Photosynthesis is the process where plants use energy from light to convert carbon dioxide to food so they can grow. When light intensity increases, net photosynthesis increases. The spectral range of solar radiation that plants actually use for this process is called the Photosynthetically Active Radiation (PAR), which ranges from 400-700 nm (nanometers). It is just about same range as what we refer to as visible light.

The human eye peaks in the yellow-green region, around 550 nanometers while there are two important peaks for plants: blue at 450–495 nm which promotes vegetative growth, and red at 620–750 nm which promotes flowering and budding. Plants need more red light than blue. Chlorophyll, the main pigment found in plants, is most proficient in converting this red and blue light into the energy needed for photosynthesis. Light from the green part of the spectrum is mostly reflected, which is why leaves appear green to our eyes.

The kelvin scale is used to measure the color temperature of light sources. Based on the principle that a black body radiator, such as an incandescent light bulb, emits light and the temperature determines its color. CRI (color rendering index) measures the ability of a light source to reproduce colors of objects faithfully in comparison with a natural light source. True artificial daylight light sources should have a CRI value above 90, but not all do.

A light source that closely imitates sunlight is referred to as natural light or daylight. Pure sunlight has a temperature of around 5,000 degrees Kelvin and a CRI of 100. Unobstructed equatorial sunlight at noon is considerably higher, but factors such as time of day, cloud cover, pollution etc. affect the color temperature in reality. When sunlight and atmosphere combine in the northern hemisphere it forms a color temperature between 5000–6500k, which is slightly bluer than equatorial sunlight due to the atmosphere.

Technically bulbs leaning heavily towards the blue spectrum (6000-6500K) promotes vegetative growth, and warmer bulbs (3000K) promotes blooming. Using daylight bulbs (5000-6500K) is really the best option for both growing and blooming your orchids.

The picture shows an extremely simplified model converting kelvin numbers to nanometers of wavelength. The astute observer will notice that the peaks don’t exactly match… On the chart “red” peaks at 2200K, in reality the chlorophyll red peak occurs at 660nm, 4400K. I am using the image, with this disclaimer, since it still helps to illustrate the point.

Lux is based on lumen, lumen and foot candle are both based on candela.

• Lux (lx) = the amount of, or the intensity of light that hits the surface below the light
• Lumen (lm) = the power of light (the light output) as perceived by the human eye (1 lumen on 1 m2 = 1 Lux)
• Candela (cd) = the power emitted by a light source in a particular direction
• Foot candle (fc) = the amount of illumination inside the surface of a 1-foot radius sphere were one candela in the exact center of the sphere.

When you read about the light requirements for orchids, you often see it listed in foot candles. Full, unobstructed sunlight has an intensity of approximately 10,000 fc, an overcast day will produce an intensity of around 1,000 fc. But since no light sources are ever referred to in those units some *fancy math is required in order for you to figure out what you need.

Supplemental lighting is most effective when the ambient light levels are low. For example: on cloudy days in the winter, and in the early morning and late afternoon. Furthermore, if you half the distance from the light source to the orchid you quadruple the light (yep – it’s physics people). By adding a reflector you can ‘roughly’ double the light simply by focusing it better. Not much growing happens by the light lost out the back of the fixture…

There are three basic types of HIDs: mercury vapor, high pressure sodium, and metal halide. Many serious growers advocate the use of these. Although quite effective, I think that they are a bit impractical for many hobby growers. But the main strike against these lights in my book is that they do not focus on the spectrum of light that plants actually need (see PAR). Instead they work on the principle that since they put out so much power there is enough of the usable spectrum to make them effective grow lights. So, not only are they expensive to run, the bulbs can be expensive to replace. They also run very hot and distort color, sodium and mercury lights especially. Read more on HID technology.

I prefer using Fluorescent and Compact Fluorescent (CFL) for a number of reasons. They are low energy which means they are cheaper to run than regular incandescent bulbs (and certainly cheaper than the HIDs). They are also much more efficient. Fluorescent lights emit about four times as much light per unit of electricity compared to incandescent lights. They also run much cooler, and the CFL’s run even cooler still. In a small growing space the amount of heat produced by the lights really matter, so that is one of the main reasons I prefer compacts over regular T5 fluorescents. Read more on fluorescent technology.

Until recently fluorescents have not been efficient enough to be comparable to HID’s, but with the developments of compact fluorescents and T5 technology this has changed. For example, each 55w CFL bulb in my Nebula produces 4500 lumens:

• 4 CFL fluorescent = 220 watts (18,000 lumens, 81.81 lm/w)
• 250 MH metal halide (23,000 lumens, 92 lm/w)
• 250 HPS high pressure sodium (28,500 lumens, 114 lm/w)

Incandescent bulbs last about 1000 hours, halogen may double that at best, but these CFLs are rated for 20 000 hours or more. Most lights will lose some performance over time due to cathode decay, and fluorescents are no different. This is an exponential decay, so most tubes should optimally be replaced once every- to every-other year or so. The ones I buy can go 2-3 years – but that means you cannot wait until the bulb fails to replace it. Look at the lifespan rating on the light source, it tells you when you should replace the bulbs for them to be effective at what you need them to do – grow orchids!

