Grow light

Indoor flower and vegetable growers typically use high-pressure sodium (HPS/SON) and metal halide (MH) HID lights, but fluorescents and LEDs are replacing metal halides due to their efficiency Archived 2019-02-23 at the Wayback Machine and economy.

[2] Metal halide lights are regularly used for the vegetative phase of plant growth, as they emit larger amounts of blue and ultraviolet radiation.

LED grow lights are usually composed of multiple individual light-emitting diodes in a casing with a heat sink and built-in fans.

Promotion of profuse flowering in select ornamentals including primula, marigold, and stock were also noticed.

However it's noted that in tests between tomatoes, mini cucumbers and bell peppers, the optimal light recipe was not the same for all plants, and varied depending on both the crop and the region, so currently they must optimize LED lighting in greenhouses based on trial and error.

They've shown that LED light affects disease resistance, taste and nutritional levels, but as of 2014 they haven't found a practical way to use that information.

To prevent leaf burn, LED grow lights should be kept between 12 inches (30 cm) away from plants for lower wattage lamps (under 300 watts) up to 36 inches (91 cm) away from plants for higher wattage lamps (1000 watts or more).

Historically, LED lighting was very expensive, but costs have greatly reduced over time, and their longevity has made them more popular.

[22] There are several different types of HID lights including mercury vapor, metal halide, high pressure sodium and conversion bulbs.

Metal halide and HPS lamps produce a color spectrum that is somewhat comparable to the sun and can be used to grow plants.

In addition to producing lower levels of light, mismatched bulbs and ballasts will stop working early, or may even burn out immediately.

Metal halide bulbs need to be replaced about once a year, compared to HPS lights which last twice as long.

Metal halide lamps are widely used in the horticultural industry and are well-suited to supporting plants in earlier developmental stages by promoting stronger roots, better resistance against disease and more compact growth.

[23] The blue spectrum of light encourages compact, leafy growth and may be better suited to growing vegetative plants with much foliage.

[24] They are now being made for digital ballasts in a pulse start version, which have higher electrical efficiency (up to 110 lumens per watt) and faster warmup.

[25] Horticultural ceramic metal halide offerings from companies such as Philips have proven to be effective sources of growth light for medium-wattage applications.

This allows for a single bulb solution throughout the entire life cycle of the plant, from vegetative growth through flowering.

As a result, the yellowish light of an HPS can make monitoring plant health indoors more difficult.

But, in the higher latitudes, there are periods of the year where sunlight is scarce, and additional sources of light are indicated for proper growth.

HPS lights may cause distinctive infrared and optical signatures, which can attract insects or other species of pests; these may in turn threaten the plants being grown.

High-pressure sodium lights emit a lot of heat, which can cause leggier growth, although this can be controlled by using special air-cooled bulb reflectors or enclosures.

CFLs have largely replaced incandescent light bulbs in households because they last longer and are much more electrically efficient.

In general, if a plant does not get enough light, it will become stunted, have reduced pigmentation, or begin shade-avoidance response.

High pressure sodium (HPS) and metal halide (MH) were and are still common supplemental lighting options for greenhouses and some sole-source operations.

As LEDs become less expensive and more efficient, an interest in studying light quality has risen in the field of plant science.

Leafy greens such as lettuce, spinach, and kale are typically considered low-light crops, requiring a DLI between 12 and 17 mol m−2d−1.

Plants are able to sense parts of the electromagnetic spectrum through a network of photoreceptors including phytochromes, cryptochromes, phototropin, and zeiltupe.

Information about the light spectrum can affect seed germination, the signal to transition from vegetative to flowering, and the production of secondary metabolites such as anthocyanins.

[40]In addition, many plants also require both dark and light periods, an effect known as photoperiodism, to trigger flowering.

The irradiance of PAR can be expressed in units of energy flux (W/m2), which is relevant in energy-balance considerations for photosynthetic organisms.

A small ficus plant being grown under a black LED light fixture emitting warm white light.
Ficus plant grown under a white LED grow light.
Example of an HPS grow light set up in a grow tent. The setup includes a carbon filter to remove odors, and ducting to exhaust hot air using a powerful exhaust fan.
Two plants growing under a magenta (red + blue) LED grow light
400W Metal halide bulb compared to smaller incandescent bulb
An HPS (High Pressure Sodium) grow light bulb in an air-cooled reflector with hammer finish. The yellowish light is the signature color produced by an HPS.
A 600W High Pressure Sodium bulb
HPS grow lights suspended over the grass pitch at Tottenham Hotspur Stadium to encourage grass growth [ 29 ]
Fluorescent grow light
Dual spectrum compact fluorescent grow light. Actual length is about 40 cm (16 in)
Standard Compact Fluorescent Light
Quantum meter to measure grow light for plants (350nm-800nm)
Hortipower spectrum meter to measure grow light for plants
Absorbance spectra of free chlorophyll a ( blue ) and b ( red ) in a solvent. The action spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.
LED panel light source used in an experiment on potato plant growth by NASA
Weighting factor for photosynthesis. The photon-weighted curve is for converting PPFD to YPF; the energy-weighted curve is for weighting PAR expressed in watts or joules.