You get a feel for it after you've grown a while.

Me? I tend to dismiss fc's and look at It through the lens of someone who grows outside during the summer: at what point during the day can the plant receive direct sunlight and for how long?

Indoors, a window that faces anywhere but north gets you what you need for a variety of plants. Lower light plants can do east/west windows. Higher light plants can do west/south windows.

Everyone will have their own way of interpreting light but in general if you don't have a light meter, trying to think in terms of footcandles is pretty useless.

Hands down, for me, it's best understood trough feeling yor way around. Trial and error.

You would think that talking in foot-candles would make orchid growing easier and help communication between those growers with sunlight excess most of the year and those growers with a sunlight deficit most of the year.

Here, the conversation is in shade-cloth ratings.

I remember years and years ago after reading some of the posts here trialling some cattleyas in direct sunlight...mid-winter. They suffered sunburn!

Many of the experienced growers, the so-called green fingered ones, go by leaf colour.
Lemon green washed out appearance = too much light
Dark green and floppy = not enough light

This post was edited by arthurm on Sat, Feb 1, 14 at 23:00

An added dimension is the concept of footcandles hours. A plant needs 1000 fc's, but doesn't have to be throughout the whole day. That's the recommended peak amount. So if you keep it at 1000fc for 4 hours, that's 4000 footcandle hours. If it spends the rest of the daylight at 500 (let's assume a total of 12 daylight hours, so here 8), it will have received 8(500)+4000=8000 footcandle hours

Each genus will have a sort of maximum they need. It's not really defined what specifically it would be for each though

The implication of this is that intensity can be substituted for duration. Let's assume our 1000 fc plant does required 8000fc hours.
Let's say it doesn't wind up receiving 1000fc, but rather 800 for 4 hours.
The math shows that

(800fc*4hours)+(500fc*8hours)= 7200fc hours. So you are short 800fc hours and it won't bloom.

So let's extend our daylight with artificial lights! Instead of 4 hours at te peak 800, let's do 5 hours.

(800fc * 5 hours) + (500fc * 8 hours) = 8000 footcandle hours.

By extending the daylight one hour, your plant will bloom!

Hope this makes sense to you. It makes the concept of footcandles a bit more helpful. But again if you don't have a meter (and in the absence of defined footcandle hour requirements), it's still more fun factoid than helpful guidance.

Thanks to both of you for the insights. I was suspecting that ultimately it's more about trial and error and experience...

arthurm, I do have a light meter but I'm really not sure how helpful it is or can be. As you note, a "fc" number isn't useful because it doesn't give you any idea about how long the plant needs that exposure... and I *never* see culture notes talk about duration or about footcandle hours, which would be far more useful than throwing out a footcandle number. I suspect that in a lot of cases these numbers are sort of random guesses anyway or things that people parrot from one another that may or may not have started out as random guesses or rough estimates a long time ago.

I had two phals sitting in my south-facing window. One of them is thriving and growing two spikes, but the other one, which was getting the same treatment, developed a washed out yellow color and the roots in the sun started deteriorating. I had to move it to where it gets less light. They are different kinds of phals, but there is no way for me ever to figure out the exact kinds---supports the idea that it's just trial and error with these plants.

What's so frustrating for me about the light requirements of different Orchids is that unless I have an amazing light meter I pretty much have to buy an Orchid and try to grow it for a year or however long to see if it had enough light to bloom! It's discouraging. I have a huge east window that gets a few hours of sun, but I'm afraid to try anything besides phals and Paphiopedilums because I don't want to grow a genus that needs medium or bright light only to find out it is not getting enough light to bloom :/

The question of light is a hard one... no easy answers.

Being a windwosill and under lights grower my whole life pretty much where you almost never have enough light (except for phals which are shocking how they can bloom with such little light, if you can get everything else right that is), when I started growing outdoors i found out that orchids can burn very easily with just an hour of direct sun...

and another curve ball, it was interesting to see one year when i was in mexico I saw some schomburkgias (whatever their name is now) growing very close to trunks in trees fully covered by the leaves of the trees... and if you didn't know, schoms are known to handle the most light of the orchids. I was just reading up on dendrochilum tennellum, a grassy orchid from philippines, and I saw images of this plant growing in full full sun at completely exposed branches... researching, knowing how these plants grow en situ becomes insightful, but you have to consider the total scope of things... on one hand tropical rainforest situations often have cloud over even in exposed situations, whereas here in southern california there's never ever really any cloud cover so of course that explains why orchids will burn in full sun here... and on other hand, certain orchid species, and especially hybrids can grow in a seemingly wide variety of conditions... hence, growing by trial and error is that much more significant.

and on the third hand, growing on windowsills is tricky because even in heavily clouded forests there is light at all directions for the full length of they day. In the house, there's only light when the sun is at angle to enter windows... let's go back to consider en situ notes... if you grow species/miniatures like i do you read things like such and such is a twig growing species... what you can take from this is that, yes they are small and can grow on twigs, but twigs generally are up higher and away from trunks of trees therefore they will probably be more exposed to consistent all day light, whether high or low... and then you read such and such species grow on trunks of trees, which you can gather these must be ok with having sunlight only at parts of the day and like having lower light rest of day, afterall trunks shade the plants for at least half of the day... many phal species grow on trunks, if i'm not mistaken, many bulbophyllums being another... these are the types then that potentially can do well on windowsills. For those found on trunks, on branchs, high and low, you have more adaptability, also candidates for windowsill.

But for me I've almost done much better growing under lights when yes you can extend daylight hours of lower light to compensate for lack of duration of natural light indoors.

and one more note, in regards to understanding light levels through leaf color... certain orchids will get purplish/reddish spots/coloring in higher light levels, having those around are a good natural meter.