The quality of a grow light can be understood by looking at the spectrum of that light source. Natural sunlight is intense and has a broad, balanced spectrum. Indoor plants require the same high intensity and broad yet balanced spectrum. 

So what is a spectrum? A light spectrum is the many different wavelengths of energy particles, called photons, produced by a light source. Those photons are measured at specific nanometers (nm) representing a wavelength of light or band of light energy. Visible light is the part of the spectrum from 380nm to 780nm. Plants utilize spectral energy from 300nm to 800nm.

The spectral distribution chart is the best illustration of the quality of the light spectrum. In the chart below you will see how the spectrum of a grow light differs from that which the human eye can see. This is why it is important to pay attention to the spectral output of a grow light source and not rely on your eyes or other metrics that describe light color or how a human perceives light (like kelvin temperature or lumens, respectively) when evaluating the quality of the grow light. Only a spectrum can provide the quality of your light source for growing plants.