A Guide to the Practical Use of Aerial Color-infrared Photography in Agriculture
Vegetative Response to Solar Radiation
When solar radiation reaches a vegetative canopy such as a forested area or a field that is cropped, predictable interactions occur. In general, all vegetation tends to react to sunlight in a similar fashion, although some specific differences may be caused by variations in plant morphology and physiology, local soil type, and the climate of the growing area.
The study of the response of vegetation to solar radiation is relatively complex but is generalized for this brief discussion. Known biophysical interactions include the fact that a small amount of incoming light is reflected from the outer surface of the plant's leaves, while a greater amount is transmitted into the spongy mesophyll tissue in the leaf where the light rays are reflected back by the cell walls toward the light source.
The spectral character of the interactions varies according to the phenology of the plant (the relation between climate and periodic biological phenomena, such as flowering and fruiting). As a typical plant begins its life cycle and is exposed to sunlight, the amount of chlorophyll in the leaf increases for a time. Red and blue light tend to be absorbed by chlorophyll, while green light is reflected, which is why plants appear green to the human eye (fig. 2).
As the plant matures, the chlorophyll content begins to stabilize, as does the corresponding green reflectance; however, the reflectance of near-infrared light increases proportional to the increasing number of cell walls and intercellular spaces and the total plant surface area (fig.2). These reflectance characteristics remain relatively constant during the mature stage unless external influences such as severe weather or disease cause a change in the overall chemistry of the plant. The external factors may result in an eventual change in plant pigmentation, mesophyll structure, water content, or the surface condition of the plant (for example, leaf mole).
When the plant reaches the senescence or deterioration stage, the cell walls of the mesophyll tissue begin to desiccate and collapse, which results in a substantially reduced intercellular surface area and air space (fig. 2). The end result is a decrease in the level of green and near-infrared reflectance and an increase in reflectance of blue and red light. By making use of the impact of invisible near-infrared energy on sensitized film and by displaying vegetation in shades of red, it is sometimes possible to correlate the variations in tone with factors such as plant species, stage of maturity, plant vitality, and even the moisture content within leaves.
"Red" Vegetation on Color-infrared Film
Whereas panchromatic film yields gray tones and conventional color film produces resembling what we view with our eyes, CIR film renders healthy vegetation in bright red tones (fig. 3).
This substitution of red for green vegetation is somewhat confusing and warrants further explanation. The red tone of healthy vegetation as seen on a CIR photo is perhaps best explained by comparing CIR to conventional color film. While normal color film contains emulsion surfaces sensitized to blue, green, and red, infrared film contains emulsions that react to green, red, and reflected infrared energy (fig. 4).
These emulsion sensitivities result in the unique capability of color-infrared film to provide the user with detailed information about growing vegetation. Because one emulsion layer is sensitive to green light and growing plants reflect intermediate amounts of green light, this aspect of the vegetation can be captured on the film. In addition, the red-sensitive emulsion layer provides information relating to normal plant cell-wall numbers and shapes, and vigorously growing, healthy plants reflect high levels of infrared energy. If vegetation becomes stressed, the result can be captured by the color-infrared film, sometimes before the damage becomes apparent to the human eye.
During the development process for CIR film, yellow, magenta, and cyan dyes are assigned in combination to the green, red, and infrared-emulsion layers (fig. 4). The assignment of these particular dyes results in a "false-color" rendition in which blue images result from objects reflecting primarily green energy, green images from objects reflecting primarily red energy, and red images from objects reflecting primarily in the photographic-infrared portion of the electromagnetic spectrum (fig. 3). Thus, vigorously growing, healthy vegetation appears in a bright red or magenta tone on color-infrared film.
Although the explanation may seem involved and difficult for most to follow, another reference to figure 4 might serve to clear up the remaining confusion. Notice that both the dye layers and resulting colors are exactly the same for normal-color and color-infrared films. Only the film sensitivities differ, and the visual colors in that category are merely shifted one position to the left with the near-infrared sensitivity added with CIR film.
The Loss of Red Color with the Onset of Stress
Perhaps the best way to explain how vegetation looses its bright red CIR signature with the onset of stress is to compare stressed to healthy vegetation (fig. 5). As noted earlier, normal growing vegetation reflects a little red energy, a medium amount of green, and a large amount of near-infrared. Because CIR is a reversal film, the final film signature for vegetated areas is the result of the high-level near-infrared reflection being replaced by only a small amount of cyan dye, and the medium green reflectance being replaced by a medium amount of yellow dye. Most importantly though, the low red reflectance results in a large amount of magenta dye in the film composite. Therefore, healthy vegetation is red.
In the case of stressed vegetation, the low near-infrared reflectance is reversed to a high level of cyan dye, and the high red reflectance is reversed to a low level of magenta dye in the final composite. Hence, stressed vegetation is not red, but often tends toward cyan.