Does light height really matter, except for coverage?

[Reefing102]

So genuine question…except for overall coverage, does the height of the light really matter par wise. I don’t have a par meter to test this out but say you typically run a light at 60% at 11 inches off the water. If you were to run the same light at 2 inches off the water, still at 60%, does par chance that much? Obviously you’d have less coverage but what about par, would par increase? I’d imagaine it would, but would it be significant?

I ask because in all lighting systems you have products designed to be hung and those that use legs. Metal halide had them, T5 had them, heck even some LEDs have them. But most of the big names do/did not have them. I think running a low profile hood with LEDs would look pretty slick you’d just have the coverage issue to contend with. Thoughts? Disagreements?

 

[Saltyreef]

As far as PAR and coverage are concerned, they work together but differ.

Your coverage at a given height will vary in par readings from the same fixture, with the same settings at a different height.

"PAR is measured by the amount of micro moles of light per square meter per second and represents the area of light that plants use for photosynthesis or to grow."

To address a low profile hood with LEDs, typically, these setups are wall to wall chip coverage with no real lens vs a single point pendant style lamp that is designed to be suspended a little above the tank due to its size and lens angles.

Par can be adjusted to liking in each of these situations though the pendant style light will suffer from par hot spots and have to contend with the light spread vs wall to wall chip coverage in a low profile hood.

 

[Bpb]

One light. One tank. One circumstance. But I tested it myself just now. Raising my light 2” higher at a given measurement point (centered a couple inches below water directly under the luminaire resulted in a 10% drop in par.

Light tested: kessil a360x with 55 degree reflector.

Apogee mq510

75% intensity 100% color

Par 2” under water at 12” light height: 462

Par 2” under water at 14” light heigh: 416

Different degree of optical delivery will impact the par drop more I would surmise. Wider spreading lights would probably have a greater falloff is the prediction. Removing the optic from the kessil and using the native wide lens spread confirmed that (with this single data set, your results may vary)

12” height: 255 par
14” height: 215 par

Roughly a 20% reduction with wide angle vs the 10% reduction with the narrow lens

 

[merkmerk73]

So genuine question…except for overall coverage, does the height of the light really matter par wise. I don’t have a par meter to test this out but say you typically run a light at 60% at 11 inches off the water. If you were to run the same light at 2 inches off the water, still at 60%, does par chance that much? Obviously you’d have less coverage but what about par, would par increase? I’d imagaine it would, but would it be significant?

I ask because in all lighting systems you have products designed to be hung and those that use legs. Metal halide had them, T5 had them, heck even some LEDs have them. But most of the big names do/did not have them. I think running a low profile hood with LEDs would look pretty slick you’d just have the coverage issue to contend with. Thoughts? Disagreements?

YES it matters!

Here’s an example why

Note the way the light casts on the tank in the two images

I didn’t understand lighting heights when I purchased my tank

As a result my lights are mounted very close to the water - maybe 5-6 inches at the most off the surface

As a result you can see significant spotlighting in the picture where the water level is full

This also results in significant hot spot areas of 300-550 par even at moderate LED settings of 50%

I am going to spend the money on the taller, uglier hood to get another 5 inches or so off the water and it will make for a much better aesthetic - like in the water change pic - where the light casts more smoothly

Learn from my error

 

[Reefing102]

Awesome feedback. Thanks guys. So it seems the only ones that may work well is the LED bars in this scenario to ensure proper coverage. I’ve seen a few tanks that have that spot light effect but honestly didn’t realize that the par difference could be so significant. It’s good information.

 

[Scott Allen]

It depends on if the light is a point source, infinite line, or infinite plane.

Point Source: Par will fall with relation to distance by 1/r^2
Line Source: Par will fall with relation to distance by 1/r^1 = 1/r
Plane Source: Par will stay the same (1/r^0 = 1/1 = 1)

Obviously there is no infinite in a finite world, but it can get close enough that it doesn't matter.

