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External Quantum Efficiency

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Loss due to Total Internal Reflection
Let us consider an emission where the light ray due to emission makes an angle θ with respect
to the normal to the interface and the angle of incidence is θi. When light travels from a denser
medium (of refractive index n1) to a rarer medium (of refractive index n2), for an angle of
incidence greater than the critical angle (θ௖ = sinିଵ(nଶ/nଵ), total internal reflection occurs,
which implies that the emitted light is completely reflected back into the same medium. In our
case, the critical angle (θ௖

) is sinିଵ(1/nଵ).

Thus, when the angle of incidence θi is greater than a critical angle θ௖

, the emitted light is not
available at the output of the device. If we consider the source of emission as a point source, it
may emit in all possible directions, but only a fraction of this is available outside the device.
The output is available in the cone of half-angle which corresponds to θ௜