PAR readings through air and water

[Alastair]

This has baffled me as when I was doing my water change yesterday I turned my lights on full and placed my par meter on a branch of the wood just underwater. As the water was dropping the par values were rising and out of water it was slightly higher than below water.

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[James O]

Index of refraction is ringing in my ear. The glasses composition and the change of the angle of incidence between air/glass and water/glass need to be taken into consideration.

From the tinterweb:

'One indicator of the optical density of a material is the index of refraction value of the material. Index of refraction values (represented by the symbol n) are numerical index values that are expressed relative to the speed of light in a vacuum. The index of refraction value of a material is a number that indicates the number of times slower that a light wave would be in that material than it is in a vacuum. A vacuum is given an n value of 1.0000. The n values of other materials are found from the following equation:

The table below lists index of refraction values for a variety of medium. The materials listed at the top of the table are those through which light travels fastest; these are the least optically dense materials. The materials listed at the bottom of the table are those through which light travels slowest; these are the most optically dense materials. So as the index of refraction value increases, the optical density increases, and the speed of light in that material decreases.

MaterialIndex of Refraction
Vacuum1.0000
<--lowest optical density

Air1.0003
Ice1.31
Water1.333
Ethyl Alcohol1.36
Plexiglas1.51
Crown Glass1.52
Light Flint Glass1.58
Dense Flint Glass1.66
Zircon1.923
Diamond2.417
Rutile2.907
Gallium phosphide3.50
<--highest optical density

This equation for the critical angle can be used to predict the critical angle for any boundary, provided that the indices of refraction of the two materials on each side of the boundary are known. Examples of its use are shown below:
Example ACalculate the critical angle for the crown glass-air boundary. Refer to the table of indices of refractionif necessary.
The solution to the problem involves the use of the above equation for the critical angle.

Θcrit = sin-1 (nr/ni) = invsine (nr/ni)
Θcrit= sin-1 (1.000/1.52) = 41.1 degrees'

Of course none of the numbers are for planted tanks. And as each one of those is different. One of my tanks empty will of course measure lower when planted because it's covered in amazon frogbit

 

[Iain Sutherland]

that cleared it right up James

having reread this i understand it a little differently....
correct me if im wrong but its saying that the light reflecting off the glass increases the par in water against just the light hanging in a room at the same distance due to light spill/ no reflection from the tank.
So the par DOES drop when filled with water against the light over an empty tank.
Is it not the measurement without a tank that is causing confusion here.... no tank so no reflection etc...??
the OP was asking if his par will drop once he flood his tank... so yes.....

Also the difference just above the water surface vs. just under the water surface are small => about 10 µmol PAR according to my findings

isnt this what we are actually getting at?! which also increases with depth....?

The air measurement is relevant if you plan to grow an emersed tank, ie dry start though.

 

[James O]

So light at an angle greater than 61.04 deg will pass from water thru the glass. Anything less will be reflected back inside (I love wolframalpha.com )

Air/glass is 41.1 deg so there is a 20 deg increase in angle of incidence between air/glass and water/glass

And I've run out of mental steam (well I'm going to the beach.....) so I'll let someone else drawer a conclusion from this info

 

[John P Coates]

Just want to mention the importance of light spectrum. Blue light is attenuated to a lesser extent than red light, for example. Based on my measurements of a full-spectrum light, PAR is less in water and decreases as depth increases.