Light spectrum, PAR and lumens
Light consists of photons, which are small particles moving at the speed of light. The photons in light rays vibrate at different frequencies and wavelengths. Humans can see photon emissions within wavelengths of 380-680 nm, while plants' sensitivity to light ranges from 200-800 nm. Not all wavelengths within this spectrum have the same effect on photosynthesis; those that most strongly activate biochemical processes in plants are called PAR (photosynthetically active radiation), whose spectral range is 400-700 nm.
Light can be measured using photometric quantities derived from the human eye's sensitivity to colour, or radiometric quantities related to the energy carried by light rays. Lumens and lux are commonly used photometric units that refer to light perceived by the human eye.
However, photometric measurements do not show how much light energy a lamp sends to a plant for photosynthesis because they do not include PAR variables. Nevertheless, lumens and lux can provide an initial indication of a particular bulb's output, and we can compare different products by measuring their photometric efficiency using the lumen:watt ratio, regardless of possible differences in light-spectrum intensity at different wavelengths.
Lux and lumens work well when measuring lights such as MH, HPS and CFL, but they will not accurately measure the effectiveness of LED light for growing plants.
Generally speaking, the minimum amount of full-spectrum light a tomato plant needs, for example, is around 9000lm per square metre, while the optimum is over 20 000lm/m². However, the growth and flowering of a vigorous indoor plant depend on light thresholds reached at particular wavelengths that trigger photosynthesis. This is why LED grow lights use radiometric systems to measure the number of photons emitted at the PAR colour frequencies mentioned earlier.
The most widely used radiometric measurement in horticulture is PPFD (photon flux density), which measures the flow of PAR photon micromoles per second per square metre (μmol/m²/s). Given a grow light's PPFD, we can calculate its radiometric efficiency and compare different lighting systems using the PPFD:watt ratio.
SUN, BULBS AND THE RIGHT PHOTON PRESSURE
In midsummer, the sun reaches the Earth with a photosynthetic output of 1200-2000 PPFD. However, seedlings, clones and mother plants are content with PPFD of only 200-400μmol/m²/s. Tomato plants in their vegetative phase need 400-600μmol/m²/s, while flowering plants generally need 600-1500μmol/m²/s PPFD .
A study found that the most productive PPFD for tomatoes is 1500-2000μmol/m²/s at a temperature of 25-30 °C with natural CO₂ increased to 750 ppm.
Although tomatoes are demanding plants, irradiance above the threshold limit for each variety, life stage or environmental condition does not necessarily increase yield. On the contrary, excessive photon pressure can cause damage to leaves and flowers. In other words, flower production increases when tomatoes receive 20-30 moles of PAR light per day, then levels off between 30-40 moles and declines above 40 moles.
HOW MUCH LIGHT DOES YOUR GROW ROOM NEED?
To determine the right amount of light for your crop, multiply the length of the grow room by its width to obtain the growing area, then multiply the resulting figure by the desired PPFD level.
If your grow room is 250 cm long and 80 cm wide, the growing area will be 2,50 m × 0,80 m = 2m². If you are aiming for a flowering-plant canopy of around 2 square metres and want to experiment with a PPFD level of 500 μmol, you simply need 1000 μmol/m²/s. This result must then be divided by the PPFD per watt of light to determine the required power.
