LED Color Modifications, Mixing, Adjustments, Filtering, etc.

For special colors, etc.

updated 3/22/2011

To get yellowish green for video display primary green usage, etc.
To get nice deep pure red with good brightness
To get a non-orangish yellow

To get yellowish green for video display primary green usage,etc.

I get asked a fair number of times about achieving or obtaining a high efficiency green LED that is less bluish and more yellowish than the usual InGaN high brightness green ones, with dominant wavelength of 540, 545 or 550 nm. The usual ones have dominant wavelength typically around 525 or 530 nm. The green primary in a monitor complying with sRGB has a dominant wavelength around 549 nm, near 550 nm.

In an LED video display where the red LEDs have dominant wavelength of 625 nm (typical for InGaAlP red) and the green ones have a dominant wavelength of 530 nm, there is a way to make the "green primary" closer to that of most non-LED video displays:

Make the red LEDs receive a signal that is 84% of the "red signal" plus 16% of the "green signal". Do this after any nonlinear circuitry/processes that are used to achieve non-unity gamma. Adjust gain for each of the three primary color signals after this mixing process to achieve a "white point" of 6500 K or whatever you want "white" to be.

When this is done, the color achieved with a "pure green signal" has a dominant wavelength of 544 nm according to my calculations, assuming the three primary color gains are adjusted to achieve 6500K for white point and the blue primary has the same dominant wavelength as that of the blue primary in an sRGB monitor (approx. 464 nm).

This 544 nm "green primary" including any of most InGaN green LEDs with dominant wavelength of 530 nm has its z 1931 CIE chromaticity less than that of the green primary specified by sRGB. Mixing light from InGaN green LEDs with 530 nm dominant wavelength with light from red LEDs does indeed achieve yellows at least as saturated as those achieved by CRT monitors and CRT television / TV receivers / TVs.

I chose 544 nm dominant wavelength for "effective green primary" to use with 625 nm dominant wavelength red because use of these in a ratio of 77.3% green 22.7% red (by photometric content) achieves a yellow that when mixed with an appropriate amount of 464 nm blue achieves "D65" 6500 K white. A monitor complying with sRGB, when accomplishing yellow (red and green signals equal, blue signal being zero) has 77.3-77.4% of its photometric output from the green phosphor and 22.6-22.7% of its photometric output from its red phosphor.

If your video display using red, green and blue LEDs is an LCD one with an RGB backlight or a tricolor projection one, then the green needs to be adjusted by filtering with a yellow filter. Wratten #12 appears to me suitable, by appearing to me to alter the light from an LED having a dominant wavelength of upper 520s to 530 nm to a "lime green" shade having a dominant wavelength in the low 540s of nm. Yellow acrylic sheet such as yellow "Plexiglas" should achieve similar results, and should have good resistance to fading when exposed to intense green LED light for a large amount of time.

As for longer dominant wavelength green InGaN LEDs: Nichia has some available with color rank of H (dominant wavelength in the 530s of nm). Nichia also has some models with typical 1931 CIE chromaticity of x,y of (.189, .718) which has dominant wavelength around 533 nm as well as saturation that I consider above-average for green InGaN LEDs.

It is also notable that green InGaN LEDs tend to have color varying with current, in a way making them less bluish and more lime green or yellowish green as current decreases. InGaN LEDs also tend to have their efficiency increased when they are moderately or moderately severely underpowered. Some InGaN green LEDs achieve a nice "lime green" color with maybe-useful brightness when they are underpowered.

To get nice deep pure red with good brightness

The LEDs that are best for this have peak wavelength around 660 nm. Their dominant wavelength is generally close to 640 nm. A slightly shorter wavelength variant notably by Avago has dominant wavelength of typically 637 nm and peak wavelength around 654 nm. Sometimes the dominant wavelength (a color specification roughly meaning hue) is overstated towards 650 nm.

As for LEDs to name:

For more output or higher efficiency, what I would suggest is specific ones that I mention here.

There is a lower power one that I like for nice deep pure red color and efficiency high enough for usefulness as a low current LED for indicator lamp duty: Kingbright WP7113LSRD. That one is useful for "star map lights", AKA "gazer lights". That one has deeper red color than usual even for GaAlAsP red LED chemistry, with dominant wavelength probably ~ 642-644 nm (updated 1/5/2014) and s/p ratio likely around .16 - extremely low.

If you want deeper pure red than that with just a little less efficiency - that can be done. However, the color can only be changed a little, since the LEDs mentioned above are already nearly essentially pure red in color.

There are deep red filters that are useful for "purifying" the light from these red LEDs:

Wratten 92 and Schott RG630. A true Wratten 92 is a "gel", as in a thin plastic sheet. I don't know how stable that is if it's blasted with intense LED light 24/7/365. The Schott RG630 is glass, presumably very durable.

The above red LEDs, after being filtered by either of these filters, have dominant wavelength around 648-650 nm, with s/p ratio around .145-.15 which is close to "rock bottom" of ~.14.

To get a non-orangish yellow

Filter white LEDs with a Wratten #12 or similar #12 filter, or with Schott GG495 glass. Yellow acrylic sheet such as yellow "plexiglas" works, but may be milder filtering that may appear slightly greenish. Use Schott GG515 glass or a #15 filter if you want a less greenish shade of yellow, but still less orangish than usual yellow LEDs.

Written by Don Klipstein.

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