Normally the voltage drop across LED while forward bias will be about 1.2 to 1.4 volts depend on type of LED by has the temperature coefficient is -1.5 mV per degree Celsius. Which similar to the temperature coefficient of junction. between a base and emitter of silicon transistors.
From this relationship can the constant current circuit that no temperature coefficient as show in Figure 1.
Figure 1 Constant current circuit using LED
From in the Figure current I flowing through the value.
(U LED – U BE) / R
And since temperature coefficient of transistor and LED fully offset.
Thus, Temperature occurs It does not affect the current flowing yet.