Photosynthesis is most pronounced in the red (600-680nm) and blue (380-480nm) wavelengths of light. Horticultural lighting, also know as High Intensity Discharge (HID) lighting is designed to cover these specific wavelengths, known as the PAR spectrum (photosynthetically active radiation). There are two types of HID lamps which emit different color spectrums. Metal Halide lamps emit a white/blue spectrum. MH lamps are best used as a primary light source (if no or little natural sunlight is available). This type of lamp promotes compact vegetative growth. There are also MH to HPS conversion bulbs available which allow you to provide MH light during vegetative growth and then switch over to the HPS for fruiting/flowering stages of growth. High pressure sodium lamps emit a yellow/orange spectrum. They are the best lamps available for secondary or supplementary lighting (used in conjunction with natural sunlight). This type of light promotes flowering/budding in plants. HPS lamps are ideal for greenhouses and commercial growing applications. The Son Agro and Hortilux HPS lamps add an additional 30% blue factor to their spectrum, making them a better choice than straight HPS lamps for solo use. There are also HPS to MH conversion bulbs available which can provide MH light during vegetative growth then let you switch back to HPS for the fruiting/flowering stages of growth.
White light Is actually a combination of all colors of light. Red + Green + Blue (and all colors in between)
Blue light stimulates hormones that trigger growth and inhibit dormancy. Blue light powers photosynthesis causing tips to grow towards the source (phototropism). Metal Halide lamps emit strong levels of blue light making them good for promoting the growth of leafy plants. Blue light also serves to keep plant growth compact and shapely by minimizing the distance between internodes (branches).
Green light is reflected, that is why plants appear green, however some green light is required for growth. HID lamps do not emit much green light, neither do high pressure sodium lamps. Red light also powers photosynthesis, aids in seed germination , helps to form pigments and aid flowering.
Red light is also responsible for triggering dormancy in some plants. High Pressure Sodium bulbs emit red light and are generally better for flowering and fruiting plants.
Far-Red light speeds up some full sun plants, reverses some red light effects. HID lighting usually doesn't emit far-red except in the case of some High and low pressure sodium bulbs, more so in the form of heat rather than photosynthetic light.
Anatomy of a grow light
The lighting system shown above consists of a reflector which is usually suspended above the garden on a chain and pulley system for easy height adjustment and a ballast which is connected to the reflector with a cord through which it supplies the power. The purpose of the ballast is to step up and maintain line voltage.
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