Suppose you wanted to model a wedged airgap between the two silica slabs.  It might seem like the easiest thing to do would be to tilt one of the slabs, by entering a small angle value for the “Tilt About Z” of the bottom slab.  Unfortunately, that won’t work because ZEMAX, you’ll recall, only sees an airgap if it’s created using a coating between two surfaces that are in direct contact with one another, because it needs all three sets of material data.

The answer here is to keep the two slabs as they are (parallel and in direct contact) and then to create a new air coating that is wedged.  We do this using the TAPER coating function, specifically the [Cartesian] Polynomial Taper (PT).

Add the following lines to your coating.dat file:

TAPR LINEAR
PT 0 1
PT 1 0.1

COAT FTIR_LINEAR
AIR 0.200 1 0 LINEAR

The first three lines define a taper shape, which is linearly increasing as a function of x across the surface. 

The second two lines define a coating that has a nominal thickness of 200nm and then varies in thickness according to the taper function defined above.  Following the description of tapered coatings in the ZEMAX manual, we see that the thickness of the coating is actually given by the following equation:

d_eff = (0.2mm) * (0.1x + 1),

where d_eff  is the effective coating thickness and x is the distance measured in the x-direction across the surface.After adding the above lines to the coating.dat file and then saving the file, go back into ZEMAX and click to “Reload Coating File” again.  Change the coating on Surface 2 to “FTIR_LINEAR.” 

Now clear the detectors and run a new raytrace.

Now you’ll see that the amount of light that is TIRed varies across the surface in the x-direction.  Note that there is still a conservation of light: the sum of the total powers on the two detectors is still 1.0 Watt (46.2% on the top detector and 53.8% on the bottom detector).