Krischan Photography
  Garden Images For Publication

High Intensity Discharge (HID)
High Intensity Discharge (HID) is an excellent lighting choice for the indoor gardener. We use a Hydrofarm "Super Grow Wing System" which consists of a 15 pound lamp fixture attached to a 30 pound ballast with an 8 foot long power cord. The lamp fixture is suspended from the ceiling by two metal chains attached to hooks screwed into the ceiling rafters. The bottom of the lamp fixture is 36 inches above the wire rack table top. Trays filled with plants are then placed on the table top surface. It's a great way to grow plants indoors.

Hydrofarm "Super Grow Wing System"

How to assemble a high intensity discharge lighting system. (click here).

BAD MANNERS: Hydrofarm is an exceptional manufacturer that produces high quality components. We wanted to include an update about their new light fixtures and share our growing results with them. Unfortunately, we must have done something that pissed off Hydrofarm. The company no longer answers our questions or responds to our mail. Bad manners appears to be a growing trend with many marketing departments. It's an unfortunate trend because we only wanted to promote their light fixtures. You would think that a company would want to talk with a garden writer who has written favorable magazine articles about their product. You might think that, but you'd be wrong. Ah ... but I digress ...

Back to artificial lighting. Over the winter months, you can grow healthy indoor plants that flower freely. The secret to success is using artificial lighting. The best artificial indoor light source is the Metal Halide (MH) lamp. We have used both "Sunmaster Warm Deluxe Metal Halide" and "GE Multi-Vapor Metal Halide #MVR400/U" 400 watt lamps with identical results. Both lamps screw into a M59 bulb socket. The Sunmaster lamp looks long and thin, while the GE lamp looks like a super giant sized light bulb.

MORE BAD MANNERS: I requested spectral analysis charts of these bulbs from both Sunmaster and GE. As you might have guessed, both companies ignored my requests. Bad manners strike again. I am beginning to think that maybe it's me. Maybe I just write requests that naturally piss people off. Ah ... but I digress ...

We purchased our fixtures and bulbs from CropKing. We purchased a 400 watt MH-HID replacement bulbs every year. CropKing shipped the system quickly and flawlessly. Great company.

Metal Halide lamps produce light by discharging an electric arc through a mixture of gases and gaseous metals. Plants require a specific intensity, quality, and duration of light to remain happy, healthy, and set bloom.

Measuring Intensity of Light
People have been growing plants in agriculture for the better part of 20,000 years. Early on, light intensity was recognized as an important factor. So, you'd think that after 20,000 years, civilization would have created an easy to use system of quantifing light intensity. You might think that, but you'd be wrong.

Typically, growing conditions for outdoor plants are horticulturally classified as "full sun", "part sun", or "shade". Indoor plants are treated little better, they are classified as "high", "medium", and "low" light plants. Not a very quantative system.

BAD TERMINOLOGY: What exactly is "part sun" anyways? Is this like what happens during a solar eclipse? For me, most of the time, I witness either "full sun" or "no sun". I call the full sun situations "day", and the no sun situations "night". Maybe only part of the sun rises on certain days. Ah ... but I digress ...

Lamp intensity, on the otherhand, is measured in footcandles. That is, the amount of light produced by the flame of one candle measured at a distance of one foot (one footcandle of light falling evenly across one square foot is called a "lumen").

So, you might ask yourself this question, "How many 'footcandles' are needed for 'sun' condition plants?" Direct sunlight, at noon on a clear day, is approximately equal to 10,000 footcandles. That is, the light given off by 10,000 burning candles placed one foot away from the plant. Create yourself a visual image of this. Imagine a plant in a little pot surrounded by thousands and thousands of flickering candles. Egads, what idiot came up with this stupid measurement standard. This is one pretty strange analogy, ah ... but I digress ...

We now need to classify some of this light intensity. What follows is my attempt of classifying light intensity. Think of full sunlight as 10,000 to 6,400 footcandles, part sun as 1,600 footcandles, shade as 400 footcandles, and deep shade as 200 footcandles or less. Footcandles of light can be approximated by using a photographic light meter. Photographic light meters also measure in a unit called Exposure Value (EV). Two tables follow. The first table shows a side by side comparision of footcandles to exposure value. The second table shows photographic light meter readings for full sun, part sun, shade, and deep shade.

