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Why is a solid state laser dioide pen shoning an undulating constant beam

  1. Feb 4, 2005 #1
    Diode laser emission dangerous to eye - Why?

    Why is a solid state laser diode pen which emits eletromagnetic radiation in the form of a constant range of undulating photons harmful to the retina?

    In other words, (Did I use the technical terms right? "Undulated" means 'wavelength' correct?)why is diode laser radiation(for example from a laser pen) harmful to the eye?

    Since the eye has a hole in it (the black part of your eye.), this hole is vulerable to the high intensity of a laser beam and cannot adjust accordingly. For example, in a dark room, wouldn't looking directly into a laser beam (common laser pen) more dangerous than in a room illuminated with light? Since in dark light your pupil is dilated more, therefore it's even harder for your eye to comphensate to the laser.

    Since photons have no mass, what exactly happens when a stream of photons goes though your pupil, into your retina? Why is this so damaging?

    Thank you.
    Last edited: Feb 5, 2005
  2. jcsd
  3. Feb 5, 2005 #2


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    The damage is basically due to the total intensity. Laser beams can be quite intense. You can also go blind by staring at the sun.
  4. Feb 5, 2005 #3
    In the case of common laser diodes, there is a risk of eye damage simply because the intensity is very bright, much like what would happen by staring at the sun or a very bright light. How many photons get in your eye depends on pupil diameter vs laser diameter. If laser diameter was very small, it wouldn't matter whether you're in a dark room or not, all the photons would get in either way (though there is slightly less chance if pupils are smaller, of course). If laser diameter is bigger than the pupil, less photons will enter in a smaller pupil.

    When photons hit the retina, they get absorbed by special molecules. If the intensity is low (few photons), this energy gets transmitted to the brain by nerve cells as color. If there is more energy than the molecules can absorb, the excess will damage the area (through heat and/or unusual chemical reactions). The area of course consists of nerve cell endings, which don't grow back very well.

    (no need to use the term "undulated" here)
  5. Feb 5, 2005 #4

    Thank you! This is the answer I've been looking for. Isn't intensity measured in lumens?

    Since laser light is coherant, the photons are alligned up and enter the pupil easily.

    So, I see. Since the suns rays is just another form of eletromagnetic radiation (in the form of light) and has a high intensity, it can do the same damage.
    That makes sense.

    Laser light is, after all, still light.

    Is a laser beam amplified by some device such as a magnifying glass to give the term "light amplified by stimulated emission radiation."

    Though experiments, I also noticed that if you take a laser beam and put it up to a magnifying glass, the dot of the laser beam is magnified greatly (depending on the power of the glass) and you can see the profile of the laser. (The pits of crevices of the laser quality)

    Does undulating mean frequency?
  6. Feb 6, 2005 #5
    Edited quote with numbers to structure answers :

    1. I think so, I have also heard "Candelas". I use power per area in [tex]W/m^2[/tex].

    2. Coherence is one thing and intensity is another. In a laser, photons travel in the same general direction, this results in great intensity, and possible eye damage. Coherence is totally different and has nothing to do with eye damage as far as I know. Only brightness = intensity = energy per area per time unit matters here.

    3. Yes, damage depends [tex]W/m^2[/tex], not on the specific differences between common light and laser light.

    4. No, not a magnifying glass. When light passes through most materials, its intensity is attenuated, or diminished, lost as heat. Some engineered substances however, are such that intensity in increased instead. This is the amplification in laser. The substance (solid, liquid, gas) has to be fed of course, by another light or electrical current (for semiconductor diodes).

    5. Yup.

    6. Undulating means wiggling, or oscillating. Any imaginable light does so. When something oscillates, you can associate to it a frequency, wavelength, phase, speed, amplitude etc. etc. all of wave theory.
  7. Feb 6, 2005 #6


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    It has nothing to do with the light being coherent. It has everything to do with how much energy is contained per unit area per unit time. This is what is meant by "intensity" and equivalent to the rate of the number of photons per unit area.

  8. Feb 6, 2005 #7


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    In this context (of laser light) it's higly reccomendable to use the unit:[itex] Js^{-1}m^{-2} [/itex] or to really express the fact that you're talking about the particle character of light, [itex] No. \ photons \ s^{-1}m^{-2} [/itex]


    P.S.I've always hated photometric units... :yuck:
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