HPS ballast lamps are designed to only operate HPS lamps and Metal Halide core & coil ballast were designed to only operate Metal Halide lamps.
Conversion lamps give you the ability to operate a HPS lamp on a Metal Halide ballast and an Metal Halide lamp on an HPS ballast. While these are used much anywhere due the popularity of LED and CFL lamps, we'll list some here to get you started if you decide to try some out.
Perhaps the best option in HID lighting is to make sure that your fixture and your Ballast can handle either HPS or MH bulbs. The best way to do this is with conversion bulbs. For example, if you have an MH system, you can use compatible HPS conversion bulbs. With this type of system, you can take advantage of the best aspects of both types of HID lighting. You can use MH during the vegetative phases of growth, and switch over to HPS during the budding, flowering and fruiting stages. While this approach may give the highest yeilds, perfectly acceptable results can be achieved with either bulb type by itself.
Regardless of which system you choose, there are a few rules of thumb that apply to wattage requirements that hold true for all of them. In general, 25 watts per square foot is considered a minimum, with 30-50 watts preferred. It is important not to go way overboard on your wattage. While your growing space can’t be too light, it can be too hot! It is also important to keep your high enough above your plants; a 175 watt light should be kept at least 12” above the tops of your plants while a 600 watt light should be kept at least 24” above the tops of plants. A simple test is to place your hand under the light at plant height. If you can’t take the heat from the lamp, your plants can’t take it either and you need to raise your fixture a bit.
While technologies are emerging which will undoubtedly unseat HID as the best option for indoor growing, for the moment it remains so. HID systems have the good spectral coverage, and the best light intensity. Their attributes make them well suited for a wide range of crops. Used in conjunction with each other, HPS and MH lighting can maximize growth and yeilds through the stages of a plants life.