Which Light to Choose for a Tent - Discharge Lamps Part 2

In the previous article we looked at what bulb power suits tents of different sizes. But which type of bulb should you choose? The basic types of discharge lamps include MH (metal halide) and HPS (High Pressure Sodium) bulb on the left and HPS bulb on the right. The fundamental difference between these two bulbs is in the composition of the light spectrum they emit.

MH bulb on the left and HPS bulb on the right

MH (Growth) - For growth a light source with a higher proportion of blue light is suitable, which plants in the growth phase can use better - this mimics the light emitted in spring, when the plant has plenty of time before "winter". This light is best imitated by the MH bulb and therefore it is always labeled as Growth. Even though it cannot emit the same amount of visible light as an HPS bulb (a value given in lumens), it can still create more of the light suitable for growth at the same power consumption than an HPS bulb with a higher share of the visible spectrum and thus a higher lumen value. MH bulbs are also characterized by a higher color temperature, which ranges between 4000K and 6000K; for cultivation values near 4000K are usually used.

HPS (Flowering) - The plant can make much better use of the yellow-red part of the spectrum, which imitates summer conditions and together with the shortening of daylight the plant flowers to ensure the survival of its species into the next growing season. HPS bulbs best imitate this period and have their spectral composition adapted accordingly. An HPS bulb usually has a color temperature (chromaticity) between 2000K and 3000K.

For comparison, an illustrative graph of the light spectrum of MH and HPS bulbs

I also promised you in the previous article a tip on how to get the most out of a bulb's energy. The principle is very simple. If you most often use a 600W bulb, buy a 600W HPS bulb and a 400W MH bulb. Seedlings in the growth phase are still small and do not need as much light as during flowering. Save money and buy a weaker bulb, and save energy further. If you use a setup with a 400W HPS bulb, you can use a 250W MH bulb for growth, or conversely with a 1000W HPS bulb you can use a 600W MH bulb for growth.

DON'T FORGET that you will need a switchable ballast that allows you to use 2 different bulbs so that you don't burn the weaker bulb before you even start. More about ballasts can be found in our piece what do I need a ballast for?