Notices
Results 1 to 11 of 11

Thread: infared detection

  1. #1 infared detection 
    Forum Freshman
    Join Date
    Dec 2005
    Posts
    8
    can an someone tell me how do the night vision devices,the ones based on thermal infared detections work


    Reply With Quote  
     

  2.  
     

  3. #2  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    What exactly do you mean by 'do'? Do you want to make one? Construct a simple example for a school project? Make one that will function practically?
    Most night visions devices use photo-multiplier arrays to amplify the small number of received photons. Infra-red devices are a different matter, converting infra-red radiation to visible radiation. Composite devices may exist, but I suspect these would be difficult to construct, and expensive to buy.


    Reply With Quote  
     

  4. #3  
    Forum Ph.D.
    Join Date
    Oct 2004
    Location
    Maastricht, Netherlands
    Posts
    861
    The human body gives off heat, and heat is really just a form of light that can not be detected by the human eye. This form of light, which falls in the infrared spectrum, can be detected by infrared cameras, and than transformed into colours that we can see. For example, 'warm' is red, 'cold', is blue.

    These are not, however, night vision goggles, that give the 'green' vision. Those are amplifiers of normal light, which can either be active or passive, the first sending out infrared light and creating an image based on the light returned. These amplify a couple of thousands times, so shining with a flashlight into them is pretty.. uhm.. dumb. I'd guess military goggles compensate that though.

    About creating them, although I don't think that was the line of your question, I wouldn't think about it. Unless you have a technical education, just buy them at Walmart :P

    Mr U
    Reply With Quote  
     

  5. #4  
    Forum Professor
    Join Date
    May 2005
    Posts
    1,893
    Quote Originally Posted by HomoUniversalis
    The human body gives off heat, and heat is really just a form of light that can not be detected by the human eye.
    Heat is not the same thing as infrared light. It is true that things warm up when they absorb IR light, but that’s just because the energy from the light is going into the material.
    Reply With Quote  
     

  6. #5  
    Forum Sophomore Stranger's Avatar
    Join Date
    Sep 2005
    Posts
    148
    But why can't we detect infrared light?

    I know that UV light can't be detected because it is absorbed by our lens before it reaches the retina, but what about the IR?
    Watch what thy eyes can't see... and live it.

    (T.B)
    Reply With Quote  
     

  7. #6  
    Forum Radioactive Isotope mitchellmckain's Avatar
    Join Date
    Oct 2005
    Location
    Salt Lake City, UTAH, USA
    Posts
    3,112
    Quote Originally Posted by Scifor Refugee
    Heat is not the same thing as infrared light. It is true that things warm up when they absorb IR light, but that’s just because the energy from the light is going into the material.
    Correct. But the main point is that all objects radiate light according to their temperature (and composition). There is a general relationship between temperature and the range of wavelenths in which objects radiate. This is described by equations for black-body radiation. The sun radiates right in the yellow range of light because it is at a temperature of 5800 degrees Kelvin. At only 305 degrees Kelvin the human body radiates in the infra-red region of the spectrum. I should explain that the bell shaped curve under which this radiation from an object is not only broad but the height of the curve increases with temperature. The first explains why the Sun is more of a whitish yellow and why there are no green stars (stars which peak in green light appear to us as pure white). The second explains why the sun radiates much more infra-red light than the human body does even though the peak of the curve for the sun's radiation is in the yellow part of the spectrum.

    Quote Originally Posted by Stranger
    But why can't we detect infrared light?

    I know that UV light can't be detected because it is absorbed by our lens before it reaches the retina, but what about the IR?
    The retina of the human eye has 4 types of cells that respond to light (rods and 3 types of cones: blue, green, red). All 4 respond according to a bell shaped curve over a range of frequencies. Together these ranges form the visible range of light. Infrared and ultraviolet light are both just outside this range of visible light. Below (in frequency) infrared are the microwaves and radio waves. Above ultraviolet are the x-rays and gamma rays. We cannot see any of these.

