Light, Wavelengths and Intensity

Light

Light is energy specifically; it is a form of electromagnetic radiation. The two components of light are the electric and the magnetic fields, which are set at right angles to each other. These fields are synchronized together and collectively are known as an electromagnetic field. Electromagnetic radiation travels in a wave from a source.

Wavelength

Although the speed of the wave is constant, the frequency or wavelength is not. The speed at which the same point on the wave passes by a point in space is the frequency. The distance between wave crests is its wavelength. A wavelength is literally the length of the wave. Even though specific numbers are often used as a boundary, electromagnetic waves are a gradient.

Where one color starts being a different color is more an approximation than a hard line. The order of longest to shortest wavelengths is: radio waves, infrared (heat), visible light, ultraviolet, x-rays and gamma rays. The human eye only uses a small portion of the electromagnetic spectrum to see.

These wavelength of visible light are between about 790 and 390 nanometers. This visible light is further broken down into colors. The long wavelength end of the visible light spectrum is perceived as red, and at the short end of the spectrum is violet.

Approximate values by color:

Red 700-625nm Orange 624-600nm Yellow 599-670nm Green 569-500nm Blue 499-465nm Indigo 464-426nm Violet 425-400nm Light waves slightly too long to be red are called infrared, wavelengths slightly too short to be violet are called ultraviolet. Objects appear to be the color of the light they reflect. If all colors are absorbed by a leaf except for green, then the leaf will appear green.

The wavelength of the electromagnetic field determines its color if visible, or its type if not. Photosynthesis in plants makes best use of light with wavelengths from peaks at the red, blue and violet bands. Chlorophyll a is the blue-green pigment in plants, and two peaks of absorption occur at 665nm (red) and 465nm (violet-indigo). Chlorophyll b is the yellow-green pigment in plants, and absorbs light at two peaks of 640nm (red) and 450nm (indigo-blue).

The valley between the two spikes - between 600 and 500nm (the green and green-yellow bands) - is not used as much, and is reflected by the plant. An ideal light source should supply wavelengths including 665-640nm (red) and 465-450nm (indigo). A rule of thumb is that 'cool' lights lean toward the blue-violet side, and 'warm' lights tend toward the red-orange side.

Intensity

Waves not only have length, but also have height, or amplitude. The greater the amplitude, the brighter the light. A light at 550nm will appear green, either dim at low intensity or bright at a high intensity; however, the color remains the same. The greater the intensity, the more energy is being transmitted. A lumen is the light of one candle. A lux is that light from one meter away spread over a square area one meter wide. A full moon on a clear night lights at about one lux; a well-lit indoor room is about 400 lux and bright sunlight is about 100,000 lux.

Lighting Efficiency

Power comes from some source, such as a wall plug, and goes through a process to give off light. How much of this power is converted to useful light is the lighting efficiency. If the process produced 683 lumens per watt used, perfect efficiency would be achieved. Fire gives off light, but most of the energy is expressed in the infrared wavelengths, and therefore it is not well-suited for growing plants.

Incandescent lighting has an efficiency of about six percent (10-30 lumens per watt). LEDs are also about six percent (20-60 lumens per watt) efficient although in theory they could be made more efficient. Fluorescents measure at 10% (30-100 lumens per watt), metal halides/MH around 14% (70-125 lumens per watt) and high pressure sodium/HPS at 15% (60-140 lumens per watt). The excess heat generated by lighting systems is often the largest source of waste heat in a garden.

Matching the Light to the Space

One number that is often listed is the lumens, which includes all the light given off (including the less-used wavelengths), but at least it is an estimate with which to work. The amount of light that hits the plants drops off as an inverse square to the distance. At two meters, the lux is reduced to 25% and at three it is reduced to 11%.

A plant that is twice as far from the light source only receives a fourth of the light. A plant three times as far only receives a ninth. If a 400w bulb gives off 50,000 lumens in an area one meter to a side, then the bulb gives off 50,000 lux.

These lights are used for areas .5m to 1m to a side. This same bulb could be used to light an area two meters on a side, but at only 12,500 lux to the plants. A 600w giving off 95,000 lumens at one meter still delivers only 23,750 lux at two. Good for 1-1.25m gardens. A 1000w bulb may give off 130,000 lux which would be overkill for a one meter, but 32,500 lux over a two meter space is more reasonable. These are well-suited for gardens of 1.5m to a side. If you measure your grow space, you can draw out different scenarios until you find one you like.

For example, in a space 1.5m x 3 m, two 1000w lights, two or three 600w lights or three 400w lights could be used as a general guideline. However, make sure you can deal with any waste heat the lights generate as cooked plants perform even worse than under-lit ones do. There are many factors involved in choosing the lights for an indoor garden; the better informed you make yourself, the better the chances are you'll be happy with the results you get. Peace, love and puka shells, Grubbycup