Measuring Greenhouse Lighting: PPFD, PAR, and DLI

PAR, PPFD, and DLI are common lighting terms that greenhouse growers will encounter. Since light is not tangible, it can be hard to understand how it is measured. Measurements of light describe its different qualities like color, intensity, and the amount of energy it can deliver to the plant.

Photosynthetic Active Radiation

PAR stands for photosynthetic active radiation. PAR are the wavelengths of light that can be used in photosynthesis. PAR sensors can be used at different levels of the plant canopy if the grower wants to measure how much useful light is penetrating through the leaves. The measure used for PAR is watts per square meter (W/m2).

Photosynthetic Photon Flux Density

PAR tells growers the wavelength or color of photosynthetic light, but to describe the photon energy or intensity of the light, another measure is needed.  PPFD describes the number of energy particles (photons) in the photosynthetic active radiation (PAR) range that falls on a one square meter area in one second.
  • Photosynthetic photon flux density (PPFD) is written as μmol m-2s-1 (micromoles per square meter per second)
  • Too low PPFD lighting may not be enough light to keep crops productive 
  • Too high PPFD lighting can be excessive and damage crops

Daily Light Integrals

Daily light integrals (DLI) are another important measure of photosynthetic light that describes the number of photosynthetically active photons that accumulate in a square meter area in a 24 hour period. Where PPFD expresses light intensity in a second, DLI expresses the sum of those measurements in a 24 hour period (86,400 seconds per 24 hours).
  • DLI is written as mol m−2d−1 (moles of light (mol photons) per square meter (m−2) per day (d−1))
  • DLI is the sum of PPFD in a day
  • Heavily influences short-day plants (the plant, chrysanthemum, poinsettias)
  • Most greenhouse crops need about 30 to 35 mol m-2 d-1 for peak productivity
Natural DLI changes based on latitude:
  • In the northern latitudes of North America days will be shorter and nights will be longer, so the DLI will be lower than in southern latitudes where days are longer.
  • Day lengths can naturally drop under 12 hours earlier in the year in the north than in the south.
  • Greenhouse growers in the north may need daylight extension lighting.
  • DLI maps can help you estimate your lighting needs.

Other Common Greenhouse Lighting Terms

Watts is a unit to measure the electric currency a bulb draws, a watt is equivalent to one joule per second. 
Amperes measure the strength of the electric current. Some electronics draw large currents that require outlets that can put out more amperes. It is very important to be sure that lighting equipment is compatible with the outlet capacity. High-intensity discharge lamps and fluorescent bulbs must also be compatible with the ballast where they are screwed in.
Radiant flux is the amount of radiation a light source emits, including infrared (IR), ultraviolet (UV), and visible light. The amount of radiant flux that is visible to the human eye is expressed in lumens. One lumen per square foot is equal to one foot-candle, which is a term that describes the density of light that reaches a surface. Lumens are not a good measure of lighting for plant growth, but many bulbs will talk about their efficiency as lumens per watt of energy consumed. 

Lumens can also help to describe the life expectancy of a bulb, they are expressed as lumens per watt (LPW). As light bulbs are used, the initial lumens will decrease. The life expectancy of a bulb is reached when lumens have dropped by 50%. High-intensity discharge (HID) lamps have a life expectancy of 10,000 to 20,000 hours, and LEDs can last up to 50,000 hours.

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