There are different types of  light bulbs to consider to use in the growing process. When considering which type of light bulb to use, the main features to look for are spectrum and output. The serious grower can get even more out of their crop by fine tuning the spectrum to the stage of plant growth, and/or spin the lights.

Light Bulb Spectrum

Full-spectrum is best. Of course the sun is full spectrum, and plants have evolved for many millions of years to adapt to sunlight.

Below, the chart on the left is what the human eye sees; on the right, what the plant prefers:

The Lumen rating of a bulb is not a good measurement for plant growth, because it is based on what the human eye can see. A full spectrum light is like music, with bass, treble, and everything in between.

High Output Bulbs

The best full-spectrum lights for digital ballasts are Pulse Start Metal Halide (PMSH). The best full-spectrum lights for magnetic ballasts are Ceramic Metal Halide (CMH). That is because in addition to being a true full spectrum, they are also high output.

Here is the Spectral Chart:

Chart for 4k CMH

High Pressure Sodium Bulbs

HPS is high output, but not full spectrum; it is mostly yellow-orange. Premium hps like Hortilux are super, that is, higher output than standard hps; but don’t last as long as standard output. HPS works well only at high levels of intensity, so it takes more watts to get the job done. HPS uses a ceramic arc tube which is glued together and leaks with heat stress

Distribution of Light onto Leaf Surfaces

While it is true that a 1000 watt light is the highest output, a single point of light would concentrate the light too much in one small spot, instead of evenly distributing over a larger area. Right below the heat zone around a bulb, is the zone of too-high intensity. This can best be measured with a light meter. Probably the optimum level of intensity is 4300 foot candles; 5500 foot candles would be the maximum and not all varieties want 5500 fc under all environmental conditions.

Light Spectrum for Different Stages

Different portions of the spectrum perform different functions. Blue light has the highest energy level, reduces stem elongation, increases rooting, and induces growth hormones. Red light has the greatest ability to bounce and penetrate the leaf canopy, increases growth of leafs and buds, and induces blooming hormones.

The plant cannot overdose on plant hormones; excessive amounts are simply stored. That is why you can use a full spectrum light for all stages. However, by enhancing relevant portions of the spectrum depending on the stage, you can increase efficiency of plant growth.

This post  barely skims the surface of all the information out there on different types of bulbs. There is even more information on how to use different bubls to their full potential. Remember when looking for a bulb that wattage, type of bulb (MH, HPS, CMH), different light spectrum, all make a difference in different ways. Get the most of your lighting system by choosing the right bulb!

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How does a plant light work?

An HID lamp operates with a properly matched transformer, capacitator and lamp bulb which consists of an inner tube filled with a chemical mixture. To simplify, it is a metal ballast with a cord that goes to the wall outlet and a cord that goes to the socket powering the bulb. A reflective hood sits around the plant light bulb and reflects light onto plants that would otherwise be lost. Reflectors are painted white inside or else will be plated with a highly reflective aluminum or brilliant chrome. Some reflectors are built to allow for air cooling which exhausts hot air out from the bulb and out of the room.

Are there any drawbacks to plant lights?

The two main drawbacks are 1) the  intense heat put out, which subsequently causes plants to dry out faster 2) they are incapable of providing the entire natural spectrum. Grow lights come as close to natural light as human technology can take us, but natural sunlight is still more full and vibrant.

You mentioned High Pressure Sodium lights and Metal Halide lights, what is the difference?

There are two types of plant lamps to be aware of, Metal Halide lights and High Pressure Sodium lights. Although high pressure sodium indoor grow lights and metal halide plant grow lights are both types of  indoor grow lights, they emit different color spectrums. High pressure sodium light bulbs emit very bright light that is concentrated in the red to yellow side of the spectrum and weak in the blue-violet end. Metal halide grow light bulbs, by contrast, emit a very balanced light, which contains all the energy peaks of the visible spectrum. Both types of lighting have their place in an indoor grow room environment.

Another difference is the lumens per watt of both bulbs.  High pressure sodium lights produce between 97 and 150 lumens per watt and Metal Halide lights produce 65-115 lumens per watt. An easy way to tell difference between the two grow lights is by appearance. High Pressure Sodium bulbs appear very yellow-orange; metal halide bulbs appear more blue-white.

What is the difference between lumens and wattage?

Lumens is the measurement for the efficiency of the bulb or how much light you’re producing for the amount of electricity you’re using and refers to the total number of light particles coming from the bulb. Wattage refers to the amount of electric power, expressed in watts or kilowatts which a bulb uses. In other words, lumens describe bulb brightness while wattage describes the power usage.

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