This is basically like a mini guide and crash course on HPS and MH lighting, benefits of each, differences to other types of lighting, and most importantly knowing the proper distance between the bulbs and your plants - regardless of heat! Some basic things to know, what is MH/HPS?: - MH: Metal Halide - HPS: High Pressure Sodium MH and HPS are far superior lighting alternatives to the sun over any other type of lighting known to man. Some people suggest that you should accommodate lighting relative to your space, I disagree. You CAN use up to 1000W MH/HPS system in a closet as long as you have a method to carry the heat from the bulb to the outside of the house, not back in to the room! For this I recommend getting a cooltube on ebay for about $100, or building one yourself as my signature will show you everything to do that, which will roughly be $35 for everything, minus the ballast. And then combined with a 160CFM inline fan, and some ducting and tape, you are on your way to a balanced temperature closet grow using 1000W HPS/MH! Please keep in mind I am not responsible for you burning your house down from lack of experience or knowledge, common sense plays a big part in this. It is critical that the plants have a hygrometer right next to the plant to monitor temperature and humidity, temp should be between 75-79 degrees F, this is when optimal photosynthesis production occurs. Regardless of what people say about the hand test method to test the heat from the bulb, this is a misconception! With MH/HPS systems you should be careful as to not get too close to the plants with the light, even if it feels cool, I have a couple of graphs below that illustrate acceptable distances from the bulb for best growth/lumens/PAR. If you get too close (regardless of heat), your plants can in fact get too much light by being too close, this will cause a plant's top to get white and feel rubbery to the touch. Too far away and you lose lumens and light output to plant, so the graphs below are to illustrate how you can maximize distance while maintaining optimal temperature. Combined with the proper nutrition, environment, and proper care, your flowering and budding will exceed that of any other artificial lighting used. You will yield higher with MH/HPS, there is no dispute, so long as the conditions are met for this plant to thrive! Always keep your temps in the grow room between: 68 and 85 degrees to avoid stress to your plants. However plants are fine for short periods of time a little above or a little below this range. MH/Metal Halide: Metal halide lamps, a member of the high-intensity discharge (HID) family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained. Originally created in the late 1960s for industrial use, metal halide lamps are now available in numerous sizes and configurations for commercial and residential applications. Like most HID lamps, metal halide lamps operate under high pressure and temperature, and require special fixtures as well as ballast to operate safely. Since the lamp is small compared to a fluorescent or incandescent lamp of the same light level, relatively small reflective luminaires can be used to direct the light for different applications (flood lighting outdoors, or lighting for warehouses or industrial buildings). HPS/High Pressure Sodium: High-pressure sodium (HPS) lamps are smaller and contain additional elements such as mercury, and produce a dark pink glow when first struck, and a pinkish orange light when warmed. Some bulbs also briefly produce a pure to bluish white light in between. This is probably from the mercury glowing before the sodium is completely warmed. The sodium D-line is the main source of light from the HPS lamp, and it is extremely pressure broadened by the high sodium pressures in the lamp; due to this broadening and the emissions from mercury, colors of objects under these lamps can be distinguished. This leads them to be used in areas where good color rendering is important, or desired. Thus, its new model name SON is the variant for "sun" (a name used primarily in Europe and the UK). HPS Lamps are favored by indoor gardeners for general growing because of the wide color-temperature spectrum produced and the relatively efficient cost of running the lights. High pressure sodium lamps are quite efficient—about 100 lm/W—when measured for photopic lighting conditions. They have been widely used for outdoor lighting such as streetlights and security lighting. Understanding the change in human color vision sensitivity from photopic to mesopic and scotopic is essential for proper planning when designing lighting for roads. Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminium oxide. This construction led General Electric to use the tradename "Lucalox" for their line of high-pressure sodium lamps. Xenon at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be easily started. At the end of life, high-pressure sodium lamps exhibit a phenomenon known as cycling, which is caused by a loss of sodium in the arc. Sodium is a highly reactive element, and is easily lost by reacting with the arc tube made of aluminum oxide and the products are sodium oxide and aluminum: 6 Na + Al2O3 → 3 Na2O + 2 Al As a result, these lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the maximum voltage output by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly. That is how you know it is time to get a new HPS bulb. More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts. LPS lamp failure does not result in cycling; rather, the lamp will simply not strike or will maintain its dull red glow exhibited during the start up phase. About MH and HPS: Both MH and HPS systems are far more efficient than other artificial light sources. MH and HPS work far better than other artificial lighting sources because they produce a spectrum closest to that of natural sunlight. MH should be used during vegetative growth due to that of the summer sun, which emits more of a blue spectrum of light. HPS lighting should be used during flowering/bloom period due to that of the fall sun, it emits more of a orange/red spectrum of light which triggers the flowering when combined with reduced lighting (12 on/12 off). Some basic things to know: Photosynthesis: How far should my lights be from my plant's you ask? MH graph for optimal bulb placement from plants: HPS graph for optimal bulb placement from plants: MH Percentage of sun power, graph: HPS Percentage of sun power, graph: Enjoy! And nothing ever beats the SUN!!!