I hope this is not a duplicate of previous post.
This calculator will help you where to place your lamp using a parabolic reflector.
This is the link: Parabola Calculator

Willy68

Im not sure Ive seen it before...looks the goods
Thanks for the post.

Hey, that looks like a great program for designing a parabolic reflector. One day I'll download it and take a look. Thanks for posting it.

Now I need a method to build the result reflector. If possible from a sheet aluminium.
Any idea?

Hello Tobias

Parabolic reflectors come with issues ..mainly a dim spot in the middle of the screen where the lamp is..to get rid of that you really need to homogenise or mix up the rays so you dont see the dim spot !

So even if you could DIY one ...this issue still remains

If you are up for building your first pj then I would advise using the normal tried and tested spherical reflector either the LL pro reflector or such like.

Unless you are keen to experiment !

Thanks! This will really help me out.

Guys, let's be practical here ... There is not going to be a dim spot in the middile of the reflector, if this were true you would see one with a spherical reflector as well. Hopefully, the following diagram will explain this:

I think people keep forgetting that the bulb creates light too, therefore replacing the light it shadows.

your right light does come from the front of the lamp .... but is not collimated light as you show..rather it is diverging..

Hey daz in that ray trace it looks like the rays reflecting off the parbolic do not even add to the theoritical projection at all cuz of the fl's are different; with the exception of the middle rays? Is that what your showing or am I blind?? Maybe I should stop stairing at my light gate....

I should have added some text.

What I was showing is that the rays coming directly from the arc don’t focus to the same point on the other side of the fresnel. So the void doesn’t get filled. So the rays from the arc focus to one point and the rays from the reflector focus to another.

Silo extreme,
the void is very real and it is very hard to get rid of, may not even be possible with the set-up we use. A spherical set-up doesn’t suffer from this for two reasons. Generally the spherical reflector doesn’t have a hole for the lamp to penetrate. And the spherical reflector isn’t the primary source of light. The fresnels use the direct arc as the source and the spherical reflector adds light to the source. In a parabolic or elliptical reflector the source of light is from a virtual source created by the reflector. So if there is a void in the reflector then the virtual source will have the void as well.

DJ

Some more info.
You can actually fill the void by moving the arc slightly forward BUT you end up with another problem, hot spotting. Parabolic and elliptical are not very forgiving. There are three things that will make them difficult to utilize. The size of the arc compared to the reflector diameter, the diverging and converging method that we use in our DIY projectors. And the size of the reflector compared to the LCD
Let’s look at a very generalized commercial projector using a parabolic.

This pic shows the arc at the FL of the reflector with the void present. If an LCD was placed in the light path then there would be a dark spot in the middle of the image.
Click to view attachment


So the trick there use is to move the arc slightly forward so that the rays at one point are more even and the void at that point isn’t visible.
Click to view attachment


If the LCD is placed at that point then the image is even as well. After the LCD the void becomes visible again. All that matters is that the LCD is even and that the rays after it are directed through the objective lens.
Click to view attachment



Some ratios of a commercial projector to get the idea:
LCD size = 18mm
Reflector diameter=50mm
Arc size= 2mm
Objective lens= bigger than the LCD

So is we wanted to copy the commercial projector we would need
LCD size 431mm
Reflector size=1200m
Arc size= ours are small enough.
Objective size= bigger than 431mm

I’ve only shown a very simple pic of the commercial set-up. In reality their use other lenses besides just the reflector to direct the light to the LCD. BTW It is also the same principal used in cinema projectors.

DJ

Great info Dazzz ...cheers...

1.2 metres for a reflector diameter..dat is BIG !..


Thinks...
If DIY can ever get smaller high res diameter affordable LCD Panels then more set up options would be available & easier to implement.

For what we do with our parts it is amazing.
DJ

I'm knew to all this so I'm having trouble understanding. In the line drawing you removed the deep part of the arc. Does that mean you are assuming that the bulb is mounted through the center of the reflector?

What if the bulb were mounted horizontal above the FL of the reflector? Say 15" dia, 3" Deep, ~4.5" FL So its 1.5" above reflector? Or are you saying that as the light reflects off the parabola at the center it collides back with the bulb leaving the voids you are talking about?

If so is that why some are starting to use precondensors to help shape the light against the rear fresnel? Or is that because of the different focal length problem at the field fresnel side?

(So many questions, I know I'm a noob)

I think I found out the answer to my own question by reading another post. The spherical is projecting back to the bulb ensuring that it follows the same path to the condensing fresnel.

Click to view attachment
I was wondering what people thought of this setup, just something I thought of while reading this thread.
The blue circle is a lens to collaminate the light come directly from the bulb... If the focal length and diameter of the lens were right, shouldn't it give a pretty uniform light distibution across the LCD panel? (aside from a little bit of blockage from whatever you suspend the lens with.) Curious to see what any thinks of that