Blue LEDs - Amazing Fact

[georges80]

The afterglow is of course due to white LEDs and CRTs using a phosphor for light conversion. It remains active for a little while even after the original stimulating source is removed.

cheers,
george.

[CatalinaWOW]

Plotted it again on a log scale.  At low light levels both green and blue will be good, within 3dB of each other.  But red will be at least two orders of magnitude worse, and possibly three orders of magnitude, depending on the exact wavelengths of blue and red.

This terrible response of the eye to red at low light is why red lights are used to preserve night vision.  It just doesn't trigger the chemical reactions in the cone cells used at those light levels.  An interesting question, which I haven't probed, is why evolution chose fewer, but higher energy photons to see in low light conditions.  The answer isn't obvious to me.

[matseng]

The afterglow is of course due to white LEDs and CRTs using a phosphor for light conversion. It remains active for a little while even after the original stimulating source is removed.

Yet the LiFi guys (at least the slightly crazy LiFi proponents does) claim that they can be used for 200+Gbit data transfers....

[Nerull]

You know that not all LEDs use phosphors, right?

[ataradov]

You know that not all LEDs use phosphors, right?

If this is a come back to LiFi mention, then you are wrong, since all lighting LEDs use some sort of compound with afterglow.

And even if it was not the case, all light fixtures in my house have an enclosure that has afterglow.

And light communication using non-lighting LEDs has been around for quite some time in form of lasers.

[matseng]

You know that not all LEDs use phosphors, right?

For sure I do.  But the LiFi people usually claims that your regular white LED lighting in your home and office will be used.

[georges80]

You know that not all LEDs use phosphors, right?

Yes, obviously. I did write WHITE LEDs (the typical ones we see from folk like Cree etc) when I stated phosphors.

cheers,
george.

[Zero999]

Plotted it again on a log scale.  At low light levels both green and blue will be good, within 3dB of each other.  But red will be at least two orders of magnitude worse, and possibly three orders of magnitude, depending on the exact wavelengths of blue and red.

Your attachment contains an error: the wavelength should be nanometres not micrometres which is in the far infrared spectrum.

This terrible response of the eye to red at low light is why red lights are used to preserve night vision.  It just doesn't trigger the chemical reactions in the cone cells used at those light levels.  An interesting question, which I haven't probed, is why evolution chose fewer, but higher energy photons to see in low light conditions.  The answer isn't obvious to me.

I don't know the exact answer but could guess it's because blue light is refracted over the horizon so at dawn and dusk there's not much red light.

The afterglow is of course due to white LEDs and CRTs using a phosphor for light conversion. It remains active for a little while even after the original stimulating source is removed.

Yet the LiFi guys (at least the slightly crazy LiFi proponents does) claim that they can be used for 200+Gbit data transfers....

That shouldn't be a problem. The phosphors only emit the longer wavelengths so add a blue filter to the sensor and the phosphor afterglow is no longer a problem.

[T3sl4co1l]

Hint: Lifi doesn't communicate in yellowish wavelengths.

Tim

[CatalinaWOW]

Plotted it again on a log scale.  At low light levels both green and blue will be good, within 3dB of each other.  But red will be at least two orders of magnitude worse, and possibly three orders of magnitude, depending on the exact wavelengths of blue and red.

Your attachment contains an error: the wavelength should be nanometres not micrometres which is in the far infrared spectrum.

Thanks for catching this.  I usually work in the infrared and my fingers entered the units without checking with my brain.  The chart is now corrected.

[Someone]

Plotted it again on a log scale.  At low light levels both green and blue will be good, within 3dB of each other.  But red will be at least two orders of magnitude worse, and possibly three orders of magnitude, depending on the exact wavelengths of blue and red.

Your attachment contains an error: the wavelength should be nanometres not micrometres which is in the far infrared spectrum.

Thanks for catching this.  I usually work in the infrared and my fingers entered the units without checking with my brain.  The chart is now corrected.

Heh, you now have two wavelength axes one in microns and one in nanometers.

