Are there RGB LEDs that can produce a pure WHITE output ???

OLDmarty

Senior Member
#1
Hi All,

As the title suggests "Are there RGB LEDs that can produce a pure WHITE output? "

I'm mostly interested in Round 5mm leds (or 3mm if they're even available).

I've tried many (diffused) RGB LEDs from all kinds of different sources and struggle to achieve a balanced pure white colour when driving all 3 RGB pins equally.
WHITE often looks more blue or slightly light Magenta (purple).

Even mixed colurs such as red/blue, , red/green or blue/green don't always show a nicely balanced Magenta, Yellow or Aqua colours.
Am i expecting too much? or are there simply BAD LEDs as opposed to Excellent LEDs to achieve a nicer White or Colour-Mix output?

I've recall seeing RGBW LEDs (RGB with a 4th internal LED substrate being pure White), but i'm trying to avoid using the 1 extra pin to drive the White LED, but i might "have to" go this way.

Can anyone advise me of certain brands or model numbers i should aim for?

Thanks in advance...
 
Last edited:

newplumber

Senior Member
#2
Hi All,

As the title suggests "Are there RGB LEDs that can produce a pure WHITE output? "
My thinking is always on the edge of disaster but I believe the only way you will be able to
make a led "pure white" is thru a filter which in turn makes it less bright. I have in theory made pure white light with 5mm leds
when I connected them to 12v dc with out resistors but it didn't last on long enough to compare what light it was :)
If you do find a way i'll be happy to know
 

techElder

Well-known member
#3
OLD Marty, I've come close with RGB, but you have to drive each color differently ... sometimes a ratio of 5:1 is necessary. You certainly won't have the same intensity per color.
 

OLDmarty

Senior Member
#4
OLD Marty, I've come close with RGB, but you have to drive each color differently ... sometimes a ratio of 5:1 is necessary. You certainly won't have the same intensity per color.
Thanks Tex, yeh, i figured since the 3 internal led colours require 3 different forward voltages and/or currents, that i would have to balance the way i drive them. This becomes a little more complicated when driving these leds with a led matrix chip like the Maxim 7219 which uses only 1 resistor to setup the drive current.

I find that the internal LEDs seem to be placed more towards the outer edge of the round LED body, instead of being closer together in the middle.
This is probably the majority of the reason why the colour-mixing isn't so good.

I've placed these RGB leds within white silicon button caps, which certainly diffuses/mixes the colours better, but, when "white" is actually a light purple, then it's simply now a softer purple and not white by any means. ;-(

Anyway, i keep searching and pondering the options until somebody provices a part number of a super fantastic LED that does white nicely, along with RGB of course ;-)

Thanks again.
 
#9
There are three aspects to getting white from an RGB led (of any size).
  1. Colour balance. Invariably, Red will be the weakest due to the lower forward voltage drop across the LED. Next will be Blue and then comes the all-powerful Green. Green can often be twice the strength of Red and Blue. You can balance the intensity of each colour using different current limiting resistors, with PWM to do the fine adjustment.
  2. Colour temperature. When using several RGB LEDs, make sure they are all the same. So many RGB LEDs have different colour temperatures (specifed in degrees Kelvin).
  3. Colour distribution. Since each colour is emitted from a different location within the epoxy "blob" that encapsulates them, it can be difficult to get a uniform distribution of colour. If you have "water clear" LEDs, you can experiment with frosting the surface with emery cloth.
 

OLDmarty

Senior Member
#10
There are three aspects to getting white from an RGB led (of any size).
  1. Colour balance. Invariably, Red will be the weakest due to the lower forward voltage drop across the LED. Next will be Blue and then comes the all-powerful Green. Green can often be twice the strength of Red and Blue. You can balance the intensity of each colour using different current limiting resistors, with PWM to do the fine adjustment.
  2. Colour temperature. When using several RGB LEDs, make sure they are all the same. So many RGB LEDs have different colour temperatures (specifed in degrees Kelvin).
  3. Colour distribution. Since each colour is emitted from a different location within the epoxy "blob" that encapsulates them, it can be difficult to get a uniform distribution of colour. If you have "water clear" LEDs, you can experiment with frosting the surface with emery cloth.
Thanks Pete, Yes, the drives all need to be different to achieve a better white balance.

However, i have frosted LEDS, but the internal substrates still seem too far apart from each other to offer any kind of 'pure white' colour mixing.
With various voltages and currents, the best i can achieve is a light purple instead of white. This indicates the red & blue substrates are closer to each other than the green substrate, giving a more purple(magenta) biassed colour temperature.

I have seen RGBB leds, (2 blue substrates) to improve overall colour balance, but sometimes even those leds start looking too blue ;-)

There seems to be a large array of SMD 5050 leds with RGBW on them (APA102 etc) but i'm really trying to find a standard round 5mm solution.
An APA102 5mm round led would be ideal, but the closest thing is an APA106 which seems to be the "impossible to drive" WS2812 chip within them.
APA102 are best for picaxe control.

The search is on....
 
#11
Given that we don't know your projects physical arrangement for the LEDs, I have a few suggestions. Maybe you could drive a set of 2x 3mm LEDs in parallel to give slightly more control/balance? Not sure how that would work unless you put different resistors on each LED in the pair?!
I know you can get 2mm LEDs (in 'standard' round/non-SMD) - have you seen any RGB 2mm ones? Then you might be able to squeeze a set of 3x together in a 6mm ish hole?!
The other idea I had was using a light pipe or a fibre link between the LED and the actual output position - then if the arrangements allowed you could have any number of LEDs tuned and mounted at the other end, without getting in the way of your 5mm hole?
 

hippy

Technical Support
Staff member
#12
Am i expecting too much?
I think you are. There's good reason why manufacturers added an additional white to create RGBW.

I have some LED stage lights which use a few hundred individual RGB LED's which give an adequate white light where one wants white but the edges of the white spot show R, G and B tinges where the convergence isn't perfect.

That's the 'colour distribution' aspect inglewoodpete mentioned and it's always going to be an issue unless a single point light source or some convoluted tricks are used which will likely reduce the brightness and increase costs.
 
#13
Hi All,

As the title suggests "Are there RGB LEDs that can produce a pure WHITE output? "

I'm mostly interested in Round 5mm leds (or 3mm if they're even available).
....
This is coming from many years ago when I was building a sock-color-sorter to assist a visually challenged person to mate their socks after wash/dry.
As you can tell from Inglewoodpete's response, there is much to consider. Most of the details are contained in #7 response here: https://electronics.stackexchange.c...ctrum-led400nm840nm-vs-grow-light-led-redblue

I finally wound up using several "white LEDs" with overlapping spectrum. Newark White LEDs

"Warm" and "Cool" are defined generally by the intensity of the output spectrum:
https://electronics.stackexchange.com/questions/125500/cool-white-led-bulbs-are-they-full-spectrum

You can also use some colored gels to modify the spectrum:
https://www.edmundoptics.com/p/15quot-x-325quot-200-filters-color-filter-booklet/4469/
The above are quality gels, easy to use, and come with a very detailed absorption/transmitance graphs... and cutting little filters out of the booklet is very easy.

Ray
 
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