ENERGY TRANSFER - light is one of the ways in which energy is transferred. 
1) conduction - molecule to molecule 
2) convection - mass movement 
3) radiation - radiant energy transferred as electromagnetic waves 

light - light is the layman's term for visible radiant energy in the 400 to 700 nm wavelength region of the spectrum. In other words, it is the form of radiant energy (i.e. radiation) that animals can see. It is also the wavelengths of radiant energy that plants use in photosynthesis and for most other reactions that require light. 

LIGHT HAS FOUR PROPERTIES
1) quantity - the intensity or amount of light 
2) quality - the wavelength or color of light 
3) duration - determines the total amount of light energy received 
                     total amount of light energy = quantity x # hours of light 
4) photoperiod- the day length, or length of light in a 24 hour cycle, regardless of quantity. 

LIGHT CAN BE AFFECTED AS FOLLOWS
1) absorbed - when radiant energy (such as light) is absorbed it is converted primarily to
                          heat energy.
    re-radiation - heat energy is converted to radiant energy as long wavelengths in the 
                             infrared (IR) region of the spectrum. 
2) transmitted - when radiant energy (such as light) passes through an object unaffected, 
                         such as glass. 
3) reflected or scattered - when radiant energy (such as light) is "bounced off" an object, 
                                         such as a solid colored surface. 

The color of an object is the color (as determined by wavelength) of light that is transmitted or reflected. In other words, your eyes see the color that is not absorbed. 

1) photometer or common light meter (cheapest) - measures amount of luminance 

Expressed as:
a) foot-candle (ft-c) - 1 lumen per square foot 
b) lux - 1 lumen per square meter  

    1 foot-candle = 10.76 lux

2) quantum sensor - measures actual light intensity or light energy in the 400-700 nm  
                                 wavelength band. 

photosynthetically active radiation (PAR) - light intensity in the 400-700 nm wavelength 
                                                                    band that is used by plants in photosynthesis. 
Expressed as
a) microEinstein per second per square meter - mEs^-1m^-2 (400-700 nm) 
b) watts per square meter - Wm^-2(400-700 nm)

1) phototropism - response of plants to light 
    a) plants bend towards areas of higher light intensity. 
2) photosynthesis  
    a) light reaction - increases with increasing light intensity 
    b) stomata - C3 and C4 plants: open in light; close in dark 
    c)  stomata - CAM plants: open in dark; close in light 
3) temperature - high light intensity increases temperature due to: 
    a) absorption of radiation, especially IR; greater with darker colors 
    b)  greenhouse effect  
4) transpiration - greater in high light intensity due to heat buildup, but transpiration may decrease 
                           if it gets too bright then too hot, which will cause the stomata to close.   
5) sun versus shade plants

a) leaf structure

sun grown leaf
- thicker, due to thicker palisade parenchyma layer
shade grown leaf
- thinner, due to thinner palisade parenchyma layer
- therefore, higher proportion of spongy mesophyll
- larger size
- softer and more pliable

b) optimum light intensity

shade plants: have a low optimum light intensity
sun plants: have a high optimum light intensity

6) photooxidation - destruction of chlorophyll by high light intensity.
7) etiolation - elongated, pale green to yellowish growth due to low light intensity.
8) blanching - lack of color development due to exclusion of light; 
                         - used on cauliflower, asparagus and celery.
9) light acclimatization - conditioning of plants to low light intensity interior environments.

