Can someone explain "kelvin" ratings to me??

[7Reefer]

What is kelvin exactly????? I always read about the "kelvin" of a bulb, but there seems to be a difference in color from 1 company to another for the same kelvin rating. Should all 20K bulbs be blue and all 10K be white? Or, are companies jumping on the "color of the week" and labeling their bulbs incorrectly? What determines a bulb's kelvin rating?

Thanks

 

[Acro76 Verified Sponsor]

Kelvin is a measure of temperature. When it is used for color temperature, the standard is heating a block of carbon, which changes colors with temperature.

I think Kelvin Temperature in lighting is just the overall "look" of the bulb. It doesn't tell you much that is useful, since even different bulbs of the same Kelvin rating can look quite different to the eye. A spectrum plot gives you more useful info.

 

[Russellaqua]

I've wondered this as well. If my memory serves (and physics is not my strong point) the color temperature of light emitted from an ideal black body (think bulb filaments) is defined as the temperature of that body in kelvins. One kelvin is the same "size" as a degree Celsius, with 0 K equal to -278.3*C (absolute zero.) Unless I misunderstand something, all 20K lights should emit the same color within a reasonable error ascribable to manufacturing difficulties, power fluctuations, and instrument error when measuring the color temperature of the bulb. Anyone else have insight on this?

 

[Me z]

In an ideal world all 20k bulbs would look blue all 10k bulbs would be crisp white and all 65k bulbs would be tinted yellow. However manufacturers mainly just use the k rating as a marketing tool not a true measurement of kelvin. Look up sanjay's spectral plot on different bulbs, this will be closer to a tru k rating than what the manufacturer slaps on the bulb. For example on my old tank one half was lit by a xm 20k and the other half was lit by a pheonix 14k. The xm was very yellow looking more like something between 65k and 10k and the pheonix was really blue in comparison. I think the pheonix may have been an accurate 14k but the xm coloring was way off the 14k should not have been more blue than the 20k. The next question your gonna ask is how do I know what bulb to buy. I would research, look up sanjay and his spectral plots and then look up the bulbs your thinking of using and see how others have rated them in terms of accurate k description, growth, etc...

 

[Troylee]

For example on my old tank one half was lit by a xm 20k and the other half was lit by a pheonix 14k. The xm was very yellow looking more like something between 65k and 10k and the pheonix was really blue in comparison. I think the pheonix may have been an accurate 14k but the xm coloring was way off the 14k should not have been more blue than the 20k.

umm i think you had a bad bulb or ballast... i have never seen a bulb blue as a xm 20k... xm20k looks like a blue led next to a phoenix on my tank haha.... you sure you didn't have the xm 15k or 10k???? a xm 10k is very yellow but a par beast!!! 20k very blue bulb with great colors but horrible growth..:sad: unfortunately i don't like my phoenix and radium don't make a 250w hqi so im prolly going back to xm20k myself....

 

[Glenn]

In an ideal world all 20k bulbs would look blue all 10k bulbs would be crisp white and all 65k bulbs would be tinted yellow.

Since the OP is trying to cut thru all the advertising hoopla and figure out "what's what", I think this comment should be clarified to let them know that there is no "65k" bulbs on the market. Using your standard numbering system, it would actually be 6.5k.

6500K bulbs are quite yellow looking - I used them on my frag tank and growth was good but viewing was 'ugly'
10,000K bulbs are whiter, good for viewing and good for growth
20,000K bulbs are very blue DEPENDING ON THE Manufacturer

anything between 10kK and 20kK will be shades of white to blue

 

[JackoChang]

To make it even more confusing, aquarium kelvin color temps are unique to the aquarium trade, they will be different if you purchase a light bulb for your house.

 

[bct15]

For example on my old tank one half was lit by a xm 20k and the other half was lit by a pheonix 14k. The xm was very yellow looking more like something between 65k and 10k and the pheonix was really blue in comparison. I think the pheonix may have been an accurate 14k but the xm coloring was way off the 14k should not have been more blue than the 20k.

There must have been something wrong with your setup, I use the xm 15k and they are about the same color as an ATI blue +. Bluest bulb I have ever used, I can only imagine the 20k...

From my understanding Kelvin rating is based off if a measurable scale,and has to do with the color emitted at a specific temperature of a certain burning gas. My knowledge base is more solid materials based then fluids based so i can't give an extensive explanation. The kelvin temp is not a marketing ploy and requires testing to verify the rating, u can't just label an iridescent bulb as a 14k bulb.

 

[bct15]

To make it even more confusing, aquarium kelvin color temps are unique to the aquarium trade, they will be different if you purchase a light bulb for your house.

Not true, 10k is 10k and 20k is 20k and so on, no matter the type of bulb it is; headlight, single ended metal halide, hqi, t5....

 

[7Reefer]

I knew I was not the only one unsure! I remember kelvin being based on temp, but of what? If it is a gas then how do you rate LEDs?

What leads me to my original question is I bought a single 50 w led that was labeled as 20K. When turned on it is a crisp white with no blue. How is the kelvin of LEDs determined. Is it based on a Color scale or what?

Thanks for the replies

 

[bct15]

From what i know (which isnt much) It is a tool used by the industry to communicate the color emitted by a light source, sort of a universal language that is based off of (datum) the color emitted by a burning source (gas I believe) ...that way you can have an idea of what you are getting without necessarily seeing the product in action. LED manufacturers label there color rating to give you an idea of the metal halide bulb that the fixture mostly resembles. A good maufacturer will perform vigorous testing to prove that it fully mimics that bulb but I haven't seen any yet, and probably won't because the waveforms emitted by led fixtures are not similar to metal halides. Generally, in the led fixture world, the Kelvin rating refers to the blue to white ratio of diodes. 20k having the highest amount of blues, this rating is completely subjective and really has no meaning, led fixtures do not have any design standards that I am aware of so LED fixtures do not fit into the normal Kelvin temperature criteria, but it is still used as a comparative universal language though.

 

[CJO]

I knew I was not the only one unsure! I remember kelvin being based on temp, but of what? If it is a gas then how do you rate LEDs?

What leads me to my original question is I bought a single 50 w led that was labeled as 20K. When turned on it is a crisp white with no blue. How is the kelvin of LEDs determined. Is it based on a Color scale or what?

Thanks for the replies

As Acro76 said, it's based off of the color of black carbon heated to a certain temperature. Also, as Glenn said, it's not 20K (the object would still be black at -250 degrees C), it's 20,000 K. Think white hot metal, then think hotter.

CJ

 

[cparka23]

Kelvin rating is the color radiated by an ideal black body at a certain temperature. An ideal black body is a substance that absorbs all EM radiation and reflects none of it back. It's an ideal that doesn't exist. Never heard anyone refer to it as carbon.

Unless I'm mistaken, it's not carbon and not any certain gas. It's just a way to say, "what color is emitted at X degrees by something that doesn't reflect 'light'?"