A Gardeners Forum Forums Gardening Reference Gardening in 2004 Q for the scientifically minded of us...

My friend (a new gardener) said he went to various sites for help starting his garden. He came across several sites that group houseplants into, well... groups according to their light requirements. <100 ft candles, 100-200, 200-500, and >500. He asked me how to go about measuring foot candles. I know how to do so on artificial light such as lightbulbs, but I don't know of any ways to measure the intensity of natural sunlight. Does anyone know how to guestimate the intensity of natural sunlight without a $200 light meter? Thanks y'all.

A quick search of the web would lead me to believe that direct sunlight (at the equator I guess) is equal to 10000 foot candles. Also 1 foot candle is equal to 1 lumen/ square foot. Don't know how much location on earth affects it, but the farther from the equator one gets, the less direct the sun there is.

Hi!

According to an old book I picked up:
Sunny South-facing windows = 4000-8000 foot candles
East or West-facing windows = 1000-3000 fc
North-facing windows = 50-500 fc

This is just a guideline and will depend greatly on the layout of your house (for instance, if you have a south facing window that has a lot of trees blocking light it will be on the low end of the range or slightly below). Also, it might partly depend on where you live. I'm not sure about this, but it stands to reason that you would get fewer fc's if you live far to the north and more if you live to the south.

Hope this helps!
Applescruff

That is a real weird way of categorizing the light plant needs. I am actually not sure if that would be very accurate. Please find a website that categorize s plant in more common sense way.