Amount of Light

Can anyone point me to a thread or explain to me the correlation between foot candles and percent light? Light meters read in foot candles, but most orchid info describes amount of light in percent. How do the two measurements relate? Thanks!

Tami

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Plants respond to the length of day and to the amount (intensity) of light received during the day. Each species, maybe even each plant, has its own minimum amount and maximum amount of daily sunlight accumulation. On sunny days, light intensity varies over time, as the sun travels from east to west. It is possible to have 900 fc in the morning, 6000 fc at noon and 900 fc in the evening with a waxing and waning in between. Also, bright full June sunlight is more radiant than bright full March sunlight. One must consider location as well. Toronto receives more of a glancing blow (less radient) from the sun while Florida gets hit on the top of the head. I gather that light exposure is cumulative. There are a couple of ways to consider exposure. One way is to estimate the percentage of the day that the plant receives the 6000 fc. Another way (photoperiodism) is to calculate exposure over time. Light intensity (measured in foot-candles) times the duration of the light intensity (measured in hours of exposure) equals foot-candle hours. A schefflera supposedly prefers bright light with some direct sunlight: a daily accumulation of 20,000 to 35,000 fc/hrs in the spring and 25,000 to 65,000 fc/hrs in the summer. An african violet likes spring 10,000 to 20,000 fc/hrs and summer12,000 to 25,000 fc/hrs.

I'm wondering about this, too. I'd like to know how to relate foot-candles to indoor lighting.

If you have ORCHIDS THROUGH OUR EYES, look on pages 99-100. It tells how to use your camera to determine the amount of light in your home or grow area. The average living room has between 30 and 80 footcandles. Typically orchids need between 800 and 3000 footcandles of light to thrive and bloom. Direct sun light would be in the range of 50,000 footcandles so you can see why orchids burn in direct sun light. Check it out - especially if you have a manual camera. It is interesting to see the differences from one area of your home to another. It is also important to note that perception of brightness is a subjective matter that varies from person to person, so using a light meter or your camera is the best option for finding the right light for your plants.

Cheers,
BD

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We could learn a lot from crayons: some are sharp, some are pretty, some are dull, some have weird names, and all are different colors....but they all exist very nicely in the same box. ...Hmmm? .

This is one of the reasons why I prefer documentation that indicates light intensity in terms of empirical measurements. Foot-candles, lumens, etc, at least those we can rely on to be consistent across the board regardless of where we are.
1500fc in Winnipeg is the same as 1500fc in New Jersey. %shade/%light however, thats an entirely different can of worms.

One of my pet peeves is someone documenting an orchid's culture sheet and provides %shade or %sunlight, and then omits where they are growing (or what is the value of their 100% direct sunlight).
"50% shade" does not help me much if I dont know what 100% light is equal to in the first place.
That is like someone telling me to "Drive along highway 50 then turn left." Turn left where?
I have yet to see any official documentation from the AOS, RHS or whoever telling us "OK everyone, 100% sunlight equals 15000fc as far as culture sheets are concerned."

Direct sunlight at a high elevation region along the equator could be in the tens of thousands of foot-candles, while direct sunlight in my local garden tops out at 8000fc. Heck, my next door neighbor's garden could top out at 7000fc courtesy of my house blocking most of his southern sun.

Some documentation could state "this plant needs 50% shade". 50% of what? This is only half the information that we REALLY need.
They may be growing in an area where full sun only amounts to 5000fc. So their 50% shade means the plant is getting 2500fc.

If I give that same plant 50% shade in MY garden, then it would be receiving 4000fc. More than likely I would burn the plant, even though I faithfully followed the culture sheet.

So going back to your original question "How do %shade and foot-candles relate to each other?"
The short answer: Its all relative.
The long-ish answer: First find out what is the value of their 100% sunlight in terms of foot-candles. Then apply the shade multiplier to calculate how much foot-candles make it through to the plant. Based on that information, and your local sunlight intensity, you can calculate how much real shade% you need to apply to your area to achieve the necessary foot-candle readings.

Thanks all, for your answers. I guess I was expecting 7000fc = 30% shade. The answer that it's relative to the growers location is something I'd not considered; just thinking of my little corner of the world. I had not considered the length of exposure as well.
So, based on your replies, I can calculate the light intensity and time of exposure and come up with a number that may or may not be accurate for my orchid based on whether or not the information I am comparing it to is relative to my location. Right?
I think I should stick to "yellow/red/burned leaves = too much light; deep, dark, green leaves = too little light" as that has seemed to work so far!
Seriously, I appreciate you all helping me better understand how light measurements are generated, and how better to apply what I can measure to my growing conditions.

Tami

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Life's a journey, not a destination.
Enjoy every day.

Knowing even more than what I posted above, I agree with and follow your approach 100%!! It's not our job, it's our relaxation, right? Still, a neat question and a fun thread. Thanks.