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post #13 of (permalink) Old 01-05-2013, 09:24 PM
Algae Grower
Join Date: Jan 2013
Location: Queensland, Australia
Posts: 17
Originally Posted by Build My LED View Post
Veryzer. Glad you think this is helpful :-) We are currently working on the under-water measurements, but the air measurements allow us to predict average PPFD (PAR) measurements for the specific tank size. Beam angles will narrow in the water, but the total amount of the light in the tank will not be impacted beyond the Fresnel reflections off the top of the water.
it's great to see someone putting in the effort to give some easy to understand data.

a couple of comments ( I hope they're a help and not a PITA)

while reflection is a major loss of light energy there are a few more factors that impact light through water. there's some quite handy information (graphs, tables, formulas, the lot) out there from oceanology studies and even industry probe/sensor setups (eg ultrasonics - sound is just a different size wavelengths and same laws apply)


Reflection & Refraction - lower angle of incence means more light will pass into the water.
using fresnel and snell you can determine how much energy has been reflected & how much has passed through & at what angle.
unfortunately these laws only apply for plane waves with a constant frequency.

Absorbtion - light becomes heat or is used in photosynthesis. this is what gives water colour. water appears blue as the blue photos travel futherest before being absorbed. after 1m 60% of all the light energy has been absorbed (as red photons only travel 1m all red has been removed after 1m)
absorption will decrease as temperature increases. salinity is also a fat or.

Scattering - light photons change direction ( often upwards)
as pure water scatters very little it appears dark ( light is absorbed) turbid water on the other hand reflects (scatters upwards) and appears lighter
2 forms of scattering are;
molecular (short wavelengths scatter more than long. the sky appears blue as blue is scattered first)
particle (wavelengths scatter equally (clouds appear white as the light is scattered equally)

well my 2 bobs worth
hope it makes sense

Last edited by -Az-; 01-05-2013 at 09:30 PM. Reason: add link
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