6500K vs 2700K; what does the kelvin number actually mean?

(Above) The Kelvin rating of sunlight changes throughout the day, depending on cloud cover, seasonality, etc.

2 Applications of the term

K rating is used to describe color temperature of black body radiators

The Kelvin unit used by scientists refers to specific blackbody radiation, where the colour temperature of the electromagnetic radiation emitted by an ideal blackbody is defined as its surface temperature in kelvins. For example, a near-perfect black body such as the Sun burns at a temperature of around 5772 Kelvin and emits a white colour. While the sun burns at a constant temperature, the light that hits the earth's surface is subject to atmospheric influences and its colour can range from 2000+ Kelvin (dawn/dusk) to 5800k (midday). It can appear bluer due to the scattering of light by cloud cover, or redder at dusk/dawn due to the Tyndall effect.

 An incandescent lamp approximates an ideal black-body radiator, so its color temperature is essentially the temperature of the filament. Tungsten filaments have to remain below 3600 kelvins to remain solid, and thus they can only produce a warm white glow instead.

For black bodies, we can also derive their spectral curves (amount of red/blue/green light produced) from the associated K rating, as the red/blue/green ratio follows predictable curves for black body lamps. Interestingly, by definition, blackbody lamps produce a CRI of 100 - so incandescent lamps produce a CRI of 100, even though their warm hues make for poor rendering of cooler colours. This would come as a surprise to people who think that high CRI lamps naturally mean attractive colour rendering (another common, high-level myth), but this is not always the case. This will be discussed in another section.

K rating for incandescent bulbs (above) actually hint at the temperatures that their filaments burn at. Not so for non-black body lighting such as Florescent/LEDs. A 6500k T8 florescent light bulb can be cool to the touch - it definitely does not burn at 6500 kevins.

K Rating is used to measure Color co-related temperature (CCT)

The K rating printed on modern commercial lamps (which applies to non-incandescent lamps such as LEDs, fluorescent lamps, etc.) differs from the Kelvin rating used on black-body lamps. In this application, the K rating indicates the average visual hue of the light (as perceived by human color vision) as approximate to a black body generator of the same Kelvin. This K rating is known as CCT (color correlated temperature).

For example, a fluorescent lamp may be labelled 5800K because it has the same visual hue as sunlight (i.e. it looks like sunlight). However, unlike a true black body radiator (i.e. the sun), a fluorescent lamp does not burn at 5800 Kelvin. Its K rating is based purely on its visual resemblance to a true black body radiator of the same Kelvin rating. Nor does the fluorescent lamp have the same spectral distribution (amounts of red/blue/green light) as the sun. In fact, most non-specialist lamps have spectral curves that are nothing like sunlight, despite the 6500k rating label - so it is pure rubbish to say that they produce light similar to sunlight. The only similarity is the visual hue.

In the graphs above we see the spectrum distribution chart comparing sunlight with standard 6500k LEDs. The difference is that the spectral distribution is huge, despite the 6500k rating label. Modern lamps have their K rating calculated using CCT* (colour correlated temperature), it was developed to give the lighting industry a way of describing the overall hue of a lamp. Lamps with a lower K rating have warmer tones, while lamps with a higher K rating tend to be bluer. Interior designers would use the K rating system to match lamps when designing interiors. The calculation of CCT is thus based on human color vision's sensitives to different color spectrum.

The CIE diagram below represents all of the chromaticities visible to the average person. These are shown in color and this region is called the gamut of human vision. The CCT (kelvin rating) of a light bulb, can simply be understood as the color tone that best matches a point on the chart. However, because the calculation is an approximation, in reality bulbs of the same CCT can still produce very different color renditions.

It is the spectrum distribution of light that affects plant growth and photomorphogenesis, not it's visual color hue. Hence, the idea that a light is great for plant growth just because it is of a certain K rating is a spurious claim. What spectrum of light is used by plants for growth is covered in this section. Many lighting salespeople make the mistake of thinking that just because their lamps have the same colour tone (CCT rating) as sunlight, their lamps produce the same spectrum as sunlight.

Due to the approximate nature of CCT, lights with the same K rating can have quite different color hues. A 6000k light can appear whitish (point B above), Greenish (point A) or reddish (point C).

Same K rating, different spectrum curves

Lamps with the same K rating label can have very different spectral distributions. Here we compare a 6500K BML RGB LED unit on the left with a pure white 6500K LED on the right. The BML unit on the left has much larger red peaks due to the mix of red LEDs used.

Here is a comparison of two 6500k T5 tubes; Giesemann on the left and Wavepoint tropical wave 6500k at center. A spectra distribution chart for sunlight is on the right.

IS 6500K THE BEST FOR PLANTS?

6500K indicates the approximate colour hue of a lamp, but has no direct bearing on whether it is a good lamp for plant growth or not. As mentioned above, it is the spectral distribution of the light that affects plant growth and photomorphogenesis, and you cannot tell the exact spectral distribution of a light by its K rating alone. Any website out there claiming that 6500K is better for plants is just a marketing gimmick.

For aquarists, the K rating is useful if you are buying domestic lighting and converting it for aquarium use. For most commercial household lamps, the convention is that buying a lamp in the 5000K - 8000K range will produce a neutral white colour tone, anything below 3000K will be warm white and anything above 10000K will be bluish. Again, this only gives the visual hue of the light and doesn't give any indication of the actual strength of the light (you'll need to read it's PAR values for this) or spectral curves (you'll need to read it's spectral charts as above).

However, if you are buying lamps from a specialist retailer or aquarium specific luminaires, the K rating label may not even be a good indication of the general colour tone of the lamp. For example, a T5 tube may be labelled 6500K but produce a pinkish light, and a 10000K light may not be bluish-white but actually blue.

BML's LED fixture is rated at 3600K. It is a red-heavy fixture with a mix of blues (from blue diodes and from white LEDs, which themselves contribute quite a lot of blue spectrum). Despite the low K rating and low CRI (78), the lamp renders colours well.

6500K is a neutral white bulb in most cases, which produces neutral colour rendering in a tank. We use a wide range of bulbs; much to the chagrin of sites that try to claim that low K bulbs don't work... take a look at my tanks below, grown with bulbs of very different K ratings.

Grown with 4200K:

Grown with 12000K:

Grown with 12000K:

Click here to learn more about reading PAR Values.

Click here to learn about spectrum curves.

Click here to learn more about aquarium lighting for planted tanks