Question

How to convert reflectance to absorbance?

Answer 1

I see this is an old question, but I don't feel it's been properly answered in this forum yet. So let me add a few comments.

Reflection (%Reflectance, etc.) is an interface, or surface phenomenon. While Transmittance (%Transmittance, etc.) is a bulk phenomenon.

For example, anti-reflection coatings are often applied to lenses to increase the amount of light that is coupled from the air, across the air/glass interface, and into the bulk of the lens. Then, on the other side of the lens, another similar coating again increases the light coupled across that interface. If the coating is efficient, only a fraction of a percent or so is reflected at each interface (for bare glass, normally about 4% would be reflected at each interface). Hence, with an AR coating, the majority of losses are those that occur in the bulk. If the lens is made of high quality optical glass, and the wavelength is in the UV/VIS/NIR region, bulk losses are very small. For example, N-BK7 (Schott glass) has a bulk absorption coefficient of about 0.0024 cm-1.

When you think about reflectance in this way, converting between transmittance and reflectance is rather simple. The transmittance (through the coating/interface) is simply T = 1-R.

Another way to view this is via the Fresnel Equations (see: https://en.wikipedia.org/wiki/Fresnel_equations) that describe the reflection of light at an interface based on the index of refraction of the two materials at the interface, the angle of incidence, and the polarization of the light with respect to the plane of incidence. Under certain circumstances the light could be coupled through the interface and into the bulk with zero reflection, even without an anti-reflection coating. Conversely, for other conditions, light could be totally reflected with zero coupled across the interface. Again, this is a description of a surface phenomenon.

On the other hand, once coupled across the interface and into the bulk, transmittance and absorbance now have a new, simple relationship. By definition, T = 10-A (-log(T) = A). You may already know of the famous Beer-Lambert law (from freshman chemistry) that relates molar absorption coefficient, concentration and path length: A = epsilon * c * l. Not all materials obey the Beer-Lambert law, but most (optically linear) materials do.

So, if you're talking about some kind of optical component, like a lens or a window, and you're using it in its design wavelength band, then you may be able to ignore bulk losses and use the %reflection figure to calculate %transmittance. Just remember there's an interface at both sides: one on the incident side and another on the exigent (cool word!) side. So Transmittance becomes: T = (1-R)2. If you like, it's easy to include bulk losses, just subtract them: T = (1-R)2 - alpha*thickness. Where alpha is the absorption coefficient.

Obviously, these examples are highly simplified. And I haven't even begun to talk about what happens with metals, or non-dielectrics in general. If you really want to get into it, Maxwell's equations and a bit of quantum mechanics enter the discussion. Not to mention physics, chemistry, materials science .... But I'm pretty sure that's not part of your question