Question about Cree 3W Cool White LED

[g8gxp]

I am using a dimmable meanwell to drive my lights. When my whites (Q5) are on low load (0-10%) the lights themselves look yellow to me. As i put a higher load on them they gradually turn more white. Is what i am actually seeing what is happening (i.e. yellow warming to white) or is it just the way my eyes perceive it?

[Chris27]

That is what is happening, depending on the LED's, the color could range anywhere from 5000 - 10000K. Take a look at the manufacturer data sheets for your LED, you should find some different graphs depicting the wavelength relative to power.

[der_wille_zur_macht]

What driver are you using? Drivers that dim via PWM'ing the output generally produce vastly less color shift. There is still some due to a change in die temp, but it's less than what you get from different currents.

This is a great read on the subject:

http://www.maxim-ic.com/app-notes/index.mvp/id/3070

Also, either you are extra-perceptive or something else is going on with your rig, as I doubt the average person would notice the color shift unless explicitly asked to look for it.

Chris - out of curiosity, where did you get that spec? I don't think I've heard of or seen changes that large for the typical HP LEDs we're using. Cree doesn't even publish the info for their cool white LEDs, as far as I know.

[Garage1217]

http://www.cree.com/products/pdf/xlamp7090xr-e_b&l.pdf

Most likely what is happening is that you are able to see the actual color of the led when it is dimmed down vs being blinded at full power. I used to put a piece of bright white paper over white leds to see the color differences at full power. Dimming also gave a general idea of what color I was dealing with. Also if you have the blue leds on, it will make the whites "even when dimmed" look like they have a yellowish tinge to them.

[der_wille_zur_macht]

Originally Posted by Garage1217

http://www.cree.com/products/pdf/xlamp7090xr-e_b&l.pdf

That shows color bins (determined experimentally for a given batch, under constant conditions), not color shifts.

[Garage1217]

Correct, sorry misunderstood your post

[Chris27]

http://www.cree.com/products/pdf/XLamp7090XR-E.pdf

[g8gxp]

I'm using the ELN 60-48D, strings of 13 with a max internal pot setting of .90A. I should clarify. I can tell the light is yellow looking at the LED itself not at the tank. Also note that it's much more noticeable with the optics on than not. So basically cool white is not really cool white?

[Garage1217]

"cool white" is a very generic term for a higher spectrum led. Where did you purchase them from / what bin were they supposedly from?

[der_wille_zur_macht]

Originally Posted by Chris27

http://www.cree.com/products/pdf/XLamp7090XR-E.pdf

If you're referring to the CCT range columns in the flux characteristics table on page 2, that is NOT intended to indicate color range for a single cool white LED as it is operated under different conditions. Instead, it is intended to specify the range over which individual color bins are available.

In other words, you will never have a single given cool white XR-E that will operate at 5kk under one set of conditions, and 10kk under another. However, you can buy about two dozen different color bins of XR-E cool white that cover that range of colors. Again, Cree doesn't specify a color shift with respect to current, at least that I know of (including the documents on the product page on their website.)

Originally Posted by Garage1217

"cool white" is a very generic term for a higher spectrum led. Where did you purchase them from / what bin were they supposedly from?

Besides being generic, we have to keep in mind that the model naming conventions (i.e. cool white vs. neutral or warm white) are done from the perspective of marketing to the general lighting products market, NOT reefkeeping. Our hobby has a very skewed sense of what is "cool" or "warm" compared to the rest of the world.

[Chris27]

In looking at the datasheets, it appears that at low power, the wavelength is more towards the yellow end of the spectrum, vice a nice crisp blue.

[der_wille_zur_macht]

Originally Posted by Chris27

In looking at the datasheets, it appears that at low power, the wavelength is more towards the yellow end of the spectrum, vice a nice crisp blue.

I don't mean to be argumentative and I know it's true in general for LEDs, but my point is that I don't see that represented in the datasheet. Can you provide a page number or exact reference?