What are the most important factors in LED lighting technology?
1) LED CHIP
LED is a semiconductor which emits light when an electric current passes through. ALL LEDs are designed to work with a particular current (not voltage). In most cases the lamp manufacturer does not produce the chip, they source it from a handful of global suppliers. Knowing the actual source of the LED chip is important but not enough. Knowing also what kind of chip BIN is used is vital for colour consistency. Binning is the process of splitting the semiconductor wafer in many smaller pieces which are then sold. The tolerance in colour variation varies within 1 BIN. The greater the tolerance, the greater the possible colour variation.
Therefore you can have 2 identical lamps (same model and spec) but because the chips are coming from lower quality bins the actual colour emitted can be visibly different (eg. 3100K & 3800K).
Going one step further, specific LED chips are being designed for high colour rendering (above CRI90), whilst other chips are designed for intelligent control of White light (able to control the white colour emitted from 2700K to 5000K from the same luminaire).
2) HEAT MANAGEMENT
LEDs do not emit heat BUT they do heat up. The higher the current the more they heat up. The heat has to be managed in order to maintain a maximum temp of around 50degC. Beyond this temperature the lifetime of the LED will decay very fast. Eg; I can pass 1A current through a chip which was designed to take 350mA. Obviously the light will be more but the lifetime will be extremely short.
This is exactly why it is not advisable to replace a conventional bulb to LED in an existing external light fitting. External fittings are sealed and therefore the heat generated by the led remains contained inside the fitting reducing the lifetime of the led bulb.
A driver is always needed to transform any voltage to the constant current led needs. LEDs ONLY run on constant current (NOT constant voltage). This is true for any LED being considered. You will find replacement bulbs such as GU10 (direct 240VAC). These would have a driver inside. Same thing with an MR16 (low voltage) halogen replacement; even in this case the LED lamp would have a driver. In other cases you have complete LED luminaire (internal or external) which would have incorporated the driver. Finally you will also find very small LED lights, too small to contain the driver and in such cases the driver would be external. ONLY in this latter case you will find the spec saying 350mA or 700mA and this means that a driver is needed and IMPERATIVE is that the wiring in this case is in series (explained below). Even in this case, the LED light manufacturer purchases the drivers.
Is the wattage of LED lamps a good measure of light output?
Absolutely NOT. The light output is much more important (measured in lumens ‘lm’). The efficiency of an LED lamp/fittings varies a lot between manufacturer and manufacturer. The efficiency (lm/W) of the LED is dependent on all factors described above.