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Solar radiation and photosynthesis

Solar radiation is a term used to describe visible light and near-visible, the latter composed by the ultraviolet and near infrared radiation from the sun. The components of solar radiation and Earth are classified by their wavelength within the band of 200 to 100,000 nm (nanometers): ULTRA VIOLET

• 250 to 400 nm VISIBLE
• 400 to 700 nm
• NEAR INFRARED 700 to 3000 nm

Above the atmosphere, solar radiation has an intensity of 1370 watts per square meter. This is an average value designated as the solar constant . On the surface of the earth at noon, the direct radiation is about 1 kW per square meter. This average value depends on the location, season and time of day, but cloud cover (weather) that affects the energy received and the fact vary according to complex parameters.

The human eye perceives this radiation depending on its wavelength. We have a peak sensitivity in the yellow-green region, centered around 550 nanometers. Is a color that actually makes very visible signs and objects. Plant photosynthesis operates a broad spectrum including lengths red and blue wavelengths. The 'peak' of absorption, or optimum values are 630 nanometers for red and blue 450 nm. Graphs placed below show the response curves of comparative perception of the eye and efficiency of photosynthesis in plants. To complete this discussion, we must add that in the case of marine organisms, penetration of the wavelengths located in the red is rapidly attenuated by the natural water filter, so that this radiation is particularly useful for the growth terrestrial species. Blue is, however, a wavelength that is effective even at high depth. Lamps for horticultural land plants favor blue and red peaks, a lamp dedicated to marine photosynthetic organisms will favor the blue peak.
Lights are characterized by their color temperature in Kelvin. This sets the tone of a lamp, from cold to hot. The higher the color temperature increases over the perceived color appears blue, for a range that goes from red to purple / blue and white through. The marine environment and use of lighting color temperatures between 6500 high (cool white or daylight) and 20 000 Kelvin (deep blue). The lamps are also characterized by the Color Rendering Index (CRI) which indicates their ability to reproduce all the colors of the spectrum. But this interest is less in the aquarium to the extent we do not seek a neutral color but rather to enhance or exploit specific radiation.

spectrum sensitivity of photosynthesis compared to that of the human eye

The measurement of light is essentially adapted to human vision. It is based on how the light source is perceived and how our eye perceives light. We are particularly sensitive to yellow light and our measuring devices promote this region of the spectrum. The blue and red radiation effective for the plants are comparatively less represented. The sensitivity of the eye is the basis for a measure of luminous flux in lumens of illumination in lux (lumen / m²) and light intensity in candela (lumen / angle). These various units are actually related to each other. 100 lux lighting lumens means that 100 are received on one square meter of surface. But this system of measurement refers specifically to human vision and not about how plants can use light, which is our concern here. Another risk of misinterpretation, the sensitivity curve of the eye is not linear but logarithmic so that the qualitative assessment of light is easily mistaken without a meter. And very low light seems sufficient or strong light may be doubled without receiving a significant change. Judgement 'firsthand is therefore unreliable in terms of efficiency for photosynthesis.

A few technical
Plant growth is directly dependent on energy from sunlight. Photosynthesis (photochemistry) begins when a photon excites an electron (Stark-Einstein law). In 1972 Keith Mc Cree has demonstrated that counting photons of light radiation, predicted photosynthesis with greater precision than with a light meter. Scientists and industrialists in connection with the plants are naturally passed to this new measurement system called the lumen, Photosynthetic Photon Flux (PPF) and illumination Photon Flux Density Photosynthetical (PPFD). The energy of a photon is a quantum so that the apparatus for measuring the photosynthetic activity are also called quantum meters. The measure of light energy for plants is called Photosynthetic Active Radiation (PAR). The reaction of a plant with a PAR eventually leads to a result expressed in Photosynthetical Usable Radiation (PUR).

lamps for domestic use and can make a lot of lumens, allowing good illumination for our comfort, and still have a result on the plants well below the light sources that are developed specifically for this purpose. In our case (reef tanks), it is much more important to measure the PAR efficiency standpoint photosynthesis, ramps lighting aquariums while manufacturers or lovers generally do not communicate as the illumination in lux or lumens.

Ramps LED light Alpheus are designed with this in mind to provide the best solutions for optimal growth of animals, plants or marine aquarium freshwater adding control energy costs.

The reader interested in this subject will find the answers to her questions by consulting the excellent work by Stephane Fournier lighting aquariums Animalia Editions.

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