astronomers used the entire
electromagnetic spectrum to
study the cosmos.
Portions of the
Electromagnetic
Spectrum that
Amateurs Can Use
Forming images requires
optics. For amateurs, the most
practical optics is the lens and
mirror. When it comes to portable
cameras for drones and BalloonSats
with their limited payload weight,
lens trump mirrors. Using lenses
means amateurs are limited to
imaging where lenses are transparent,
or between the infrared and
ultraviolet portion of the
electromagnetic spectrum.
A digital camera makes a
convenient imaging system since its
lightweight, uses no film, and
produces images easily manipulated
with software. The CMOS imager in
a modern digital camera can detect
light in the visible, infrared, and
ultraviolet portions of the spectrum
where most lenses are transparent.
That means a digital camera is
capable of imaging part of the near
infrared (NIR) between 850
nanometers (nm) and 750 nm, the
visible spectrum between 750 nm
and 400 nm, and the near ultraviolet
(NUV) between 400 and 250 nm.
Near infrared (NIR) imaging —
which spans the range of
wavelengths between 750 nm and
1,400 nm — is useful for detecting
the chlorophyll of plants. Chlorophyll
is green because it reflects green
photons, and absorbs red and blue
photons. While we can’t see it,
chlorophyll also reflects near infrared.
By comparing the amount of red and
near infrared light reflected by a
plant, we can assess the presence
and health of plants.
Satellite images make a
mathematical comparison
between the intensity of red
and NIR called the
Normalized Difference
Vegetation Index (NDVI).
NDVI has a value between -1
to +1. NDVI values of -1
indicate water; values near 0
indicate barren land; and +1
indicates thick foliage. You
can read more about this
interesting technique at Open
Lab’s website at http://infragram.org.
An easy way to convert a digital
camera to NIR imaging is to remove
the IR blocking filter inside the
camera and replace it with several
sheets of red and blue theater gels.
Two blue gels will block all visible
light except for a small amount of
blue. A single red gel will block the
remaining blue light, and thereby
stop all visible light from reaching the
CMOS imager.
Theater gels are designed to
transmit infrared light because of the
high temperature of theater lighting.
Indeed, if the theater gels also
blocked infrared, they would get hot
enough to melt or even catch fire
(kind of a bad thing to happen). So,
APPROACHING THE FINAL FRONTIER
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December 2016 17
Visible light is only a tiny portion of the entire
electromagnetic spectrum. Science has discovered
many of the ways that the universe works using
tools to measure the electromagnetic spectrum at
various wavelengths.
A comparison between a tree viewed in near infrared (L) and visible light (R). The tree’s leaves are so bright
in NIR that it appears the image on the left was recorded on a snowy winter day rather than in the middle of
summer. Also, notice how dark the sky is in NIR. That’s because molecules of oxygen and nitrogen are
approximately the same size as the wavelength of blue light. Their close match in size to the wavelength of
blue makes these atmospheric molecules very effective at scattering the Sun’s blue light, but note its much
longer red and infrared light. Therefore, when an NIR camera is not aimed at the Sun, it sees no scattered
sunlight. Since the air does not emit NIR, it leaves the sky very dark.
