Multispectral Effects

As humans, we view the world through a narrow range of electromagnetic radiation, the visible spectrum (0.40.7 mm wavelength; see Fig. 2-1). Film photography extends this range from near-ultraviolet to near-infrared (0.3-0.9 mm). Black-and-white infrared film was developed in the 1920s and was utilized for aerial photography already in the 1930s (Colwell, 1997). World War II spurred a great need for aerial camouflage detection, and color-infrared (CIR) film was perfected. Nowadays both CIR film and digital cameras are available for small-format aerial photography.

The "color" of objects depends upon what parts of the spectrum are observed. This is obvious, for example, when comparing panchromatic (gray tone) images with equivalent color images. However, objects that appear similar in visible light often have quite different appearances in other portions of the spectrum. Diffuse reflectivity or albedo for most common surficial materials ranges from less than 10% for clear water to nearly 100% for fresh snow. Photosynthetically active vegetation typically has an albedo of 50-70% in the near-infrared portion of the spectrum (Fig. 4-15).

FIGURE 4-14 Multi-directional reflectivity for spruce forest in red (A) and near-infrared (B). In each case, the peak in reflectivity represents the hot spot at the antisolar point. The bowl of low reflectivity values indicates increased shadowing looking toward the sun. Adapted from Schaaf and Strahler (1994, fig. 3).

Artificial Turf Spectral Reflectance
FIGURE 4-15 Comparison of reflectivity for common surficial materials in the visible and near-infrared portions of the spectrum. Note the distinctive spectral response curves for grass vs. artificial turf. Taken from Haack et al. (1997, fig. 15-5).

Different parts of the spectrum may be photographed by using various combinations of films, electronic detectors, and filters. Photographs are routinely taken in b/w panchromatic, b/w minus blue, b/w infrared, color-visible, color-infrared (minus blue), and multiband types. Near-ultraviolet photography is also possible for special applications. As an example, color-infrared film is exposed to green, red, and near-infrared wavelengths, which are depicted, respectively, as blue, green, and red in the photograph. This shifting of bands to visible colors is called false-color imagery (see Fig. 2-5).

The case of vegetation is most instructive. Compare, for example, grass with artificial turf (see Fig. 4-15). Both appear nearly identical green in visible light; however, grass is highly reflective in near-infrared, whereas artificial turf is not. Distinguishing between real and artificial turf requires photographs in the near-infrared portion of the spectrum. Photosynthetically active "green" vegetation has a unique spectral signature (Fig. 4-16). Leaves selectively absorb blue and red light, weakly reflect green, and strongly reflect near-infrared radiation. No other materials at the Earth's surface have this spectral signature. On this basis, CIR imagery plays a key role for analysis of all types of vegetation—crops, prairie grass, emergent aquatic plants, and forests.

Water, likewise, has a distinctive appearance in CIR images. Although clean and turbid water differ in their visible reflectivity (Fig. 4-17), both strongly absorb near-infrared radiation. Thus, water bodies typically appear dark blue or black in CIR photographs, unless photosynthetically active vegetation is floating on the water surface. CIR images are typically taken using a yellow or orange filter in order to remove blue light from the image. This renders shadows much darker, as scattered blue light illuminates

£IGMENT£

STRUCTURE . |

LEAF WATER CONTENT

I

HEAR INFRARED PLATEAU

- RED EDGE \

GREEN REFLECTANCE PEAK V

—Í-1-L

RED WELL _!_1.1,,,,

-1-1-i-1-,-1-,-i-1—J-

9 10 11 12 13 14 15 li 17 16 1? 20 21 22 23 24 25 26

WAVELENGTH (nm) X 100

9 10 11 12 13 14 15 li 17 16 1? 20 21 22 23 24 25 26

WAVELENGTH (nm) X 100

kRED i I _^ I_SHORT WAVE INFRARED_

FIGURE 4-17 Visible spectral response curves for two lakes. Crater Lake (gray) is clear, deep water; San Vicente (dashed) is turbid, phyto-plankton-rich water. Note that both trend toward zero reflectivity in the near-infrared (>700 nm). Adapted from Wiesnet et al. (1997, fig. 6-3).

FIGURE 4-18 Special lighting effects are enhanced in color-infrared imagery. (A) Sun glint (*) and glitter from fish hatchery ponds; water is dark blue. Pueblo, Colorado, May 2003. (B) Hot spot at scene center on canopy of deciduous trees; shadows are black. Elkhorn Slough, California, November 2002. Kite aerial photographs by SWA and JSA, United States.

FIGURE 4-16 General spectral reflectance curve for a green leaf. Note blue and red absorption, weak green reflection, and strong near-infrared reflection. Adapted from Murtha et al. (1997, fig. 5-11).

FIGURE 4-18 Special lighting effects are enhanced in color-infrared imagery. (A) Sun glint (*) and glitter from fish hatchery ponds; water is dark blue. Pueblo, Colorado, May 2003. (B) Hot spot at scene center on canopy of deciduous trees; shadows are black. Elkhorn Slough, California, November 2002. Kite aerial photographs by SWA and JSA, United States.

Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

Get My Free Ebook


Post a comment