"Pure Red" LEDs Good at 1-2 Milliamps
InGaN Green LEDs Good at .22-.5 Milliamp
Yellow LEDs Good at 1.1-2 Milliamps
Kingbright WP7104LSRD (3 mm / "T1") and WP7113LSRD (5 mm /"T1-3/4")
Both of these are diffused red tinted, lower cost LEDs. Both are characterized to have intensity of 20 mcd at 2 mA. I find them good at 1 mA and fairly usable at .5 mA, though the rated viewing angle is only 30-40 degrees. Although the viewing angle is notably narrow, the performance is so good and the cost is so low that these can be useful in many applications.
All 3 of the "pure red" LEDs in this category are available from Digi-Key.
Nichia NSPG320C / NSPG320CS (3 mm / "T1")
This LED is a clear one rated to have a 45 degree viewing angle, but in my experience (only one lot) it is almost as "wide angle" as many rated to have the usual 60 degree viewing angle of diffused LEDs intended for use as indicator lamps.
My actual measurement of a small number of units of only one lot is 25-40 millicandela at .21-.22 milliamp. I found brightness to be typical of LED indicator lamps at .1 mA with this lot, despite efficiency tending to decrease as current decreases below 1.5-2 milliamps. Expect other lots to have brightness "at least sufficient for use as indicator lamps" at .25 mA.
Nichia NSPG520AS (5 mm / "T1-3/4")
This LED is a clear one rated to have a 45 degree viewing angle, but in my experience (only one lot) it is almost as "wide angle" as many rated to have the usual 60 degree viewing angle of diffused LEDs intended for use as indicator lamps. It has slightly less "wide angle" than the 3 mm one above.
My actual measurement of a small number of units of only one lot is 25-40 millicandela at .15-.16 milliamp. I found brightness to be typical of LED indicator lamps at .07 mA with this lot, despite efficiency tending to decrease as current decreases below 1.5-1.8 milliamps. Expect other lots to have brightness "at least sufficient for use as indicator lamps" at .22 mA.
UPDATE 7/28-29/2009 on Nichia NSPG520AS - I tested one at 1.69 mA and achieved .58 lumen and about 800-900 millicandela. I found it generally visible in direct sunlight and usually uncomfortably bright to look at in normal room lighting (to extent of mildly putting spots in my eyes) at 1.69 mA.
Nichia LEDs are best available in USA from their own USA sales office.
TT Electronics / Opto Technology OVLLG8C7 is a much lower cost LED. It generally achieves at least 3 mcd at .35 mA according to my testing of units probably from only one lot. However, I expect that part number to reliably produce at least 2 mcd at .44 mA unless "static electricity damage" has been incurred.
My experience with this LED has intensity at 20 mA being 1500-plus mcd, as opposed to the 670 mcd mentioned in the datasheet. Maximum rated current is 20 rather than the usual 30 mA.
Both are clear yellow-tinted LEDs. The 3 mm one has a viewing angle of approx. 60 degrees. The 5 mm one has a viewing angle of approx. 40 degrees.
The 3 mm one achieves usable brightness at 1.5 mA, and the 5 mm one achieves usable brightness at 1.1 mA.
CAUTION on use at elevated temperatures: There is an aging mechanism in InGaAlP LEDs caused by a combination of temperature and amount of time with forward bias. This aging harms efficiency in general, and especially harms low current performance. If InGaAlP LEDs are expected to endure high temperatures despite low current, then it is recommended to pulse them with instantaneous current of around or over 10 milliamps. This reduces forward bias time and avoids having the LED perform at low instantaneous currents. It may help for the pulsing circuit to discharge stray capacitance in and across the LED when a pulse ends to reduce forward bias time.
Please read my Copyright and authorship info.
Please read my Disclaimer.