- 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 spectrumThe 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.
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HID lifespan comparisonIt is difficult to compare most bulb types lifespan projections because most manufacturers do not publish these figures.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.