NextPreviousTable of Contents

The concept of a "spectral signature", another name for a plot of the variations of reflected (or absorbed) EM radiation as function of wavelengths, gives rise to the widely used approach to identifying and separating different materials or objects using multispectral data obtained by remote sensors.


Electromagnetic Spectrum: Spectral Signatures

For any given material, the amount of solar radiation that it reflects, absorbs, transmits, or emits varies with wavelength. When that amount (usually intensity, as a percent of maximum) coming from the material is plotted over a range of wavelengths, the connected points produce a curve called the material's spectral signature (spectral response curve). Here is a general example of a reflectance plot for some (unspecified) vegetation type (bio-organic material), with the dominating factor influencing each interval of the curve so indicated:

The spectral signature of the general class 'vegetation' plotted over the wavelength interval shown on the abscissa.

This important property of matter makes it possible to identify different substances or classes and to separate them by their individual spectral signatures, as shown in the figure below. *


Typical continuous spectral curves or signatures of the four classes shown.

For example, at some wavelengths, sand reflects more energy than green vegetation but at other wavelengths it absorbs more (reflects less) than does the vegetation. In principle, we can recognize various kinds of surface materials and distinguish them from each other by these differences in reflectance. Of course, there must be some suitable method for measuring these differences as a function of wavelength and intensity (as a fraction [normally in percent] of the amount of irradiating radiation). Using reflectance differences, we may be able to distinguish the four common surface materials in the above signatures (GL = grasslands; PW = pinewoods; RS = red sand; SW = silty water) simply by plotting the reflectances of each material at two wavelengths, commonly a few tens (or more) of micrometers apart. Note the positions of points for each plot as a reflectance percentage for just two wavelengths:

Two-dimensional plot in spectral reflectance space of the values for the 4 classes shown in the previous illustration.

In this instance, the points are sufficiently separated to confirm that just these two wavelengths (properly selected) permit notably different materials to be distinguished by their spectral properties. When we use more than two wavelengths, the plots in multi-dimensional space (3 can be visualized; more than 3 best handled mathematically) tend to show more separability among the materials. This improved distinction among materials due to extra wavelengths is the basis for multispectral remote sensing (discussed on page I-6).

I-11: Referring to the above spectral plots, which region of the spectrum (stated in wavelength interval) shows the greatest reflectance for a) grasslands; b) pinewoods; c) red sand; d) silty water. At 0.6 micrometers, are these four classes distinguishable? ANSWER

I-12: Which material in these plots is brightest at 0.6 micrometers; which at 1.2 micrometers? ANSWER

I-13 Using these curves, estimate the approximate values of % Reflectance for rock (sand), water, and vegetation (choose grasslands) at two wavelengths: 0.5 and 1.1 micrometers, putting their values in the table provided below. Then plot them as instructed on the lower diagram. Which class is the point at X in this diagram most likely to belong? (Note: you may find it easier to make a copy of the diagram on tracing paper.) ANSWER

Framework for recording and plotting values for the reflectances of the 3 indicated classes at the two wavelengths indicated. If need be, print this figure out.

I-14: Presume that two unknown surface features in an image or photo, which actually are a forest and a field crop with the plants close-spaced, are measured for their spectral values, and both display quite similar reflectances at three chosen wavelengths. How might these be separated and perhaps even identified? (Hint: think spatially.) ANSWER

Spectral signatures for individual materials or classes can be determined best under laboratory conditions, where the sensor is placed very close to the target. This results in a "pure" spectral signature. But what happens if the sensor is well above the target, as when a satellite remote sensing device looks down at Earth. At such heights the telescope that examines the scene may cover a large surface area at any moment. Individual objects smaller than the field of view are not resolved (this is akin to spatial resolution limitations). Each object contributes its own spectral signature input. In other words, for lower resolution conditions several different materials/classes each send (unresolved) radiation back to the sensor. The resulting spectral signature is a compound of all components in the scene. Analytical techniques (e.g., Fourier analysis) can extract individual signatures under some circumstances. But the sampled area (corresponding to the pixel concept introduced on the next page) is usually assigned a label equivalent to its dominant class). This integration of several signatures is inherent to the "mixed pixel" concept examined in the bottom half of page 13-2.


* The principles of spectroscopy in general, as well as a survey of imaging spectroscopy and hyperspectral remote sensing, are explored in greater detail in Section 13 (pages 13-5 through 13-10). Also treated in that part of Section 13 is a brief review of the concept of "spectral resolution".


NextPrevious



18 comments:

Anonymous said...

http://d.hatena.ne.jp/huangshumei/20130128/1359362456
http://cinecloudfilms.com/index.php?do=/blog/93531/if-you-go-online-and-search-for-a-spyware-virus-remover-or-some-type-of-spy/
http://www.blurpalicious.com/xmyshang/tory-burch-selma-riding-boots
http://heraldbulletin.neighborsink.com/node/247693
http://nen360.nenonline.org/blog/always-be-aware-where-you-search-your-spybot-virus-protetion
http://aaa333333.cocolog-nifty.com/blog/2013/01/the-implementat.html
http://www.toma.jp/blog/aaa333/?entry_id=870937
http://ludhianacity.net/blog/57192/three-mobile-phone-deals/
http://sns.cam111.com/blogs/entry/including-their-target-audience
http://thundernetworks.info/jcow/index.php?p=blogs/viewstory/28091
http://archive.remdublin.com/blog/xmyshang/2013/01/28/always-be-aware-where-you-search-your-spybot-virus-protetion
http://ludhianacity.net/blog/57433/choose-spyware-protection-prevent-spyware/
http://www.bloglog.com/blog/fashion4/126323/in-your-e-book-and-or-on-your-web-site-with-the-authors-resource-box-included-and-without-any-changes
http://blog.qlep.com/blog.php/216559/718423
http://socialcirclessite.com/blogs/viewstory/1229594

Free Antivirus Download said...

