The electromagnetic spectrum is categorized by wavelength. The longer the wave, the less energy it holds. So blue light, which has a short wavelength of 475 nanometers, has more energy than red light, which has a wavelength of 660 nm. Infrared light is just beyond the range of human vision at 730nm. Although we cannot see it, we can feel infrared radiation as warmth. For instance, a glowing charcoal emits red light in the visible spectrum and infrared light that we sense as heat.
Every object emits infrared radiation from its surface. So an object with a cold surface emits less and an object with a warm surface emits more. That's how an infrared camera works. It records a photo of the infrared rays emitted by the objects in the picture. Plants are usually a little warmer than the surrounding environment, so they are lighter. Water and moist objects such as soil are often cooler and darker. Even a totally dark room yields an infrared picture.
Since the camera is only recording waves that are already there and is not projecting any light into the space, it has no effect on the plants. If you take an infrared photo outside your space, you can see how much heat is escaping and how it's leaving. You might find that of interest also.
The plants use red and infrared light to regulate stem growth and flowering response. Plant cells produce a chemical called a phytochrome, which has two versions. One version, PR, is sensitive to red light (660 nm). Red light converts PR into PFR. PFR signals the plant to grow short stocky stems and also helps the plant grow into specific shapes. The plants also use red and infrared light to measure uninterrupted darkness. As far as plants are concerned in terms of flowering, if there's no red light, it's dark.
PFR is sensitive to infrared light (730 nm), which converts it into PR. When PR levels build to a critical amount, scientists hypothesize that a hormone called floragen becomes active and induces the plant to flower. The reason floragen is called hypothetical is that researchers can see its effects, but they haven't found it yet.
PFR reverts to PR naturally. For PFR to be present, it must be renewed continuously by the presence of red light. When plants are shaded, they get less of the needed red light. In the absence of red light, the PR version predominates and the stem stretches to reach the light. Lower side branches shaded by leaves from above have PR and grow longer until they reach the light. Then they modify their growth in the presence of PFR.
Outdoors during the day, there is more red light than infrared. However, at dawn and dusk the first and last light from the sun isn't the visible red of the rising or setting sun, but infrared, which is at the far end of the electromagnetic spectrum. The infrared converts the PFR to PR and the critical dark-time begins or ends its countdown.
This has too many implications for them all to be discussed here. For instance, it explains why plants grown under incandescent lamps stretch (more infrared than red light). The effects of the two spectrums can also be used in innovative indoor lighting programs.
Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA
All featured questions will be rewarded with a copy of Ed's new book, Best of Ask Ed: Your Marijuana Questions Answered. Sorry, Ed cannot send personal replies to your questions.