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Light intensity is measured / expressed in various ways.  One of the oldest measures of light is the foot-candle (abbreviated fc).  
A fc is an old English measure. It defines the light projected by a standardized candle over an area of one square foot from a distance of one foot.  Back then, before electricity, that’s how people read, by the light of a candle.  
Another way to measure light is by Klux (stands for kilo lux or 1,000 lux). The lux (plural luxes, abbreviated lx) is the International System unit of illumination, equal to one lumen per square meter.
One fc is also one lumen but per square foot.  One Klux equals approximately 92.3 fcs.
Other methods for measuring light will be in mE/m2 = milli-einsteins per square meter per second, or in Watts/m2 = watts per square meter,...
Most books about growing orchids refer to light levels in fcs, so that’s the measure we’ll use for this chapter.
Orchids in the wild are widespread, growing in the dim light of a rain forest or in the bright light of a mountain slope. Therefore we’ll encounter orchids that need relatively low light conditions, other that need moderate light and some that need bright light.
I often hear people say “I have my orchids in a very bright location, it’s by a north facing window” or “my orchids get plenty of light, there are in the middle of the room under a skylight”.
Sorry folks but what your plants are getting is what we, humans, consider bright light, but this is far from being bright light for your plants.
On a bright early September day, at noon,  I measured the outside light and the light in a south facing kitchen.  The outside light was 8,000 fcs. The south facing kitchen  has a sliding door opening to the south.  It has a 2 1/2 feet by 5 feet window facing south and it has a 3 feet by 5 feet skylight in it.  The walls are light cream and the floor is light green-white tile. There are no trees or tall buildings to reduce the light entering this room. This is a bright room by any measure.
Here are the results in fcs:








I also measured the light at a north facing window in the living room.  The living room has an true east window, a south facing sliding door and a large window on the north side.  Here are the light levels:




As these measurements show the light levels drops very fast with as you move away from the sources of light and there is a vast difference in the amount of light depending on the exposure of the window.  Remember none of these windows is shaded by trees or other buildings.
Let me try to illustrate how light diffuses with an example. Let’s assume we build a pyramid whose base is 8 feet by 8 feet and it’s height is 8 feet.  The construction is such that no light escapes from the inside and no light penetrates from the outside.
At the inside top of this pyramid there is a 1,000 watts light bulb emitting approximately 9,230 fcs of light (now remember that’s the measure of light on a one square foot area at a distance of one foot from the light source).  
At the base of the pyramid the area illuminated by this bulb is 8 feet by 8 feet = 64 square feet.  As it is the same amount of light (9,230 fc) that reaches this 64 SF area, the illumination in fc at the base of the pyramid is 144 fc (9,230 : 64).
Here is another way to illustrate this. Let’s assume we have a square basin 1 foot wide and 1 foot tall. The volume of this bucket is one cubic foot of water. Let’s imagine for the sake of our discussion that the height of the water can be converted into imaginary foot-candles and that 1 foot of water height equals 1 imaginary foot-candle. So if we measure the height of the water in the bucket we’ll find it to be one foot and therefore one imaginary foot-candle.
Now let’s pour our bucket of water in a square basin whose sides are two feet wide. The surface of this basin is 4 square feet. Simple math’s will show that the height of the water in this basis will only reach 1/4 a foot which translates into one quarter of our imaginary foot-candle.
And if we poured this same bucket of water in a square basin whose sides are 3 feet wide, thus with a square surface of 9 square feet, the height of the water will only reach 1/9 of a foot which translates into 1/9 of our imaginary foot-candle.
Light, like the water in our basin, will fill all the available space at any given area. Since the total amount of light is the same at any given area, the further away we are from the light source, the larger the illuminated area is and the less amount of light will fall on any given square foot of area.
You can measure light with a light meter. Good ones are available between $ 80.00 and $ 150.00.  Some measure light in foot-candles and others in luxes of Klux’s  
You can also use a 35 mm camera to measure light.  Here is how to do it:
• Set the camera at 25 ASA.
• Set the shutter speed at 1/60 of a second.
• Place a white sheet of paper where the leaves of the plants would be.
• Focus on the white sheet of paper from a distance of 1 foot.
• Make a note of the f/stop shown in the viewfinder.
Here are the approximate foot candles corresponding to various f/stops:
f/2 =    100 fc
f/2.8 =    200 fc
f/4 =    375 fc
f/5.6 =    750 fc
f/8 = 1,500 fc
f/11 = 2,800 fc
f/16 = 5,000 fc.
Remember the maximum light should be at noon on a bright summer day.
Let’s start with the maximum light : at noon, on a bright, cloudless summer day, the light level outdoors (in the northeast) will top 10,000 fc.
We usually consider the light in a supermarket to be bright.  If we measured that light level with a light meter we’ll find it’s intensity is about 200 to 300 fc, and for us, humans, these 200 or 300 fc of light intensity in the supermarket are quite comfortable.
But 200 to 300 fcs of  light is just a pittance for orchids.




