Hi Anna and Mark,

For me that is a MOST interesting question.

And why do flourescent lamps work better when VERY close to the soil to start seeds and grow small plants?

Flourescent lamps use a high voltage to initiate the arc that drives them. Also, they emit quite a bit of ultraviolet. The ultraviolet seems to disturb / kill many small insects.

That said, the only thing I can see really changing as the lamps get close is the electric field. [The light intensity stays more or less the same.] This also makes sense if one looks at the claims some of the radionics folks make.

So I think your experiment could be most interesting comparison.

Lots of fun :).

John

why do fluorescent lamps work better when VERY close to the soil to start seeds and grow small plants?

The closer you are to the light, the higher the light intensity. Say the fluorescent light is an inch in diameter. It gives off a certain amount of light energy from its surface. Imagine a cylindrical surface 5 inches away from the lamp, so with a diameter of 11 inches. The surface of that cylinder is 11² or 121 times bigger than that of the fluorescent tube, but the same amount of energy goes through it. So the amount of energy per surface area is reduced by a factor of 1/121. This is called the inverse square law.

they emit quite a bit of ultraviolet

They actually create only ultraviolet light. The phosphor coating on the inside of the glass converts most of it to visible light. IIRC, the UV exposure of sitting for several hours under a fluorescent light equals that of one minute of sunshine. (source) So it's not that bad.

the only thing I can see really changing as the lamps get close is the electric field

Electromagnetic fields follow the same inverse square law, so it falls of quickly. If you look at a Sankey diagram for a fluorescent light, most of the energy is lost as heat by the phosphors that convert the UV to visible light.

Regarding John's question: I'm wondering if it has something to do with the difference in light spectrum between the LEDs and daylight fluorescent bulbs. Daylight fluorescents mimic sunlight, but plants use a lot of light in the red and blue spectrum, reflecting green light which is why plants look green. LEDs, from what I can find, have a similar light range. However, you only need two fluorescents as opposed to many LEDs to cover the area you need. Maybe it has something to do with the small amount of heat being produced by the fluorescent bulbs in addition to mimicing sunlight?

A neighbor considered using LEDs but when he figured out how many bulbs he would need and what the cost was it was basically prohibitive - in the range of hundreds of dollars vs. $60 or so. ::shrugs::

Roland! Thank you for the explanation. I smacked myself in the head when I read it thinking I should know that! I studied physics! Okay it was 40 years ago but...

Thanks again for the very succinct explanation.

Nayan

It's been a few years since I did the research, so unfortunately I don't have my sources handy to cite, but I read several articles that outlined the use of warm vs cool fluorescent bulbs when growing plants.

Basically what it boiled down to was that neither provided the full light spectrum ideal for growing plants, whereas "grow" lights do. But if you can mix your bulbs and have a warm and a cool bulb shining on your plants, then you get the full spectrum needed for healthy plants, at a much cheaper cost than buying specialty grow lights.

Based on this, I mixed warm and cool bulbs, and I've had healthy, vigorous seedlings. That being said, I've never experimented with just using one or the other, so I can't say for sure that it's the mixing of the bulbs that is responsible for my seedling quality (because if it ain't broke, don't fix it). But, since light quality is such a big contributor to plant health, it stands to reason that the bulbs are doing what they need to do in providing the plants with the resources they need.

There are differences among fluorescents as well, T12, T8, T5, compact... I've replaced some of my double T8s with a single T5 for seedlings and things grow just as well. Off the top of my head I think T5s are considered more energy efficient and produce a more complete spectrum of light. I use 2x4' length T5s for a 2'x4' area, which holds 4 of the 10-20 sized tray. Also, the ends of the bulbs aren't as bright as the centre, so plants should be rotated, you probably do that in the window anyhow...I bet in a few years LEDs take over completely.

Remember another factor to use is some type of fan, allowing the seedlings to maintain their stem strength. I used this method last year. Just a thought.

I'm glad to see such an interesting discussion on lights! And Rose Nell is entirely right about the fan. We plan on one as an anti-fungal measure since we're going the electric route this year.

Daylight fluorescents mimic sunlight

It's a rather poor mimicking. Fluorecents tend to give off most of their light in a limited number of frequencies. See e.g. the spectra for different light sources shown here. If you shine the light from a fluorescent lamp on a CD you can see the actual colors it emits as shown here.

By adding certain elements to the phosphor coating inside the tube, the frequencies can be customised somewhat. Hence the different colors. It seems that T5 format fluorescent tubes are popular as grow lights.

I wonder in how far all of those grow lights now available are tuned for producing weed.

I'd like to ask if anyone knows how often grow lights should be replaced??