EMR01-P01: Reflectance Curve Assignment

Assignment explanation on EMR: Spectral reflectance Curves

NOTE: This assignment is advance. You have a paper copy of this page but the links can be accessed only from CANVAS. So it is better that you keep CANVAS opened in this page (minimize) so you can use the links when necessary.

Explanation video of the practical:

Learning aim

Calculate reflectance curves from laboratory measurements and compare with other sources and materials.

Expected Completion time

3 hours

Resources needed for the exercise

PC with MS EXCEL spreadsheet
Data required to download: EMR01-P01_EM_Radiation_Exercise_reflectance.xlsx Download EMR01-P01_EM_Radiation_Exercise_reflectance.xlsx
file containing lab measurement results at the ITC.

Product

Students should answer the questions in the assignment in a separated sheet, Later there will be an assignment quiz to input the answers individually.

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Spectral reflectance curves: Theoretical refreshment.

Introduction

Incident Electro-Magnetic (EM) radiation on the Earth’s surface goes through three fundamental interactions with objects on the surface: the radiation is partly absorbed, transmitted and/or reflected. This is shown in the figure below, with the example of a lake.

What you see with your eyes is in fact only the reflected fraction of the incident radiation in the visible. You don't see other wavelengths, but they are there and also affected in the same way. To quantify the EM radiation we use spectral radiance, expressed as the amount of radiative energy per unit of area per unit of solid angle (steradian) per unit of time per unit of wavelength [watt µm^-1 m^-2 sr^-1]..

The reflectance property of the object is thus defined as::

ρ(λ)= (E_R(λ)⁄(E_I(λ)),

where:

ρ(λ) is the reflectance,
E_R (λ) is the spectral radiance of reflected EM radiation,
E_I (λ) is the spectral radiance of incident EM radiation, also called irradiance.

All the three are functions of (depend on) the wavelength of the radiation. In some wavelengths objects reflect more than others. That is unique per object.

Since reflectance for the same material depends on the wavelength, it is a good idea to have a look at a graphical representation of such a function of a given material. We can use this graph as an identifying tool for surface materials in remotely sensed imagery. It can also help us to realize changes in the curve when they occur, and to monitor them. Take for example agricultural crop vegetation. A farmer (who is also a RS fan) looks at two reflectance curves of his wheat taken on two different dates, can recognize if there is a drop in the near infrared NIR range of the spectrum (where reflection is usually high for vegetation) and through this, he can realize that something is wrong with the crop in the field.

The graph that represents reflectance as a function of wavelength is called spectral reflectance curve, sometimes referred to as spectral signature. One has just to bear in mind that two samples of the same material will similar but they will never be identical, and therefore even within spectral ‘signatures’ we should always expect some variation.

Spectral reflectance curves are measured by using ‘spectrometers’ at a continuous (but limited) range of wavelengths. This is done by measuring in the lab the incident and reflected radiation and then applying the expression given above. In the rest of this exercise you will generate such curves from the spectrometer field data of ITC. Later you will compare them to the ones generated for the same material at the spectroscopy laboratory of the Jet Propulsion Laboratory (JPL), NASA. Again, notice that ITC measurements are in the field and JPL's are in a controlled environment at the lab... so it should be differences not only because the materia is slightly different but because of the procedure.

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Assignment Section: Part 1

Do this task.png Open the spreadsheet EMR01-P01_EM_Radiation_Exercise_reflectance.xlsx Download EMR01-P01_EM_Radiation_Exercise_reflectance.xlsx

. Open the worksheet “data”. This contains the field data of ITC.

Column A shows a sequence of wavelengths for which measurements were taken.
Column B shows the spectral irradiance from the sun E_"I" (λ) (measured in spectral irradiance units: W m^-2 μm^-1)
Columns C and E show the spectral radiance E_R (λ) from the mineral Gypsum, measured in two locations in the field (units: W m^-2 μm^-1)
You may use columns D and F to calculate the reflectance of the mineral Gypsum (in %)
Columns G shows the reflectance of olive trees in an orchard (in %). [Note: In reality, measurements from above an orchard possibly will contain radiation reflected from olive trees and soil, but for sake of this exercise, we assume that this data set contains information only from the olive trees].