[Someone]

Off-topic (sort of) factoid:
Do you know why the telescope operators use red LED lights to read their maps/etc when they are out with their telescopes at night?
True, our eyes are more sensitive to green (thus you would think green is preferred).
However: They use red because the human eye's iris reacts the least to red....thus when the light is needed to read a star map or find an eyepiece, the light will not cause their "night vision" sensitivity to detail to be compromised.
It can take 30 minutes or more for a telescope operator's eyes to totally acclimate to the dark.   

You would totally be schlonged (Dont you love the election season?) in that would would have to wait for 30 minutes to catch a good view of a DSO such as the Messier 101 pinwheel galaxy (or other DSO) if you used a green light to find that 8mm eyepiece.

v/r,

P

You've got quite a bit of information lost in your retelling of this, the iris is very quick to respond (seconds) but only has an order of magnitude or so of range. The full range of human vision extends from around 1:100,000 instantaneous out to 1:1,000,000,000 or more through biochemical adaption (the slow process) with some individuals exceeding 10^14 dynamic range.

https://en.wikipedia.org/wiki/Adaptation_(eye)
https://en.wikipedia.org/wiki/Purkinje_effect
https://en.wikipedia.org/wiki/Accelerating_Dark_Adaptation_in_Humans

The sensitivity to bleaching is dependent on wavelength and you can maintain much of the dark adaptation even in the presence of yellow and green monochromatic light (LEDs can be broad enough they extend into the blues, or just have odd emission bumps), but red is the least bleaching. Its not as simple as the sensitivity curves presented here so far.

[dom0]

Another good read just about how sensitive the eye is: http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

... sounds a bit like a biochemical PMT, doesn't it?

[CatalinaWOW]

I agree, the whole subject of eye response is very complex.  FOVs, surrounding fields, illumination history, individual differences and a whole host of other factors.  Measurements are difficult.  Think of what the CIE engineers had to do in the 1920s and 1930s to try to generate calibrated amplitude monochromatic light sources to generate the originals of these curves.  But engineering is powered by useful approximations, and if you are looking for qualitative explanation of why a blue LED might seem surprisingly bright at low power levels compared to a red LED these curves go a long way, even to the point of roughly explaining the power differences observed.  If you are looking for time-nut or voltage-nut accuracy human visual response will be a very frustrating field to pursue.

[Someone]

I agree, the whole subject of eye response is very complex.  FOVs, surrounding fields, illumination history, individual differences and a whole host of other factors.  Measurements are difficult.  Think of what the CIE engineers had to do in the 1920s and 1930s to try to generate calibrated amplitude monochromatic light sources to generate the originals of these curves.  But engineering is powered by useful approximations, and if you are looking for qualitative explanation of why a blue LED might seem surprisingly bright at low power levels compared to a red LED these curves go a long way, even to the point of roughly explaining the power differences observed.  If you are looking for time-nut or voltage-nut accuracy human visual response will be a very frustrating field to pursue.

Except that the steady state relative sensitivities you show ignore the huge photochemical effects which dominate the dark adaptation, and are an additional 3 orders of magnitude to consider/correct/understand.

[CatalinaWOW]

Since we know little of the original posters experimental conditions we don't know if dark adaptation plays a role or not.  Agree that if he is comparing a red LED viewed after a few minutes in a bright normally illuminated room to a blue LED in a dark room after his eyes have acclimated for half an hour there are several orders of magnitude more to try to account for.  I assumed since he was commenting on the nanopower output of the blue LED that that observation was in a dark room and then further inferred that he was comparing a red LED in similar conditions.  There are an infinite number of other possibilities.  Which is why I try to do quantitative calculations of illumination, power, reflection etc in MKS units, and try to leave lighting evaluations to something on the order of: If the spousal unit likes it, it is good.  Otherwise it is not good.  if it is not good I ask questions like more?  Redder?  Successive approximation leads to an answer which is valid for a while.

