Plant Stress Detection Filters

Plants use red and blue light to photosynthesize.  If these were mixed (explore how with Color Vectors), we would see magenta.  If a plant is unhealthy or stressed, it absorbs less light for photosynthesis, so it reflects more red and blue light.  A magenta filter lets red and blue light through but absorbs green light.  If you looked at plants through a magenta filter, healthy plants would appear dark (little reflected red and blue light), and stressed plants would appear brighter.  Foresters use plant stress detection glasses to monitor the health of trees.

A photograph of a mix of healthy and stressed leaves in visible light.

Near-Infrared Goggles

Build near-infrared goggles for under $10! Yes, our eyes can see just into the near-infrared (NIR) spectrum, but we need to block out most of the visible light. Here is an inexpensive way to see things we usually can’t!

The landscape image below left is what you would see when looking through a red filter, and the image on the right is what you would see using the NIR goggles. None of the objects in the picture are emitting any red light or NIR. Dark red colors show materials reflecting little of that spectrum (so are highly absorbent), and bright red indicates that the material was quite reflective. Notice how the leaves, sky, and pavement reflect different amounts of red light and NIR.

Why do we see red when using the NIR goggles? Infrared doesn’t have a color in human vision! But our eyes have three color detectors: red, green, and blue. And only the red sensors can see a portion of the NIR, so our eyes tell our brain that everything is in shades of red. Use the goggles to look at the black fabrics you use in the experiment available below. You will then be able to use them to pick the clothes you wear outside to keep warm or cool, depending on the season and your preference.

Red light reflected in a landscape.  Dark objects either reflected little red light or are in shadows.

Low-Cost Filter Booklets

Buy a booklet containing over 100 color filters for as little as $1.50 (search the Internet for the best price) and explore your world.  Some examples of color lenses sold commercially are:

• Reduced eye stress while working on your computer,
• Improve your vision while playing golf, and
• Improve vision in low-light and hazy conditions.

Photometers

Photometers measure the intensity of light reaching the sensor.  They may be classified as broad-spectrum, or their capabilities may be limited to a small range of wavelengths.  An inexpensive device that measures light intensity for a relatively narrow range of wavelengths is a light-emitting diode (LED) connected to a voltmeter. We usually use LEDs to generate light by running a current through them.  Interestingly, shine a light of similar wavelengths onto the LED to produce electricity.  This is how our remote controls work: one LED emits pulses of near-infrared, and a receiving LED converts the light pulses to electric signals.

Explore measuring reflected and transmitted light with LEDs with these activities.

Read more on using LEDs as light detectors.

Digital Cameras

If you try the LED photometer activities, you will appreciate how much time it takes to collect and analyze several measurements.  A digital camera contains millions of photometers that collect spatially organized red, green, and blue intensity measurements in a blink of an eye.  These data are easily shared with and analyzed on your computer.  Depending on your creativity, study the reflection, transmission, absorption, scattering, refraction, diffraction, and polarization by objects.

And we aren’t done – digital cameras may be modified to photograph in the near-infrared and ultraviolet wavelengths.

Plus, most digital cameras have a video mode, so you are able to capture 30, 60, or more frames per second, all providing spatially organized light intensity measurements.  Extremely modified digital cameras capture the propagation of light!

Because the data are spatially organized, we can do more than just study light intensities.  We can study the size, shape, orientation, movement, acceleration, texture, position, and behavior of an object with digital cameras.  Not bad for a device that is found on every smartphone today!

Is a Digital Camera the Ultimate Experimental Tool?

Using the concept map showing the ways a digital camera may be used to collect light measurements, you can design over 2300 experiments to study not just light, but the processes that created the patterns of light captured in the image.  Not too shabby!

Key Concepts to Consider

• When using visible light, a digital camera captures intensities of red, green, and blue light simultaneously.  See Color Vectors to explore how red, green, and blue light create the colors we see.
• A digital camera’s light sensor detects ultraviolet (UV), visible light, and near-infrared (NIR), but internal filters block the UV and NIR from reaching the sensor.  These filters may be removed so the camera may take photos in the UV or NIR, opening up ways to view (and measure) what humans cannot do with their eyes.
• The ways light interacts with matter (reflection, transmission, absorption, and emission) depend on the chemical and physical properties of the material.  Therefore, the light measurements captured by the camera tell us about these properties.

The video that follows provides a few examples of how to use a digital camera in science.

Unfortunately, with changes in computer operating systems, the MovieTracker software referenced in the video no longer works on today’s computers.

A landscape photographed in visible light.  There are three overlapping layers of color intensity measurements in the image: red, green, and blue.

Three black fabrics in visible light (top) and near-infrared (bottom).  Dark objects absorb those wavelengths, and lighter objects reflect more.