Below is a quote from Texas A&M's site regarding lighting requirements for plants:
Day and Night:
Day length or duration of light received by plants is also of some importance.Poinsettias, kalanchoes and Christmas cactus flower only when days are 11 hours or less (short-day plants). Some plants only flower when days are longer than 11 hours (long-day plants), while others are not sensitive to day length at all (day-neutral plants).
Increasing the time (duration) plants are exposed to light can be used to compensate for low light intensity, as long as the plant's flowering cycle is not sensitive to day length. Increased light duration allows the plant to make sufficient food to survive and grow. However, plants require some period of darkness to properly develop and should be exposed to light for no more than 16 hours per day. Excessive light is as harmful as too little.. When a plant gets too much direct light, the leaves become pale, sometimes burn, turn brown and die. Therefore, protect plants from too much direct sunlight during summer months.
Additional lighting can be supplied with either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and do not use electricity very efficiently. If artificial light is the only source of light for growing plants, the quality of light or wavelength, must be considered. Plants require mostly blue and red light for photosynthesis, but for flowering, infrared light is also needed. Incandescent lights produce mostly red and some infrared light, but very little blue light. Fluorescent lights vary according to the amount of phosphorus used by the manufacturer. Cool-white lights produce mostly blue light and are low in red light; they are cool enough to position quite close to plants. Foliage plants grow well under cool-white fluorescent lights, while blooming plants require extra infrared light. This can be supplied by incandescent lights or special horticultural fluorescent lights.This phrase in particular caught my attention: " Plants require mostly blue and red light for photosynthesis"
The LED grow light I am planning will only produce red and blue light. The theory being; why waste energy producing green, orange, yellow and violet, when the plant does not use, or need it? Plants reflect green light, that is why they look green to us..
As an experiment I have added a small project box containing six UV LEDs to the lighting supplied to the dianthus and ornamental pepper. The spectrum is in the UV-A range of near visible light. I have read several studies regarding the effect of UV light in plant growth. Some studies have concluded UV is detrimental, and some conclude it is beneficial. The LEDs I added have a wavelength between 395 and 410 nm. This wavelength is not considered harmfull and is very near the wavelength of blue light where photosysthesis begins.
Again, time will tell....