What full sun actually delivers
Peak noon
Clear early-summer sun can hit about 15,000 foot-candles (161,000 lux)
Plant ceiling
Cannabis likely cannot use more than about 7,000-7,500 fc (75,000-80,000 lux)
Day average
Dawn-to-dusk average is much lower, about 1,000-2,000 fc (10,700-21,500 lux)
PAR share
Only about 20% of sunlight is PAR. The rest becomes heat, infrared, or biochemical dump
Sun-loving crop
Fastest, strongest outdoor plants get unobstructed light all day. Move containers out of new shade
When sun is enough
Five-hour rule
Unobstructed sun at least five hours a day usually needs no summer supplement
Bright-enough check
About 4,500 fc (48,375 lux) for five hours or more is the book's bright-space target
Below that
Growth, ripening, yield, and quality all drop
Window and terrace
Direct autumn sun can be enough; bright-but-indirect light still needs a lamp
Seasonal angle
Some summer-shaded windows pick up direct sun in fall as the sun lowers
Autumn problems and fixes
Ripening clash
Day length and intensity both fall while flowers finish, so yield undershoots potential
Loose buds
Weak, oblique, or overcast autumn light makes airy, weaker flowers
Three options
Force flower earlier, add electric light, or grow earlier-finishing plants
Supplement rate
About 20-30 watts per square foot (215-320 W/m²) for about five hours during daylight
Coverage
A 1,000 W lamp can lift a square about 6-7 feet on a side
Lamp choice outdoors
Metal halide blends with daylight and is less obvious than amber HPS. Run it in the brightest hours
Camera as a foot-candle meter
Setup
White card at 45 degrees at canopy height. Adjustable 35 mm camera
Low range
ASA 100, 1/15 s. f/4 = 10 fc, f/5.6 = 20, f/8 = 40, f/16 = 160 (book table; f/11 row is printed as f/1 = 80 fc)
High range
ASA 400, 1/125 s. f/4 = 128 fc up to f/32 = 8,192 fc
Use
Rough-check a patio or window before you buy a dedicated meter