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I am trying to understand what causes variability in the color temperature of white LEDs.

I recently got some white LED strip lighting and it was sold to me as having a color temperature of 3000K. I also have some overhead LED downlights that were of the recessed bulb type and they also are supposed to be 3000K.

I used a high-quality spectrometer to measure the color temperature of all the lights. I found that the down lights varied from 3000 by about +/- 50K. Also, if I measured the same light multiple times I got a variation of +/- 5k, which I guess is the limit of precision for the instrument. The down lights where never less than 2975K or more than 3080K.

The LED strip, however, was a completely different story. Depending on where I measured on the strip it varied anywhere from 3200K to over 3400K. If I measured in the same spot, I got the same +/- 5k, just like the overhead lights. Why would the strip be so much more variable? Also, how do they control the color temperature of the manufactured LED in the first place?

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  • \$\begingroup\$ the color temperature of the manufactured LED is dependent on the formulation of the phosphorus strip in the LED \$\endgroup\$
    – jsotola
    Mar 11 at 22:27
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LED strips vary a bit down the length because of the voltage drop in the conductive traces of the strip. This results in a slightly lower current for LEDs further from the power supply. This can be fixed by applying power in more than one place on the strip.

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Did the supplier spec a range on the led strip temperature? A lot of cheap strips are just whatever junk is left over or failed QC for higher paying customers. If you bought cheap lights made from reject parts, color temperature is going to be all over the place.

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Unlike many LEDs which have the colour determined by the semiconductor materials, the white LEDs use a blue LED which illuminates a mixture of rare-earth phosphors.

If the quantities are not mixed in exactly the correct proportion the colour temperature varies.

Eg. enter image description here

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When many electronics parts, including LEDs are made, even with best manufacturing practices, not all of the devices made are the same. They may vary in quality factors(Output at rated current, color) that don't affect longevity and thus don't prevent sale, even for a reputable company. As the devices are made, they are tested and "binned" into groups based on their properties. The highest quality bins are sold at a premium and the lowest quality bins more cheaply. If you're buying LED's online and you want to know why two sites with reputable CREE XP-G3 LEDs have a significant difference in price, often checking the bin number of LED will give you the answer. In addition to varying by brightness at rated output, LEDs can be binned by Chromatic range index, which is a quality measure of the breadth of spectrum the light is composed of. XP-G3s come in 70 CRI typical, 70 CRI minimum, 80 CRI minimum and 90 CRI minimum. You get less luminous efficacy(Brightness to the human eye per Watt) but better color representation at higher CRI.

There's a good chance your downlights have better CRI than your strip lighting, and it's also possible the manufacturer of the strip lighting has worse manufacturing tolerances and therefore higher variation, although phosphors are already covered in another answer.

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So there is two sources of issues here.

First is application/installation. Led strips tend to be on fairly thin Flexible Printed Circuits leading to higher than wanted resistance paired with lax application where strings run longer than ideal. This means that sections farther from the source experience voltage droop and that affects their output. Consider even a tenth of a volt can affect the nominal forward current and thus light output you will not get an equal reading from different sections. Led segment 1 will see the full 12V while Led segment 100 may be seeing 11V. Did your readings change equally as you go down the strip?

Ways to avoid that is by running parallel power conductors and injecting power every 2.5 or 5 meters to ensure minimal voltage difference along the strip.

Second is quality/construction. The cheaper the source components and questionable the retailer the less likely you will have consistent output. Leds may not have matching characteristics in VF/IF curves or color temperature. Material or production process may use cheaper components so they burn out quicker. Cheap led strips will overdrive the leds to produce a brighter output but will cause heat damage reducing their lifespan and cause color shift early in their life. They might even be mixing leds from different manufacturers let alone production runs or bins.

Retail products in store may have higher quality than some random internet vendor. I'd say you get what you pay for but price isn't exactly a measure of quality because they will charge you a premium for crap.

Finally, I know I said two but third is measurement issues. You need to ensure there is no stray lighting and quality instruments for accurate measurement. Ignoring that and going with a good enough attitude could lead to false results.

So basically you need to ensure a good install, equal quality and trustworthy exam to be able to tell. You can't really compare retail lighting to iffy import products.

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