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
Photosynthetic Photon Flux Density
- 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
- 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
- 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
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.