PAR intensity correlation with mounting height

[Flatlandreefer]

Is there any correlation between a light fixtures mounting height and par reading at a specific spot in the tank as the light fixture is raised or lowered above the tank?

The reason I ask is I rented a par meter and obtained par measurements at every spot in my tank and the readings are lower than I would like them to be so I am currently lowering my light to increase par.

If I have a spot in the middle of the tank, let's say that spot is 12 inches below the fixture and is currently receiving 100 par. If I decrease the distance between that point and the light source by half, 6 inches between that spot and the light, would my PAR roughly double to 200 par?

 

[Crabs McJones]

Yes, the further away from the light the lower the par, and the closer to the light the higher the par. However cutting your height by half wont double your par, it'll increase much more than that.

 

[Flatlandreefer]

Yes, the further away from the light the lower the par, and the closer to the light the higher the par. However cutting your height by half wont double your par, it'll increase much more than that.

That's what I'm wondering, is there any way to get a close estimate of what the new par would be if I have a starting point.?

Also I am not talking about height above the water I am referring to height above a point in the water column.

 

[Crabs McJones]

That's what I'm wondering, is there any way to get a close estimate of what the new par would be if I have a starting point.?

Also I am not talking about height above the water I am referring to height above a point in the water column.

Yup, regardless where you're measuring from as the light gets further away the par decreases. With the differences in light fixtures there is not a way that I am aware of to accurately measure increases or decreases in par without at meter.

 

[oreo54]

Inverse square rule states that at 2x the distance you get 1/4 of the light.. for a point light source..
Which lights really aren't..

I've found that within a fairly narrow range "PAR" falloff is fairly linear..like between 15 and 25 inches is "fairly" linear..

Lights are different though..at at best only a crude approximation..
Actually one could use one point and get the "par" to inch ratio..

for fun use the below chart..
ATI 4 bulb t5HO orange line..
138 PAR @ 15" =9.2 "par" per inch..
go to 20" and theoretical PAR is 138-46 = 92 (138 - (9.2 x 5"))
chart shows 96...

Nice picture of the idea..
Going from like 10 to 40 doesn't work well..
Nor using mixed lighting w/ different "par" to inch numbers..(well I've never checked it, may crudely work in a narrow range of change)
Nothing beats real numbers though.
Using 2 data points and getting the slope and then calculating the value of the new Point on that line is a bit more accurate..
My math is really rusty so if someone knows of "errors" (or a better approach for humans..) please chime in..

 

[Ben Pedersen]

Inverse square rule states that at 2x the distance you get 1/4 of the light.. for a point light source..
Which lights really aren't..

I've found that within a fairly narrow range "PAR" falloff is fairly linear..like between 15 and 25 inches is "fairly" linear..

Lights are different though..at at best only a crude approximation..
Actually one could use one point and get the "par" to inch ratio..

for fun use the below chart..
ATI 4 bulb t5HO orange line..
138 PAR @ 15" =9.2 "par" per inch..
go to 20" and theoretical PAR is 138-46 = 92 (138 - (9.2 x 5"))
chart shows 96...

Nice picture of the idea..
Going from like 10 to 40 doesn't work well..
Nor using mixed lighting w/ different "par" to inch numbers..(well I've never checked it, may crudely work in a narrow range of change)
Nothing beats real numbers though.
Using 2 data points and getting the slope and then calculating the value of the new Point on that line is a bit more accurate..
My math is really rusty so if someone knows of "errors" (or a better approach for humans..) please chime in..

Are the readings dry or wet?

 

[oreo54]

Are the readings dry or wet?

Usually wet...Anyways 1) Lights are not a "point" light source.

The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated outward from a point source in three-dimensional space

https://en.wikipedia.org/wiki/Three-dimensional_space

2) Refraction/reflection counts
Keep in mind I'm not using "extremes" just measurements in limited ranges.
like 15"-25" range is fairly linear..Error due to local "bowing" seems minimal..

Lighting an Aquarium with PAR instead of Watts

www.plantedtank.net

not my stuff but plenty of graphs to browse through..

your error at the midpoint may be 20% but it's easy enough to guesstimate..
closer the 2 points are together the less the error..

inverse sq rule w/ 80 "PAR" at 15 inches .."PAR" at 30" should be 20
Measures 34.. See Catalina t5ho 2 54w tube (magenta line).
Linear "predicts" 40..

Keep in mind all these are best guess estimates.

ATI 138 @ 15 to 59 at 30
138 / 2....69 "PAR"- linear 2x distance 1/2 intensity
Inv sq 34.5....1/4 of 138

Probably a better mathematical way to describe this behavior but beyond me..