HID and Fluorescent Lighting information

General Indoor Lighting Basics


Horticultural Lighting Information

Horticultural lighting allows you to grow plants without any sunlight. This is a fantastic way for someone who might like fresh cut flowers, year round fresh, natural, and organic fruits, vegetables, and herbs to grow them themselves without a greenhouse. Indoor grow lights can also be used to get an early start growing young plants which you intend to place outside immediately after the final frost. This will increase yield and decrease time to harvest. Lastly, they can be used as a secondary light source in a greenhouse or sunroom to manipulate photoperiod and increase productivity.

There are several types of lighting to choose from when we are discussing horticultural lighting options. There are fluorescent, H.I.D. (which includes Metal Halid - the preferred choice for vegetative growth, and High Pressure Sodium - the preferred choice for flowering and fruiting,) LEDs, and plasmas. A brief explanation of each type will follow.


Glossary of Terms

Ballast - The group of electrical components that energize a fluorescent or HID bulb. There are two general types:

Magnetic – Generally consisting of 3 parts, a transformer, igniter, and a capacitor. This type is generally much less expensive and easier to fix but is heavier, produces less lumens per watt, tends to run hotter, and may produce more noise.
Digital/Electronic – Uses circuit boards and electronic components to strike and ignite the bulb. They tend to cost a bit more but produce less heat, more lumens per watt, and run almost silently.
Color Temperature - Also known as Kelvin temperature or correlated color temperature. A description of the color of light emitted by a bulb. Color temperatures over 5,000K (K is for Kelvin) are called cool colors (appearing blue/white), while lower color temperatures (2,700–3,000 K) are called warm colors (appearing yellow to orange through red).

Footcandle - A unit of illumination equal to the intensity of one candle at a distance of one foot. Footcandles are usually used as a measure of light received.

Lumen - A unit of illumination equal to the light emitted by one candle that falls on one square foot of surface located one foot away from the candle. Lumens are usually a measure of light output.

Photosynthetically Active Radiation (PAR) – The spectral range between 400nm-700nm (nm = nanometers) that plants are able to utilize for photosynthesis. PAR is expressed as either Micromoles per meter square, per second or as Microeinsteins. PAR is the most scientific way of evaluating a light's ability to drive photosynthesis. Many companies use lumens for their measurements, but they it is not accurate for plants because lumens correspond to the human eye's sensitivity to light not a plants ability to photosynthesize. There are 2 PAR numbers to consider:

500 Micromoles - The average optimum level of PAR for most garden plants
2,000 Micromoles – Micromole output of the sun on a clear summer day
Photoperiod – The relative hours of light compared to darkness in a 24-hour period to which a plant is exposed. Some plants respond to a change in day length (photoperiodism) in order to grow or flower.

Photosynthesis - A process used by plants and other organisms to convert the light energy captured from the sun into chemical energy that can be used to fuel the organism's activities. The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2.


General Info

Fluorescent Lamps - These lights are perfect for young plants, cuttings, clones and seedlings. They are generally not the best choice for fruiting or flowering because of their lower lumen output (lumens of light created per watt of electricity used.) Although they are not as efficient as HIDs they may be used as a primary light source in some systems for lower light plants, i.e. lettuce or artichoke etc. or for illuminating mother plants. Plant growth will not be as rapid as under HID lighting due to the lower light levels. Fluorescent bulbs are sized based on diameter in relation to 8ths of an inch so a T12 is 1.5 inches in diameter (12/8=1.5) a T5 is 5/8ths of an inch in diameter. As a general rule the smaller the diameter of the bulb the higher the lumen per watt output. The standard choice in the horticultural industry is currently the T5 fluorescent light.


Metal Halide lamps and High Pressure Sodium lamps all belong to the H.I.D. (High Intensity Discharge) family of lights. Metal Halide and High Pressure Sodium lamps are used for garden lighting because of their high light output per watt, and the effective spectral distribution of their light.

Metal Halide - MH (Metal Halide) bulbs are very efficient and produce between 70 and 115 lumens of light output per watt of electricity used. MH (Metal Halide) bulbs produce a light that is very close to full summer sun, with a spectrum rich in the blue light. This promotes fast vegetative growth and compact, stocky, well branched plants with short internodal leaf spacing. MH bulbs are considered the ideal choice for the vegetative stage of plant growth.

High-Pressure Sodium - These lamps produce light that is primarily red to yellow in spectrum. The spectrum is ideal for fruiting and flowering plants as it promotes some stem elongation, rapid fruit formation and large harvests. They are an ideal choice for supplemental lighting in a greenhouse. They are more efficient than Metal Halides at creating light and as such have a higher lumen output per watt. They can produce up to 140 lumens per watt. HPS bulbs are considered the ideal choice for the fruiting and flowering stage of plant growth.

There are color corrected HPS (High Pressure Sodium) bulbs (such as the Son Agro) available for improved growing which will reduce inter-nodular elongation of plants. These “color corrected” or “blue enhanced” HPS lamps are designed specifically for indoor horticulture, and have a more balanced color spectrum.

Conversion Bulbs - There are two types:

1) Sodium bulbs which run on Metal Halide ballasts - available in 250, 400, & 1000 watt ballasts.

2) Halide bulbs which run on Sodium ballasts - available for 250, 400, 600, and 1000 watt systems.

LED Grow Lights (Light Emitting Diodes) – were introduced to horticulture over the past several years. A LED is a semiconductor light source which utilizes small amounts of electricity to produce light in very specific wavelengths; in the case of horticulture, those wavelengths targeted are those that drive photosynthesis. Early attempts with LEDs used 5mm diodes (it would have taken 50 of these diodes to use 1 watt of electricity,) which did not have the intensity to compete with HIDs. More recent and effective models have evolved and now ranging from 1 watt to 5 watt individual diodes. LED’s ability to produce a specific wavelength of light combined with their electrical efficiency are making them one of the hot prospects to eventually replace HID lights.

Plasma Grow Lights – Plasma lamps are a type of electrodeless lamp energized by radio frequency or microwaves. The interest in these lights by horticulturists is driven by two factors:

The spectral output of the light created is almost identical to the light spectrum of the sun!
They are capable of producing large amounts of light from small amounts of electricity.
If this type of lighting is perfected it will revolutionize the world of indoor horticulture! There are different lamps or “bulbs” that contain different mixtures of gasses which influence the spectrum of light emitted. Currently there are models out for sale but they are still in their respective infancy as a concept.