[Someone]

Since we know little of the original posters experimental conditions we don't know if dark adaptation plays a role or not.  Agree that if he is comparing a red LED viewed after a few minutes in a bright normally illuminated room to a blue LED in a dark room after his eyes have acclimated for half an hour there are several orders of magnitude more to try to account for.  I assumed since he was commenting on the nanopower output of the blue LED that that observation was in a dark room and then further inferred that he was comparing a red LED in similar conditions.  There are an infinite number of other possibilities.  Which is why I try to do quantitative calculations of illumination, power, reflection etc in MKS units, and try to leave lighting evaluations to something on the order of: If the spousal unit likes it, it is good.  Otherwise it is not good.  if it is not good I ask questions like more?  Redder?  Successive approximation leads to an answer which is valid for a while.

You've also used normalised curves which hide the relative wavelength dependencies of the eyes components, here is a modern reference with much better presented data:
http://webvision.med.utah.edu/book/part-viii-gabac-receptors/light-and-dark-adaptation/
and this is still an interesting area for research.

[NiHaoMike]

I have one of those cheap alarm clock/mini digital picture frame combos (separate blue LED backlit LCD for the clock) that runs off an internal Lipo battery and can be charged over USB. I put a 100k resistor across the backlight transistor but it turns out that made the clock too bright at night! I added a piece of pink post it note between the diffuser and LCD both to change the color (much easier than trying to change the tiny SMD LEDs) and attenuate the light. Still too bright, so I increased the resistor to 470k and it was just right.

[CatalinaWOW]

Since we know little of the original posters experimental conditions we don't know if dark adaptation plays a role or not.  Agree that if he is comparing a red LED viewed after a few minutes in a bright normally illuminated room to a blue LED in a dark room after his eyes have acclimated for half an hour there are several orders of magnitude more to try to account for.  I assumed since he was commenting on the nanopower output of the blue LED that that observation was in a dark room and then further inferred that he was comparing a red LED in similar conditions.  There are an infinite number of other possibilities.  Which is why I try to do quantitative calculations of illumination, power, reflection etc in MKS units, and try to leave lighting evaluations to something on the order of: If the spousal unit likes it, it is good.  Otherwise it is not good.  if it is not good I ask questions like more?  Redder?  Successive approximation leads to an answer which is valid for a while.

You've also used normalised curves which hide the relative wavelength dependencies of the eyes components, here is a modern reference with much better presented data:
http://webvision.med.utah.edu/book/part-viii-gabac-receptors/light-and-dark-adaptation/
and this is still an interesting area for research.

I've not used normalized curves to hide the relative color sensitivity, the relative color sensitivity is fundamentally unknown.  For a brief explanation see (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html).  We can say with some certainty that a blue light under some well specified set of conditions causes a perceptual response that can be related to the perceptual response to a red light under the same set of conditions.  We can't say whether that is because the blue cones responded differently than the red ones or because the downstream processing provides different emphasis.  Much of human vision is unknown physics, or perhaps psychology or even training.  Think of the phenomenon of color constancy.  Agree that there is much room for research.

[Someone]

I've not used normalized curves to hide the relative color sensitivity, the relative color sensitivity is fundamentally unknown.

You have normalised the photopic and scotopic spectral sensitivities, when they should be relative to each other (both changing in absolute and relative by amount of dark adaption).

[CatalinaWOW]

I've not used normalized curves to hide the relative color sensitivity, the relative color sensitivity is fundamentally unknown.

You have normalised the photopic and scotopic spectral sensitivities, when they should be relative to each other (both changing in absolute and relative by amount of dark adaption).

I now understand your comment and agree.  I should have placed them on two separate graphs, each of which would still be normalized to one for the maximum response.  The concept is so deeply engrained for me that I didn't imagine that they would be interpreted as directly comparable.  Sort of inherent in the definition.  I can think of no correct way to represent both responses with an appropriate relative scaling on a single graph since each curve applies over a multi-order of magnitude brightness range even without the added complication of adaptation.