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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I bought a green led to have a 'play' with, HLMP-CM2A-120DD (about 75% of theoretical max. efficiency !) I'm running it at 20mA and it's very green and very bright. Pressing the led on a fingertip, looking from the other side I see RED patterns, presumably blood vessels, WHY? In the dark I illuminated all kinds of stuff and for example red inks/pigments look grey/black so there's little or no red in the original spectrum, is blood fluorescent under green light?
I'm guessing it is due to blood being very thin, with red blood cells not as frequent as the pigments in ink, so inks would appear dark (more light blocked out) while blood appears red (more light admitted).
I am going to go on a wild speculation that is likely to be wrong.
Is red the colour we see because red lights reflected off blood? Or that blood absorb all other colours and only let red light passes through it?
The two modes are different. In this case, you are seeing the lights after it passes through your blood.
Also from here: >Below are images of a green LED for comparison. The peak is located at 559 nm, and its width is 24 nm. : >Students may be surprised to see that emission from a green LED is so wide that a bit of red is visible at one end of the spectrum.
And lastly how do we know if your blood is really red?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The datasheet spectrum of the led indicates virtually no red in it's spectrum, I think that is correct because as I mentioned red dyes/pigments appear black/grey in the green led light and the 'brightness' of the red light through my fingertip is significant - not a vague trace the red is so strong that you would not know the led was green.
I'll just mentally file it under 'miscellaneous data' until I research it more.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Surely somebody here must have a green laser that they can try on themselves? That's guaranteed to have no red light in its spectrum. (Infrared maybe, if it was a DPSS )
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
LEDs are not sources of monochromatic light as often quoted, There is indeed a very little bit of red radiation from green LEDs, enough to make the red visible when all other colors are blocked out.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have bright pure green LEDs that cause fluorescence in different materials so it might be that.
Checking the spectrum of two of my green ones show ouptut all the way from UV to IR. Looks almost like a white lightsource behind my thumb. So better check the output of the LED to be sure.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
OK, I just used a cd as a diffraction grating and to my surprise (vs. datasheet) my 'green' InGaN led covers the whole visible spectrum ... mystery solved..Thanks.
Registered Member #4362
Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98
Question. Could you see the freq. of the light by monitoring the input voltage with an oscope? Next question does the freq. change if light is reflected back into the led? By your finger?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
My 'scope bandwidth doesn't extend to the 100's THz so I can't answer that.
BTW I see (!) where I made a significant mistake in interpreting the datasheet for the led, the intensity scale for the led intensity vs. wavelength has a linear intensity scale, whereas I believe that my eyes have a nearly logarithmic response, so what looks to be negligible intensity on the graph is still quite visible.
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Seeing red with a green led ... why?
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