Color Temperature is not how hot the lamp is. It represents the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature. It is important to remember that the CRI readings of two lamps cannot be compared if their color temperature is unequal. For example, The CRI 22 of HPS incomparable to CRI 70 of MH because the color temperatures are different, such as 2200K of HPS and 4500K of MH. Lumen is the unit used in measuring the amount of light given off by a light source. The higher the lumen, the more efficient the source is producing light. Traditionally, lumen efficacy has been the benchmark of a lamp’s ability to grow plants; the brighter the lamp the better the plant. However, studies have shown that a broader color spectrum lamp performs better for plant growth than a lamp with high lumen output.