Please keep in mind these levels of light are for mature plants and the maximum light is for a limited time at the brightest moment of the day (noon in a bright, cloudless, summer day).
Please also keep in mind you can acclimate some of your orchids to grow a little outside of these ranges, within reason of course.
The book “An Introduction to Orchids - A Guide to the Growing and Breeding of Orchids” published by The South Florida Orchid Society(Tel 305-226-4757) and the book “All About Orchids” published by Ortho Books both have very clear charts about light requirements of orchids.
In practical terms what this means is that to provide the minimum light our orchids need to grow and bloom we’ll have to grow them :










Please keep in mind we are talking about windows that are not shaded by trees or other buildings.
Your orchids will, to some degree, adapt to their environment.  You may be able to coerce a medium light plant to grow and bloom on a bright east windowsill but you’ll have a real hard time blooming a high light plant on an east windowsill and very few, if any, will bloom on a north windowsill.
Orchids will tolerate levels of light on the higher end of their range provided the plant receives more water (more frequent watering), more fertilizer and, if possible, better air movement to keep their leaves a little cooler.
When we say more frequent watering we mean maybe every five days or so instead of every seven days, not every other day instead of every seven days.
This does not mean you should subject your plants to excessive light. If your orchids get too much light and not enough water and fertilizer, they will be stressed.
Stressed (weakened) plants have less defenses against pests and diseases the same way as weakened humans have.
Visible signs of stress are shriveling pseudobulbs and / or leaves, drying buds, prematurely wilting flowers,...  Note that this kind of stress can also result from unproper potting, decaying potting material, insufficient watering,....
Watch the foliage of your plants. If the leaves stay green, are crisp and firm, then the light is probably right.
If the foliage is dark green, then the light is too low.
If the foliage shows purplish marks or coloration, then the light is probably too high.
Sometimes if the light is too high the tips of the leaves will dry up.
Very few orchids will tolerate direct sunlight, except maybe for an hour or too after sunrise and an hour or two before sunset.
If you grow your orchids at a south or southeast or southwest location, in a sunroom or in a greenhouse, you’ll have to provide some shade, at least for the brightest part of the day.
Surrounding trees or tall buildings may provide enough shade, sometimes too much shade.
And remember, there is a substantial reduction in the light from summer to fall to winter. Provide more light / less shade as from mid October to mid February to compensate for this natural reduction in light.
Finally remember that although plants will adapt to changing environments, they will adapt better, with less stress if you gradually ease into the new conditions (such as summering them in bright outdoors light).  
My personal experience is limited as the only plants I grow under lights are my flasks, but here is some information to get you started.
If you look at a rainbow which diffracts the components of light you’ll see the various colors of the spectrum of light.
Natural light comprises ultra-violet, violet, blue, green, yellow, orange, red and far-red light. And there also is invisible infra-red light.
To us, humans, the green and yellow parts of light are what is important for our vision, but these colors are not that important to plants.
For plants, blue, red, far red and infra-red light are the important parts of light.
The blue and red rays of light are necessary for plants to photosynthesize.
