Absorptance in multilayer dielectric I have a simple question regarding absorptance in the 2nd of two back to back dielectric slabs:
If I know the total reflectance R, transmittance through the first slab t1, and total transmittance T (transmittance through both slabs), then my absorptance is:


*

*A(first slab) = 1 - R - t1

*A(both slabs) = 1 - R - T


What would be the Absorptance through the second slab?
 A: Your question is not entirely clear, and it is not obvious to me that you have provided enough information to answer the question. However, there are two or three different methods you might use to find information about a second slab given the transmission, reflection, absorption of the first slab, and the corresponding information for the joint system of two slabs.
At interfaces between different materials, light is reflected as predicted by the Fresnel reflection coefficients. The coefficients in general depend on the polarisation of light incident on the interface. For an explanation of the Fresnel coefficients, see Section 7.3 of "Classical Electrodynamics" by Jackson.
In the bulk of the material, light propagates and gains a phase of $\exp(iknx)$ where $k$ is the magnitude of the wave vector in the direction of propagation, $n$ is the refractive index and x is the thickness of the layer. The refractive index for a system that absorbs light would be complex. The absorption would be given by $\exp(ikn_ix)$, where $n_i$ is the imaginary part of $n$.
In order to work out what is happening in a composite system you have to take into account the effects of the interfaces and of multiple rounds of reflection between each pair of interfaces. For a brief account of multiple beam interference see this document and references therein:
http://www.physik.uni-siegen.de/quantenoptik/lehre/fpraktikum/fabry-perot.pdf. 
The formulae given in the document above might help you solve your problem, but they are likely to be cumbersome for the problem you have in mind since it presumably involves at least three interfaces: air and first slab, first slab and second slab, second slab and air. If you want to do the algebraically by hand or using Mathematica, you may find it easier to use transfer matrices, which are explained here:
http://arxiv.org/abs/1205.1318.
If you want to do the calculation numerically, then you are probably better off using S matrices for reasons of numerical stability:
http://inside.mines.edu/~ayuffa/Research/Yuffa_J_Comp_Phys_231.pdf.