IMPORTANT NOTE: in the following, write down the answers in a separated paper to all questions written in red, later will be asked in a quiz for correction]

Give Answer.png Q1) What is the λ range (from min to max, in nm) of all measurements?

Give Answer.png Q2) Why are radiance values (C, E) always smaller than irradiance values (B)?

Do this task.png Generate a chart with spectral radiance and spectral irradiance curves in the worksheet “curves”. You may refer to Support sheet if you don’t know how to plot data in MS EXCEL. Download Support sheet if you don’t know how to plot data in MS EXCEL.

Example of a result for the sample gypsum of ITC (g1):

Do this task.png In the spreadsheet calculate reflectance values for the first measurement of Gypsum (column D). The equation is given in the beginning of this exercise. Refer to Support sheet if necessary on how to calculate in a spreadsheet if you do not know how to do it Download Refer to Support sheet if necessary on how to calculate in a spreadsheet if you do not know how to do it

Do this task.png In the spreadsheet calculate reflectance values for the second measurement of Gypsum (column F).

Give Answer.png Q3 - For the column F, should you use the same irradiance values as before? Radiance values of the second measurements are listed in column E.

Do this task.png Once calculated, visualize in a new chart the reflectance curves of Gypsum (both measurements) together with the reflectance curve of olives.

Give Answer.png Q4) What is common to all curves?

By now you should have 2 charts in sheet “curves”.

2.3. Comparison with an external source

Do this task.png Open the site of the Jet Propulsion Laboratory (JPL), NASA with the Internet Explorer: https://speclib.jpl.nasa.gov/library (Links to an external site.)

Use the spectral type: “minerals”, class: "sulfate".

You can see in the output of your query a long list of materials with some description of the material, chemical composition, particle size, and the spectral range of the available measurements (in microns, as X Start, X Stop. "X Start" means the wavelength at which the measurements of reflectance starts. "X Stop" is the wavelength at which it finishes). Note that different samples were used for the spectral measurements (different minerals with the same chemical composition). Spectra was measured on minerals with different particle size (fine, medium, coarse, etc) and for two solid surfaces.

NOTE: For the questions, concentrate only in the "Name" column containing "GYPSUM" of different kind. (Note: using CTRL+F in your browser you can use the finder to locate all NAMES starting with "GYP" for example. The search is easier in this way.

Give Answer.png Q5) There are many Gypsum samples. What are the 4 ranges of wavelengths (X Start, X Stop) available for the samples (in micrometers)? (Be aware that there is more than one page of results. You need to search both).

Give Answer.png Q6) How many of these ranges cover also the visible range of the spectrum?

Do this task.png Open the reflectance curve (view plot) of the "coarse" particle size sample with "sample No"= gypsumc. Reduce the window size of your browser so you can see both the ITC and the JPL signatures of Gypsum, side by side and compare them visually.

Give Answer.png Q7) You are now looking at reflectance curves of similar materials. Can you confirm the similarity between the JPL measurement and one of the two ITC measurements in this way?

Give Answer.png Q8) What would be the proper way to compare the reflectance curves?

The part 2 of the exercise will be guided by the staff. Keep the 8 answers of part 1 for later!!!

Assessment and feedback

Along the assignment there are a number of questions (about 8) that you have to answer in a paper of your own. After the assignment is finished and you have the answers to the questions, you can enter them in the quiz of the next section. After you submit the quiz, you will get the right answer of those questions that are not essays. To get he answers of all questions, including the essays: click this link Download click this link
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Assignment Section: Part 2

This part is guided by the staff.

Up to now you have practiced the concept of reflectance with data from the lab (JPL) and the field (ITC) using a spreadsheet as a tool. It is time to see reflectance in an image.

In this part we will attempt to build portions of spectral reflectance curves from satellite imagery. Those portions correspond to the (central) bands of the satellite.

Guided by staff (no written guide).
Data: build a working directory. Download this file Download this file , and unzip in the working directory.