The Human Eye

Discussion in 'General Electronics Chat' started by cjdelphi, Apr 3, 2010.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    It turns out that in mankinds evolution we did have the ability to see ultraviolet light, but it harmed our eyes quickly and for the lifespan we have, would mean loss of sight well before expected death.

    For example a hawk can see UV to track animals urine the stronger the patch the more brightly blue it would glow on the ground indicating an animal's presence allowing it to swoop down and get it... it also turns out humans can see ultraviolet light too but the lens in the back of our eye has a filter to filter out UV to protect our eyes from exposure, scientists claim this is due to the fact we live much longer and require our eyes for much longer, hawks and other animals don't live as long thus die before losing their eye sight.

    So, how come when i look at a UV source, UVA for example why do we see that dark blueish glow at all? why when you point a UV source against a wall we can see any marks or stains at all?... if we don't see UV, but obviously we must see UV how else would we see the light bounced back off of surfaces?....
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    It's not UV bounced back, it is fluorescence.

    Atoms in the substance are exited to a higher energy level, when they collapse back to normal a photon of light of a specific colour (for that type of atom) is emitted.

    It's not specific to UV, think of the phosphor layer in a Cathode ray tube (Old TV or Oscilloscope, excited by an electron beam), X-Ray intensification film (in an x-ray cassette with the photographic film, fluoresces under x-ray illumination to convert to visible light), the little testers for IR Remotes that glow visibly when exposed to IR light etc.

    Do NOT look at high power UV sources. The eye can focus it but cannot detect it so there is no blink reaction and the iris does not close. Homan eyes do NOT hav a UV filter!

    It can burn holes in your retina and blind you.
  3. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    Well yeah, the eye does have an UV filter hence we don't see it, but what is that dark blue/purple with a fuzzy glow to it, if that's not UV what is it?... UV LED or Fluro Tube, I understand what you say the photos excite for example when it hits a white-tshirt, the t-shirt glows bright blueish (so that's not UV we see, that's the T-Shirt emitting photons?)

    And when i do look into a small keyring UV LED, i see a blueish LED glowing, but to a person who has no filter in the lens behind the eye, they see a brilliant beam of light, the filter simply removes it from view and we're left with a small dull blue/purple visible light..

    I saw a doc from Richard Hammond's Invisible World where a person who had an artificial lens fitted and he now sees really bright light emitting where as his other eye sees the dark blue glow...

    So what is that dark blue glow we do see? did you say that was just phosphorus same with the LED? I can't get over the fact if we don't see UV what do we see exactly from the light source?
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    In Starks page there is a link to an article "Let the light shine in

    You don't have to come from another planet to see ultraviolet light, says David Hambling"

    In the fourth paragraph they write about how we "see" uv from a tee-shirt:

    it is pretty interesting:
  5. Markd77

    Senior Member

    Sep 7, 2009
    Some of what you can see is probably plain old violet because most if not all UV lights have a fairly broad spectrum. Keychain "UV" LEDS may not even have any UV output, it might all be violet but still causes fluorescence. Even blue LEDs make some things fluorese and there definitely isn't any UV coming from them.
    (Presumably you were watching the same Invisible Worlds program I was).
  6. someonesdad

    Senior Member

    Jul 7, 2009
    It's difficult to say what other people "see" with respect to UV radiation. One of the problems is it's non-trivial to provide the proper stimulation. A good tool would be a UV monochromator, but most of us don't have access to such things, as they are expensive. I would also be a bit leery of doing such experiments with my own eyes. One or two short (a few seconds) exposures to low-intensity sources in the 300-400 nm range wouldn't concern me too much, but shorter wavelength stuff would. And one must realize that current wisdom on UV exposure is that the damage is cumulative, similar to x-rays and ionizing radiation sources. It's not stuff you want to fool with.

    When you look at a UV fluorescent tube or a "UV" LED, you can see some of the radiation; I've attached some measurements of the spectra of some UV tubes I measured a few years ago. These were UV tubes used to visually examine people's skin. These were conventional mercury discharge tubes (you can see the Hg peak around 365 nm).

    I've also measured the spectra of research-grade UV LEDs with peaks around 350 nm (and FWHM values of 5 nm or so) and I can definitely state I cannot see any visible radiation from them at all, even in the pitch dark. If you choose to do something like this, wear polycarbonate safety glasses when testing (I measured the UV attenuation of the glasses first to prove to myself they were safe), then get your eyes accommodated to the dark, then raise the glasses briefly (a second or two) to look at the UV source, then drop the glasses again.
  7. peck68


    Nov 27, 2009
    Ye, the eye was really only designed to see visible light between roughly 500nm and 600nm (ofcourse going beyond this range, but it fades out until you cant see it at all)

    Try it yourself, get say a red filter and then a blue filter - you'll notice that no light is getting in, thats because the red filter only allows red light to pass through it, the blue filter allows the blue light to pass but since there is no blue light (since it was blocked by the red filter) no light gets passed at all :)

    Most digital cameras can see infrared, however there is a filter on the CMOS that prevents it getting to the sensor
    Last edited: Apr 4, 2010