3 Watt Cool White LED on star heatsink with yellowish light output

Thread Starter

PeteHL

Joined Dec 17, 2014
473
Seven years ago I bought a large quantity of the 3W cool white LEDs on a star heatsink and all of them had a cool white light output. But then in replacing a few of these LEDs purchased seven years ago with the same LED from the same vendor purchased about 2 years ago, the replacement LEDs have a yellowish output. Has anyone else experienced this? If the unlighted LED has a yellowish appearance, does that mean that when powered that it will produce a yellowish output? How can I avoid purchasing a LED described as producing a cool white illumination but in reality it has a yellowish tinge to its light output?

The LEDs that I purchased seven years ago that have not failed (most of them) continue to produce cool white light.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
473
if the product is described as cool white but you get warm white version, there has been mistake. you should contact seller
The light output of the LEDS that I purchased about 2 years ago to replace failed ones I would say is more yellowish than what would be called warm white. Earlier this year I did return LEDs to the same vendor complaining that they produced the yellowish type of light. To get a refund, I did not power them and returned them on the basis that they had a slight yellowish color of the casing. Yesterday looking at some 3W LEDs for sale from other vendors, in some cases I noticed that the photo of the LED for sale said to produce a cool white output not powered had a yellow tint.
 

Ya’akov

Joined Jan 27, 2019
9,078
The phosphor that produces the white light when excited by the blue LED under it will always be yellow. How yellow will depend on the formulation, but it is not a filter, it is the light source.

So, seeing yellow in a white LED doesn’t tell you the color when it is powered.
 

MrSalts

Joined Apr 2, 2020
2,767
More simply, if. you mix yellow light with the blue light used to excite it you get white light.
That's very simplified!
a bit less simplified...
A mixture of fluorescent materials are used to make the "phosphor" (they typically look yellow by the time all the materials are present). When excited with the underlying blue LED, the some ingredients fluoresce green, some yelloish-green, some orange and some red and some deep red (with tails into the near-IR). Since the human eye can only detect red, blue and green - the density of the phosphor determines how much blue gets through and ratios of the recipe determine the rest.

There is no way to determine the color of a single LED by looking at the phosphor. However, if you look at two LEDs of the same style from the same series from the same manufacturer, you can do pretty accurately sort them from cool white to warm white by the color of the phosphor (light yellow will be a bluer, cold white" (higher Kelvin temperature), darker yellows or more orange color will be warmer looking (lower Kelvin temp) - more incandescent like).
 

DickCappels

Joined Aug 21, 2008
10,152
The point is, if you buy from an authorized distributor there should be a data sheet that gives the color coordinates for white LEDs, and the dominant wavelength for LEDs of other colors.

If you are buying bargain-basement LEDs from the odd dealers, like many of those on eBay, "You pays yer money and you takes yer chances".
 

MisterBill2

Joined Jan 23, 2018
18,179
The numerical value of the color temperature specification, if accurately given, is the simplest way to determine the yellowness of the light. The higher values are whiter/bluer. And like already stated, reliable sellers have a better idea of what they are selling you. It really is true.
 

BobTPH

Joined Jun 5, 2013
8,813
However, lI doubt that any of them accurately produce the spectrum of a black body at the given color temperature.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
473
More simply, if. you mix yellow light with the blue light used to excite it you get white light.

Is it yellow when power is applied?

One specification to check is the color temperature or even better when you can get it, the color coordinates.
https://en.wikipedia.org/wiki/CIE_1931_color_space
The seller was claiming to be selling a LED that produced a cool white light output. That was good enough for me, I wasn't going to be picky, I just wanted a LED that made white light.

What I did notice was that the LEDS that I purchased seven years ago not powered did not have a tint of yellow, whereas the ones that I purchased about 2 years ago also not powered had a slight yellowish color. So when I ordered more of the same LED from the same seller this year, I figured that If the new ones had a slight yellowish color when not powered, then probably they would produce the prominent yellowish output that I didn't want. I didn't want to give the LEDS that I ordered this year power so that there would no question that I could return them.
 

DickCappels

Joined Aug 21, 2008
10,152
I remember about 20 years ago Kingbright came out with a shockingly highly priced white LED made with three separate dies: Red, Green, and Blue, to make white. That was before the UV/Blue with phosphor approach was introduced. Just had a thought: those patents should be expiring soon, at least in the U.S.
 

Boggart

Joined Jan 31, 2022
82
The seller was claiming to be selling a LED that produced a cool white light output. That was good enough for me, I wasn't going to be picky, I just wanted a LED that made white light.

What I did notice was that the LEDS that I purchased seven years ago not powered did not have a tint of yellow, whereas the ones that I purchased about 2 years ago also not powered had a slight yellowish color. So when I ordered more of the same LED from the same seller this year, I figured that If the new ones had a slight yellowish color when not powered, then probably they would produce the prominent yellowish output that I didn't want. I didn't want to give the LEDS that I ordered this year power so that there would no question that I could return them.
A while back there were also white LEDs that emitted white light directly from the chip, they were zinc selenide units, I used to sell them when they were available, but there was only one manufacturer and they disappeared quite a few years ago. You may have had some of those, although not sure they came in 3W versions.

All modern white LEDs use a phosphor that gives the LED die a yellow look. Some have the phosphor applied to only the top surface of the die, so it's less obvious, while others have a big blob of phosphor over the whole die cup. You can tell which are cool and which are warm whites just by the colour of the phosphor, once you get a bit of experience with them.

[Edit] I remembered I sent some of those zinc selenide LEDs to LED Museum, they are in the discrete LEDs category...
 
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