I bought two lots of these 5mm white LEDs, mentioned in
another thread ( 26Kmcd on Ebay, anyone try these? Comparison pics).
They are very bright indeed at 20mA, but didn't seem equally
dazzling at lower currents, so I made some measurements
just to see. My findings follow.
I gauged relative outputs by applying a given current
to the LED under test, then measuring the photocurrent
produced in a solar cell illuminated by the LED.
Please note that the comparison to the Nichia LED is not
completely fair, as this setup did not catch all of the
light from either LED type. [In a setup designed to do
that, with a forward current of 20mA, the new LEDs
produced twice the photocurrent of the very best Nichias
(NSPW500BS, circa 2002-2003) I could find in my extensive
stash.]
NOTES:
<ul type="square"> [*]LED #1 = eBay 26,000mcd, from warden_jp2002
[*]LED #2 = Nichia NSPW500BS, from 2002-2003
[*]Luminous efficacy ('n') units are 'uA of photocurrent per mW of LED power'
[*]missing data points were filled in by linear interpolation,
redundant data points were combined by averaging.[/list]
<font class="small">Code:</font><hr /><pre> --------- LED #1 ---------- ------ LED #2 ------
i(LED) iPhoto (uA) iPhoto Ratio
Run #1 Run #2 Vf(1) n(1) Run #1 Vf(2) n(2) (n2/n1)
------ ------ ------ ----- ---- ------ ----- ---- -------
25 uA 0.29 0.28 2.346 4.9 3.33 2.510 53.1 10.9
50 0.78 0.82 2.401 6.7 7.00 2.551 54.9 8.24
100 2.53 2.49 2.447 10.3 14.3 2.604 54.9 5.35
250 13.1 11.8 2.494 20.0 35.5 2.693 52.7 2.64
500 42.8 38.2 2.560 31.6 69.1 2.779 49.7 1.57
750 79.0 40.7 101 47.6 1.17
1.0 mA 117 109 2.622 43.1 132 2.878 50.3 1.17
1.5 157 2.666 39.3 191 2.943 47.8 1.22
2.0 274 254 2.701 48.9 248 2.994 45.9 0.939
4.81 681 692 2.825 50.5 529 3.153 38.9 0.771
9.62 1320 1330 2.988 46.1 966 3.328 33.6 0.729
14.4 1870 1870 3.108 41.7 1340 3.449 29.9 0.717
</pre><hr />
I fully tested only one of the wardenjp2002 LEDs, but
then checked 5 samples at random from 2 lots to confirm
the fully tested unit was indeed closely representative
of the entire batch. It was. It seems wardenjp2002's
LEDs are closely matched.
In another test for longevity, I ran a single unit for ~300
hours, after which time the output was unchanged, +/-
a few percent.
Summary: the 26,000mcd LEDs are quite a bargain, but
inefficient at low currents. I suspect the dice used
are either leaky, or have been slightly damaged, neither
of which affects their usefulness when driven hard.
Jamaica
(edit 2/28/05: corrected a Vf data point)
another thread ( 26Kmcd on Ebay, anyone try these? Comparison pics).
They are very bright indeed at 20mA, but didn't seem equally
dazzling at lower currents, so I made some measurements
just to see. My findings follow.
I gauged relative outputs by applying a given current
to the LED under test, then measuring the photocurrent
produced in a solar cell illuminated by the LED.
Please note that the comparison to the Nichia LED is not
completely fair, as this setup did not catch all of the
light from either LED type. [In a setup designed to do
that, with a forward current of 20mA, the new LEDs
produced twice the photocurrent of the very best Nichias
(NSPW500BS, circa 2002-2003) I could find in my extensive
stash.]
NOTES:
<ul type="square"> [*]LED #1 = eBay 26,000mcd, from warden_jp2002
[*]LED #2 = Nichia NSPW500BS, from 2002-2003
[*]Luminous efficacy ('n') units are 'uA of photocurrent per mW of LED power'
[*]missing data points were filled in by linear interpolation,
redundant data points were combined by averaging.[/list]
<font class="small">Code:</font><hr /><pre> --------- LED #1 ---------- ------ LED #2 ------
i(LED) iPhoto (uA) iPhoto Ratio
Run #1 Run #2 Vf(1) n(1) Run #1 Vf(2) n(2) (n2/n1)
------ ------ ------ ----- ---- ------ ----- ---- -------
25 uA 0.29 0.28 2.346 4.9 3.33 2.510 53.1 10.9
50 0.78 0.82 2.401 6.7 7.00 2.551 54.9 8.24
100 2.53 2.49 2.447 10.3 14.3 2.604 54.9 5.35
250 13.1 11.8 2.494 20.0 35.5 2.693 52.7 2.64
500 42.8 38.2 2.560 31.6 69.1 2.779 49.7 1.57
750 79.0 40.7 101 47.6 1.17
1.0 mA 117 109 2.622 43.1 132 2.878 50.3 1.17
1.5 157 2.666 39.3 191 2.943 47.8 1.22
2.0 274 254 2.701 48.9 248 2.994 45.9 0.939
4.81 681 692 2.825 50.5 529 3.153 38.9 0.771
9.62 1320 1330 2.988 46.1 966 3.328 33.6 0.729
14.4 1870 1870 3.108 41.7 1340 3.449 29.9 0.717
</pre><hr />
I fully tested only one of the wardenjp2002 LEDs, but
then checked 5 samples at random from 2 lots to confirm
the fully tested unit was indeed closely representative
of the entire batch. It was. It seems wardenjp2002's
LEDs are closely matched.
In another test for longevity, I ran a single unit for ~300
hours, after which time the output was unchanged, +/-
a few percent.
Summary: the 26,000mcd LEDs are quite a bargain, but
inefficient at low currents. I suspect the dice used
are either leaky, or have been slightly damaged, neither
of which affects their usefulness when driven hard.
Jamaica
(edit 2/28/05: corrected a Vf data point)