Lab 5: Field Spectroscopy – Measuring Reflectance Manually
One way to generate reflectance spectra is to measure them in the lab or outdoors
using instruments called field spectrometers, sometimes called radiometers or
spectroradiometers. These devices measure the reflectance of a target in different
wavelengths just as a multispectral satellite does. The difference is that in the field you
don’t have any atmospheric effects to deal with. This is why relating satellite data to
field reflectance requires the researcher to make atmospheric corrections on the
satellite data, a topic we will discuss later in the semester.
Field spectrometers range in price and functionality from very inexpensive (like the ones
you will use today) to very expensive (~$40,000) like the one that I will demo and you
will play with. UW acquired the research spectrometer as part of the Berry Biodiversity
Center gift. You will work in small groups on the data collection portion of today’s lab.
There is no worksheet today, but at the next class your group will hand in (or e-mail)
graphs showing the reflectance curves for the materials you measured! You may be
asked to draw these on the board and interpret them.
Part I
First I will demo the ASD FieldSpec4 full spectrum radiometer. This instrument is
available for research use (and for students!) through the Berry Center Ecology Lab.
There is an online form that one can fill out to borrow it. In the past, some students
have used this for their semester projects.
After I have demo’d the radiometer, each group will use it to generate a spectral
reflectance curve for at least one of the materials that you examine in part 2 below.
Part 2
1. Familiarize yourselves with the inexpensive field spectrometers. These also measure
spectral reflectance, but with much less precision and over a narrower range of
wavelengths than the ASD instrument. Notice the hole on the back of the unit that
contains 11 diodes surrounding a central diode. Watch the diodes as you push each
button on the front of the unit (you have to push and hold). What happens when you
push the IR 1-4 buttons? Why?
Each diode emits light of the wavelength printed on the button that turns it on (units
are nm: 1 nm = 10-9 m). The sensor in the center measures the amount of light that is
reflected back. This amount is displayed on the front of the spectrometer in units of
1
millivolts (mV) (a photoelectric cell is converting the amount of light received into
electric potential, measured in millivolts).
2. To convert the numbers that appear on the display of the spectrometer to
reflectance, we will calibrate the spectrometers using a quantity called the “dark
voltage” (the reading when you put the unit over a dark target and don’t light any of the
diodes—reflectance = 0%) and the reading from a white (bright) “standard” (reflectance
= 100%). The white standard must be measured separately for each wavelength! This
process allows us to determine the range of intensities that the spectrometer can
measure (from darkest to brightest) for each wavelength, and then to figure out where
the reflectance from our samples fit into that range. You will only need to calibrate the
spectrometer once, and then you can apply the calibration to all of the materials you
look at today. Detailed field spectrometry with an expensive instrument usually
requires calibration before every reading to increase accuracy.
a. Measure the dark voltage for your spectrometer by putting it over the black
posterboard and recording the display number (mV) in the data sheet (provided on
page 4 of this lab) without pushing any of the colored buttons. Be sure not to let any
light leak in around the edges of the instrument!
b. Measure the amount of light from each diode that is reflected off of the white
posterboard standard and record on the data sheet. Be sure to hold each button down
until the number stops changing substantially. These values are the maximum
reflectance (100%) for each wavelength.
You will eventually correct the voltages you measure for various objects using the
equation:
%Reflectance = 100*(sample voltage – black voltage)/(white voltage – black voltage)
3. Head out to Prexy’s pasture and measure reflectance for a variety of objects that you
find there (e.g., green leaves, yellow leaves, flower petals, grass, concrete, soil, etc.) in
all of the wavelengths on the spectrometer (see data sheet on page 4). For each
sample, measure the amount of light reflected in each wavelength and record the
values (mV) on the data sheet.
4. Before the next lab, use the equation (from #2 above) to calculate the %Reflectance
for each object in each wavelength (you might want to use Excel). Fill in the data sheet
(pg. 4). Plot the reflectance for each object on graph paper to create reflectance
spectra for each sample.
What are key differences between the reflectance curves for the materials? Can you
explain them? Be prepared to show some of these curves to the class and to compare
2
to the curve for at least one of the materials that you generated using the ASD
spectrometer.
6. The Landsat 8 satellite has a pixel size (spatial resolution) of 30 m. This means that
the satellite measures reflectance from a square on the ground that is about 100’ on a
side. With your group, pace out the edges of the approximate area covered by a TM
pixel (1 big step is equal to about 1 meter). Leave a person on each corner of the
“pixel.” What surface materials are included inside the area you defined??
How might this affect the reflectance that the satellite “sees” compared to the
reflectance that you measured with the spectrometer?
7. Before you leave today be sure to return all of the spectrometers and posterboard to
Ken.
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Data Sheet 1
Spectroradiometer Measurements
Dark Voltage (mV):
Color
Blue
Cyan
Green
Yellow
Orange
Red
Dp. Red
IR1
IR2
IR3
IR4
Color
Blue
Cyan
Green
Yellow
Orange
Red
Dp. Red
IR1
IR2
IR3
IR4
Wavelngth
(nm)
470
525
560
585
600
645
700
735
810
880
940
Wavelngth
(nm)
470
525
560
585
600
645
700
735
810
880
940
White
Std.
Voltage (mV)
Material
Material
Material
#1
#2
#3
% Reflectance (using calibration data)
Material
Material
Material
Material
#1
#2
#3
#4
4
Material
#4