Re: LED engineers, what would cause a green LED to shift toward blue?
- From: don@xxxxxxxxxxxxxx (Don Klipstein)
- Date: Sun, 30 Jul 2006 05:23:16 +0000 (UTC)
In article <1154161771.956603.96520@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
itsme.ultimate@xxxxxxxxx wrote:
I have some LEDs rated at 525nm green @ 20mA.
When it is driven from a resistor limited DC power source and pushing
27mA, it looks green (see left)
I took apart a 2AA powered LED flashlight that drives the LED with a
distorted squarewave at 100KHz and the same LED looks turquoise(see
right)
http://img135.imageshack.us/img135/8747/ledcolorshiftfw3.jpg
Why is this?
I suspect the reason is usage of InGaN chemistry, where some shift in
color and wavelength as a result in major or sometimes even moderate
change in instantaneous current is rather common. Higher instantaneous
current can shift wavelength and color towards blue-violet and away from
yellow or yellow-green (shorter wavelength), while lower current can shift
color away from blue-violet and towards yellow or yellow-green (longer
wavelength).
I have found this fairly common in many blue, blue-green and
non-yellowish-green LEDs. I have even noticed impact of such spectral
shifts in a few white LEDs, and most white LEDs are based on blue-emitting
chips of InGaN chemistry.
Also note - sometimes and/or in some LEDs of this chemistry (especially
when peak wavelength at "characterizing current" is close to or less than
450 nm deep blue) the wavelength and color do not shift as much, although
I have found broader band mid-1990's 450-nm-peak GaN LEDs to shift more
subtly (with less-visible impact on visible color) to violet/UV.
One other LED chemistry where I have seen color shift result from
varying current: a "low current red" chemistry, as in red version of GaP
active chemistry (as opposed to only the chip substrate), and with peak
wavelength usually noted to be 690-700 nm. That is a "high efficiency
red"/"low current red" formulation developed in the 1970's, with
efficiency being high only at low currents and with the spectrum having
two peaks - one in the deep red 690-700 nm and the other in the green near
560 nm. As current increases, the main longer wavelength peak loses
efficiency.
- Don Klipstein (don@xxxxxxxxx)
.
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