PPFD, PPF, PAR and Lumens
Today we are gonna look at PPFD and how we measure light in a garden. In the past, we used lumens, but unfortunately that wasn’t telling us the whole story. As you will read below lumens only measure visible light, now this information is valuable when designing lighting for a warehouse or chemical plant, or even a classroom, but plants use a larger spectrum than we can see. Photosyntheticlly Active Radiation is how we typically measure intensity from a grow light today but is in usually represented as PPFD, this information tells us the intensity at a given distance over time. Most growers today use these numbers to dial in the intensity of a high yielding crop and maximize the canopy space. A wise man once said:
“Lighting is only as powerful as your A/C.”
What this gentleman was implying was that with typical HID lighting a substantial amount of heat is generated by 1000W lights, ~4,000BTU’s, meaning gardeners are limited to the number of lights in a garden and how much air conditioning they can provide. Large commercial gardens with new DE lighting have switched to using High Temperature Refrigeration units to cool these large grow rooms.
First, lets look at what wikipedia has to say about all these word we are using to measure just how bright a grow light is.
“The lumen …is… a measure of the total quantity of visible light emitted by a source per unit of time.”
PAR (Photosynthetically Active Radiation)
“Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place…
PAR measurement is used in agriculture, forestry and oceanography. One of the requirements for productive farmland is adequate PAR, so PAR is used to evaluate agricultural investment potential. PAR sensors stationed at various levels of the forest canopy measure the pattern of PAR availability and utilization. Photosynthetic rate and related parameters can be measured non-destructively using a photosynthesis system, and these instruments measure PAR and sometimes control PAR at set intensities. PAR measurements are also used to calculate the euphotic depth in the ocean.
In these contexts, the reason PAR is preferred over other lighting metrics such as luminous flux and illuminance is that these measures are based on human perception of brightness, which is strongly green biased and does not accurately describe the quantity of light usable for photosynthesis.”
DLI (Daily Light Integral)
“The daily light integral (DLI) is the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day. It is a function of photosynthetic light intensity and duration (day length) and is usually expressed as moles of light (mol photons) per square meter (m−2) per day (d−1), or: mol·m−2·d−1.
DLI is usually calculated by measuring the photosynthetic photon flux density (PPFD) in μmol·m−2·s−1 (number of photons in the PAR range received in a square meter per second) as it changes throughout the day, and then using that to calculate total estimated number of photons in the PAR range received over a 24-hour period for a specific area. In other words, DLI describes the sum of the per second PPFD measurements during a 24-hour period.”
Next, lets look at what our leading manufacturer a Light Emitting Diode technology has to say about how these terms apply to our garden.
Horticultural Lighting Metrics
“If you have been researching LED horticulture lighting systems for your plant growth facility, you have likely been bombarded with a variety of metrics that lighting manufacturers use to market their products. Some terms and acronyms you are likely to see include: watts, lumens, LUX, foot candles, PAR, PPF, PPFD, and photon efficiency. While all of these terms do relate to lighting, only a select few really tell you the important metrics of a horticulture lighting system. The purpose of this article is to define these terms and acronyms, correct some common misunderstandings, and help growers understand which metrics are applicable to horticulture lighting systems, and which ones are not.