Optical calculations for SSL applications

June 12th, 2014, Published in Articles: Vector


Rough calculations are useful for estimating LED type and count at the beginning of a project. This article provides some typical techniques for calculating optical quantities and estimating LED count.

A basic understanding of optical principles is necessary before any calculations can be started. The most important principles for SSL applications are reflection, refraction and total internal reflection (TIR).


The law of reflection states that, for a ray striking a reflective surface, the angle of incidence is equal to the angle of reflection.


A reflection which obeys this law is called a specular reflection. If the reflective surface has some roughness (which is always the case), some portion of the incident light will be reflected at some angle other than the specular angle of reflection. Under extreme conditions, the reflective surface is perfectly diffuse and the reflected light scatters in all directions. In practice, reflections are somewhere between perfectly specular and perfectly diffuse (see Fig. 1). Light passing through a surface can also be transmitted diffusely.

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