to provide reference spectra of minerals and other materials that could be encountered in nature and need to be distinguished in remote-sensing data. The spectral library is included as a key reference library in commercial spectral image processing software packages including ENVI.
Interactive Spectral Reflectance Graph
Use the below interactive graph to look at the spectral profiles of common surface materials. Data from the USGS Spectral Library.
Select Material to Graph