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Understanding Light Measurements



PAR and Plant Response Curve


  • Wiki Page - https://en.wikipedia.org/wiki/Photosynthetically_active_radiation
    • PAR measurement is used in agriculture, forestry and oceanography. One of the requirements for productive farmland is adequate PAR, so PAR is used to evaluate agricultural investment potential.
    • PAR sensors stationed at various levels of the forest canopy measure the pattern of PAR availability and utilization.
    • PAR measurements are also used to calculate the euphotic depth in the ocean.
      • The photic zone, euphotic zone (Greek for "well lit": εὖ "well" + φῶς "light"), or sunlight zone is the depth of the water in a lake or ocean that is exposed to such intensity of sunlight which designates compensation point, i.e. the intensity of light at which the rate of carbon dioxide uptake, or equivalently, the rate of oxygen production, is equal to the rate of carbon dioxide production, equivalently to the rate of oxygen consumption, reducing thus the net carbon dioxide assimilation to zero.

Just as humans need a balanced diet, plants need balanced, full-spectrum light for good health and optimum growth.

  • The quality of light is as important as quantity. Plants are sensitive to a similar portion of the spectrum as is the human eye.
  • This portion of the light spectrum is referred to as photosynthetically active radiation or PAR, namely about 400 to 700 nanometers in wavelength.
    • Nevertheless, plant response within this region is very different from that of humans.

The human eye has a peak sensitivity in the yellow-green region, around 550 nanometers. This is the "optic yellow" color used for highly visible signs and objects.



Plants, on the other hand, respond more effectively to red light and to blue light, the peak being in the red region at around 630 nanometers. The graphs below show the human eye response curve and the plant response curve. Note the vast difference in the contours.


  • In the same way fat provides the most efficient calories for humans, red light provides the most efficient food for plants.
  • However, a plant illuminated only with red or orange light will fail to develop sufficient bulk.
    • Leafy growth (vegetative growth) and bulk also require blue light. Many other complex processes are triggered by light required from different regions of the spectrum.
      • The correct portion of the spectrum varies from species to species.
      • However, the quantity of light needed for plant growth and health can be measured, assuming that all portions of the spectrum are adequately covered.

Light for plants cannot, however, be measured with the same standards used to measure light for humans. Some basic definitions and distinctions follow that are useful in determining appropriate ways to measure the quantity of light for hydroponic plant growth.



Measuring Light for Humans: Lumens and Lux

First, how do we measure light quantity for humans? The obvious way is based on how bright the source appears and how "well" the eye sees under the light. Since the human eye is particularly sensitive to yellow light, more weight is given to the yellow region of the spectrum and the contributions from blue and red light are largely discounted. This is the basis for rating the total amount of light emitted by a source in lumens.

The light emitted from the source is then distributed over the area to be illuminated. The illumination is measured in "lux", a measurement of how many lumens falls on each square meter of surface. An illumination of 1000 lux implies that 1000 lumens are falling on each square meter of surface. Similarly, "foot-candles" is the term for the measure of how many lumens are falling on each square foot of surface.


Quick Guide - Lux Levels for Optimal Cannabis Growth

Information from http://www.growweedeasy.com/lux-meter

Life Stage





70,000 lux

40,000 lux

15,000 lux


85,000 lux

60,000 lux

35,000 lux

  • 15,000 lux - sparse or "stretchy" growth - plant isn't getting enough light
  • 15,000 - 50,000 lux - good amount of light for healthy vegetative growth
  • 45,000 - 65,000 lux - optimal amount of light for cannabis plants in the flowering (budding) stage
  • 70,000 - 85,000 lux - a lot of light, some strains do okay at this light level, but some plants lose their top leaves early under this light intensity, especially plants that are not resistant to heat/light (like many indicas)
  • 85,000 lux - at this light intensity, you've hit the plant's "saturation point" which means your plant can't use all the light (watch for light bleaching)

On a clear summer day, the sun gives off something close to 32,000-130,000 lux in the direct sun. This is relative where you are located on a longitudinal axis. The closer you are to the equator, the more direct sun your plants are getting.