400 watt HID Lamp
Exposure Value (EV)
Of Light
8"EV 93200 FC
16"EV 81600 FC
32"EV 7800 FC

Exposure Value (EV)
(FC) Of Light
Full SunEV 10.06400 FC
Part SunEV 8.01600 FC
ShadeEV 6.0400 FC
Deep ShadeEV 5.0200 FC
Actual Daylight Measurements - readings taken 17 February 2002 at 3PM CST. Sunny day with light wispy clouds, air temperature mid 40s F. Full Sun - reading taken in open area without any tree canopy or nearby trees. Part Sun - reading taken in forest with decidious tree canopy. Shade - reading taken in the shadow of a pine tree, but otherwise in an open area with no tree canopy overhead. Deep Shade - reading taken in the shadow of a pine tree and directly beneath a full pine tree canopy.

400 watt Sunmaster Warm Deluxe Metal Halide
We measured the light intensity from a new 400 watt Sunmaster Warm Deluxe Metal Halide grow lamp. Readings were taken directly below the lamp fixture. We took EV readings using a Gossen Luna-Star F2 photographic light meter and FC readings using an Extech meter.

    How to measure:
  1. EV (camera meter method) - Set the incident meters film speed to ISO 3. The film speed value is zero at ISO 3. Take readings in inches and measure directly below the lamp fixture. Record both distance and Exposure Value (EV) at different points. Use a ruler for accurate measurements.
  2. FC (volt meter method) - Set the Fluke VOM meters selection to mV and set the Extech to the 5000fc range. Take readings at the same points as where EV readings were taken.

We found that a new 400 watt Sunmaster Warm Deluxe Metal Halide grow lamp produced 3200 footcandles at 8 inches and 1600 footcandles at 16 inches below the lamp fixture (our older lamp produced slightly lower readings). Replace your HID lamps about every year (3,000 hours), rather than waiting until it burns out (20,000 life hours on average).

We also measured the light produced from a one year old 400 watt High Pressure Sodium (HPS) lamp. The lamp was encased within an opaque glass fixture. We found that the High Pressure Sodium produced only 200 footcandles at 12 inches below the lamp fixture. No doubt, the bulb was old and the opaque glass had greatly reduced the amount of transmitted light. We have since discarded this old style HPS lamp fixture. But the moral to the story is, measure your light output with a photographic light meter and change the bulb when necessary.

Incident Meter Readings
Exposure Value (EV) to Footcandles (FC)
Exposure Value (EV)
Of Light (FC)
EV 106400 FCFull Sun
EV 93200 FCPart Sun
EV 81600 FCBright Overcast
EV 7800 FCOpen Shade
EV 6400 FCShade
EV 5200 FCDeep Shade
EV 4100 FCTwilight

Photosynthetically Active Radiation
Visable light is generally accepted to fall within the wavelength of 400 to 700 millimicrons (also called a nanometer). Millimicrons are one thousandth of a micron, a millimicron is abbreviated as "mu". Lightwaves smaller then 400 millimicrons are called ultraviolet (not visable). Lightwaves larger then 700 millimicrons are called infrared (also not visable).

Photosynthetically Active Radiation (PAR) is defined by the same wavelength of 400 to 700 millimicrons. While convenient, it's not entirely accurate. Part of PAR spectrum is not useable by a plant. That is, some of the photosynthetically active spectrum is not photosynthetically active. Also, there is experimental evidence that the additional far-red spectrum, at 730 millimicrons, is useful to the plant. However, as scientists, we never let the truth get in the way of a good acronym. PAR is 400 to 700 millimicrons.

Photosynthesis is the absorption of spectrals of light to produce chemical energy. Generally speaking, lightwaves of 400 to 492 millimicrons (visable light, violet to blue) and lightwaves of 586 to 700 millimicrons (visable light, orange to red) are absorbed. Absorbed light is used to produce chemical energy. Lightwaves of 492 to 586 millimicrons (visable light, green to yellow) is reflected. Reflected green light is why plants appear green. No spectrum is 100% absorbed or 100% reflected. I've oversimplified the facts to make a point. The real numbers are closer to 95% of blue light and 75% of red light is absorbed, and 70% of green light is reflected. Of course, not all kinds of plants absorb exactly the same spectrum. There is a bit of play in all of these numbers.

Plant cells contain pigments such as chlorophyll, anthocyanin, xanthophyll, and carotene. Chlorophyll is the only light absorbing pigment (and associated with photosynthesis). There are four different forms of chlorophyll, some are found in all green plants, others are found in algae. There is a slight difference in the absorption spectrum of these chlorophylls. Anthoocyanin, xanthophyll, and carotene are pigments that are not light absorbing (and not associated with photosynthesis).

Red light induces blooming. Blue light induces vegetative growth.

Visable Light Spectrum versus Useful PAR Spectrum
Spectrum of Daylight, 400-700 millimicrons

Spectrum Visable to the Human Eye, 492-586 millimicrons

Spectrum Useable to Plants, 400-492 millimicrons & 586-700 millimicrons

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