After studied of your blog i get more knowledge about uses of radiation. free antivirus download

Anonymous said...

Remove Toxins From Your Body diets are filled with foods which help to flush
out toxins is one of the first dayyou must take a muti vitamin.

In healthy cells, the? But how do the toxins affect our body actually?

Produced in FDA-approved laboratory conditions and made from pharmaceutical - grade
ingredients, Phen375 is the safest and most easily utilized by your body.


Feel free to visit my web blog - how to detox naturally

Anonymous said...

Hey, I think your blog might be having browser compatibility
issues. When I look at your blog in Chrome, it looks fine but when opening in Internet Explorer, it
has some overlapping. I just wanted to give you a quick heads up!

Other then that, terrific blog!

my homepage; Cosima revival

Anonymous said...

An outstanding share! I have just forwarded this onto a friend who
was conducting a little research on this. And he in fact bought me dinner because
I found it for him... lol. So allow me to reword this.... Thanks for the meal!

! But yeah, thanks for spending the time to talk about this
issue here on your internet site.

My webpage; Zinn collection reviews

Anonymous said...

Greetings from Colorado! I'm bored at work so I decided to browse your blog on my iphone during lunch break. I love the information you provide here and can't wait to take a look when
I get home. I'm shocked at how fast your blog loaded on my phone .. I'm
not even using WIFI, just 3G .. Anyways, very good site!


Look into my web site :: Goji Berry Diet

Anonymous said...

Why people still make use of to read news papers when in this technological globe the whole thing is presented
on net?

Visit my web-site Test Force Xtreme

Anonymous said...

It's nearly impossible to find well-informed people for this subject, however, you sound like you know what you're talking
about! Thanks

My homepage: Nuvocleanse Review

Anonymous said...

This article will assist the internet users for building up new blog or even a blog from start to end.


My web blog :: Rvtl Skin Care

Anonymous said...

Do you mind if I quote a couple of your posts as long as I provide credit
and sources back to your weblog? My blog is in the very same niche as yours and my visitors
would definitely benefit from some of the information
you present here. Please let me know if this alright with you.

Thanks a lot!

Also visit my web blog; Beyond Raspberry ketone Review

Anonymous said...

I will immediately grab your rss as I can not in
finding your email subscription link or newsletter service.
Do you've any? Please let me understand so that I may subscribe. Thanks.

Also visit my web blog :: Mito slim reviews

Anonymous said...

Hello, I think your website might be having browser compatibility issues.
When I look at your blog in Chrome, it looks fine but
when opening in Internet Explorer, it has some overlapping.
I just wanted to give you a quick heads up! Other then that, very good blog!


My web blog - Buy Chronic profits

Anonymous said...

Spot on with this write-up, I truly think this website needs
a great deal more attention. I'll probably be returning to read through more, thanks for the information!


heating and cooling minneapolis

Anonymous said...

This post will assist the internet users for building
up new webpage or even a blog from start to end.

Also visit my web site - Eden

Anonymous said...

Hi there, I discovered your web site by way of Google whilst looking for a similar subject, your site came up, it seems to be great.
I've bookmarked it in my google bookmarks.
Hi there, just become alert to your weblog thru Google, and located that it's
truly informative. I'm gonna watch out for brussels. I will be
grateful if you happen to continue this in future. Numerous other people shall be benefited from your
writing. Cheers!

my blog ... Nintendo 3Ds Emulator Download

Anonymous said...

It provides tailor-made travel packages for all overseas away
matches, including the 2010 World Cup. I guess all of this procrastination can be forgiven, only if things like Gerard Pique and Lisandro
Lopez’s facial hair are represented in a befitting manner, hopefully EA
Canada have picked up on their respective grizzly aspirations.
The second half however would prove to be my first
heartbreaking experience in football as five minutes
in to the second half Brandts would redeem himself by
scoring for the Netherlands to make the score one, one.

Look into my blog post: Watch 2014 FIBA Basketball World Cup Live Stream

Anonymous said...

Thankfulness to my father who told me on the topic of this website, this web site is in fact
awesome.

Also visit my web blog: buy jogger pants

oakleyses said...

lancel, swarovski, hollister, swarovski crystal, moncler uk, juicy couture outlet, montre pas cher, doudoune moncler, links of london, ray ban, ugg uk, moncler, louis vuitton, moncler, hollister, moncler outlet, moncler, barbour uk, louis vuitton, replica watches, nike air max, louis vuitton, toms shoes, barbour, canada goose outlet, converse outlet, canada goose outlet, marc jacobs, louis vuitton, ugg, juicy couture outlet, pandora jewelry, wedding dresses, canada goose, louis vuitton, ugg,uggs,uggs canada, pandora charms, canada goose uk, pandora uk, pandora jewelry, moncler, canada goose outlet, gucci, moncler outlet, ugg pas cher, converse, thomas sabo, karen millen uk, ugg,ugg australia,ugg italia, canada goose, supra shoes, canada goose jackets, vans