When growing cannabis, you don't really get any additional gains by adding more light to get over 85,000 lux. Not only is it the extra light wasted by your plants, too much light can actually give your plants unsightly light burn or cause it to lose its leaves early!

Clearly, both lumens and lux (or foot-candles) refer specifically to human vision and not to the way plants see light.

How then should the rating for plant lighting be accomplished? There are two basic approaches to develop this rating: measuring energy or counting photons. 


PAR Watts for Plants

  • Watts is an objective measure of energy being used or emitted by a lamp each second.
  • Energy itself is measured in joules, and 1 joule per second is called a watt.
  • A 100 watt incandescent bulb uses up 100 joules of electrical energy every second.
    • How much light energy is it generating? About 6 joules per second or 6 watts, but the efficiency of the lamp is only 6%, a rather dismal number.
      • The rest of the energy is dissipated mainly as heat. Modern discharge lamps like high pressure sodium (HPS) and metal halide convert (typically) 30% to 40% of the electrical energy into light.
      • Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts.
    • This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.
      • The output of a 400 watt incandescent bulb is about 25 watts of light,
      • a 400 watt metal halide bulb emits about 140 watts of light.
      • If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR.
      • A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye). "Illumination" for plants is measured in PAR watts per square meter.
        • There is no specific name for this unit but it is referred to as "irradiance" and written, for example, as 25 watts/square meter or 25 w/m2.




Another means of measuring light quantity for plant growth involves the understanding that light is always emitted or absorbed in discrete packets called "photons." These packets or photons are the minimum units of energy transactions involving light.

  • For example, if a certain photosynthetic reaction occurs through absorption of one photon of light, then it is sensible to determine how many photons are falling on the plant each second.
  • Also, since only photons in the PAR region of the spectrum are active in creating photosynthesis, it makes sense to limit the count to PAR photons.
  • A lamp could be rated on how many actual tiny photons it is emitting each second. At present no lamp manufacturer does this rating.

Instead, plant biologists and researchers prefer to talk of the flux of photons falling each second on a surface.

  • This is the basis of PPF PAR with PPF standing for Photosynthetic Photon Flux, a process which actually counts the number of photons falling per second on one square meter of surface.
  • Since photons are very small, the count represents a great number of photons per second, but the number does provide a meaningful comparison.

Another measure appropriate for plant growth, called YPF PAR or Yield Photon Flux, takes into account not only the photons but also how effectively they are used by the plant.

  • Since red light (or red photons) are used more effectively to induce a photosynthesis reaction, YPF PAR gives more weight to red photons based on the plant sensitivity curve.
  • Since photons are very small packets of energy, rather than referring to 1,000,000,000,000,000,000 photons, scientists conventionally use the figure "1.7 micromoles of photons" designated by the symbol "µmol."
    • A µmol stands for 6 x 1017 photons;
    • 1 mole stands for 6 x 1023 photons.
  • Irradiance (or illumination) is therefore measured in watts per square meter or in micromoles (of photons) per square meter per second,
    • abbreviated as µmol.m-2.s-1

The unit "einstein" is sometimes used to refer to one mole per square meter per second.

  • It means that each second a 1 square meter of surface has 6 x 1023 photons falling on it.
  • Irradiance levels for plant growth can therefore be measured in micro-einsteins or in PAR watts/sq. meter.

These three measures of photosynthetically active radiation, PAR watts per square meter, PPF PAR and YPF PAR are all legitimate, although different, ways of measuring the light output of lamps for plant growth.

  • They do not involve the human eye response curve which is irrelevant for plants.
  • Since plant response does "spill out" beyond the 400 nanometer and 700 nanometer boundaries, some researchers refer to the 350 – 750 nanometer region as the PAR region.
  • Using this expanded region will lead to mildly inflated PAR ratings compared to the more conservative approach in this discussion. However, the difference is small.





Photosynthesis and Photomorphogenesis

Plants receiving insufficient light levels produce smaller, longer (as compared to wide) leaves and have lower overall weight. Plants receiving excessive amounts of light can dry up, develop extra growing points, become bleached through the destruction of chlorophyll, and display other symptoms of excessive stress. Plants are also damaged by excessive heat (infrared) radiation or extreme ultraviolet (UV) radiation.

