Help with LED

[rshayes]

This web page (http://www.otherpower.com/otherpower_lighting.html) does some comparison of LED and fluorescent tube efficiency.

It appears that the efficiency of fluorescent tube is still around two to three times higher than white LED's. This is based on a measuremet of the total light output rather than the light intensity in a small area. Fluorescent lights radiately fairly evenly in all directions. Reflectors can redirect the light from the other side of the bulb, so practically all of this light is usable if you are trying to illuminate a large area.

A combination of cool white bulbs, which tend toward a bluish light, and warm white bulbs, which are definitely pinkish, might be a fairly good match for the response of a plant.

The lifetime of the fluorescent bulbs will be shorter than the LEDs, but the replacement and operating costs are probably enough lower to give the fluorescent lighting a strong economic advantage over LEDs.

A single fluorescent tube will save between 40 and 80 watts compared to an LED array putting out the same amount of light. If the lamp is on an average of 12 hours a day for 365 days a year, the saving in energy would be between 175 and 350 kilowatt-hours per year. At 10 cents per KWH, the savings in electric power would be between $17.50 and $35.00 per lamp per year. Replacing the lamp once per year would cost about $2.00. Replacing the fixture every 5 years might add $3.00 per year to this, leaving a savings beteewn $12.50 and $30.00 per year per lamp.

Even if the LEDs were free and never required replacement they would still be substantially more expensive than the fluorescent lamps.

<small>[ November 24, 2005, 12:54 AM: Message edited by: stephen ]</small>

[Mike6158]

Yeah but... Somewhere back there someone mentioned using solar energy to power the leds. Also, as the post author mentioned, plants like a specific temperature range of light. From what I've read the flourescent tubes would have to be replaced at least every 6 months (cathode decay?).

Interesting side note: I have a circuit board that is 0.75" wide by just a shade over 6" long. There are 18 LED's. (3) red, (3) blue, (3) red, (3) blue, (3) red, (3) blue (each group of LED's is connected in parallel with each other. The LED's in each group are connected in series). The circuit draws ≈190mA @ 13.8VDC. At ≈3' above the floor the array produces an ≈18" diameter circle of red and blue light. πD²/4 = (3.14 * (18²))/4 = 254 in² = 1.77 ft² of area lit by this array. I was surprised that the lighted area was circular. I expected "circles along a pseudo rectangular area". The board isn't warped and the LED's are inserted flush with the surface of the board so they aren't "aimed".

All things considered I think that LED's could work as supplementary light (ie not a full 12 hours worth but something to supplement the gloom of winter) but not on a large scale. I still have a question as to how much light energy is actually getting to the plant as well.

The blue LED's that I used have a dominant emission wavelength of 470nm (4,600 mcd / IF=20mA) and the reds have a dominant emission wavelength of 628nm (12,000 mcd / IF=20mA). IFM for the both LED's is 50mA so I suppose that one could get more light out of the array by pulsing the power to it. I could have used 5,500mcd blue LED's but this was just an experiment so I used what I had at the time.

<small>[ November 24, 2005, 01:49 PM: Message edited by: NE5U ]</small>