Author Topic: White light hi-output LED system: Why phosphor and not RGB based?  (Read 2053 times)
lights*plus
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White light hi-output LED system: Why phosphor and not RGB based? « on: September 15, 2020, 12:10:29 AM » Author: lights*plus
Can a knowledgeable person answer me: why do lighting companies exploit the YAG:Ce phosphor incorporated over an InGaN diode for most if not all lighting needs for white light, instead of mixing discrete diodes as in an RGB lighting system? Aren't discrete diodes more efficient than a phosphor-based diode? Wouldn't glare be less of a problem with a mixed RGB system?
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dor123
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #1 on: September 15, 2020, 12:38:35 AM » Author: dor123
InGaN blue LED + Ce:YAG phosphor is cheaper than RGB LED and simpler. RGB LED also have the same glare and awful white light as InGaN blue LED + Ce:YAG phosphor.
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #2 on: September 15, 2020, 12:49:50 AM » Author: lights*plus
I don't know, why is it "cheaper & simpler"? In the manufacturing process?
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #3 on: September 15, 2020, 10:35:29 AM » Author: Lumex120
I'm no expert here either but one thing I do know is that white light produced by RGB means typically has horrible, and I mean HORRIBLE CRI.
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dor123
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #4 on: September 15, 2020, 01:32:43 PM » Author: dor123
The white light created by RGB LEDs looks like that pinkish white. I've seen this on a color changing RGB LED nightlight once.
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #5 on: September 15, 2020, 09:20:16 PM » Author: Xytrell
Some thoughts:
1) Color rendering of RGB is rather poor since the spectral linewidth of a LED is somewhat narrow.
2) Red and green LEDs aren't as efficient as blue. Even with the phosphor losses, final efficacy is usually higher.
3) the voltage drop is different among the colors, which either increases the complexity of the driver, or results in a very off-white.
4) BOM list is longer
5) optics will often have fringes of color because of the different points of light. If you focus a RGB LED, you don't get a white beam, you get the three colors in overlapping beams.
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #6 on: September 15, 2020, 09:59:16 PM » Author: lights*plus
Point 3 - I didn't know this.
Point 4 - What is BOM?
Point 5 - Like in a phosphor-based LED chip, couldn't a maker miniaturize the discrete R-G-B diodes on one chip?
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #7 on: September 16, 2020, 04:57:59 AM » Author: Medved
Point 3 - I didn't know this.
Point 4 - What is BOM?
Point 5 - Like in a phosphor-based LED chip, couldn't a maker miniaturize the discrete R-G-B diodes on one chip?

4) "Bill Of Materials" - all components needded to assemble the thing and to make it working. In other words it is complex.

The RGB concept works only when you want to display majority of all colors (you can not display all in the "horseshoe" but only a triangular part of it, but that contains the ones mostly present in scenes). So the only thing you need is they excite the eye sensing cells the same way the real scenes do.

But for an illumination you need something else: You need to make sure the light reflected off objects illuminated by that light source excites the eye the same way as when the object is illuminated by a daylight.
It may seem there is no difference at first, but if you dig into it, you see the difference is VERY significant:
Assume you have monochromatic RGB LEDs, so each one emits one wavelength, so it excites its corresponding eye cell type. So by varying the intensity you may go across most of the visible colors (most, because the spectral sensitivity of the eye cells is pretty wide and overlap a lot, but that is a detail now).
But if you shine on an object reflecting just one narrow line between those LEDs (e.g. yellow), this object will be yellow on a broadband sunlight, becauise the sunlight spectrum does contain emission matching the reflected one, so that part reflects off. But this hypothetical object will look completely black under that RGB, because the LEDs radiate at red, green and blue lines, but not at the yellow one.
This example is a hypothetical extreme, in reality the reflectivity of real objects is more broadband, but having just 3 lines is too few so the color gets distorted.
Normal CRI80 rare earth (tend to generate narrow spectrum lines) fluorescent phosphors have to radiate on 5 wavelengths to work well with most real life objects.
LEDs use wide spectrum phosphors covering red to green, the blue region uses to have just the primary blue LED line so has gaps there. But nature does not offer much narrow reflected spectrum objects, so for normal use it is OK.

But there is another problem with RGB:
The LEDs tend to loose efficacy as their temperature rises, but each color does that with different slope.
So when blue degrades with 10..20% when warming up to ~60degC, the red tends to "fall down" to 1/4. That means the ratios among the color components changes as the LEDs are warming up. This effect needs to be compensated by the driver to get reasonably stable colors (most applications don't care, if all that is needed is changing colors), which need good LED temperature sensing and good performance charts of the used LEDs. And the accuracy of this data is usually the main cause of the off-color output of the RGB sets.

And for the multiple colors on one chip: This is impossible because of the laws of physic. The thing is, the color depends on the electronic band gap of the semiconductor material. So for any given wavelength you need dedicated material. But if you want a single chip, you need to select one material for that chip.
The manufacture makes many inch diameter wafer (old used 4", modern are up to 20") of that material first, the structures of many 1000's (typical 1W LED is 0.5x0.5mm, so we are talking about 70k 1W LED dies on a single 6" wafer; a manufacturing batch tend to contain 24 wafers processed at once) dies are made on it and only at the end it is then sawn to the individual chips.
So most process steps, which are expensive, make 100000's chips at once, so even when the process is very expensive ($12k to process the batch, so $500 per wafer), this cost gets diluted among all the product units, so those units then seem to be dirt cheap (0.7 $-cents per a single LED).
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #8 on: September 16, 2020, 03:18:33 PM » Author: lights*plus
Thanks for these points and issues. After the great successes in converting traffic light systems to diodes (staring way back in 1990 for California), I thought it was strange that lighting companies didn't pursue RGBs for general white-light lighting. I can see now how they're quite a deal breaker for such a system. 
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Re: White light hi-output LED system: Why phosphor and not RGB based? « Reply #9 on: September 30, 2020, 02:22:46 AM » Author: Medved
And there is one more reason:
It happens, so far the only really efficient (now I mean really an efficiency, so how much percentage of the input electrical power they convert into the output radiation) LED chips are those in the blue and deep red/IR region. Going closer to green, the efficiency get very bad. So bad, so a native green LED uses to have lower efficacy than a blue chip with a green emitting phosphor and a heavy blue suppression filtering (both with considerable energy losses, even in the "pass" spectrum) on it when aiming for a green light source.

And because the green is the major contributor to the lumen output, the low efficiency in generating it "hurts" a lot in the final efficacy figures. You get almost 5x higher efficacy from a blue+phosphor than from native RGB. In fact an RGB LED combination displaying white barely reaches efficacy of an incandescent, while the "blue+phosphor" use to be 8x above (yet with better color rendering).

But don't be fooled, attempting to generate somewhat usable scene colored light from incandescents (so using gels,...) still yields way lower efficacy, mainly due to extremely low incandescent output in the blue part and generally all saturated colors (as you filter out most of the other light power)...

So the RGB is used only when displaying wide variety of colors is the main task (there the narrow spectrum of its components is essential in maximizing the area of attainable color space) - e.g. the color effect stage lights or display screens (either the home OLEDs, or mainly the large LED matrix panels).
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