Red light affects maturation, ripening and dormancy of plants. Far red is necessary for  plants to grow.  Far red and infra-red affect cell activity, stimulate plant growth and development of new leaves and roots.
If we plan on growing under lights we have to pay attention to the spectrum of light emitted by the lamps.
Horticultural lamps have been devised to meet as best as possible the types of light necessary for plant development which of course is not true for lamps developed for human use as our light needs are not the same.
Horticultural lights are available as fluorescent lights and bulbs.
Fluorescent lights are mostly rated in the 20 to 40 watts which translates into something like 185 to 370 fc of light.
Using several fixtures together will barely produce enough light to grow low light orchids (Phalaenopsis, mottled leafed Paphiopedilums,...) and then only if  the lamps are set only 8” to maximum 12” above the foliage.
As their name indicate these lamps emit substantially more light than fluorescent lights. There are lamps of 100, 250, 400, 600 and even 1,000 watts, emitting anywhere from 1,000 to over 9,000 fc.
These bulb lamps require special lighting fixtures that can cost anywhere from $ 150 to over $ 500.
Replacement bulbs cost anywhere from $ 35 or so for a 100 watt lamp to over $ 100 for a 600 watt lamp.  Most of them are rated for 10,000 hours of use which at 14 hours per day will last more than 2 years, so the cost of a 600 watt bulb is something like
$ 0.15 per day)
There are several brands available (HydroFarm, Sun Systems, Philips Lightning,...).
These fixtures can be combined with a rail system which moves the fixtures back and forth on a straight line or that rotates the fixture allowing a greater growing area.
Keep in mind the properties of light : at 2’ from the lamp you’ll get 1/4 the light emitted by the lamp and at 3’ you’ll get 1/9 of the light emitted by the lamp.
Many people have great success growing under lights, in poorly lightly rooms, in basements, ...
The biggest challenge of growing under light is managing the day & night temperatures, the humidity and the air movement which is necessary to prevent hot spots on the leaves.
Some of the information provided above was derived from the book “Growing Orchids Under Light” (by Charles Marden Fitch, published by the American Orchid Society, Tel 561-404-2000, web: orchidweb.com) which I suggest you buy if you are interested in growing under lights.
Also you may want to ask for the Charley’s Greenhouse and Garden catalog ( Tel 800-322-4707, web : charleysgreenhouse.com) or for the Worm’s Way catalog (800-274-9676, web : wormsway.com) as both feature an extensive line of horticultural lights and light fixtures.

Against the unscreened pane
Against the screened pane
1 foot away (in the sunlight)
2 feet away (in the shade)
3 feet away (in the shade)
4 feet away (in the shade)
10 feet away, under the skylight
By the sliding door
6,000
4,000
3,000
600
500
400
less than 200
By the window
6,000
4,000
3,000
600
500
400
less than 200

Against the unscreened pane
Against the screened pane
1 foot away from the window
By the east window
800
500
450
By the north windo
400
200
Not measurable
To grow and bloom properly :
• Low light orchids need  
• Moderate light orchids need
• High light orchids need
Low end
1,000 fc
1,500 fc
3,000 fc
High end
1,500 fc
3,000 fc
4,500 fc.
Low light orchids 



Medium light orchids


High light orchids
-on the windowsill of an east window
-or within 1 foot of a west windowsill
-or within 2 feet of a south windowsill
-or within 1 foot of a south windowsill if it is shaded by other plants or by shade.
-on the windowsill of a west window
-within a foot of a south windowsill

-
on the windowsill or within a foot of a south windowsill,
 (as long as they do not get direct sun in the middle of the day).