What is the best HID grow light?

This is a guide to help you decide what HID technology to chose for your grow. We compare the spectrum output, efficiency and lifetime cost of the most common types of High Intensity Discharge (HID) grow lights:

• Metal Halide (MH)
• High Pressure Sodium (HPS)
• Ceramic Metal Halide (CMH)

HID technology overview

Inside the protective glass tube are HID light bulbs filled with gas and other particles. When an electrical current is passed through the gases they emit light. Different gases and particles emit different light wavelengths or colours.

The gas is initially ignited by a high voltage spark across the inner tube. The Ballast is a power supply and controller combined and is needed to generate the starting voltage spike and control the current and voltage to the bulb. Metal Halide and High Pressure Sodium bulbs of the same power ratings can be run on the same ballast.

Ceramic Metal Halide bulbs run at a different wattage and need a dedicated ballast. Both Metal Halide and High Pressure sodium bulbs have the same screw fitting, E40.

The connector for the Ceramic Metal Halide is called a PGZ 18 socket. It is a push and twist fitting and is not compatible with a MH or HPS fitting.

 

The most common bulb sizes of each type are as follows:
Metal Halide: 400W, 600W, 1,000W High Pressure Sodium: 150W, 250W, 400W, 600W, 750W, 1,000W Ceramic Metal Halide: 315W

HID light spectrum

The spectrum output of the HID bulb differs depending on the type. Let's look at the spectrum output of each bulb and compare them. The Metal Halide bulb emits a broad spectrum light and is suitable for growing from seed to harvest. However the relatively high amount of blue in the spectrum at 23% is more than needed. Increasing the amount of blue over 15% reduces the efficiency of the light for growing. The High Pressure Sodium bulb produces an efficient spectrum for growing but with only 5% blue the plants are likely to stretch. Stretching means the plant will grow tall with large internodal distance (distance between branches on the main stem). This is not ideal for growing indoors in confined spaces but has been tolerated in the past as it is a more efficient spectrum for growing, especially in the flowering stage when the plants demand a lot of light. The Ceramic Metal Halide is a perfect spectrum for growing plants from seed to harvest. The level of blue light is ideal at 15% and the balance is red and green in equal measure. The comparison of the spectrum composition in blue, green and red light demonstrates that CMH is the most balanced spectrum for growing.

HID efficiency comparison

To give you a like for like comparison we used the same reflector, light hanging height, grow tent and testing method for each HID technology. We have tested the Photosynthetic Photon Flux Density or PPFD output of each bulb and divided by the power consumed. The efficiency result can be compared just like the fuel efficiency of cars can be compared using kilometers per litre. Ceramic metal halide is the clear winner.

 

HID lifespan comparison

It is difficult to compare most bulb types lifespan projections because most manufacturers do not publish these figures. However Philips publish the data for their full range and we have used their data sheets for the comparison purposes.

HID Cost of running

We compare the three year cost of ownership for the the different systems including the original purchase price and running costs.

The High Pressure Sodium is the benchmark for PPFD output and the Metal Halide and Ceramic Metal Halide are multiplied by a factor to level the output of all the lights. For example 1.7 CMH systems are required to match the 600WHPS so that factor is applied to the CMH fixture and running costs so the two systems can be compared.

The three year running costs for the CMH is within 7% of the running cost of the HPS system. As the CMH has a much more desirable spectrm it is clear that the CMH is the best overall choice for growing with HID equipment.