Virtually the final component in the conversion of radiometry to photometry for any light engine is the secondary optic. Most LED sources have either a sealing compound or primary lens already built in to their package. However the emission of these components, in their raw form, tend to have outputs of 2θ½ 120°~140°. This is, of course, dependant on the physical height of the chip within the package itself. The outputs of these sources can sometimes be used "as is", however, in many applications, secondary optics are required to allow the light engine to fulfill a dedicated task.

Although it sounds obvious, the first steps in selecting the best secondary optics for an application are to determine what illumination levels you want to achieve and over what area.  From these two figures the total amount of light required can be calculated.  Once you know how much light you need, it is then possible to decide what type of LED is required and how many.  Only when the inputs to the optics, (the type and number of LEDs), and the outputs, (the illumination level and beam shape), have been defined can the secondary optics be specified.