Acquisition of Multispectral Images Using Digital Cameras
2. SPECTRAL SENSITIVITIES OF CAMERAS AND FILTERS
In general, digital cameras record image with CCD or CMOS sensors (Lillesand et al., 2004).
If a silicon based CCD sensor is used, one may notice that this digital camera is an excellent
sensor for the near infrared wavelength range of 750nm to 1000nm, as shown in Figure 1
(http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fip-Spectral.html).
However, digital cameras are generally not designed for taking near infrared images. Their
sensors may not record infrared images without using IR filters. In other words, if an IR filter
which can block wavelength below 700nm (visible portion), such as written 89B et al. shown
in Figure 2, is used, one may examine whether the digital camera can record near infrared
images or not.
A remote controller of television can be used to test the near IR sensitivity of a digital camera.
One can take pictures of the IR beam of a remote controller with an IR filter placed in front of
digital camera lenses. A light dot shown on digital image indicates that the digital camera can
sense the near IR energy.
Figure 1. Spectral Characteristics of Silicon, The
Sun, and The Human Visual System.
(http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fi
p-Spectral.html)
Figure 2. A Comparison of Infrared
Filter Absorption Plots.
(http://www.photo.net/photo/edscott
/ir000020.htm)
3. MULTISPECTRAL IMAGES ACQUIRED BY DIGITAL CAMERAS
As stated before, accompanied with an IR filter, an IR sensitive digital camera can take near IR
images. The IR filter also blocks some lightness. Therefore, exposure time should be long
enough for showing the IR images somehow. According to various experiments, one to two
seconds of shutter speed may be adapted. A tripod should also be used for stabilizing the
camera.
In this study, the normal color images for three scenes were taken as shown in Figures 3(a),
4(a), and 5(a) respectively. For each scene, near IR images were also taken using the approach
described above. The near IR image of scene 3 is shown in Figure 5(b). Both normal color
image and near IR image can be split in three primary colors of blue, green, and red. As a
matter of convenience, they are demoted as B, G, R, IR1, IR2, IR3 bands in sequence. Image
data can also be presented in the intensity-hue-saturation (IHS) coordinate system. As
explained below, the most useful band of near IR image is intensity one. Therefore, color IR
images were generated as shown in Figure 3(b) and Figure 4(b) respectively. The results are
“false color” images in which blue images result from green bands of normal color images,
green images result from red bands of normal color images, and red images result from
intensity bands of near IR images.
