Modern Colored LEDs

One of the long-standing facts about LEDs (Light-Emitting Diodes), was that for any color (wavelength) of light to be emitted, a different semiconductor material was needed, each with a different gap-energy, and therefore also each with a different forward voltage-drop. (:1)

What seems to have happened in more recent times, is that in the cheap, mass-production of LEDs, the Industry is realizing cost-reduction, by always producing the famous Blue LED (InGaN), but putting a different phosphor coating onto it, which absorbs most of the blue light, and which emits a mixture of wavelengths to taste.

But this is also the reason for which, when we buy glow-sticks for camping for example, we may no longer find the traditional, deep red glow-sticks as easily as we could before. What has happened is that here too, a Blue LED is used, but a phosphor put onto it that emits red light. And then, a small amount of the blue light passes through the phosphor without being absorbed, so that a ‘pink’ shade of light emerges.

At least, if the ones I just bought are any indication.

One disadvantage to this is the fact that deep-red light tends to blind human vision the least at night, so that assuming night-vision, there used to be advantages to pure red light for signaling purposes. Because the newer LEDs still contain some light-energy at the (shorter) wavelength that looks blue, those LEDs will also have a stronger, undesirable tendency to ‘dazzle’ a person’s night-vision.

My favorite color of glow-stick for camping is actually the green one.

And the reason for that is the fact that several times, I’ve gone camping as part of a group, and on some occasions, certain campers actually tented. When this was being done, we decided on a different color for each camper, and a blinking glow-stick would have signaled to a supervisor, that one camper was in some sort of trouble.

What I was able to do with battery-powered, LED glow-sticks quite easily, was to leave mine on steadily, throughout each night, and still to be able to fall asleep. Like that, the glow-stick was also a source of illumination for my tent. And at each morning, my source of illumination was still working.

And ‘green’ suited me best for that.


 

If the reader finds a glow-stick in a retail store, in a package that invites “Try Me,” where, the button on the glow-stick works, without the packaging having to be opened, then the reader should expect that this type of glow-stick has been factory-programmed, to shut off automatically after 1-2 hours. This beats any store-clerk having to watch out when the store closes, whether there are still any blinking products on the shelves.

And, this can hinder in certain camping-uses.

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These two were hanging on the same item-hook at a Walmart today, and according to their packaging, came from the same company, under the same brand: “Life+Gear”. Each only cost C$ 7.98 +taxes. I know that the 3 additive primary colors are Red, Green and Blue. I’m making the far-out-assumption that, the circuit-diagram of each is essentially the same, meaning that the two colored light-emitters operate at the same voltage, and that the two quasi-white emitters, operate at the same voltage. And yet one looks green, while the other looks pink to the eye. Red ones were not available, even though the stock was plentiful.


 

When I bought glow-sticks on the Internet in the past, those had thin plastic strips between a battery terminal and the device, which are pulled out before first use, to avoid discharging the battery before sale. In my experience, those will stay on for the duration of a night, or, until turned off by the user.

BTW: ‘Pink’, or ‘Purple’, has ‘Green’ as its complementary color. It would in fact be possible to add a green phosphor to a pink lighting-device, but due to the additive mixing of colors, doing so would just make this light-source slightly less pink.


 

(Update 6/13/2021, 18h50: )

1:)

One fact which has become apparent to me is that, since I posted this in 2018, the information is already a bit dated. One anchor within the WiKi page about LEDs no longer exists, which used to list which semiconductor material was once needed, to produce light output at different (and narrower) wavelengths. Instead, there is now a WiKi section about how ‘A layer of InGaN can be sandwiched between two layers of GaN,’ and how different wavelengths can be obtained by varying the thickness of the InGaN layer. Further, greater total light output can be obtained, through the use of more than one layer of InGaN.

What I find most peculiar about the WiKi article on LEDs for illumination would be, how they treat the light output of each LED as not being completely monochromatic, thereby leading to the effect that tetrachromatic LEDs can achieve good colour rendering. Yet, the WiKi also writes, that such tetrachromatic LEDs have poor energy efficiency. This poor energy efficiency would only set in, as long as the amount of current which passes through each LED is accurately regulated (the reciprocal of photon energy each time), effectively putting all 4 in parallel. The lower-photon-energy LEDs in such a circuit would end up wasting a sizeable amount of voltage, through whatever regulation was applied in series with each… (:2)

Well, back in my day, what circuit designers would have done was, just to put all 3 or 4 LEDs in series, thereby ignoring that the combined light output would have progressively more power at the shorter wavelengths. The light would not even be close to white, but according to older metrics, would be close enough for some uses.


 

(Update 6/13/2021, 20h25: )

One concept which the WiKiPedia doesn’t need to bother to explain is, that it’s possible to use the same semiconductor composition, but to re-tune it numerous times, resulting in different chips, so that different wavelengths of light emerge, but in this case, a wider band of wavelengths each time as well.

If a manufacturer needs to change the way individual chips are made, then he still ends up having to set up a separate assembly line for each type, and there is no economization. In that case, the only real advantage of the newer semiconductors over the old would be, higher amplitude of light output.

However, if a manufacturer has one assembly line, that manufactures one exact chip, then the wavelength of light output will be the same for those, and the manufacturer can put different droplets of phosphor on them, thus achieving different, approximate colours of light. This way, 1 manufacturer can produce packages that generate 5 different colours, but not at 5x the cost.


 

(Update 6/13/2021, 23h35: )

2:)

In contrast with the subject that this posting began on, I could now do some speculating, as to how tetrachromatic LEDs – that have enhanced bandwidth per diode – could be controlled, in order to generate whatever colour temperature was desired, as precise amounts of current flowing through each diode, and, without wasting much of the supplied energy.

One way in which such inefficiency can be minimized is, that there could only be one ‘Blue’ and one ‘Green’ LED diode in one package, but the same package could contain two ‘Yellow’ and two ‘Red’ LED diodes in series each. That way, one regulating current input could have its current mirrored, and multiplied by the following series:

6, 8, 5, 6  … x25

In order to evoke a quantity of ‘white light equivalent’. And a second regulating current could be mirrored and multiplied by the series:

4, 8, 6, 8  … x26

In order to evoke a quantity of ‘pink light equivalent’.

This way, one LED package could hypothetically be designed, that offers a range of added light mixtures, that loosely correspond to different colour temperatures.

 

Dirk

 

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