METHOD 1.  GROW PLANTS UNDER REDUCED LIGHT INTENSITY FOR THE ENTIRE PRODUCTION TIME (most common method used)
Common Name	Scientific Name	Suggested Light Intensity (ft-c)	% Shade (in summer)
Zebra Plant	Aphelandra squarrosa	1000-2000	80-90%
Chinese Evergreen	Aglaonema spp.	2000-2500	80%
Prayer Plant	Maranta spp.	"	"
Stromanthe	Stromanthe amabilis	"	"
Pilea	Pilea spp.	2000-3500	73-80%
Spathiphyllum	Spathiphyllum clevelandii	"	"
Peacock Plant	Calathea spp.	3000-3500	73%
Parlor Palm	Chamaedorea elegans	"	"
Ferns	Nephrolepis exaltata cv,	"	"
Peperomia	Peperomia spp.	"	"
Philodendron	Philodendron spp.	"	"
Christmas cactus	Zygocactus truncatus	"	"
Dumbcane	Dieffenbachia spp.	3000-4500	63-73%
Dracaena	Dracaena deremensis	"	"
Fragrant Dracaena	Dracaena fragrans	"	"
Arrowhead Ivy	Syngonium podophyllum	"	"
Asparagus Ferns	Asparagus spp.	3500-4500	63%
Ti plant	Cordyline terminalis	"	"
Weeping Fig	Ficus benjamina	"	"
Indian Laurel Fig	Ficus nitida	"	"
Splitleaf Philodendron	Monstera deliciosa	"	"
Snake Plant	Sansevieria spp.	"	"
Pothos	Epipremnum spp.	"	"
Norfolk Island Pine	Araucaria heterophylla	5000-6000	55%
Schefflera	Brassaia actinophylla	"	"
Madagascar Palm	Chrysalidocarpus lutescens	"	"
False Aralia	Dizygotheca elegantessima	"	"
Corn Plant	Dracaena marginata	"	"
Fiddle Leaf Fig	Ficus lyrata	"	"
Lg Leaf Philodendron	Philodendron selloum	"	"
Rubber Plant	Ficus elastica	5000-8000	30-55%
Croton	Codiaeum variegatum	7000-8000	30%

Recommendation: 
Greenhouses are covered with shade cloth to yield the desired % shade.  In the Rio Grand Valley of south Texas, the greenhouses commonly are covered with 63-73% shade.  The plants are grown the entire time under these shaded conditions.  The production time is a bit longer than growing in higher light.  The plants are typically a bit taller with larger, deeper green leaves.  They can be used immediately indoors and in interiorscapes. 

In this method the plants are grown in a greenhouse at very bright light intensity, often full sun.  The plants grow fast, are short and compact with lighter green leaves.  If placed directly indoors or in an interiorscape.
a) growth stops
b) leaves turn yellow, especially the older leaves, and
c) leaves fall off, especially the older leaves

So to acclimatize the plants, they are placed in a very heavily shaded greenhouse or a lighted warehouse in order to acclimatize them to low light intensities. 

Notice on the graph below that each plant started out with a certain light compensation point.  For example, Dracaena was the highest at about 120 ft-c and Schefflera was the lowest at about 15 ft-c.  Over time in the acclimatization treatment, each plant's light compensation point decreased.  This is showing their rate of acclimatization.  Notice that all the species acclimatized, i.e. even very shade tolerant plants acclimatized to even lower light intensities.    After about 4 to 6 weeks, the light compensation point does not decrease much more, so the light acclimatization process is virtually complete. 
 

 CONCEPT OF THE "ATMOSPHERIC WINDOW" 
The 400 to 700  nm wavelength band of radiation (visible light) from the sun passes through the atmosphere relatively unaffected before reaching the earth's surface, whereas other wavelengths (especially UV and  IR) are absorbed, reflected or scattered by the atmosphere. 

• ultraviolet light - absorbed by ozone 
• visible light - not selectively affected 
• infrared light - absorbed by carbon dioxide and water vapor 

Thus, the atmosphere acts as a window allowing visible light through relatively unaffected.  Plants and animals are adapted to utilize this relatively stable source of radiation in their photosynthesis, photomorphogenic responses, sight, etc.

civil twilight - reflected sky light that occurs approximately 1/2 hour before sunrise and 1/2 hour  
                      after sunset. 

Plants can detect civil twilight, so it must be taken into account when determining the photoperiod that plants perceive.  Plants cannot detect moonlight, so it does not effect the photoperiod plants perceive. 

photoperiod - the day length a plant perceives, which will be the absolute day length (time from 
                       sunrise to sunset) plus 1 hour of civil twilight. 
critical photoperiod - the photoperiod (absolute day length + civil twilight) above or below
                                         which the photoperiodic response is turned-on or turned-off.
                                      -each species has its own unique critical photoperiod that it "looks" for.