Objective:The objective of the lesson is to have the students learn how to use an ALTA® Reflectance Spectrometer. The lesson will introduce the student to the concept that different earth materials reflect differing amounts of electromagnetic radiation at various wavelengths.
Materials:
ALTA® Reflectance Spectrometer can be purchased from the Lunar and Planetary Institute for about $130 + $8 for shipping and handling. See details below for purchasing.
Operation of the Spectrometer
On/off. The on/off switch is on the side of the ALTA® spectrometer. To turn the ALTA® on, slide the switch toward the display panel of the ALTA®. When the switch is turned on, the lamps in the bottom of the ALTA® will not turn on. However, the numerical display should show a number between 1 and 1999. Under no circumstances should you open the spectrometer case.
Color lamps and light detector. The colored lamps are visible in the hole in the bottom face of the ALTA®. Each lamp is controlled by its own color-coded switch pad on the top face of the ALTA®. The switch pads turn on little LED lamps - the number on the front measures how much light hits the spectrometer's light detector sensor, which is in the center of the LED lamps on the underside of the ALTA®.
Measuring reflectance. With the ALTA® turned on, place the instrument on the object to be measured so that the hole in the bottom is facing the object. Flat, matte-surface objects work best; do not let objects project into the hole. With no lamps illuminated, the display will show a number less than 150 - this is the dark voltage. Press down, and hold down, a colored switch pad on the ALTA® top face to turn on a colored lamp. (Using a pencil or your fingernail to hold the switch pad down may be more comfortable.) The number in the display will become larger - this number (minus the dark voltage) is a linear measure of the amount of light that is striking the detector.
Standards. The amount of light represented by display numbers of the ALTA® is not calibrated. To calibrate this number, take reflectance measurements (as above) on an object with known reflectance values as standard. A good standard is white poster board, which reflects about 85-90% of the light that hits it.
Engagement
Get the students excited about understanding the operation of the spectrometer. You can mention that the instrument is looking through different sunglasses that can see different wavelengths than what their own eye can see.Calibrating the instrument. To measure the reflectance standard, put the spectrometer on a piece of white paper and measure the spectrometer's output voltage for each lamp. Record these numbers in a table in the column labeled Standard White Paper.
Outdoor activity. Find a green leaf - larger than 1" square. Remember that leaves use sunlight, water and air to make food for plants. In most plants, the chemical chlorophyll absorbs light energy and converts it to the chemical energy of sugars. Chlorophyll absorbs and uses both blue and red light to make sugars; green light is not absorbed very strongly and thus reflected, giving rise to the green color of leaves. Leaves also contain other chemical dyes: carotenes and xanthophylls give red, yellow and orange colors; and flavonoids give pink to purple colors. The colors of leaves, when seen from a satellite or airplane, allow scientists to tell how ripe, how healthy, or what type of stress the plant is undergoing.
Place the spectrometer, lamp side down, on a green leaf so that the lamp/sensor array is over the leaf. Note the display number (in millivolts) when no ALTA® lamps are on; record this on the example data sheet as the dark voltage. Starting with the blue lamp, turn it on by pushing the blue switch pad on the ALTA® face and holding it down. The display number will change from its dark value and will become constant (except for random variations in the last few digits) within a few seconds to a minute. When the display number remains nearly constant, record it on the reflectance calculation worksheet in the blue row and the Sample green leaf column. Using the same procedure, work through the rest of the lamp colors on the ALTA®, recording the display number on the data sheet.
The display numbers for green and infrared-1 will vary, which is principally due to the variation with the ALTA® spectrometer. Because of variations in the manufacture of the electrical components, lamps and light sensor, each ALTA® has its own unique sensitivity to light (within limits, of course).
Measurement of light reflectance is given as the percentage or proportion of light (for each wavelength or color) that reflects from the leaf. The display number measurements indicate how much light (of each color) has reflected from the leaf, but we need to know how much light hits the leaf to start with.
With the Standard data, we can now calculate the proportion (or percentage) of light reflected by the leaf. For each color, simply divide the display voltage number for the leaf by the display voltage number for the white paper. This value is called the reflectance.
Reflectance = (Display voltage for sample) / (Display voltage for standard)
To convert this reflectance proportion to a percentage, simply multiply by 100.
However, the spectrometer display number is usually not 0 when there is no light on the sensor. The display value is usually between 0 and 150 - this is called the dark voltage. It comes entirely from the sensor, not from light hitting the sensor. To get a real reflectance value, you should subtract the dark voltage from the measured voltages, and then divide the sample by the standard.
Reflectance = (Display voltage for sample - Dark voltage) / (Display voltage for standard - Dark voltage)
Graph your standardized reflectance data (as proportions), as Reflectance vs. wavelength. This graph is called a reflectance spectrum.
Repeat this process for three other materials - water, concrete sidewalk, and bare soil. The measurements that you made for the standard white paper can be re-copied to the new tables.
Exploration
Additional activities include determining the reflectance of other materials. For example, not all leaves will reflect the same amount of electromagnetic energy. Leaves will show different hues of green, younger leaves will be different colors from more mature leaves, and dying leaves will be different from healthy leaves.Extension
The NASA JPL maintains a spectral library. There are over 2000 spectra of various materials. Select the appropriate spectra to compare with materials that the students measured and plot the data values corresponding to the wavelengths of the ALTA spectrometer.Evaluation
Be sure that the students complete the worksheets and plot their data on the graphs.Tips for Teachers
This exercise introduces the students to the concept that objects reflect different amounts of electromagnetic radiation.Pertinent Internet Sites
To purchase the ALTA® Spectrometer, contact the Order Department, Universities Space Research Association, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058-1113 (1-888-801-9895). Lesson plans are included with the instrument.Privacy Policy and Important Statements We welcome your questions and comments. Send e-mail to: merry.1@osu.edu