Before we get too far into this, there are a few terms that need to be broken down first:

• Photon: A quanta, or particle, of light
• Micromole: a way to measure the amount of a substance, or in this case the number of photons passing through a target area, one micromole of light equals just over 62 quadrillion photons
• Photosynthetically Active Radiation (PAR):  all radiation with a wavelength of 400nm-700nm that is used for photosynthesis
• Photosynthetic Photon Flux Density (PPFD): a measurement of PAR; the number of photons falling on a given area in a given amount of time, typically measured in micromoles per square meter per second

Because plants have evolved over thousands of years using sunlight, their preferred spectrum for growing mimics the spectrum of sunlight, also known as full-spectrum light. Full-spectrum light includes wavelengths between 380 and 700 nanometers (nm). Nanometers measure wavelengths of light in billionths of a meter, the smaller the number, the shorter the wavelength. As stated above, plants have evolved to grow best with light within this spectrum, specifically light between 400 and 700nm; however, Cannabis also uses some Ultra-Violet (UV) components below 400nm for increased essential oil production and some Near Infra-Red (NIR) wavelengths above 700nm to help the plant compete with neighboring plants that may be shading it from the light source.

If you are growing indoors, it is important to mimic the plant’s preferred spectrum as closely as possible. But what’s the best way to ensure the proper spectrum for plant growth is being utilized? Most bulbs will tell you right on the box what spectrum the bulb is designed to emit.

However, it is not as easy as that. Another measurement is typically included that can be misleading: lumens. Lumens measure relative brightness as observed by humans.. While it may seem obvious that the brighter the light, the more light that is being put off, lumens are based on human perception and plants view light differently than humans.

Humans cannot see as much of the full-spectrum of light as easily as plants. In fact, the human eye is sensitive to light in the middle of the spectrum, 500-600nm, and is most equipped to use light at 550nm. As mentioned above, plants have a larger range of ideal spectrum, but they react most strongly to light at 400-460nm (blue spectrum) and 580-700nm (red spectrum). As you can see, the light plants prefer, for the most part, is not within the same range as the light humans prefer. This is why using a subjective measure of light, such as lumens, an ideal way to measure the amount of light reaching the plant canopy.

Another Way

Light that is useful for photosynthesis is not necessarily bright, it should be dense. Light given to plants should be measured in micromoles and PPFD (see definitions above) which describe the density of light on the target are, rather than the brightness. This provides a more accurate measurement of the amount of light available for photosynthesis.

Micromoles and PPFD are what actually cause plants to react to the light reaching them. As PPFD increases, the light that is reaching the plant is more dense, and thus actually provides more light for the plant to use for photosynthesis, creating larger plants. As a quantifiable measurement, PPFD is a better way to determine how much light plants are receiving than lumens.

Next time you shop for a light or reflector, be sure to ask about the PPFD it puts out. And remember, a bright light for humans is not necessarily a bright light for plants.