Within the acceptable range, however, plants respond very well to light with their growth rate being proportional to irradiance levels.

  • The relative quantum efficiency is a measure of how likely each photon is to stimulate a photosynthetic chemical reaction.
  • The curve of relative quantum efficiency versus wavelength is called the plant photosynthetic response curve as shown earlier in this section.

It is also possible to plot a curve showing the effectiveness of energy in different regions of the spectrum in producing photosynthesis.

  • The fact that blue photons contain more energy than red photons would need to be taken into account, and the resulting curve could be programmed into photometry spheres to directly measure "plant lumens" of light sources instead of "human lumens."

The main ingredient in plants that is responsible for photosynthesis is chlorophyll. Some researchers extracted chlorophyll from plants and studied its response to different wavelengths of light, believing that this response would be identical to the photosynthetic response of plants.

  • However, it is now known that other compounds (carotenoids and phycobilins) also result in photosynthesis.
  • The plant response curve, therefore, is a complex summation of the responses of several pigments and is somewhat different for different plants.
  • An average is generally used which represents most plants, although individual plants may vary by as much as 25% from this curve.
  • While HPS and incandescent lamps are fixed in their spectral output,
    • metal halide lamps are available in a broad range of color temperatures and spectral outputs.
      • With this in mind, the discriminating grower can choose a lamp that provides the best spectral output for his specific needs.

In addition to photosynthesis which creates material growth, several other plant actions (such as germination, flowering, etc.) are triggered by the presence or absence of light. These functions, broadly classified as photomorphogenesis, do not depend much on intensity but on the presence of certain types of light beyond threshold levels.

  • Photomorphogenesis is controlled by receptors known as phytochrome, cryptochrome, etc., and different plant functions are triggered in response to infra red, blue or UV light.


Summary of Light Measurement

Plants "see" light differently than human beings do. As a result, lumens, lux or footcandles should not be used to measure light for plant growth since they are measures used for human visibility. More correct measures for plants are PAR watts, PPF PAR and YPF PAR, although each in itself does not tell the whole story. In addition to quantity of light, considerations of quality are important, since plants use energy in different parts of the spectrum for critical processes.



Photosynthetic Considerations - for Horticulture Lighting


There are certain important considerations when choosing a light source for horticulture lighting. The first group relates to the wavelength range and amount of light needed for photosynthesis, which is the fundamental metabolic process in plants.

  • A common misconception is that since chlorophyll absorbs light predominantly in the red and blue parts of the spectrum (leading to the green color of plant leaves), green light is not used by plants in photosynthesis.
  • In reality, precise and independent measurements of the photosynthetic activity under different wavelengths by McCree and Inada have demonstrated clearly that green light is nearly as effective as blue light for a considerable number of crop plant species, with only small differences between their respective photosynthetic action spectra.
    • The short explanation for this experimental fact is that higher plants have evolved both biochemical and biophysical solutions (e.g. “antenna” molecules and light-trapping structures) to utilize green light better.

It is important to know that red light (600-700 nm) is almost twice as effective as blue (400-500 nm) light per incident watt, with green (500-600 nm) light in between.

  • This is primarily due to the fact that the number of photons each unit of light energy carries is directly proportional to its wavelength.
  • If the radiant energy is converted to photon counts and the photosynthetic yield is plotted in terms of the latter, the resulting curve becomes considerably flatter.
  • The explanation for this fact is that similar numbers of photosynthetically active photons are needed at various wavelengths to transform biochemically one molecule of carbon dioxide.

For this reason, botanists routinely use quantum meters, which measure the spectral region between 400 nm and 700 nm to obtain the incident photosynthetic photon flux density (PPFD). PPFD is typically expressed in micromoles of photons in the above wavelength range, falling each second per 1 square meter of area at the plant canopy level. This effectively means that the square is taken instead of the actual plant response curve.  

  • Although this approach over-estimates somewhat all of the blue and part of the green spectral regions (from 400 to about 550 nm) and entirely ignores the spectral regions below 400 nm and above 700 nm, it is adequate for most practical purposes.



You earned for finishing Section II



Click for - Section IV - Advanced Lighting Information and formulas.




~ Hempyfan, A proud HD writing.


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