led
I read these terms in various places and every single time the LED's are just used as a signaling LED in a small circuit, requiring only a few milliamps to light up.
What is the difference? Or is it just marketing speech from different manufacturers?
The amount of light LEDs put out has a very linear relationship with the current they are driven with, and the voltage to current graph is relatively constant as they are diodes with a characteristic junction voltage. Taken together, this means that the efficiency can rise about 25% if an LED is driven at 10% of rated brightness (in this LED, but others should be fairly similar).
In any case, regardless of the level you choose to drive the LED at, what you want is a "single cell LED driver". These are usually current-regulated, eliminating the need for any resistors which do nothing for your efficiency, and also have the ability to step-up the voltage, so you can drive a 3 V nominal LED with one or two 1.2 V NiMH cells.
Linear, Micrel, and Diodes Inc all make drivers for this purpose, and you can find many more in a search for "single cell LED driver".
What comes to my mind for minimum headroom is a current mirror circuit:
Caveat: I haven't tried this myself.
This should work for battery voltage down to 3.8 V or a little bit more.
It probably works best with a matched transistor pair. These are available under $0.40 each in small quantities (I found NXP's BCM61B in a quick search), so this is fairly low cost.
The resistor in the emitter on the left is there so that the LED gets some multiple of the current driven in on the right (roughly (2.5 - 0.7)/R ), which will improve the power efficiency of the circuit. I suspect, though, that it will contribute some variability in the LED drive current from location to location, so you'd need to do some experimentation to see if this is a problem.
Best Answer
The terms listed do have relevance, albeit limited: There are no globally mandated benchmarks, I believe, for what constitutes low current, or high intensity in an LED.
In general, "middle-of-the-road" LEDs are usually specified at 20 to 25 mA. Thus, an LED that glows at nominally "full" intensity at 5 mA (maybe even 10) would be considered low current.
Over successive generations of LED technology, the luminosity per Watt for LEDs has increased steadily - to the extent that "Haitz's law" observes that light generated per LED in each color increases by a factor of 20 per decade. Some manufacturers market their 100 Lumen/Watt and better LEDs (post 2010) as "High intensity", but that too is clearly a moving target.
High efficiency in LEDs - (warning, moving fully into speculation-space) would be a measure of the light generated in ratio to wasted heat - Of special significance for large high-current LEDs as used in lighting fixtures, where significant design effort goes into cooling / heat-sinking design for the LEDs. Higher the efficiency, lower would be the cooling cost per lumen of light. Where does one draw the border on this? The marketing folks would have to say.
In summary: Until global standardization of these terms, and compliance with such standards, this is essentially marketing-speak.