Reflectivity in Remote Sensing

Transcription

1 Reflectivity in Remote Sensing The amount of absorbance and reflection of white light by a substance is dependent upon the molecular makeup of the substance. Humans have used dyes for years to obtain colors- not realizing that the molecular properties of that substance meant that the reflected wavelengths were those seen by the eye, while the other wavelengths were absorbed. White light (visible light) from the sun is composed of all visible wavelengths, which appear to us to be different colors when separated by a prism or diffraction grating. Solids, liquids and gases tend to absorb frequencies which will cause electrons in their atoms to jump up to the next energy orbital (quantum energy). For solids, this results only in vibration and for liquids and gases this energy usually translates to movement, although some atoms and molecules may produce light or sound energy as well. By definition, a transparent substance is not absorbing visible wavelengths at all. The wavelength of the light is the wave distance from crest to crest, usually in nm. The energy is measured in joules, the same unit for work. Frequency is measured as cycles/sec or hertz, or the number of wave crests which pass a given point per second. Frequency is directly related to energy and indirectly related to wavelength. This means that very short waves are high in energy and have high frequencies (such as gamma rays).

2 All of these parameters are tied together in several equations: V = λ ν where wave velocity is equal to wavelength x frequency And ΔE = h ν where energy change (of electron) is equal to Plank s constant x frequency. Is it possible for substances to absorb electromagnetic radiation in other than visible wavelengths? Yes, and the infrared or ultraviolet radiation that is absorbed can be used to identify substances. It takes special instruments to detect the absorbed frequencies since we cannot see any of this radiation (IR and UV spectrometers). Interestingly, satellite images commonly show not only visible light reflectance but shortwave infrared reflectance as well, giving interesting coded color images. Longer-wave infrared radiation is also known as heat radiation and sensors can pick up on radiated heat, generally called emissivity. The wavelengths reflected by earth substances (and manmade) can help to identify them. The following list provides some reflectivity values for commonly found earth materials: Percent reflected energy from Earth's surface objects/features

3 Below is a graphical representation of average vegetation, soil and water reflectivity in visible and infrared wavelengths ( Note that the visible wavelengths range from approximately 0.4 to 0.7 um. Plants tend to absorb particularly in the red and somewhat in the blue wavelengths, reflecting in the green. Note also that plant reflectance rises sharply in the near infrared. Soil reflectivity appears to be low in red and violet wavelengths, and higher in central wavelengths. Water reflects primarily in blue wavelengths and somewhat less in all other wavelengths. Near infrared reflectance is very low for water and moderate for soil. Typical spectral signature of vegetation, soil and water

4 Reflectivity laboratory A simple laboratory setup can give relative reflectance of a number of substances. Materials: 1. Light box with 5 cm aperture attached downwards on ring stand (we used commercial optics box, but any box would work). 2. Cardboard platform held up by clamps to two ringstands about 10 cm below light source. 3. Light source of about 40 W (any power will work since readings are relative). 4. Six-volt power source with sufficient current for light source (1.5 V batteries in series not recommended). 5. Light meter with sensor fixed next to light box, but not exposed directly to light source. Note angle of reflection.

5 6. Several materials for reflectivity testing. (see table for our list). Procedure: Materials were sequentially placed on the platform, and the illuminance read from the light meter for each of them and recorded. The entire procedure was carried out in the DARK, since ambient light will interfere with light meter readings. The DARKER the better, but get a flashlight for moving around. Data Optimum angle of reflection was noted. In this case, this occurred at an angle of 17.5 degrees from a normal directed perpendicular to the platform. This angle was found by moving a mirror along the platform until the highest reflectance occurred. A mirror was the most highly reflective material used in this experiment, at 98% reflectance, and the value of reflectance at the optimum angle, 115 lux, was taken as maximum reflectance. All other reflectance values were compared to this on a percentage basis.

