Plant Science


Light and Plant Growth

Photosynthetic Active Radiation spectrum

Artificial light is used for plant growth in three different ways: 1. To provide all the light a plant needs to grow, 2. To supplement sunlight, especially in winter months when daylight hours are short. 3. To increase the length of the “day” period in order to trigger specific growth and flowering. While visible light provides plants the primary energy to support photosynthesis, energy wavelengths slightly beyond visible (including UV and IR) also play a role in plant development and their respective health. Sunmaster lamp spectrum are specifically tailored to deliver weighted areas of visible, UV and IR in order to support specific growth phases of plants.

Visible Energy

  Photosynthesis involves the green pigment chlorophyll and generates oxygen as a byproduct. Plants use a spectrum slightly wider than the human eye can see, from 400 nm (blue light) to 700 nm (red). Different wavelengths of light are used for different stages in the growth cycle of plants. For photosynthesis, light is captured by chlorophyll A and B, primarily from the red and blue portion of the spectrum. Light absorption by chlorophyll A peaks at 430 nm in the blue band and 662 nm in the red while chlorophyll B peaks at 453 nm in the blue and 642 nm in the orange-red bands. Chlorophyll synthesis peaks at 435 nm and 445 nm in the blue spectrum and 640 and 675 nm in the red wavelengths.

Chlorophyll is not the only light-sensitive part of the plant. Carotenoids, are a group of orange pigments that capture light in the blue portion of the spectrum, primarily at about 450 nm in the blue spectrum and 475 nm in the blue-green range. Carotenoids not only contribute to photosynthesis but also protect the chlorophyll from excess light that could have destructive effects.

Another pigment that appears to play a role in plant health is xanthophyll. This yellow pigment captures light in the range from 400-530 nm, but is usually hidden from our view by the green of chlorophyll. If a leaf loses its chlorophyll – because of a nitrogen deficiency, for instance; xanthophyll’s bright yellow color becomes apparent. Xanthophyll has several functions. First, it acts as a light and heat regulator. At dawn, it is in its low-energy form, violaxanthin, which has peak reactions to light at 480 nm and 648 nm. As the light increases to levels that might hurt the thylakoids, this leads to photo-oxidation of the chlorophyll.

Infrared and Ultraviolet Energy

   Infrared radiation (IR), more commonly referred to as heat, exists in the electromagnetic spectrum between 700nm and 1mm, which is just outside of the visible spectrum. Just like other variables that affect plants, IR’s effect on plants is a delicate balance. Infrared light plays a part in blooming as well as in the speed at which plant stems grow. Correct infrared balance is important for indoor gardening.

Ultraviolet (UV) radiation has long been known to be quite important in biological photochemical reactions. Anthocyanin and other flavinoid pigments absorb blue and UV light to protect chlorophyll from photo-destruction. UV-A, or near UV, is comprised of energy between 315 nm and 400 nm and has been found to support various plant functions as well as help to control various mildews. Lower wavelengths of UV (UV-B and UV-C) have been shown to be harmful to plants, damaging DNA, proteins, lipids and membranes.

Full Spectrum Advantages

    Plants have evolved for millions of years under natural sunlight which has a very balanced spectrum. Most artificial light sources are either fully or partially deficient in certain areas of this spectrum. As an example, while producing heavy red/orange content, high pressure sodium lamps are very deficient in blue and violet.

Many hydroponic growers begin their growth cycle using a strong vegetative light source, heavy in blue/violet, to encourage young plants to grow healthy and strong. After the plants have achieved a specific height or stage in the growth cycle, the light source is then switched to a more full spectrum, or well balanced, light source.

Sunmaster FULL NOVA spectral output

Finally, many growers then switch to a light source, heavy in red/orange content, to encourage budding and flowering. In a way, this process ‘pieces together’ the various spectra needed, varied during each stage of growth.

While no light source exactly replicates the sun, Sunmaster’s FULL NOVA product family is the industry’s best replica of natural sunlight, providing a very consistent and broad spectral output with high energy (µmole/sec). Based on the type of plant, FULL NOVA lamps are used by many industry leaders, either as an all-purpose, single-lamp source or as a ‘middle stage’ lamp for crops grown under multiple lamp types.


Plants and Light

       Plants ‘consume’ light, converting the energy through the process of photosynthesis.  Light spectrum , duration and intensity are all key components  to support photosynthesis.  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.” This is likely to happen at some point in the future. In fact, manufacturers like Venture Lighting International provide PAR watt ratings for their Sunmaster line of lamps designed for the plant growth market.

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 somewhat fixed in their spectral output, metal halide lamps and ceramic metal halide lamps are available in a broad range of spectral outputs.  With this in mind, the discriminating grower can choose a lamp that provides the best spectral output for their 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.

Photosynthetic Active Radiation (PAR Watts)

A ‘Watt’ 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. They are significantly more efficient than incandescent bulbs.

Plants use energy primarily between 400 and 700 nanometers.  Light output in this wavelength region is called Photosynthetically Active Radiation or PAR.  According to the International Commission on Illumination (CIE), photosynthetically active radiation (PAR) for plants should be reported as the total photon exposure in the 400-700 nm waveband.   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’ 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 lamp 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 can be 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.

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.  While some lamp manufacturers currently advertise total micromoles of output, they do not properly limit their measurements to only account for energy between 400-700 nm.  In other words, many lamps are measured for total micromoles of output including outputs in infrared (700+ nm) and UV (less than 400 nm).

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 meaning 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, yet 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. Sunmaster lamps are designed to this weighted PAR standard, or YPF, based on plant’s photosynhetic response.

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 micro-moles (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.