Cold/Hot Mirrors
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Cold Mirror - Dichroic reflector that reflects visible light and transmits infrared (IR) light producing a cool beam of reflected visible light.

Hot Mirror - Dichroic reflector that reflects infrared (IR) light and transmits visible light.  Can be used as a filter to create a cool beam of transmitted visible light.

Cold Mirror Design

The cold mirror utilizes a long-wavelength pass filter stack design
of the basic form:

where,     H = QWOT of high index material, or TiO2 (nH = 2.4 @ 550nm)
                L = QWOT of low index material, or SiO2 (nL = 1.46 @ 550nm)
                and, the substrate is glass (nS = 1.52 @ 550nm)

If the designer desires high reflectivity over the entire visible spectral region
(400 - 700nm), one stack of the above design
(where the QWOT is centered at l=550nm)
will not be sufficient...see below.

Instead, the designer needs to overlap stacks so that the high reflectivity regions overlap to cover the entire visible spectrum...see below.

overlaps a stack at longer wavelengths

overlaps a stack a shorter wavlengths

So, if we put all three stacks on our substrate we get:

***Here's a little designer tid-bit...
If one adds a HWOT (see terminology) layer of L to the top of the design,
the IR reflectivity will be minimized.  See below.

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Hot Mirror Design

  The hot mirror utilizes a short-wavelength pass filter stack design
of the basic form:

where,     H = QWOT of high index material, or TiO2 (nH = 2.4 @ 550nm)
                L = QWOT of low index material, or SiO2 (nL = 1.46 @ 550nm)
                and, the substrate is glass (nS = 1.52 @ 550nm)

If the designer desires low reflectivity over the entire visible spectral region
(400 - 700nm), one stack of the above design
(where the QWOT l=550nm) is about all that one can use. 
There is a harmonic of high reflectivity in the ultraviolet region
that prevents the designer from overlapping stacks...see below.