Some grow lamps advertise their excellent PAR, and others advertise their superior spectrum. While we’re not going to be able to tell you what kind of bulb you should buy, because research has shown it depends on each individual garden, it helps to understand some of the terms manufacturers use to rate their products.
PAR stands for photosynthetically active radiation, and refers to all the wavelengths of light between 400 nm (violet, blue) and 700 nm (deep red). This area of the electromagnetic spectrum is also the visible region. Below 400 nm electromagnetic waves are in the ultraviolet region, and above 700 nm they in the infrared. So if PAR comprises the visible region, why is it important to plants? Check out the graph below:
You can see a curve of relative efficiency versus the wavelength. The curve is highest between 400 nm and 700 nm, so light between these wavelengths has a high capacity to drive photosynthesis.
Plant biologists figured out that the quantity of light delivered to the plants is more important than quality. In order to measure PAR, detectors look at the amount of photons (particles of light) in the PAR range. Photosynthetic photon flux, PPF, has units of μmol/s-1, or micromoles of PAR per second. Photosynthetic photon flux density measures the PPF per square meter, and has units of μmol/s-1/m-2.
In order to gauge the amount of light plant in a garden receive in a day, crop scientists use the daily light integral or DLI. DLI is amount of photosynthetically active radiation falling on a square meter in a day and has units of mol/m-2/d-1. If you multiply the PPFD value a lamp has by the number of seconds you have it turned on in a day (3600 in an hour, 18 hours in a typical vegetative grow cycle).
Researchers at Purdue suggest DLI levels between 22 mol/m-2/d-1 and 30 mol/m-2/d-1 are ideal for growing tomatoes in greenhouses. Tomatoes have long been considered as the ultimate comparison crop for cannabis, but it might not be so. Tomatoes, which grow low to the grow ground, may respond better to those levels of light, whereas cannabis might like even more. Looking at the DLI that Northern California receives in July, cannabis plants over there might be basking in 65 mol/m-2/d-1. According to the brochure for the Gavita Pro line e-series, their 1000-Watt double-ended high-pressure sodium lamp (a workhorse in the indoor scene) delivers 64.8 mol/m-2/d-1 in the two square meters directly below the lamp at the typical height a grower would hang it above the plants.
While DLI is great, it’s best for comparing to outdoor locations. Ratings for lamps generally come in PPFD. In order to determine the best grow lamp for your garden, you need to first figure out what is your biggest concern. If money is no concern, focus on yield. If you want to save money you need to get a cheap lamp that still has decent efficiency. When it comes to LED versus HPS, both can be highly effective and efficient, but they deliver light in different ways. The direct light of LED’s may be better for smaller spaces, and the expansive HPS could be better for larger setups. It all depends on you.