A point source is the easiest to think about, its just a single light source.
A line source would be like a giant warehouse with all the lights in a single line.
A plane source would be like a giant warehouse with the lights arranged in a grid pattern.

Lights over a singular tank are a bit more complicated. A singular light will always behave like a point source. But, if you have lights spread out, it would at first behave like a plane source, and as you bring it up toward the ceiling it would transition behaving like a point source.

If you line the walls of your hood with a reflective material (or at minimum, paint it white), it will help maintain the plane like ability of the light source.

 

[Dburr1014]

My tank, Reefbreeders, low canopy, NO LENSES, 100% all channels but r and g @10%, great spread, lower par than someone using the lenses.

No disco effect. Par taken about 4" under water, lights hang maybe 4" above(shielded).

 

[paragrouper]

Awesome feedback. Thanks guys. So it seems the only ones that may work well is the LED bars in this scenario to ensure proper coverage. I’ve seen a few tanks that have that spot light effect but honestly didn’t realize that the par difference could be so significant. It’s good information.

I’m going through a light/canopy upgrade right now and similar height constraints pushed me to select LED bars.

 

[oreo54]

It depends on if the light is a point source, infinite line, or infinite plane.

Point Source: Par will fall with relation to distance by 1/r^2
Line Source: Par will fall with relation to distance by 1/r^1 = 1/r
Plane Source: Par will stay the same (1/r^0 = 1/1 = 1)

Obviously there is no infinite in a finite world, but it can get close enough that it doesn't matter.

A point source is the easiest to think about, its just a single light source.
A line source would be like a giant warehouse with all the lights in a single line.
A plane source would be like a giant warehouse with the lights arranged in a grid pattern.

Lights over a singular tank are a bit more complicated. A singular light will always behave like a point source. But, if you have lights spread out, it would at first behave like a plane source, and as you bring it up toward the ceiling it would transition behaving like a point source.

If you line the walls of your hood with a reflective material (or at minimum, paint it white), it will help maintain the plane like ability of the light source.

The point light source inverse square law rarely if ever applies to aquariums.
The light is not allowed to spread out due to water refraction and constraints by glass reflections.


Many times it is a linear relationship in normal tank ranges I e 100 par at 10 inches 50 at 20 or closer than the expected 25.

Sand bed par can exceed some mid level reading due to the same refraction/reflections

Now if you just include the free air part of the measurement it is closer to inverse square law but with multiple points overlapping which you eluded to.

As you said, it's complicated.

Strip light behavior . Par at 6" depth 230.
At 12" 125.
60 degree lens.

https://reefs.com/magazine/product-review-build-my-led-48-strip-light-custom-led-lighting-for-freshwater-and-reef-aquaria/

Low wide beam lights with multi- colored diodes are harder to "tune" than narrow beam high hung lights in my opinion.
Poor blending until deep, larger par changes from top to bottom.
May have less shadowing though.

Not to argue just to expand..

 

[Reefing102]

Now that’s some interesting additional information. I’ll have to check out that reefs.com article

 

[MartinM]

So genuine question…except for overall coverage, does the height of the light really matter par wise. I don’t have a par meter to test this out but say you typically run a light at 60% at 11 inches off the water. If you were to run the same light at 2 inches off the water, still at 60%, does par chance that much? Obviously you’d have less coverage but what about par, would par increase? I’d imagaine it would, but would it be significant?

I ask because in all lighting systems you have products designed to be hung and those that use legs. Metal halide had them, T5 had them, heck even some LEDs have them. But most of the big names do/did not have them. I think running a low profile hood with LEDs would look pretty slick you’d just have the coverage issue to contend with. Thoughts? Disagreements?

T5's work well in low profile hoods also. I'm considering using T5s and multiple VolxJapan GrassyCore fixtures in a Giesemann Sphera hood on a UNS 120U. Still pondering...