HID (High Intensity Discharge) lights are the most popular grow lights for now, mostly used by professionals. They are highly intense and provide a very high quantity of visible light per watt of electricity consumed meaning they are highly efficient. They are classified into different types based on the material used to make them and the type of light they produce. Their types include Metal halide(MH), High pressure sodium(HPS), Conversion bulbs and Mercury vapor, of which, High pressure sodium(HPS) and Metal halide(MH) are most commonly used as they produce a color spectrum that is similar to that of the sun. Excessive heat production is a drawback of these lights. They are screwed into a hood looking fixture so that they can reflect light onto the plant. HID lights require a ballast to operate. Each ballast has a particular wattage. Popular ballasts are of 150W, 250W, 400W, 600W and 1000W with 600W being most electrically efficient.
High-pressure sodium bulbs deliver more energy in the Yellow/Red part of the light spectrum which is why they help promote blooming and fruiting(at later stages of plant life). They can be used as a standalone source of light indoors or in supplement to natural daylight in greenhouse lighting and to full-spectrum lighting(metal halide). If used alone, some drawbacks may include elongation of plants due to lack of blue/ultraviolet radiation, and difficulty in monitoring of plants due to yellow light. These problems can however be resolved by supplementing them with source of blue light like Metal Halide(MH) or by better adjusted spectrum of HPS light and by use of supplementary lights to offset the yellow light respectively. High-pressure sodium lights are a more efficient type of HID lighting than metal halides. They also last twice as long as Metal halide bulbs which need to be replaced about once a year. HPS lights are the industry standard for most indoor growers.
Metal Halide lights
Metal halide bulbs are a type of HID light that emit light which is similar to the light that is available outdoors during spring because they focus on the blue and violet parts of the light spectrum. They are well-suited to supporting plants in earlier developmental stages by promoting stronger roots, better resistance against disease and more compact growth. The blue spectrum of light encourages compact, leafy growth and is better suited to growing vegetative plants with lots of foliage. They are commonly used when the plants are on display in the home (for example with ornamental plants) and natural color is preferred because plants look more pleasing under a metal halide than other types of HID lights such as the High-pressure sodium bulbs which distort the color of plants. There are color corrected MH bulbs available which give off more red/orange light than regular metal halides. This helps boost flowering in addition to supporting compact foliar growth.Their life span is around one year.
LED Grow Lights
LED(Light Emitting Diodes) grow lights are often used for supplemental lighting in home and office spaces but can also be used alone. They provide plants a balanced spectrum of red, blue and green. Green, Red, far-Red and Blue light spectra have an effect on root formation, plant growth, and flowering. Output waveband of single color LEDs is much narrower than that of traditional sources of lighting used for plant growth. LED grow lights are available in different colors depending on the intended use but there are not enough scientific studies or field-tested trials using LED grow lights to recommended specific color ratios for optimal plant growth under colored LED grow lights. They produce less heat than many alternative lighting systems and have a built in cooling system. They have a long lifespan extending up to 5 years. They are popular especially with hobbyist growers that are looking to minimize their long term costs without sacrificing a lot of value. In the past, they were expensive and users were disappointed with their performance but with improving technology, cost reduction, better adjusted spectrum and option to select specific wavelengths for a targeted plant response, they are becoming a more viable option for the growers. LEDs do not require a ballast to operate. They may cost more at the time of buying but they prove cost efficient in the long run.
Fluorescent Grow Lights
Fluorescents are also widely used as grow lights.They are not as intense as HID lights and are usually used for growing vegetables,herbs,vegetative-only plants like micro-greens or salad greens or for starting seedlings to get a jump start on spring plantings. Fluorescent lights come in many form factors, including long, thin bulbs as well as smaller spiral shaped bulbs (compact fluorescent lights). The two main types of fluorescent lights used for growing plants are the tube-style lights and compact fluorescent lights.
Tube Style Fluorescent Lights
Tube style fluorescent lightings are low intensity lights. Due to their less intense light, they are not very well suited for flowering and fruiting stages of plants. They are good for leafy greens, seedlings and cloning as they do not have to go through flowering stage and need less intense light. They are available in many types including the T5, T8 and T12. T5 is the brightest and most popular version among them. The T8 and T12 are less intense and are used with plants with lower light needs. T5 is available in different sizes. High-output fluorescent lights produce twice as much light as standard fluorescent lights. They provide high intensity light and are an improvement in the fluorescent light. A high-output fluorescent fixture is thin and occupies less space, making it useful in areas where space is vertically limited. T5 high output fluorescents can produce light up to 5000 lumens per tube. They produce very little heat which is why they can be placed close to the plant. Fluorescents have an average usable life span of up to 2 years. A ballast is needed to run these types of fluorescent lights.