Spec Lab 3    Taking a Reflectance Spectrum

Your Task
      In this lab you will learn how to use the ALTA Spectrometer to make a reflectance spectrum.  You will make the spectrum by gathering data from a green leaf and making a graph.

Background
Green leaves use sunlight, water, and air to make food (sugars) for plants.  In most plants, the chemical pigment called chlorophyll absorbs light energy and makes sugars (sugars have chemical energy).   Chlorophyll absorbs and uses  blue and red light to make sugars;  green light is not absorbed very well.  Leaves also contain other chemical pigments:  carotenes and xanthophylls give red, yellow, and orange colors; flavoniods give pink to purple colors.  Scientists study the colors of leaves from satellites or airplanes.  By looking at the color of vegetation scientists can tell many things, such as how ripe a crop is, how healthy a forest is, and where pollutants come from.

What happens to most of the green light that shines on plant leaves?   __________________________________

Scientists study vegetation using a spectrometer to find out the range of wavelengths the leaves are reflecting.   Scientists calculate reflectance, which is the percentage of light reflected by a leaf.   A reflectance spectrum is a graph that shows the percentage of all the colors of light (wavelengths) being reflected.

Suppose we discover that green, yellow and orange light are being reflected from a leaf.  What is happened to the red, blue, and indigo parts of the spectrum?   ___________________________________________________________________
 

Materials
 

1 large green leaf 1 sheet thick white paper 1 ALTA Spectrometer student calculator

Lesson 3 Work Sheet (better) Graph Template 1 Graph Template 6 Reference Spectrum sheet

Procedure
1.  What kind of leaf do you have?  _______________________

2.  Turn your ALTA spectrometer on.  Place the spectrometer on the green leaf so that the lamps are over the leaf.  Read the display without pushing any buttons.   Record the display number on your data sheet under "dark voltage".

What is "dark voltage"?  ____________________________________________________

3.  Start by pressing with the blue lamp button.   Wait until the display numbers become constant.   Record this number in the blue row and in the 'sample' column on your data sheet.     Do these same steps for all the wavelengths (buttons) on the spectrometer.

4.   Graph your results on 'Lesson 3: Graph Template 1'.

Your results may not be perfect.  One reason is that the ALTA detector has different sensitivities.  Remember your results in Lab 2?  Which wavelengths is the detector most sensitive to?

How does the leaf's display number for infrared light compare to other colors?
 

4.  Standardize your results.   Write your display number for green and infrared-1 one on the chart on the blackboard.  Do all of the spectrometers read the same measurements?  ________________   Usually you will see a lot of variation.  That is because of the way the ALTA spectrometers are made... there are variations in the manufacture of the electrical components, lamps, and the light detector.

This variation is natural but, how can we compare our results with other students???   To correct for the differences between the spectrometers we must standardize the data.  We need to calculate the reflectance as a percentage of all the light reflected from the leaf.

Here's how to standardize:
4a.  We know that heavy white paper reflects close to 85% of the light that hits it.    White paper can be used as a standard since we know it's reflectance.    Put the spectrometer on the white paper and read the display for each color button.   Record these numbers on the worksheet in the column "Standard White Paper".

Look up standard in the dictionary and write it's meaning as it is used here:
 
 
 

4b. Calculate the percentage of light reflected by the leaf using this equation:
 

reflectance =      Display number for the leaf - dark voltage      X 100                      Display number for the Standard - dark voltage

Note:  Remember dark voltage?  Dark voltage is the display value when there is no light on the detector  (no buttons pushed).  Dark voltage is usually between 0 and 150.  It comes from the detector itself.  To get the most accurate results you subtract the dark voltage from both of the display numbers.

4c  Use the equation to calculate reflectance for light at each wavelength and record your results in the 'Reflectance' column on your worksheet.

5.  Make a reflectance spectrum:   Graph your standardized reflectance data on 'Graph Template 6'.
 

Summarize your results
         You have a Reference Spectrum for spinach.  The spectrum was made in a government lab using high-tech equipment.  They took measurements at hundreds of wavelengths, from violet through infrared.  Compare your reflectance spectrum with the spinach Reference Spectrum.   Are your measurements similar?    Is your leaf lighter or darker than the spinach?  (i/e/ are your leaf's reflectance values larger or smaller than the spinach's)?   Why does the reference spectrum show many more wiggles and bumps than yours?  Do you think your leaf spectrum would have similar detail if you too measured at hundreds of wavelengths?