Angraecum leonis

Saturday, July 14, 2012

What's Light Got To Do With It?


     NOTE: This post is to give anyone an idea how I determine the quality and quantity of light for my plants.  The conditions that you reside in will vary in different amounts.  Use this as a stepping stone to determine the light for your plants!
     At every lecture/presentation that I've done over the last four years, one of the most common questions has dealt with light.  As I have said numerous times, "Give an Angraecum what it requires and it will reward you with years of some of the most unique flowers you can encounter in the orchid world".  So keep in mind that there are Angraecums that require bright light (direct sunlight to some extent), there are some that thrive in medium to dark shade and then there are some that don't really seem to care and will do well in just about any type of light.  Use a specific culture sheet to help guide you!
     The easiest way to read the light reaching a plant would be to use a light meter of some sort.  There are several that will give you an accurate and/or usable reading in foot candles (the measurement that seems to be used by many professional growers when needing a unit measurement).  I have one called the "Rapitest" that has been dependable and did not break the bank as far as cost was concerned.  I purchased it at an orchid grower's nursery.

"Rapitest" Sun Analyzer

     It is easy enough to use.  Point the meter towards your light source, multiply the number the needle shows by 1,000 and you have a usable unit of measurement in foot candles.  Keep in mind that the numbers will vary depending on the time of year (seasonal changes in the position of the sun).  The north and south light will change the most.  Summer light from the north will be higher than winter's light and the light in the south will increase with the sun in the southern sky.  East and west light will have a minimal amount of change to the light. [Added May 12, 2013 - one of the problems that has come to my attention with the "Rapitest" Sun Analyzer is that it seems to loose some of its ability to accurately read the light after about 12 - 18 months; both of the "Rapitest" meters have dropped about 10% of the reading it gathered when newly purchased.]
     Another way to measure light is with a traditional photographic light meter that can give you an EV (evaluation) reading.  I use Gossen's "Luna-Pro F".  The meter is used in the ambient light mode (light that will reach your plant).

 Gossen "Luna-Pro F" Photographic Light Meter

     Set the ASA/ISO at 100.  Face the white dome toward the light source.  Press the larger red button on the left side of the meter in once (the smaller red button should be in the "IN" position) and then turn the dial until the needle is at the center mark "0".  The EV number will be at the bottom of the dial under the down pointing diamond mark.  The chart below contains the EV conversions to foot candles.
EV 9 - 119 fc
EV 10 - 240 fc
EV 11 - 476 fc
EV 11.5 - 673 fc
EV 12 - 951 fc
EV 12.5 - 1,345 fc
EV 13 - 1,903 fc
EV 13.5 - 2,691 fc
EV 14 - 3,805 fc
EV 14.5 - 5,382 fc
EV 15 - 7,611 fc
EV 15.5 - 10,763 fc
EV 16 - 15,221 fc
EV 16.5 - 21,526 fc

     Then again, you can always set the technical information aside and just read the light by eye.  The following list is what I use with most of my plants.
Heavy Shade; 150 - 275 fc
Medium Shade; 350 - 500 fc
Bright Shade; 600 - 900 fc
Heavy Spackled Sun; 600 - 1,200 fc
Medium Spackled Sun; 1,200 - 1,600 fc
Light Spackled Sun; 1,600 - 2,000 fc
Full Summer Sun AM; 2,500 - 5,500 fc
Full Summer Sun NOON; 5,500 - 8,000 fc
Full Summer Sun PM; 8,000 - 6,000 fc
*NOTE: These numbers represent summer foot candle measurements and will vary at various altitudes, various seasons and air qualities.  Be sure to keep a close eye on your plant placements and to watch for sun burn.  These numbers were also derived for my plants in South Florida (a sub-tropical climate in the northern hemisphere).  


  1. Thanks for the translations on converting a light meter reading to a footcandle measure. I like the regular light meter method, but I'm glad you posted the cross-over wording. I'll see you at OSCG in September ! Keep up the work, I'll be reading !

    Craig Morell
    Pinecrest Gardens

  2. There's an app for iphone users that detects light in footcandles for plant growers,


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