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There are three main areas to consider when looking at PAR and it's influence on growth, both plants and algae: phototropic response, photosynthetic response and chlorophyll synthesis. All are these types of growth work with PAR values ranging from the ultraviolet side to the infrared wavelengths. To check it out in action watch the following meter info video.
Sun Master Grow Lamps: http://www.sunmastergrowlamps.com.au/technology/PAR-and-Plant-Response-Curve.html
Iquatics Online: http://www.iquaticsonline.co.uk/blog/what-is-par