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Choosing the right optic for DIY LEDs


Milad LEDGroupBuy.com

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Milad LEDGroupBuy.com

The below information was taken from here

 

I've seen alot of posts about optics so this is to help others choose the correct optic for your tank.

You can easily do this on a piece of paper to adjust it for your tank.

 

Some quick notes on optics:

1. If you look at the spec sheet for your optics, most of the light is within the first 50% of the degrees. for instance a 60degree optic has most of the light in the first 30degrees. Its not linear.

2. Current CREE LEDs (XP* family) have 125degree optics built in

3. Optics are like reflectors on T5s. Using good reflectors rather than just putting the T5 inside your canopy makes a huge difference.

4. Choosing your optics depends on the depth of your tank and the height above the tank as im about to show

 

So taking the above 4 points into consideration, here is what happens when you go with no optics on a 180g tank, 12in above the tank centered

 

NO OPTICS

125d.png

60 degree OPTICS

60d.png

40 degree OPTICS

40d.png

 

 

 

Now your first thoughts are probably like, HOLY beep beep 40 degree is the way to go.

If that is your first thought you didnt read #1 above. Yes 40 degrees doesnt have spill off but you need a little so everything in your tank is covered. Here is the same optics at another angle

 

NO OPTICS

125d-2.png

60 degree OPTICS

60d-2.png

40 degree OPTICS

40d-2.png

 

As you can see, 60 degree on this particular setup is the way to go. Any higher and you would be look at the 40 degree optics.

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Coverage is not the only factor that has to be considered when choosing optics. Because they affect the spread of light, they affect the intensity, and the PAR levels as a result. Secondary optics can be used to hit certain performance objectives, and the LED array size and spacing has to be adjusted to accomodate. Much if this is an issue with arrays using non-dimmable drivers, but it's still something that should be weighed into the decision.

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Milad LEDGroupBuy.com
Coverage is not the only factor that has to be considered when choosing optics. Because they affect the spread of light, they affect the intensity, and the PAR levels as a result. Secondary optics can be used to hit certain performance objectives, and the LED array size and spacing has to be adjusted to accomodate. Much if this is an issue with arrays using non-dimmable drivers, but it's still something that should be weighed into the decision.

 

Good point. The intensity is going to be greater using optics so if you do not have dimmable drivers and you end up with high intensity because you have more than enough LEDs it will be harder to adjust the light so your corals don't get bleached.

 

I think you forgot something. PICS. :P

 

You can't see the pics?! or you being sarcastic because they are big.

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I can't see them either. Use Photobucket or Tinypic instead of Dropbox. That service probably gets filtered by corporate servers like mine.

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Milad LEDGroupBuy.com
I can't see them either. Use Photobucket or Tinypic instead of Dropbox. That service probably gets filtered by corporate servers like mine.

 

Pictures updated with photobucket.

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Much better.

 

One thing I've mentioned to others in the past is, don't use optics just to control spill. Like I mentioned before, reducing the emission angle of the LED increase intensity. Unless you have an easy way to reduce the LED output, it can cause problems with excessive intensity. We sometimes forget that the primary role of these fixtures is to support the corals that we keep. Setting the intensity to a level where the tank occupants are happy is the primary objective. Everything else is secondary, even if that means that you have light spill outside the tank. At least you don't have to clean up that kind of spill ;) There are other mechanical ways of controlling spill that doesn't require the use of optics.

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Much better.

 

One thing I've mentioned to others in the past is, don't use optics just to control spill.

 

I agree with EvilC on this one.. the pictures are nice to see a relation on the spread of different optics but the situation is totaly un-realistic as who would mount 1 line of LEDs on a 180 gal tank?

 

a better example would be to show end views with 2 rows, 3 rows 4 rows ect to show people how many rows they need to good coverage and minamal light spill with a fixture 12" above the tank, while hitting a certian par level. so lets say a set up showing a few different row amounts all with 40 degree optics, another one for 60 degree ect.

 

Steve

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Milad LEDGroupBuy.com
I agree with EvilC on this one.. the pictures are nice to see a relation on the spread of different optics but the situation is totaly un-realistic as who would mount 1 line of LEDs on a 180 gal tank?

 

a better example would be to show end views with 2 rows, 3 rows 4 rows ect to show people how many rows they need to good coverage and minamal light spill with a fixture 12" above the tank, while hitting a certian par level. so lets say a set up showing a few different row amounts all with 40 degree optics, another one for 60 degree ect.

 

Steve

 

Sorry I wasn't trying to show one line. The images are to show that the height of the LED compared to the optic makes a big difference. You can tell by the cones that if the aquarium was half the height and the 40degree optic was at same height shown in the picture, it would be the perfect optic instead of the 60.

 

Adding several rows and more LEDs will just clutter the picture and not show that calculating the optic is about height of fixture + depth of tank + width of tank.

 

What evil is saying is that 12in from the source of a 40degree optic is more intense than 60degree optic. Hes statement eludes to the fact that you shouldn't put wide optics in the middle and tighter optics to outside as most people jump to thinking its obviously better. You end up with hot spots you aren't expecting because on each different optic the farther away from the source it gets, the intensity is different so you are now mixing intensity at same depths in different areas of the tank compared to the same intensity at different depth in different areas of the tank if you used the same optics all around.

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  • 1 year later...

There is a very important aspect of lighting that is missing from this discussion. Refraction of light.

 

Refractive index of air 1.00

Refractive index of glass (for 420nm light ) 1.54

Refractive index of salt water (medium salinity 420nm light) 1.347

 

Light entering at an incident angle from normal at 20 degrees (half of 40) ( I wish I had a diagram here. I'll try to post one later) will refract to an angle of only 28.5 degrees once is passes through the water/air boundary. This layer will have an additional focusing effect on the pendant light angles. The following shows the angle of refraction at the different angles"

 

Lens | Effective angle after hitting water (degrees)

---+----------------

40 | 29.4

60 | 43.6

80 | 57.0

 

Furthermore the worry that light will escape (and be wasted) out of the water to glass to air boundaries are also mostly unfounded with any of the lenses due to total internal reflection. After the light enters the water it will strike the glass at a minimum incident angle of 61.5 degrees (for the worse case 80 degree lens). The refraction between water and glass bends the light upward slightly to a 49.9 degree angle. But it doesn't matter since the critical angle to induce total internal refraction between the glass and air (higher index going to lower index) is 40.5 degrees. This means that any light with an incident angle higher than this will all bounce inward like off of a mirror. All this light gets to be used by the corrals for growth.

 

This is the same way fiber optics keep all the light energy contained in a single fiber and can travel for miles. Since the arraignment of the glass and the water are the same for each of the lenses and since the other lenses produce higher incident angles, any of these lenses will keep all the light in, the focusing behaviors are just more complex than proposed.

 

The main difference is that the path taken by the reflected light will 1) be a little longer than the direct light, 2) possibly be interrupted by growth of algae on the front and side glass. It should be noted that opting for lower angle lenses should reduce the amount of direct lighting the front glass gets and may help reduce algae growth on these surfaces.

 

Anyways in conclusion the path the light takes needs a little more discussion. It would be nice to see a calculator to put this all together so that someone could enter the height and width of the aquarium, the pendant height, and the choice of lens and see the light dispersal patterns of each of the lenses accounting for refrection and total internal refraction. Maybe some day when I have more time...

 

I did just start looking at the RapidLED kits for a tank I'm setting up (getting back into the hobby). They look like a nice kit, I think I'll be ordering a 24 LED kit for my 40 gallon breeder. Picking lenses will be harder than I thought.

 

- Kyle

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