    However, infra-red light is quite abundant in the human environment and it is a common means of heat transfer. When you sit in front of a fire you are getting most of the heat from it in the form of infra-red radiation. About half the heat which escapes the human body is in the form of infra-red radiation and that is why they make these emergency blankets of a mirror like material to reflect this radiation back into the body. Ultra violet light in the same quantities would be very dangerous to our health, for it has more energy and would burn and damage us rather than simply transfer heat. It is now quite common to design eyewear to block out the ultraviolet light in order to protect our eyes from the damage it can cause. "Black lights" emit ultra violet light in much lower amounts and many substances absorb it and re-emit that light in frequencies which we can see.

    Here are some links for further reading.
    http://hyperphysics.phy-astr.gsu.edu...n/rodcone.html
    http://hyperphysics.phy-astr.gsu.edu...colcon.html#c1
    http://hyperphysics.phy-astr.gsu.edu...on/bright.html
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

    I now have a blog too: http://astahost.blogspot.com/
    Reply With Quote  
     

  8. #7  
    Forum Sophomore Stranger's Avatar
    Join Date
    Sep 2005
    Posts
    148
    thanks. I read the articles you gave me, but check this :

    "Individuals who have had the lens of an eye removed because of a cataract are able to see into the near UV region because the major absorber is no longer present."

    That's from my Biophysics book. That's crap or what?
    Watch what thy eyes can't see... and live it.

    (T.B)
    Reply With Quote  
     

  9. #8  
    Forum Professor
    Join Date
    May 2005
    Posts
    1,893
    Quote Originally Posted by mitchellmckain
    Correct. But the main point is that all objects radiate light according to their temperature (and composition).
    Yeah, but he seemed to be implying that IR-based night vision devices work by detecting the black body radiation of warm objects, which (so far as I know) isn’t true. They usually have accompanying “infrared flashlights” that illuminate objects so that the goggles (or whatever) can image them.
    Reply With Quote  
     

  10. #9  
    Forum Radioactive Isotope mitchellmckain's Avatar
    Join Date
    Oct 2005
    Location
    Salt Lake City, UTAH, USA
    Posts
    3,112
    Quote Originally Posted by Scifor Refugee
    Yeah, but he seemed to be implying that IR-based night vision devices work by detecting the black body radiation of warm objects, which (so far as I know) isn’t true. They usually have accompanying “infrared flashlights” that illuminate objects so that the goggles (or whatever) can image them.
    Uh... that does not make any sense. If you are going to use a light to illuminate things then visible light is easier and much more efficient. Trying to use IR light to make things shine brighter than they already are in IR frequencies would require a fantastic consumption of energy. Think about it. Your IR light would have to be like a heat ray gun. Most object absorb pretty good in the IR frequencies and the only way to make them shine brighter in the IR range is to increase their temperature. Humans and other animals are already quite bright in the IR range (compared to, for example, brown dwarfs and cool L dwarf stars which are detected by IR astronomy).

    IR goggles would use an array of IR sensors which sends data through a computer chip that sends the processed data to a video display which produces visible light which your eye can see.
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

    I now have a blog too: http://astahost.blogspot.com/
    Reply With Quote  
     

  11. #10 image intensifiers 
    New Member
    Join Date
    Sep 2006
    Posts
    1
    Can anybody tell me why night vision goggles are so big?
    why can't they just put all the layers closer (photo cathode, MCP & phosphore screen) and have them of a width similar to basic spectacles (eye glasses). I would suppose that it is theoreticaly possible but just to complicated to manufacture, no?
    Reply With Quote  
     

  12. #11 Re: image intensifiers 
    Guest
    Quote Originally Posted by Edwood
    Can anybody tell me why night vision goggles are so big?
    why can't they just put all the layers closer (photo cathode, MCP & phosphore screen) and have them of a width similar to basic spectacles (eye glasses). I would suppose that it is theoreticaly possible but just to complicated to manufacture, no?
    If you guy's have NOT found this yet, turmn on your web-cam and point your tv remote at it, press a few buttons and watch what happens! - it's a good trick for testing remote's and will help show that light exists outside the range humans can see.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •