Spherical Reflectors

[eponymous]

Hi all,

I wonder if any of you can help me with this.

I've purchased this reflector:

G&P Optoelectronics GmbH & Co. KG - Reflector 68mm f=40mm dichroic 06-04-02 (is this perhaps the same as the LumenLabs Pro Reflector?)

diameter: 68 mm
focal length: 40 mm

I'm wanting to see if it will work ok with an Osram Powerstar HQI-TS which has a radius from the centre of the bulb to the outer edge of 16.5mm.

As this is a spherical reflector I assumed that I should place the bulb at the centre of the effective sphere which is actually 25mm from the back of the reflector - not 40mm!

How can the focal length be 40mm if the centre of the sphere is at 25mm?! Am I missing something here?

I've attached a diagram to show what I mean.

As I only seem to have 5mm from the edge of the reflector to the bulb centre I don't think this will work. Does anyone have any ideas?

Thanks!

 

[eponymous]

I've also attached a picture showing the bulb. It is so long that it can't fit inside the reflector.

If I put the reflector right up against it, the centre of the bulb is still (16.5mm - 5mm) = 11.5mm too far out from the reflector's focal point

 

[TESCORP]

This is the same reflector as Lumenlab had and I have in stock as well, the focal length is 40mm as this is a portion of the sphere and not a hemisphere. The best way to place the bulb is to move it forward or back from the reflector until the image of the arc chamber fills the reflector while you look at it from the front (around 40mm). You will see what I mean as you do it. You have plenty of room without having to move the bulb so close to the reflector.

 

[eponymous]

Thanks for your reply Tescorp. It's also good to know there's another store out there since LL went away

So I'm a little confused still. I realize this is not a hemisphere so what I did was draw an outline round the reflector on paper. Then I used a compass to draw a complete circle which followed the same arc as the reflector.

The circle had a radius of 25mm and that's where I got that value from.

Is that not correct?

Cheers.

 

[eponymous]

Incidentally if I put my finger at 25mm it does seem like the image reflected back fills the whole reflector.

Can I see the reflector you have to see if it's exactly the same? I'm guessing it is as I doubt there are many places you can get these dichroic ones of that diameter...

 

[eponymous]

I did some experimentation and have attached photos. I'm now fairly convinced this isn't a 40mm focal length reflector. It's more like 25mm.

So now I'm wondering why LL/G&P etc are selling this and claiming it will work with the larger 400w bulbs which are far too large..

I've attached some images. The first one shows the bulb reflection at 40mm, then at 25mm and the last two are how I'm trying to use the reflector.

 

[eponymous]

Ok I think I see the problem here.

This reflector is being sold as a 40mm Focal Length reflector of diameter 68mm.

However, the focal point of spherical reflectors for DIY projector builders is of little interest.

Remember that the focal point is where the light rays will emerge parallel from the reflector if a point light source is placed there. We don't want this.

We want to know, what is the the radius of curvature for the reflector (the radius of the sphere the reflector is a section of).

That 40mm advertised focal length is actually the radius of curvature I think and this is indeed the point where we want to place our lamp so that the rays reflect back directly through the that point.

This means the focal length (= 1/2r) should be advertised as 20mm. At 40mm it should look the same but upside down.

 

[TESCORP]

I did a drawing for you to show how I place this reflector to my bulb, hope it helps!
I also attached a few pics of stuff I have done.

 

[eponymous]

Some nice ideas there

Yep your picture is exactly the same as mine hehe