# [Wet Thumb Forum]-Building my own parabolic reflector--technical questions.



## Gomer (Feb 2, 2004)

I have some 19watt 6500k screw ins that I really like, but the lighting isn't as even as I want.

...so, I have decided to build a parabolic reflector. I will be using some 1/64" thick mirror finish aluminum (looks just like my AH reflectors)

Does anyone have any recommendations for parabolic coefficient(y=ax^2 + b) "a" with respect to optimal lighting?

I understand that the all light entering a parabola will cross the focus point on the first reflection. it would make sense to me that I hsould have the axial center of the light at this point. Is this correct, or should I offset in a particular direction?

Also, I will be using a smooth parabola rather than faceted like the AH reflector. Any reason to go faceted other than easier (I have the tools to fold the angles).

Thanks!


----------



## Gomer (Feb 2, 2004)

I have some 19watt 6500k screw ins that I really like, but the lighting isn't as even as I want.

...so, I have decided to build a parabolic reflector. I will be using some 1/64" thick mirror finish aluminum (looks just like my AH reflectors)

Does anyone have any recommendations for parabolic coefficient(y=ax^2 + b) "a" with respect to optimal lighting?

I understand that the all light entering a parabola will cross the focus point on the first reflection. it would make sense to me that I hsould have the axial center of the light at this point. Is this correct, or should I offset in a particular direction?

Also, I will be using a smooth parabola rather than faceted like the AH reflector. Any reason to go faceted other than easier (I have the tools to fold the angles).

Thanks!


----------



## Gomer (Feb 2, 2004)

not that I got any help here on it







, but I thought I'd share









ok, here is the schematic. As you can see, when modeling as a point source radiation, there is very very little restrike on a NON point source light.


----------



## Justin Fournier (Jan 27, 2004)

Not speaking for everyone of course, but I think this is over most of our heads.....


----------



## Vinlo (Jul 27, 2004)

That's pretty seet gomer. I have two questions though. 1. How are you going to bend it to exactly those angles? and 2. What program is that? Rhino or something? I really want to see how well it works in real life.. looks great on paper.. err.. screen. 

_____________________________
30g, 2wpg (NO Fluorescent), 90% Flourite, DIY CO2, HOB


----------



## Gomer (Feb 2, 2004)

I have a machine shop in the basement. I just set the angles, mark the lengths and fold









I already made the reflectors and set them um in a temp arrangment. I'll need to take some before and after pictures with the shutter and aperature constant. I am not sure if the results are brighter (it is close to the 92 bright paper), but I am getting a much more evel light from what I can tell. Before I was having dark areas. now, the intensity of the "once bright spots" is now even across the tank.

All in all, it looks like it has worked!


----------



## Vinlo (Jul 27, 2004)

Are those units inches or centimeters? Centimeters I am guessing. Also, where did you plan on fastening it to the hood? At the ends as to not mess of the geometry of the reflector?

_____________________________
30g, 2wpg (NO Fluorescent), 90% Flourite, DIY CO2, HOB


----------



## Vinlo (Jul 27, 2004)

> quote:
> 
> Originally posted by Vinlo:
> Are those units inches or centimeters? Centimeters I am guessing. Also, where did you plan on fastening it to the hood? At the ends as to not mess of the geometry of the reflector?
> ...


_____________________________
30g, 2wpg (NO Fluorescent), 90% Flourite, DIY CO2, HOB


----------



## Gomer (Feb 2, 2004)

Those are ratios. For me, it is inches. This is a little large, but it was made done to allow the largest spread of light while still fitting on my 10g. If your bulb is smaller in diameter, you will have even less restrike as long as the location of the central axis of the bulb is the same. I'll be fastening it at the two peaks. As for two bulbs, I am using this reflector with 2x19watt bulbs. Since the incandecent sockets are T'd, it is easy. If you mean 2 parallel, then you need a COMPLETELY different bulb. I didn't have access to the large 3ft folder at the time, so I made a split reflector. 

I'll be taking pictures so you can see how it all works.


----------



## david lim (Mar 30, 2004)

you should post pics of the final product if possible! the schematics look great too. Are there large differences in effectiveness between your reflector and AH?


----------



## Gomer (Feb 2, 2004)

I'll post pics tonight when I get home to take them







it won't be in a hood, but I have it rigged up in a temp setup that works for the tank.

It is hard to do a direct comparison with the AH since my AH uses thin linear bulbs with 55watt tubes. My 10g has spiral compacts. The AH reflectors should be designed with bulb geometry in mind, so it is kinda hard to compare farely.


----------



## Slappy (Mar 2, 2003)

Interesting...how would a rounded/half-circle reflector work?

----------------------------
Fish Slapping Extraordinaire


----------



## Gomer (Feb 2, 2004)

Short answer:
Not nearly as well to very to very bad

Short explaination:
Restrike is not addressed at all. 


Complicated explainations:
A point source at the center of a circle is at its focal point. This means that all light reflected will return exactly to the point source. Since we so not have a point source, all light with a virtual source at the center will COMPLETELY restrike.

Lets define the X-axis as the line tangent to the circle. As long as the curve is sufficiently small (ie small dx, ie small section of the arc), a circle will approximate a parabola with a focal point at R/2. While being a mathematical solution, it is not a realistic geometric solution (would reaquire a really complicated explaination that would bore the hell out of all but maybe 1 person on this forum). For the sake of arguement, lets say that it is a realistic solution, you will still have to deal with the "primary" restrike that is in the vacinity perpandicular to the focal axis.


----------



## Gomer (Feb 2, 2004)

and pics:


----------



## Slappy (Mar 2, 2003)

Interesting, indeed. That pretty much was the answer I was expecting.

----------------------------
Fish Slapping Extraordinaire


----------



## Gomer (Feb 2, 2004)

if you want to get more indepth on the math behind the paraxial approximation and when this approximation fails, you can look here
http://www.rit.edu/~bekpph/Optics/2E.pdf

it isn't the best explaination out there, but in my quick search, it looked sufficient to get the idea across for the technically inclined


----------