I actually called and talked to a Mr. Lombard (the guy in charge of fluorescent lights at Philips here in Sweden), he explained that the lifespan rating for fluorescent lights are not calculated the same… Low energy CFLs (according to EU standards) are rated in the average life expectancy of the bulb (basically until it dies), while regular fluorescent tubes are rated to service life – in other words how long the tube performs as expected, not until it dies! Service life is obviously more interesting to know.

When calculating service life a good fixture with soft start technology is used and the light is left on for at least 3 hours every time it is turned on. Since we keep our grow lights on much longer (typically 12-16 hours per day) it increases the service life quite dramatically since the degradation effect is more limited.

My growing room gets very little direct sunlight, so my supplemental lights are on year round. Research has shown that a period of dark (minimum of 6 hours) is important to allow the plants to incorporate the food it has made during photosynthesis (when the lights are on). So I adjust my artificial lights according to the daily rhythm (night/day). Most of the orchids I grow come from Latin America where there is very little difference in light levels seasonally, so I just keep my lights on for about 12 hours a day year round.

In nature the sun is up for say 12 hours each day, but the light varies in intensity over the course of the day – compare dusk and dawn to mid day light levels with some cloud coverage or fog for example. Keeping the artificial lights on at a constant level for a longer period of time each day compensates for the lack of intensity by the lights compared to that of the sun. Keeping everything on timers is essential for your sanity. A cheap mechanical timer will do fine. I connect all my 7 fixtures to one timer using a power strip.

Ok, I am ready to figure it all out… how much power do I need? Simply put you could never have too much light. But for most windowsill gardeners a few CFL’s will do the job – we are talking about supplemental lighting after all – hopefully there is also daylight available to help pull the weight. But if you are planning something more involved these numbers could come in handy. The theory change all the time along with technology advances, but a rule of thumb is that in order to sustain a good rate of growth you will need:

• Minimum: 21 500 lm per square meter (2 000 lm per square foot)
• Mid range: 54 000 lm per square meter (5 000 lm per square foot)
• Optimal: 75 000–80 000 lm per square meter (7 000–7 500 lm per square foot)

Since I grow in a pretty dark north/east facing window, my supplemental lights are on year round, but are especially needed during the darker months out of the year. We only get about 6 hours of daylight in Gothenburg in January, so supplemental lights are a necessity.

I use two Nebula Hobby light fixtures in my tall window. One cover the top two shelves and a second one hangs about half way down and cover the lower two shelves and the nursery bench for the flask babies. I use the same fixture for my warm vivarium – what can I say, I like it. The reflector design creates a positive airflow, which helps to keep the heat away from both plants and gear. Complete with warmstart system for optimal lamp life as well as instant bright running. Unfortunately they do not come with a reflector, the fixture is painted white inside. But I just got some diamond mylar and glued them in place as reflectors for mine.

For the warm vivarium (75cm wide 80 cm tall 40 cm deep) I place synthetic wine corks between the light and the glass in the summer to help ventilate some of the excess heat generated by the light away from the viv. For really hot days I place an 80mm computer fan by the end of the light as well for the same purpose. In the winter the light help heat the small growing space.

Each Nebula fixture came with two 55W CFL tubes from Osram (Dulux L 55W/954) which amounts to about 6000 lumen at 5400K (slightly cooler than daylight) with a CRI value of 93% per fixture. They are rated for 10 000 hours, but has a service life of about 16 000 hours, in other words 3 years at 14 hr per day.

For the large cool vivarium (175 cm wide, 160 cm high and 65 cm deep) I use 4 Dulux 80W lights. Same technology as the 55W lights in the Nebulas, but they are quite new and I could not find any proper fixtures for them, so I made my own. I got electronic ballasts then bought reflectors for regular T5 tubes, but them in half and put them together to make a perfect reflector for the Dulux lights. The new 80W Dulux L 840 are very intense and especially suited for tall vivaria – up to 180 cm tall. Color rendering is 85% accurate. Each light put out 6000 lumen, so I get a total of 24000 lumen spread out over 175 cm width. At 4000 K the white light is close to the natural daylight range of the color spectrum. I would prefer a color temp closer to full spectrum, but with a tall vivarium like this I still feel this is my best option. They have a rated service life of 8000 hours, but with the kind of use I do, light replacement is recommended every 2,5 years.

• 1 fc is = 10.764 lux = 10 lumens per square meter (cool online converter)
• Lux is the amount of light that hits the surface below the light, Lumen is the light output of the light source
• Candela is the power emitted by a light source in a particular direction, and Foot candle is the amount of illumination inside the surface of a 1-foot radius sphere
• Adding a reflector “roughly” doubles the light by aiming the light better
• The service life is what matters with modern fixtures and bulbs… find out what it is and replace the light sources accordingly – not yearly “just because”…
• Distance matter even more… if you half the distance you quadruple the light