Light Mismeasurement (Part 1)

Most farmers today use some sort of ‘light sensor’ to optimize plant
placement and spacing.  Everybody knows plant quality increases as the daily
light integral (DLI) in the greenhouse increases, meaning more light equals better
crops providing other environmental variables are in balance such as
temperature, humidity, and CO2 concentration.    When using a meter to
measure foot candles, remember the meter is gathering mostly green and yellow
wavelengths, which are the colors humans perceive as ‘brightest’.  When using a
meter to measure PAR, remember green and yellow hog up to 50% of your reading.
Light meters express the amount of measured light using different variables
including PAR (measured in micro-moles of quanta per second per square meter
µmol s-1 m-2 (measuring 400nm-700nm) for plants, Foot candles Lux (for
people), and in Watts/m2 (for solar radiation).
Meters displaying foot candles are measuring mostly the green and yellow
quanta from a light source.  Meters displaying µmol s-1 m-2  for PAR are
measuring all the visible light  between 400 and 700 nanometers, as this is the
range of quanta used by plants for photosynthesis.   A 42-LED Panel filled with
green “550nm” LEDs will register up to 20 times the FCs as the same Panel filled
with “680nm” red LEDs.  Light meters are designed to measure broad spectrum
light (Sunlight, HID, Fluorescent), and not LED grow lights.
Green is perceived brightest by the human eye, so it makes sense to measure
foot candles for offices, classrooms, and assembly benches, but not for
greenhouses.  Since HIDs  and fluorescent tubes were  originally developed  as
area lighting, these less absorbed wavelengths came along for the ride when the
lights were re packaged for the greenhouse industry in the 70′s.  LGM-5 covers
up to 12sqft per 9 watt bar in a greenhouse, and up to 1ft x 3ft over plants
indoors.  The new lighting variable is LGM5. Get it, grow it, love it.

Advertisement