Light Measurements in Horticulture

If you are involved with horticultural lighting, you have likely heard of the measurement micromole, often written as umol or μmol. Micromoles are widely used to measure the number of photons or ‘light particles’ within the visible light emitted from a grow light. A photon is the smallest discrete amount of electromagnetic radiation that transmits light. It is the basic unit of all light.

Micromole is a sub-measurement of a mole. So why don’t we measure light in horticulture in moles? Well, the micromole is equivalent to one-millionth of the unit mole, approximately 602 quadrillion photons or 6.022140857 x 1023 – That is 602 followed by 15 zeroes. As you can imagine, it would be difficult to work coherently with such large numbers. So, the horticultural industry began using the much more manageable micromole.

Why not use Lumens?

LED lighting is often measured using Lumens (lm), which provide an indication of brightness. To learn more about lumens as a measure of light, see our other article here. This is a measurement used widely across the general lighting industry.

Although lumens do give an accurate representation of brightness, this is based on human sight – not plants. The higher a lumen value the greater the light output and brightness. But plants view light differently to humans. Therefore, as a measurement in horticulture, lumens can be misleading. We humans do not perceive as wide a range of light as easily as plants do. Human sight is receptive to light within the middle of the spectrum, in what we refer to as the visible spectrum (400nm-700nm). While most plants have a wider range and react most strongly to light wavelengths in blue (365nm-470nm) and red (625nm-740nm). In horticultural lighting, the radiation between 400nm and 700nm, is known as photosynthetically active radiation or PAR. This is the region considered most effective for plant photosynthesis.

To successfully grow plants without natural sunlight, it is vital to replicate their specific preferred light spectrum. Light is subjective depending on who is perceiving it. The spectrum of light preferred by plants, is different from humans. Hence why lumens are a great unit of measure for brightness in general lighting but is too subjective to accurately portray the light received by plants.

Micromoles are a great alternative to give an accurate indication of light reaching your plants. Another is photosynthetic photon flux density (PPFD). This refers to the radiation and number of photons between the range of 400nm and 700nm, that land per second on a target area (measured per square metre). Both PPFD and micromoles more accurately convey the density of light supplied to target plants, rather than general brightness as perceived by human sight. These provide more reliable key measurements on the quantity of light available to your plants for photosynthesis.

At IHS (Intelligent Horticultural Solutions) we have the equipment on hand to take PAR and PPFD readings of existing lighting, which you might be looking to upgrade to LED. After taking readings, our specialists can produce a light plan to determine how many grow lights are needed to hit the micromoles you require. The team at IHS can conduct site surveys, please do not hesitate to get in touch.

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