Colorinfrared Photography

Color-infrared film is sensitive to visible and NIR portions of the spectrum. In normal practice, a yellow filter is employed to eliminate blue and UV wavelengths. In some cases, orange or red filters may be used to further restrict visible light from reaching the film. Color-infrared film carries no ISO number; nor do conventional light meters provide correct indications of NIR radiation. Without an ISO bar code on the film case, most cameras cannot make automatic settings. Therefore, taking photographs with color-infrared film requires manual settings for exposure based on estimates of available light. When using a film without an ISO rating, most cameras default to ISO 100 for setting adjustments (Table 6-3).

Given that most SFAP takes place under bright, sunny conditions, color-infrared film can be treated as ISO 200 according to Table 6-3. Following the sunny f/16 rule of thumb, the camera setting should be equivalent to shutter speed 1/250 and f/16. However, other camera-lens-filter combinations may produce different results. For example, best settings for a Canon Rebel SLR camera with a zoom

TABLE 6-3 Manual compensation for SFAP color-infrared film for default value of ISO 100.

Lighting conditions

Exposure correction

Bright sun, mid-day-clean, dry atmosphere

ISO 200 (+1 f-stop)

Light, but not bright sun-hazy, humid, dusty atmosphere

ISO 160 (+1/2 f-stop)

Slightly overcast, indirect light, thin clouds

ISO 100 (no correction)

Pale, diffuse light-early morning or late afternoon

ISO 80 f-stop)

Overcast, indirect light, rather dark-heavy clouds

ISO 50 (-1 f-stop)

Based on Pentax SLR camera with 50-mm lens and orange filter. Adapted from Marzolff (1999, Table 4-2).

Based on Pentax SLR camera with 50-mm lens and orange filter. Adapted from Marzolff (1999, Table 4-2).

lens and yellow filter are 1/250 shutter speed and f 11 for full sun and active vegetation (Aber, Aber, and Leffler, 2001). Apart from these empirical results, aerial photography with color-infrared film remains an uncertain proposition—considerable trial-and-error testing is necessary, and results cannot be predicted well. A final and nearly insurmountable problem is that since about 2005 most commercial photo laboratories no longer process color-infrared film.

For digital SFAP, the primary challenge is to identify a suitable color-infrared digital camera of relatively small size and weight at a cost that could be justified. One commercial camera that meets these requirements is the Agricultural Digital Camera (ADC) by Tetracam. This camera employs a 3.2 megapixel CMOS sensor, which operates in the spectral range of 0.52-0.92 mm wavelength (green, red, and near-infrared). A permanently mounted long-pass filter behind the lens blocks blue and UV light, and the camera has a robust machined-aluminium body (Fig. 6-10).

The primary applications for the Tetracam ADC camera are, as the name suggests, agriculture as well as forestry and other studies involving vegetation, soil, and water. The camera is designed to be operated on the ground or from manned or unmanned aircraft either by hand or remote control. Its size, shape, weight, and operating characteristics place this camera within the normal range for DSLR-type cameras. The camera produces results that are quite comparable with color-infrared film photography (Fig. 6-11), and functions well from remotely operated aerial platforms (Fig. 6-12).

Nir Adapted Powershot
FIGURE 6-10 Tetracam ADC digital, color-infrared camera in a remotely operated radio-controlled rig for kite or blimp aerial photography. R, radio receiver; P, pan servo and gears; B, NiMH battery pack; A, antenna mast; S, shutter miniservo; and T, tilt servo. Camera rig built by JSA.

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FIGURE 6-11 Digital ground photographs of a late-summer garden scene in color-visible (A) and color-infrared (B) formats. Active vegetation appears in bright red-pink colors in the latter. Also some artificial fibers and dyes are highly reflective for near-infrared (Finney, 2007), as seen in the flags. Compare with Figure 6-2.

FIGURE 6-11 Digital ground photographs of a late-summer garden scene in color-visible (A) and color-infrared (B) formats. Active vegetation appears in bright red-pink colors in the latter. Also some artificial fibers and dyes are highly reflective for near-infrared (Finney, 2007), as seen in the flags. Compare with Figure 6-2.

Near Infrared Vegetation

FIGURE 6-13 Typical response curve for CCD and CMOS image sensors without NIR blocking filter showing the transmission response after the light passes through the mosaic color filter over the image sensors. A ''hot mirror'' usually added to such a sensor blocks wavelengths above 700-750 nm. Adapted from sensor specifications given by Prosilica.

FIGURE 6-12 Color-visible (A) and color-infrared (B) digital images of marsh at the Nature Conservancy, Cheyenne Bottoms, central Kansas, United States. Active vegetation appears in bright red-pink colors in the latter. Kite aerial photographs from Aber et al. (2009, fig. 5).

Another possibility for taking pictures in the NIR spectrum is the modification of a customary digital camera. All digital camera sensors are sensitive not only to visible, but also to NIR light (Fig. 6-13). In order to prevent NIR light from degrading the quality of normal color images, a blocking filter (''hot mirror'') is placed in front of the sensor that allows only visible light to pass. By removing this hot mirror, the spectral sensitivity of the sensor cells to NIR light can be employed for photographs in two ways. The blocking filter is replaced either by an infrared (visible-light blocking) filter for pure NIR photography or by a clear filter for preserving the whole spectral sensitivity of the detector (UV to NIR). The latter option will merge NIR energy with each of the visible primary colors. While this may offer unusual artistic possibilities, it is of little interest for scientific use as it does not separate NIR reflectance in a single image channel like color-infrared film or the Tetracam ADC camera.

The modification for converting a camera for infrared photography is no trivial task and therefore increasingly offered as a commercially available infrared-camera

FIGURE 6-13 Typical response curve for CCD and CMOS image sensors without NIR blocking filter showing the transmission response after the light passes through the mosaic color filter over the image sensors. A ''hot mirror'' usually added to such a sensor blocks wavelengths above 700-750 nm. Adapted from sensor specifications given by Prosilica.

conversion service. Successful SFAP with NIR-converted digital cameras is reported by Jensen et al. (2007) for crop yield studies and by Verhoeven (2008) for archaeological reconnaissance. Both studies used a double-camera system, combining the natural color image of the original camera with the NIR image of the modified camera for a four-band color-infrared image.

Digital Cameras For Beginners

Digital Cameras For Beginners

Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.

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