If it’s mostly vegetative growth you’re after, metal halide lighting is the type of HID lighting you need. Similar to the sun’s rays in spring and summer, metal halide lamps produce light which is mostly in the blue range of the light spectrum—that’s what you need to grow lettuce, basil, and other leafy greens.
These lamps give you MH light from an HPS system and vice-versa. Because the conversion process itself takes some energy to accomplish, conversion lamps aren’t quite as efficient as regular metal halides or high pressure sodium’s, but they will give you the best of both worlds with just one light system.
If the plants that you’re growing now is the same as in the past year, then it’s likely that your bulb needs replacing. HID (High Intensity Discharge) grow bulbs such as HPS (High-Pressure Sodium and MH (Metal Halide) need to be replaced approximately every year in order to perform at optimum levels. Even more often, if they run 24 hours per day and not on the 12/12 on/off lighting cycle. HID lamps will show decreased lumen output with prolonged usage that is not visible to the human eye. Worn out bulbs will still send out light but at diminished levels, and replacement bulbs are fairly affordable, making it a no-brainer to replace the bulbs for more light and bigger yields quicker.