Throwing Light on Light Emitting Diodes - William-Robert-Robertson/WildCamera GitHub Wiki

Early-on in their development, LEDs had fairly limited light output - the output of the first visible LEDs was so limited that it was difficult to see them in daylight.

The range of colours was also limited - the first commercial LEDs produced only IR (Infra Red) light not visible light - then gradually red and green also became available - with yellow achieved by mixing red and green light - but for many years the light output of LEDs remained vary limited and much lower than could be achieved by conventional light bulbs.

Modern semiconductor structures have brought LEDs that are much brighter than before, much more efficient than before and available in a much wider choice of wavelengths - and so a much wider choice of colours - than before.

Wildlife cameras typically use IR (Infra Red) LEDs because infrared light is not visible to most mammals and so is thought not to cause disturbance to them.

Early LEDs could handle only small currents so were usually driven from a fixed voltage via a resistor. The light intensity could be adjusted by applying pulse to mark modulation (usually referred to as PWM - Pulse Width Modulation) directly to the LED and relying on human persistente of vision.

As LEDs became more and more powerful and able to handle larger and larger currents the need for more specialised drive circuits grew. LEDs function best when driven from a supply which aims to maintain a fixed output current by adjusting its output voltage - referred to as a "fixed current supply" - to adjust the intensity of the light this current can be adjusted by supplies referred to as a "programmable current supply" or "programmable fixed current supply". This is in contrast to most electronic equipment which requires a fixed voltage supply.

The terminology can be confusing - a "programmable current supply" can also be referred to as "programmable fixed current supply".

Driving LEDs this way has a number of advantages over PWM. By driving the LEDs at less than full rated output the efficiency of the LEDs in converting electricity into light and their lifetime are both optimised.

Programmable LED driver chips and modules can accept a power supply and an intensity modulation input signal as a PWM signal, an analog signal or a value transmitted over a bus like I2C or SPI.