Working with a Broader Spectrum of Light
John Pickle, Don Blair, Mort Sternheim
Student Guide, June 2013
Objectives:
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Explore the Visible and Near Infrared spectrum with modified digital cameras.
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Use ADI tools to create useful images.
Overview: Infrared Light (from http://dew.globalsystemsscience.org/key-messages/nearinfrared-and-the-electromagnetic-spectrum)
Wavelength bands within the visible spectrum
are defined by color. The range of wavelengths
that span the infrared spectrum is roughly 40
times greater than that of the visible spectrum.
Because the infrared region is so large, there is
quite a bit of confusion when one uses the term
"infrared" or IR. Technologies that deal with
the near infrared, which is quite close to the
red visible spectrum and quite far from the
thermal infrared wavelengths, which is
commonly referred to as "heat."
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Although our bodies do emit IR, the
wavelengths are in the far or thermal IR region.
The IR detected with digital cameras and light
emitting diodes (LEDs) in TV remote controls
is in the near IR region, which is generated
from very hot light sources. All objects emit
light, but cooler objects emit light of longer
wavelengths. Hotter bodies emit light of shorter
wavelengths. For example, blue stars are hotter
than red stars (blue is a shorter wavelength than
red). The wavelength corresponding to the
maximum emission is inversely proportional to
the absolute or Kelvin temperature. The sun is
at 6000 K, while our bodies are at about 300 K.
Overview: Sensing Light with a Digital Camera
(from http://dew.globalsystemsscience.org/tools/digital-cameras-overview)
Near infrared doesn't make it to the camera sensor because it is blocked by the infrared-blocking
filter, which lets the visible light through.
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The visible light reaching the
sensor is controlled by small
color filters. The red filter
lets red light through, so only
this light is hitting the small
sensor underneath it.
Similarly, the green and blue
filters control the
wavelengths of light reaching
the sensors beneath them.
Effect of Removing the Near
Infrared Filter from a Digital
Camera:
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Activities:
Step 1: Observe your world using the book of filters and the infrared goggles. Record your
observations.
Step 2: Observe your world with a digital camera that sees in both the visible and near infrared.
First, connect your USB camera to your computer and open Google Chrome. Use the filters and
the infrared goggles to take photos with your camera.
Examples: Visible light only, with
proper balance of red, green, and
blue light.
Visible and near infrared light are
reaching the camera sensor.
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Only the near infrared light is
reaching the camera sensor. The
color is caused by the three sets of
small colored filters inside the
camera letting different amounts of
near infrared through. The reddish
tint suggests that the red filters
pass greater amounts of infrared than
the other two, with the blue filter
letting more through than the green,
as indicated by the purplish tint to
the sky.
Only the near infrared light is
reaching the camera sensor. The
color tinting caused by the colored
filters letting different amounts of
infrared through has been removed
with the ADI software.
Step 3: Expanding on Mort's Albedo analysis. Now we are going to expand to include visible
and near-infrared light (which are each nearly 50% of the sun's spectrum). Take photos of the
same location (preferably one with grass, dirt, and pavement in the scene) using the WebCam
Toy plug-in while running Google Chrome using the infrared goggles or the exposed film, the
blue filter (handed out), and the infrared-blocking filter (we only have a few, so these need to be
shared). Tip 1: place your camera in such a way as it won't move when photographing a scene.
Tip 2: As with the earlier experiment, try to have 50% of the scene covered by the white copy
paper. Suggestion: Include a fake/plastic plant in your scene.
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Step 4: Using ADI and either the line or area tools, repeat the calculation of albedo of each
surface/land cover in the photographs. Assume the paper's albedo in visible and near-infrared is
0.6. How did the values change compared to only visible light? Consider the effects on Earth's
energy budge.
Step 5: Using ADI, create a Red vs. Blue (normalized) image of the photo that used the blue
filter. Save the image using the option in the File Menu, Save Picture.
Step 6: Using ADI, create a Red vs. Blue (normalized) image of the photo that used the infraredblocking filter. Save the image using the option in the File Menu, Save Picture.
Step 7:
A) Using ADI, create an average gray-scale image of the photo that used the infrared goggles
or the exposed film. Save the image using the option in the File Menu, Save Picture.
B) Using the photo the used the infrared-blocking filter, save the enhanced image created
using Red as Gray.
C) Use the Utilities option, Combine Images, to add the Near-IR image (step A) to the red
layer, and add the Red as Gray image to the Blue layer. Use this combined image to create a
Red vs. Blue (normalized) image and save.
Step 8: Compare the 3 images created using Red vs. Blue (normalized). Note: Steps 5-8 are
research, so we don't know the final results! Two of these images, the ones that include nearinfrared, are called Normalized Difference Vegetation Index, or NDVI.
Applications: Assessing the Health of Plants
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Arrow width of the spectra above are proportional to the amount reflected from a healthy leaf.
Image: publiclab.org
NGB = Near Infrared intensity measurements shown in the Red layer, Green intensity
measurements displayed as Green, and Blue intensity measurements are shown in the Blue layer.
NDVI image uses a different enhancement than what we will create with ADI. Image:
publiclab.org
For more information about NDVI, see:
- Public Lab’s DIY near-infrared camera designs and background information on NDVI:
http://publiclab.org/wiki/near-infrared-camera
- A list of inexpensive digital cameras that are easy to modify for near-IR imagery:
http://publiclab.org/wiki/near-infrared-camera
- Lawrence Hall of Science’s NDVI guide:
http://www.lawrencehallofscience.org/gss/dew/software/guides/NDVIfromDigitalCamera.pdf.
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