What's the difference between a regular white light bulb and a grow light?

Question

I know that LED bulbs can vary along two important dimensions, lumens and "temperature." In particular, the temperature is important to make sure the plant gets the frequencies of light it needs. One source I read said

As a general recommendation, choose a full spectrum grow light with a color temperature in the blue range (5,000 - 7,000K) to promote vegetative growth and choose a color temperature in the red range (3,500 - 4,500K) to promote fruiting and flowering.

This source goes on to say

Many grow lights are labeled as "full spectrum," meaning they emit light across the full-color spectrum, including not only blue and/or red light, but also enough green light for the overall output to look like natural light to the human eye. While plants primarily use blue and red light, research suggests that some species perform best with full spectrum light. Therefore, it can be helpful to choose a full spectrum light.

Source: https://www.johnnyseeds.com/on/demandware.static/-/Library-Sites-JSSSharedLibrary/default/dw818e2afc/assets/information/grow-light-guide.pdf

I am confused by a couple things:

1. How do I tell whether a given species of plant needs full spectrum, red, or blue light?
2. I thought white light is a combination of all colors of the spectrum. Why wouldn't that be sufficient if it carries all of them?
3. They sell "grow lights" online, some of which have "full spectrum", "red" and "blue" bulbs all together. But they are obviously much more expensive then just buying a single bulb. Are there any disadvantages to first identifying my plants need and then choosing the bulb type it needs? For example, suppose my plant needs red light, wouldn't it make sense to buy a red bulb instead of a grow light?

Answer 1

It would require a book to answer all your questions. Green chlorophyll absorbs ( needs) two general frequencies (colors) of light , one blue around 450 nm, one red around 650 nm. There are different types of green chlorophylls that absorb slightly different frequencies; also red and brown chlorophylls (we don't care about). Incandescent white puts out a full spectrum, usually higher wattage have more blue so they were recommended more for growing. That is, a low wattage like 40 watt has essentially no blue. Fluorescents look white but they do it by blending colors ( different phosphors - the powder inside the tube). So a "white" looking fluorescent may have no blue or red needed by plants. Fluorescents for pants like Grolux have the needed blue and red but to the eye they are an unnatural color. Metal halide almost put out a full spectrum but with definite color peaks depending on the "metal" . They were the best for growing but are out of favor now because of high power usage. They were used for growning pot and the FDA would investigate any large purchaser of metal halides. I once saw an acquaintances' 100 gallon aquarium with 2000 + watts of metal halides , very impressive. Now we have LEDS , I never understood them . I have several on aquariums and seem to work OK.

Answer 2

Regular household fluorescent lighting (including CFLs and other fluorescent lights) and fluorescent growlights are the same thing if they have the same color temperatures, but even if they don't, they still work. I've tried various color temperatures, and they all worked (although plants didn't grow the same way with all of them, but they did grow).

Household incandescent lights can work as growlights (I've tried it), if they're bright (and close enough; proximity is a big concern for all kinds of lighting).

The question seems to be about household LEDs vs growlights, though. Here's a link that gives us the wavelengths of a regular white household LED light:

• https://electronics.stackexchange.com/questions/149683/do-standard-white-leds-produce-a-full-spectrum-of-light

While it doesn't appear to be full-spectrum, most of the light seems to be about 450nm, which in theory makes it similar to a blue LED growlight, with lower levels of other wavelengths thrown in (so, they're probably better for plants than blue growlights--but they're not full-spectrum).

Household LEDs might look white, but they're mostly blue. The light they produce also hides some imperfections on clothes (which could be a problem if you take those clothes outside or to somewhere with fluorescent lights).

Answer 3

I'll answer your questions first, but let me preface this with the fact that more expensive doesn't always mean better quality--you'll want to look for a reputable seller if you decide to buy more expensive equipment. With pricing the way it is, there are a lot of sellers passing cheap parts with substantially higher pricing than it's worth.

1. You'll have to research your plant individually. (Warning: for plants that grow through an annual cycle from sprouting to flowering, red vs blue needs change as they go through their growth stage) If in doubt, full spectrum is a safe bet because it covers a wider frequency. Anecdotally, Red/Blue by themselves aren't very common anymore as grow lights. Most of the new ones I see nowadays has a combination mix of both.

2. This is true, but not all white light has the same intensity across all spectrums. Think of it this way: you can split light with a prism from the sun in the morning, noon, evening to make a rainbow. It's full spectrum because all colors of the rainbow will show up from the prism, but the intensity of each color in the rainbow will be different depending on time of day (eg, blue will be brighter in the morning rainbow and red will be brighter in the evening rainbow). You can kind of already see that as the color of the sun.

This is true with a "full spectrum" LED also, it has all the colors, but some colors like the greens are not quite as bright as if they were outside. You can see all colors of the rainbow, but some colors aren't as bright as others. Warm white will have more reds and look oranger. Cool white will have more blues, so the blue is brighter.

Compared to if you looked at a picture of a rainbow under only a red led, where you can't tell apart any colors at all. Everything is shades of red because those other colors just aren't there.

3. Growlights have more to them than just the color.

The color mixing is for energy efficiency. It's trying to push out more red/blue intensity than the full spectrum led can output by itself. It's about trying to get more intense reds+blues. If your plant can do well in only red light, then yes, you'll get the most electricity cost to photosynthetic efficiency ratio from running only red. Although, I'd like to point out that it's easier to see problems (eg, yellowing leaves) if you have a full spectrum light.

Growlights also have different hours of usage rating, size of the light emitting element and heat sinking than regular led bulbs. An led bulb for a bedroom lamp isn't meant to run 12-24 hours a day. So cheaper bulbs can be driven a little harder, let it heat up a little more, knowing that it'll probably be off in a few hours. Growlights are expected to be on for a long time, so they can't be driven the same way otherwise they won't last as long. That's why they tend to have such huge metal enclosures, to take a way heat from the light emitting element.

What makes the most sense here will be based on your budget and needs. Just keep in mind, as I mentioned above, make sure you're purchasing from a reputable dealer. Price isnt always an indicator of quality.

Good luck!