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Color IR LiDAR

  1. Mar 9, 2010 #1
    Is it be possible to produce from the Intensity return of reflective IR laser the true color of the object causing the reflection of the laser? My though is that surfaces of the same material, but of different colors would cause slight changes in wave length of the laser light. These changes then could be map into a color.
     
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  3. Mar 10, 2010 #2

    matthyaouw

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    My short foray in to infrared photography whould suggest no. The reflectiveness of surfaces to infrared seems to bear little resemblance to its reflectance of light. Most manmade dyes for example aren't made with IR in mind so the majority appear ghostly pale. I'm not confident there's no way to tell, but none that I've seen.


    I still have an IR-pass filter so if you need any test images to help you find out then just ask. Bear in mind that a lot of filters do still allow some visible light to pass. so any colour information recorded may be from that

    edit: Of course not all digital sensors are made the same and most hav some pretty aggressive IR blocking filters so maybe that affects the result.
     
  4. Mar 10, 2010 #3
    Thanks for the Quick reply. Let me ask a slightly different questions. If I had a red laser would there be a differance if I reflected it off a red surface as compare to a blue surface? Could this difference be recorded?
     
  5. Mar 10, 2010 #4

    mgb_phys

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    No you can't - to measure the color you need white light, or at least a couple of different colored lights.
    You can't measure the color with only one illumination wavelength whether visible or IR.
    Under a red light, both red reflecting and white objects will look bright, while blue and black objects will both look black.

    The situation is a little worse with IR lidar - most of the signal return you see is due to the IR absorbtion/scattering nature of the surface. So (dry) concrete gives very strong returns, while anything wet will absorb the IR and look black.

    High end Lidar usually include a digital camera to get the color and map the point color from the camera image
     
  6. Mar 10, 2010 #5
    It sounds implausible to me. Afterall what is colour? It is the frequency of a ray of light (which is basically a single photon). If we shine a beam of light containing rays of different frequencies on an object, then we may well find that some of those rays are reflected. We may even find that only those rays of blue colour are reflected. To us, as we perceive the object, we say that it is blue.

    I fail to see how we can infer (without any prior knowledge) the colour of an object without actually probing the object with light rays pertaining to that colour.

    Also, the wavelength of the laser light would not be changed upon reflection. The only property you could hope to detect with a monofrequency source beam is a change in the intensity of the reflected signal.
     
  7. Mar 10, 2010 #6

    mgb_phys

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    Not quite correct.
    Your eyes only measure the relative amounts of red, blue, green in the image.

    But you can make a color image of yellow object by taking narrow red/green/blue images - you don't need an image in the color of the object. That's how almost all color cameras work.
     
  8. Mar 10, 2010 #7
    I have read about the use the use of digital camera along with LiDAR on the internet, but could not locate the source. Then when I went off on tangent thinking that the same information could be obtain from the changes in intensity of the laser it self. Any ways thanks for the education, as I can see from the replies using digital camera would be much easier. Also if I am reading your post right that it possible to merge the data from the camera sensor with that from the LiDAR sensor, thus creating a colored LiDAR Intensity image
     
  9. Mar 10, 2010 #8

    mgb_phys

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    Almost all the high end systems come with a digital camera ( except oddly the Leice one - I think Leica Geo and Leica Camera don't get along )

    In theory you could use multiple lasers at different wavelengths to get a color image, but for it to be 'true color' they would have to be in the visible - which has practical problems.
    The IR return signal is useful for finding the edges of a road surface, where it changes from concrete/tarmac to grass or detecting very small but highly reflecting things like powerlines - but in general it takes specialist skills to make any use of it.

    You can make 'IR color' images, with different infrared wavelenghts. They are very useful for vegetation, eg. a particular plant might more strongly absorb wavelength 'A' than 'B' while trees reflect 'A' and 'B' equally - so you fly an IR satelite over certain coutries in south america and find large plantations of a particular plant in the jungle. While on a true color image they would all just look green.

    They are also used to measure the water content in other crops to know when they are ready to harvest.
     
  10. Mar 10, 2010 #9
    You can make an image of yellow to fool the eye, sure, but can you measure the reflection of yellow light without actually probing with yellow light?
     
  11. Mar 10, 2010 #10

    mgb_phys

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    If the yellow surface was ONLY capable of reflecting a narrow yellow wavelength then you need to have yellow in the illuminating wavelength.

    But a real yellow surface would reflect A% or red light, B% of blue light and so you could measure a color without having any yellow information. Actually for only two monochromatic sources you would have a line of possible colors - you need three points in a color diagram to pin down a single color so you also need C% of some other color.
    For an imaginary yellow only surface then A,B,C would be zero and you couldn't see it.

    this is like a pure monochromatic source (eg a sodium streetlamp). If you had a camera with narrow RGB filters, none of which overlapped yellow - then you would have no info about yellow and the streetlamp would look black.
    That's why camera filters are broad enough so that green overlaps red and blue - and you still need an illuminating sources that covers all three.
     
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