Photons are the tiny particle(ish) waves of energy that are more commonly referred to as a group with the term “light.” Plants use photons, when they’re in usable wavelengths, in order to first split the hydrogen from the oxygen in water and then combine the freed hydrogen with carbon dioxide to create the sugar glucose needed for growth. This process is known as photosynthesis.
One common unit of measurement for light strength—or, intensity—is lux (lx). A lux is the amount of light given off by one candle at a distance of one meter spread over a square one meter to a side. Depending on location, direct outdoor sunlight is around 100,000 lx, and many full-sun garden plants need at least 25,000 to 50,000 lx to do well.
All other things being equal, a properly lit plant will perform better than an under lit plant will. One way to determine how much light is reaching your plants is to measure it. Inexpensive light meters can provide valuable information about a garden, especially indoor gardens.
A light meter held at the tops of the plants can determine how much light they are receiving. If measurements are taken in exactly the same place each time, records can be kept to show how much the lamp is degrading over time, or how much difference a new bulb makes.
For example, readings taken from a 400 W lamp with a dirty lens and an old bulb were 25.6 klx (kilolux) at one foot away. By cleaning the lens, the reading improved to 30 klx; replacing the year-old bulb brought the lux up to 39.2 klx. That’s an overall improvement of 13.6 klx—a fair improvement that illustrates the importance of proper lighting maintenance.
Now for the bad news: even though at one foot the lamp now gives off 39.2 klx, at two feet that falls to 16.4 klx. Distance is extremely important in indoor gardening because the amount of light that hits the plants drops off as an inverse square to the distance. At two feet, the lux is reduced to approximately one-quarter. (The averages of the actual readings I took were 41.4 klx at 1 ft. and 13.3 klx at 2 ft., which is only three lux off from the 10.3 klx I expected from using the calculation—this was in part due to the light reflector.) Even worse, at three feet it is reduced to 1/9th. Again, the actual readings taken at three feet (5.0 klx) were close to the theoretical (4.6 klx).
Compare the results taken from both a 400 W lamp and a 1,000 W lamp (in klx) at 1 ft., 2 ft., and 3 ft.:
400 W 39.2, 16.4, 5.3
1,000 W 100, 31.5, 15.6
Notice that a 400 W light at 1 ft. is comparable to a 1,000 W lamp at 2 ft. away (although for a smaller area). This is one reason why air-cooled and vented hoods that allow for the fixture to be placed closer to the plants can make a large difference.
Naked bulb fixtures generally have to be placed further away from the plants because of the heat they emit, and plants therefore suffer more from the distance drawback. However, if a 1,000 W light runs cool enough to be within a foot or so of the plant, the tops of the plants will be exposed to near outdoor levels.
Light moves in a straight line unless acted upon by an outside force. Gravity can bend it, but gardeners are more interested with light’s ability to bounce (reflect). Indoor gardeners often use reflective materials on garden walls to reflect escaping light back to the garden.
Lamp hoods are constructed to reflect light from the tops and sides of the bulb into the garden, hopefully without adding too much distance to the path. Indoor light that is reflected travels a further distance than light that strikes directly, and as discussed above, distance diminishes intensity. Still, reflectivity is helpful as it helps to recover light that otherwise would be lost or would have to travel even further to be reflected.
This exponential falling of light with distance is one of the most important differences between indoor and outdoor light. Outside, the intensity of the light at ground level and the light several feet higher is negligible due to the intensities and distances involved with natural sunlight—for example, meter readings showed 100.6 klx at both ground and at 3 ft. above. The cheap, plentiful light radiated by the sun is the most cost-effective light source (remember, greenhouses can extend the growing season for many areas).
Light meters can help illustrate the intensity of available light in various parts of the garden and can help with ensuring proper lighting levels. Nonetheless, it is in the best interest of indoor gardeners to understand at least some of the science behind lighting. Light is one of the most expensive requirements to fulfill in an indoor garden, so use it wisely and to good effect.
A TASTY, HOME-COOKED MEAL:
Speaking of light, hoods and reflectivity, you can flip a lamp hood out into the sun and run it “backwards” to make a tasty, solar-powered treat. Here’s how: Gather up a glass-lens light hood, a cardboard box big enough for the hood to fit in, some aluminum foil, wide tape, cardboard, an oven thermometer, a few aluminum cookie sheets, cupcake tins (preferably dark colored) and cupcake batter.
To prepare the hood, remove the bulb and socket. Use foil to seal the vent openings. Place it lens-side up into the cardboard box, and pack cardboard around the space between the hood and the box as insulation. Attach the cookie sheets to the sides like the petals to a flower, at an angle of about 60 to 67 degrees.
Point the box directly at the midmorning sun, tipping as needed, and blocking it into place. Use the thermometer to track the internal temperature. Aiming and adjusting the “oven” takes a bit of practice, and must be done every hour or so. However, with good sun, the internal temperature should rise to 200ºF or better (under ideal conditions, temperatures can reach 300ºF, so make sure all materials exposed to heat are safe at those temperatures).
Put the batter in the cupcake tins, and place it into the hood. Depending on conditions and the size of the cupcakes (smaller is better), there should be solar cooked cupcakes ready to eat in an hour or two (just don’t forget to adjust the box to keep it pointed at the sun).
Many foods may be cooked in this fashion, which is similar to a slow cooker, and can be cooked for as long as the sun heats the box. Noon to early afternoon are the best cooking times. For food safety, monitor to ensure the temperature does not drop and stay below 160ºF while cooking.
The principle is simple and similar to that behind a greenhouse. UV radiation passes through the glass to the space inside, where it becomes shifted and partially trapped as infrared (heat). As long as energy from the sun is put into the box faster than it escapes (which is the function of the cardboard insulation), a net gain will result and the temperature of the interior of the box will rise.