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7 Sample illuminance (lux) % reflectance value from literature ( Cardboard Black fabric 8 7 Rusty metal Aluminum Brass Nickel White Styrofoam Ice cubes snow 50-95% White paper Silica mesh Glass over cardboard Navy blue vinyl Glass over white paper Petri dish and 1 cm water water with sun from zenith to horizon: 3-80% Petri dish and 1 cm water water with sun interpolated at 17.5 degrees: 25% With blue dye Dry hay crops 10-25% Wet sand wet soil: 15 25% Dry sand dry soil: 20 25% Dry black potting soil dark soil 5 15% Wet black potting soil 10 9 White marble, polished Unpolished white ceramic grit sandpaper 24 21

8 Relation to remote sensing Visible and near-infrared satellite images can be used to show ground cover reflection and illustrate how contrast helps create the imagery. Information on type of groundcover, reflectance and emissivity often accompany the images. Vegetation is usually represented via NDVI- Normalized Difference Vegetation Index. The NOAA AHVRR instrumen has detection capability for wavelengths of light ranging from um (visible) and um (near infrared). Vegetation density is calculated for each pixel (an AVHRR pixel is 1 square km of land surface) using an equation base on the principle that healthy photosynthetic vegetation absorbs strongly in the visible wavelengths (and reflects little) while reflecting strongly in the near infrared. NDVI = % reflected NIR - % reflected VIS % reflected NIR + % reflected VIS For instance, a dense vegetative index might show 60% NIR reflectance and only 10% VIS reflectance. NDVI = / = 0.71

9 Damaged or limited vegetation (shrub or grassland) might show only 40% reflectance NIR and 30% VIS reflectance. NDVI = / = 0.14 Desert, snow, tundra or bare soil would show a low ratio near zero, since reflected percent of NIR and reflected percent of VIS would be similar. NDVI = / = 0.02 A pixel shading will range only from 0 (no vegetation) to 1 (extremely dense vegetation). With the launching of the Terra spacecraft in 1999, a MODIS sensor (MODerate-resolution Imaging Spectroradiometer allowed a similar method of imaging vegetation density, but with better resolution (250 meter) and with a greater array wavelength measurements. The EVI (enhanced vegetation index) which results from this improved capacity also adjust for distortions due to air particulate matter and is not as readily saturated by high reflectance levels. NDVI anomaly: Trends in vegetation are perhaps more revealing, where yearly NDVI is compared to an average NDV of several years duration for a particular season. If NDVI is less than average, the pixel coloring is changed typically to brownish hue and if NDVI is more than average, greenish hue is applied to the pixel. No change results in a neutral color. The satellite image is presented as a drought or excessive water index (although other stressors may be at work such as cold temperatures or cloud cover). Image courtesy of earthobservatory.com, showing 1993 anomaly.

10 Tasks: 1. Define the following terms: Reflectivity: the fraction of incident radiation reflected by a surface. In full generality it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength Reflectance: The ratio of the total amount of radiation, as of light, reflected by a surface to the total amount of radiation incident on the surface. Emissivity: (usually written ε) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to absorb and radiate energy. Kirchhoff's law of thermal radiation: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body reflectance. Albedo: The ratio of the amount of electromagnetic radiation reflected by a body to the amount incipient upon it, commonly expressed as a percentage. The albedo is to be distinguished from the reflectivity, which refers to one specific wavelength. Heat radiation: The energy radiated by solids, liquids, and gases as a result of their temperature. Such radiant energy is in the form of electromagnetic waves and covers the entire electromagnetic spectrum, extending from the radio-wave portion of the spectrum through the infrared, visible, ultraviolet, x-ray, and gamma-ray portions. From most hot bodies on Earth this radiant energy lies largely in the infrared region 2. Find NDVI images which show North Dakota for at least two years and also an NDVI anomaly image which also shows North Dakota (images may be national). The following web sites can be used to locate imagery: Naturalhazards.nasa.gov Earthobservatory.nasa.gov Science.nasa.gov nationalmap.gov seamless.usgs.gov ned.usgs.gov soildatamart.nrcs.usda.gov

11 Vegetation anomaly 2000 above and 1988 below, courtesy earthobservatory.com (NASA)