Opie Yutts, Hey you asked for a short definition, and you said that it wasn't right. So I dug-out a Horticulture book from when I took it at Texas A@M. I'll still try to do it short, but mabe explain it better OK. Plant growth, harvest, potency and even the time to Flower are all dependent on the light they receive. Light quality, intensity and duaration are all important. The following is a brief introduction of plants, light, lumens, and PAR. Light is a plants food. Nutrients are only building blocks for the plant cells, but it is light that provides the energy! So how does it work? When light falls onto leaves it triggers the process of photosynthesis, which in simple terms is the process of turning light, which is radiant energy, into chemical energy. The amazing process of photosynthesis turning light energy into chemical energy, is one of natures wonders. This energy transfer happens inside the plants cell structures called chloroplasts. The basic components of chloroplasts are individual membranous, sacs which contain fats, proteins and pigments. Pigments play a important part, They absorb light in the photosynthesis process of turning light energy into chemical energy. Choraphyll, For example is an important pigment which absorbs red and blue wavelengths of light. The light absorbed by the pigment causes a reaction, which produces chemical energy( it makes electrons out of the light, and the electrons use thier charges to make sugar energy for the plant). The chemical energy produced by the chlorophyll (pigment) from light is sufficient to split the water molecules apart. This provides units of hydrogen(H) and hydroxide(OH). The hydroxide combines with carbon dioxide, Which provides the energy for plant growth.( And you thought there was nothing going on in your plants!!) Light, its intenisty, quality, its colour, spectrum, wavelength are therefore all-important factors, but how do we measure light and what are the most important components. Light is measured in photons, Light actually hits objects just like a spray of water, and the sun emits lots of light photons, to give you an idea of how many; the sun hits our body with over 12,000,000,000,000,000,000,000, photons every second and a plant needs about 20 photons to make a finishedmolecule of sugar. So our scientists can count the number of photons hittng the plant and even predict how much of this energy will convert into FLOWERS of FRUIT!!! Each industry has its own way of measuring light. Photographers use a light meter, the lighting industry uses lumens or lux and the gardening industry uses PAR. All are only measurements. Light from the sun is ideal but its not the same as artifical light, where output, quality, spectrum ect; raries upon the type of lamp and how it is used. Many growers think that more lumens= better growth/yields, When in fact artifical light, even at its BEST in a HID @ HPS lamps, but is not so good in terms of colours, much of the light from the bulb is not used by the plant, mainly because it is not in the 400 to 700 NW ( Nanowave) spectrum, and plants can only see and use light in this range. Light quality and its colours are as important as lumens. Light, as seen by plants is not a single colour but separate bands of active colour-band of light as a separate signal. Each band of colours has a different effect on plants and the following are only a few of the functions which each band of light promotes. 400 to 700 nw spectrum Blue light (350-500nw) powers chlorophyll production, powers cell actively, energies the stonata movement and makes the plant follow light. Green/Yellow light (500-650nw) not much action from these bands of light, but needed for good health. Red light (600-700nw) makes sugar from CO2. powers chlorophyll production , signals light and dark times among other functions. Strong blue and red light photons (as above) are also needed for good carbon dioxide uptake. The PAR scale measures all these colours, photons between 400nw @ 700nw, The critical range that plants can use of light. If its not in this range then its wasted light. For growers PAR is all impotant and as important as lumens. PAR stands for Photosynthetic Avtive Radiation. Photosynthetic, the sensed by a leaf pigment. Active, the light that causes the leaf pigment to become active for making energy. Radiation, another way for light @ photon energy. PAR is the measurement scale used internationally as a metric light measurement and is becoming more and more relevant to Growing and Greenhouse light measurement. PAR is the measure of light that a plant actually senses and uses, and it is the light the plant sees and can use that is more important then actual output lumen of the HID @ HPS Grow Lamps. A Large HID Lamp may give out loads of lumens, but if its to far away from your plants most lumens are wasted, the light intensity diminishes with distance and in addiction the light is limited of what the plant needs because it is in the wrong spectrum!! So the main value of the PAR measurement is that it is the only measure that takes into account the actual light and light Colours that the plant uses to energies its pigments and generate sugar energy, and its the sugar that makes your plants grow and produce SUCH SWEET FRUITS OF OUR LABOR!!! You should have taken my short definition. Hope that explains where the exrta potency comes from using the LED's, correct Colours and no waisted spectrum. Fuck I need about 20 Bong-Hits. Later