Lumens, Kelvin Temperature, CRI

All these terms are standard lighting measurements that tell us how the light looks to the HUMAN EYE. These terms do not represent the quality of a light spectrum nor do they take into account the McCree Curve.


Lumens are for humans. This measurement is strictly for human eye sight and has nothing to do with what plants see. But if you really want to know what it is, then here is the definition: “A lumen is a unit of luminous flux, equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela.”

Kelvin Temperature

Kelvin temperature is the unit of measure used to describe what color a lamp appears to be when it is lighted. Kelvin temperature is a lighting term used to relate the color appearance of a lit lamp to the color appearance of a glowing hot piece of metal. An example is hot steel which glows bright yellow when heated to 1200 degrees Kelvin. A lamp may be called 1200K if it looks like that same shade of yellow when lit.

Kelvin temperature has no relation to light spectrum, light quality or plant growth. It is used only to describe color appearance. The examples below show the spectral distribution charts of two lamps with the same Kelvin temperature but very different spectral outputs. So never purchase a grow lamp based upon its Kelvin temperature, especially if there is no spectral distribution chart to display the quality of the light emitted by the lamp. If you choose a lamp based on its Kelvin temperature alone, you may be depriving your plants of the wavelengths they need to reach their maximum potential. Always choose a grow lamp based on its spectrum, not its color temperature.


The Color Rendering Index (CRI) is a rating system that measures the accuracy of how a light source makes specific colors look.

CRI tells us how well a light source displays specific colors. 

If a light source displays colors perfectly, the corresponding CRI would equal 100. If the light source did not display any color correctly its CRI would be 0 (zero).

The CRI index was created to help companies sell products. If a light source does not accurately display colors, the carpet or paint that you purchased in the store may look completely different at home. A red shirt may look orange or pink. A white car may look grey or cream.

Do your plants care about CRI? Only if they are buying carpet, paint, shirts, or cars. You get the idea.

Though some horticulture lighting brands publish the CRI values, those values have little to do with spectrum or the light’s ability to grow plants.

Ceramic Metal Halide

The latest advancements in Ceramic Metal Halide technology provide lamps with high color temperature and a variety of high CRI values available for various applications, vastly improving visual acuity.

High Pressure Sodium (HPS)

The light from sodium sources is predominantly emitted in the longer wavelengths, between yellow and red. This light source typically puts out a low CRI value and makes items appear warmer.

For more terms used in the general lighting industry, but not in the indoor growing industry, visit here.