CHAPTER 10 ■ LEDS
To determine which lead is the cathode, look for a flat or notched side on the bottom of the otherwise
round lens. Also, the cathode lead wire is usually shorter than the anode lead wire.
Most often (but not always), the cathode is connected to the side with the reflector cup (see Figure 10-8
and Figure 10-9). The reflector cup is a tiny, rounded mirror that aims light forward.
Figure 10-9. View of a reflector cup and die
Figure 10-10. Glowing die with a view of the bonding wire and bonding site
Inside the reflector cup is a die (see Figure 10-10), also called a chip or pellet. This is the part of the LED
that actually emits light. The manufacturer adjusts the chemistry of the die to control the LED’s color.