Aquarium LED Lighting Terms: Part 2
Posted By: Matt Allen
The next group of terms we would like to discuss in part 2 of our series about common aquarium lighting focuses on the light required for photosynthesis. Providing light required to maintain photosynthetic organisms, like plants and corals, is the next most important thing a lighting system can do over your aquarium after providing the necessary illumination for viewing purposes. Let’s break down terms you should know when buying aquarium lighting.
PAR (Photosynthetically Active Radiation): the spectral range of light from 400 to 700nm that organisms are able to use for the process of photosynthesis. PAR is the type of light we are interested in measuring if we want to understand how well a light fixture will maintain photosynthetic organisms. Just as lumens and lux are metrics used to measure light that the human eye is sensitive to, PAR has a set of its own metrics that follow a similar format.
PPFD (Photosynthetic Photon Flux Density): to understand PPFD we must first discuss photosynthetic photon flux (PPF). PPF is similar to lumens in that it measures the total amount of light being emitted (PAR output) by a light source in all directions. In this case, the type of light we are measuring is PAR. PPF is measured in micromoles of photons per second. PPFD is similar to lux in that is measures how well a surface at a given distance from the light source is illuminated (PAR intensity) by looking at the density of photons hitting it, and again the type of light we are measuring is PAR. PPFD is measured in micromoles of photons per square meter per second. In the aquarium hobby when most people are talking about PAR or PAR intensity they are actually referring to PPFD at a specific distance from the light fixture, but the terms are often used interchangeably. Like lux, measuring PPFD or reviewing PPFD data at various distances from the light fixture will tell you how well the fixture directs the PAR it is producing into the target area and onto the various photosynthetic organisms in the aquarium.
PUR (Photosynthetically Usable Radiation): While PPFD allows us to measure the quantity of light available to an organism for photosynthesis, the concept of PUR allows us to consider the quality of that light for photosynthesis. The other system for measuring light has a similar metric you may be familiar with, CRI or Color Rendering Index, which measures the quality of a light source at revealing true colors to the human eye when compared to ideal natural sunlight. Although PAR ranges from 400 to 700 nm, certain wavelengths of light across that spectrum are more efficient at generating photosynthetic activity in the plants and algae than others. For example, the pigment chlorophyll a, one of the most common photosynthetic pigments, absorbs the most energy from violet-blue light and red light. Although the pigment can also absorb some energy from green and yellow light, it does so much less efficiently. This means that a light source that emits more of its PAR in the violet-blue and red ranges would have higher PUR for the pigment chlorophyll a.
PUR is difficult to quantify and is more of a concept for reviewing the quality of light spectra than a true metric. Since there are many different types of photosynthetic pigments found in plants and algae, each with different peaks in their absorption spectra, PUR is specific to a certain type of photosynthetic pigment. To further complicate things organisms often have multiple types of these pigments present. We are just beginning to understand the types of photosynthetic pigments present in symbiotic algae inside coral polyps, and the complex relationship that exists here. For the average aquarium hobbyist the idea of PUR is one of quality versus quantity, and understanding that having a lot of photons is great, but it is also important that they are the best types of photons (wavelengths) for photosynthesis in our aquarium inhabitants.
Next week we will discuss wattage draw and efficacy of LED light fixtures and provide a nice summary of what to look for in an LED light fixture for your home aquarium using the terms we have discussed.
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