Spherical Reflectors

[Luca Brazzi]

Im using the SOARE Napkin ring from IKEA as a reflector for my 250W HQI bulb.

http://www.ikea.com/webapp/wcs/store...roductId=32487

I was wondering....

1) Should the arc be above the rim? Below the Rim?

2) I know this kind of reflector will shorten my bulb life due to heat being reflected back through the arc. However, has anyone tried using a piece of IR glass between the Bulb and reflector? The way I see it the heat would be leave the lamp, go through the IR glass hit the reflector, be reflected back, then be reflected back to the reflector because of the IR glass.

[cbm5]

Or, use a spherical mirror that passes IR but not visible light. Yes, you're not the first one to think of it: these are called dichroic reflectors and are usually much too expensive to worry about using them in order to protect a $40 bulb. They are needed in close-quarters situations with tiny panels, but with these DIY projectors we can build an addition and jack up airflow, no problem.

[Squalish]

The arc should be at the center of the sphere - this doesn't depend on where the rim is, it's the intersection of line perpendicular to the surface.

[Rox]

Originally posted by Luca Brazzi
has anyone tried using a piece of IR glass between the Bulb and reflector? The way I see it the heat would be leave the lamp, go through the IR glass hit the reflector, be reflected back, then be reflected back to the reflector because of the IR glass

this is not how IR would work. IT is true that there is a proper way to use ir glass (coated side facing where the ir comes from) but even if you place the coating facing the reflector, the ir coming from the bulb would mostly be reflected by the ir glass returnig to the bulb so it wouldn´t work as you say.

cold mirror and hot mirrors are opposed things. Check what do you need and then go for it.

cold mirror= pass IR / reflect light
Hot mirror= reflect IR / pass light