LED Grow Lights for Greenhouse Toplighting

Greenhouse supplemental lighting

A greenhouse is a structure enclosed entirely by transparent materials such as glass, rigid plastic, or a thin plastic (polyethylene film) that protect plants from the effects of wind, cold, dehydration, and insects while still admitting the light they need to grow. Greenhouses allow for regulation of microclimatic parameters such as temperature and humidity, giving plants the most optimal environment in which to grow. However, the single most important variable with respect to plant growth and development—light—often becomes a limiting factor.

The influence of light on the production of various plants, such as vegetables, fruits, flowers and herbs has been well documented. Plant cultivation in greenhouse heavily relies on sunlight, of which the availability depends strictly on weather conditions. Natural sunlight is not always attainable in sufficient quantities. Supplemental lighting is often used in permanent rigid plastic and glass greenhouse structures to extend daylight hours during winter seasons with short periods of solar radiation or to fill in the natural fluctuations in sunlight due to weather.

Horticulture lighting is meant to provide a source of energy and information

The use of artificial lighting in horticultural applications generally serve three purposes: transplant production and propagation of seedling that involve both photosynthetic and photomorphogenic lighting; supplemental lighting to enhance photosynthesis and thereby improving growth and quality of plants, especially during light-limited periods of the year; and photoperiodic lighting to control developmental phases, e.g., inducing early or out-of-season flowering.

Photosynthesis, photomorphogenesis, and photoperiodism are the three basic processes related to interaction between optical radiation and plants. Photosynthesis is the process by which plants harvest energy from photons to synthesize sugars, for example glucose, from carbon dioxide and water. Photomorphogenesis is about the development of plant morphology (plant architecture) under the influence of light spectrum. Photoperiodism relates to the reaction of the plants to the light-dark cycle.

At its most basic, supplemental greenhouse lighting should address the minimum requirements on daily light integral (DLI) from plants. DLI quantifies the total number of photosynthetically active photons that a plant receives over the 24 hours of a solar day. It’s especially important to measure DLI in every greenhouse where plants are receiving varying amounts of sunlight. In addition to the seasonal fluxes in solar radiation and unpredictability of weather, light transmission inside a greenhouse is usually reduced by 35-50%.

The amount of photosynthetically active radiation (PAR) received each day can have a profound effect on plant growth, development, yield, and quality. There is a direct correlation between DLI and shoot mass, root development, transplant propagation, as well as finish plant quality attributes such as stem thickness, plant branching and number of leaves and flowers.

The deficiencies of HID lighting

High intensity discharge (HID) lamps, such as high pressure sodium (HPS) and metal halide (MH), have been the usual choice of light source for greenhouse lighting systems because of their ability to deliver adequate photosynthetically active radiation (PAR) with economic viability. An HPS lamp emits in the red-orange region of the visible light spectrum, which can power the flowering and fruiting processes. The blue-white emission of metal halide lamps may be better suited for vegetative growth and seedlings. However, the spectral power distribution (SPD) of both MH and HPS lamps do not exactly match chlorophyll absorption spectrum. About 20% to 30% of the light is emitted outside the PAR region. Radiant energy that is not absorbed by the plant photoreceptors is simply wasted.

HID lighting requires considerable separation between light fixtures and plants to ensure uniform light distribution as well as to avoid thermal stress from infrared radiation. Achieving a very high photosynthetic photon flux density (PPFD) with HID lighting has always been limited by the distance that the lamps need to be kept from the plant canopy. Furthermore, limited dimming ability and fixed light spectrum of HID lamps deprive conventional greenhouse toplights of the flexibility to develop a customized light recipe that works for each stage of plant growth.

LED technology arrives at the forefront of horticulture lighting conversations

The advent of solid state lighting based on LED technology has sparked a revolution in horticulture lighting. Capable of delivering electromagnetic radiation that plants need in the exact spectrums across all stages of crop production from, seedling, cloning, all through vegetative to flowering, fruiting and harvest, the benefits of LED lighting in the cultivation of herbs, vegetables, fruit, flowers and other plants in