How can a camera with one objective (lens) split a beam to two detectors - one for IR and one for visible?

Question

The problem is with separating the wavelengths to each detector from a single lens. Is there a coating I could use to reflect one of the wavlengths while the other passes through? Would it be best to split the beam first then use filters before each detector to single out the IR/visible?
My goal is to image both a visible AND thermal scene with one objective. I can handle everything from the detector to the display but I need a bit of help with the optics. Of course I'm looking for the simplist and cheapest method (IR and cheap don't really go together), but any REAL solutions would be great. Thanks

Answer 1

Your idea of splitting the beam is likely the right direction to head. A "beamsplitter" is the best choice for doing that. You can obtain a number of types, but the cheapest ones are either a sandwich of two prisms glued together or a partially reflective mirror coating. The second type is a simpler choice. Try Edmunds Optics, or their cheaper cousin, Anchor Optics. If you can find one, a partially gold coated mirror will work best because it reflects well across the whole spectrum. Believe it or not, Gold is one of the less expensive optical coatings. You will want to pick up the IR off of the reflection and not off of the transmission through the beamsplitter because the glass used in beamsplitters may not transmit IR very well. Depending on your detectors, you will then need to install the appropriate filters to pass only the wavelengths you are looking for. Filters can be very cheap.

Answer 2

This is usually done with a dichroic beamsplitter. That's a mirror with a dielectric interference coating that reflects part of the spectrum and transmits the rest. you put it behind your lens at a 45° angle. There is a common variety of this called a cold mirror (reflects visible, transmits IR) that might work in your application.

Answer 3

Usually there is a use of prism to split different wave lengths after the lens (this is how its done in 3 CCD digital camera).