1. What type of light meter are you using? Incident or in-camera?
2. According to optimara, av's need between 900-1200 foot candles, which is roughly 1/10th of bright direct sunlight.
3. DH's a photographer, I can ask him to explain it properly (my attention span is too short to ever actually 'get' the light metrics :-p )
4. What does the meter read where you currently have your plants?
5. Light decreases exponentially, so if you take a reading twice as far from the source, you (should) have 1/4 the light - which means it really, really matters to measure in the actual spot.

Karin

Here is a link that might be useful: Optimara plant care (lighting) guide

Karin:
Are you saying that if I had a 1000 foot candle artificial light setup, x distance above the plants then the light measured at the plant would be 1/4 or 1000/4 - 250?

Does anyone know if that could be correct?

"According to Optimara" may be for florescent lights. That is the question.

I read on other University sites that light should be 300 foot candles at the point where the light hits the plant,

Anyone know? Anyone growing in natural light?

This post was edited by lucky123 on Thu, May 29, 14 at 23:14

Foot candles are measured where it hits the plant. (Incident light)
If you have one plant at 12" from the light source in 1000 foot candle intensity light, then the plant at 24" away from the light sits in 250 foot candles.
I think in indoor gardening some people consider "window" to be a light source, rather than "sun" which would make calculations ridiculous.

If you go by the power emitted by your light setup, then you have to ask someone else for the proper calculations....

When you read up on it, make sure to pay attention as to where thy measure, and which unit of measurement they use.

As for natural vs artificial - natural light has a broader spectrum, so it would probably pack more energy - whether all that energy is in a range the plant can use is another question. Natural indirect light would be stripped of some energy though.
So if you want to get technical, find out what range of light av's are able to use, then calibrate your light meter to that.

This post was edited by froeschli on Fri, May 30, 14 at 8:16

Karin
Getting Technical Using Low Tech
The canary in the coal mine is the wax begonia.

Basically the wax begonia and AV's have similar temperature and humidity requirements (and similar bug issues)

The wax begonia requires abt X2 light more than AV's.

Therefore if the wax begonias are growing, blooming and generally thriving at 500 foot candles of my sun measured on my meter,

then in my sun, measured on my meter, my AV's should grow and bloom abt 250-300 foot candles

What do you think? I am wary of trusting my plants to reckless experiments.

too little light is easily remedied, so i wouldn't call it a reckless experiment if you went ahead and tried it....

i have my "nursery" (leaves and small plants) under a small kitchen fluorescent, and they seem to do ok, even though i can tell by eye that it is a warmer light and less intense. (ha! i should try the ones that bleach easily under that light...)

ok, back to work...

I grow my plants under artificial lights and the lux meter reads 2000-3000 lux level with the plants. So it is 200-300 foot candles. Yet the plants flower and even grow seeds normally. For this I had written to Philips since I use their T8 lighting and the reply was 1000 foot candles. I guess the lighting requirement for AVs is as diverse as the different species.

I actually ended up doing a google search on this topic, and the suggested numbers vary greatly (200-1200 foot candles). A few articles that make a bit more sense though, relate the light intensity with the duration. (Summing up the total amount of light the plant gets - probably not a linear equation though)

So if your window light peaks at 1200 foot candles, that is fine if you don't have too many daylight hours. Remember, natural light changes constantly throughout the day, so the high would be 1200 but the average probably quite a bit less.
If you have an artificial setup at a lower intensity, you can make up for it by adding extra hours. The opinions differ, but apparently av's need a total dark time from 6-8h a day.
That's how i understood It anyways. I had a link but lost it... I'll post t tomorrow...

I use an old fashioned method to tell how bright the light is.
I just check the light by holding my hand about 20cm / 10" away from the plant, between it and the window.
If the shadow is obvious, but diffuse, the light is good.
If the shadow doesn't make much difference, then it's too dark.
It's not very technical, but it seems to work.

If one of my plants looks like it's not getting enough light, I move it to a slightly brighter spot.

Great suggestion