Choosing an LED for your lighting system is a critical decision and will affect all other elements of your design. You will need to balance initial efficacy against initial light quality; and both against cost. As you will see in the other sections of this brochure, your LED choice will influence the total system cost, final light quality and total efficacy (Illuminance) of your design by impacting your optics, driver and thermal management options.

Illumination requirements – the amount of light required over a specified area – will determine the type and number of LEDs you will need. The important LED specification to consider is the luminous flux (lumens), or the amount of light emitted by the LED at specific drive currents. Be aware that all the light that can be generated from the LED might not reach the target due to unavoidable inefficiencies in each of the subsystems (optics, drivers and thermal management). The better the individual subsystems work together, the more light will reach the illumination target area. However, there is almost always a trade-off between cost and efficiency.

Specific color requirements will determine which LED color characteristics are most critical. For example, if the illumination requirement is for white light, the Correlated Color Temperature (CCT) or the Color Rendering Index (CRI) of the LED may be important. Keep in mind that there is a trade-off between CCT and luminous efficacy (lumens per Watt). Warm white LEDs (2500K~4000K) with light resembling incandescent bulbs are less efficient than cool white LEDs (5000K~8000K) with light resembling fluorescent bulbs.

If your illumination must be uniform, the quality of the LED and variation from LED to LED become very important and "binning" options must be considered. Manufacturers test, sort, then sell high-power LEDs by their color and luminous flux (lumens). LEDs with similar color and brightness are placed in the same "bin". The "smaller" the bin, the more uniform the LEDs in each bin will be. Bin sizes vary from manufacturer to manufacturer.

Since there are currently no standards for testing LEDs on any of these performance characteristics, it is impossible to accomplish an apples-to-apples comparison of multiple manufacturers' LEDs from published specifications alone. It is even more difficult to gauge the impact each subsystem will have on total performance. Therefore, it is necessary to build and test evaluation circuits before choosing LED.