Making a white LED yellow-green

Making an Ultra-Efficient Yellow-Green LED Lamp

Updated 5/11/2009.

CAUTION - This involves hacking/homebrewing with possible hazards. I disclaim adequacy/completeness/etc. of hazards/safety information for the following hacking/homebrewing. Do anything mentioned below only at your own risk!

Here is how to make yellow-green LED lamps that have an overall luminous efficacy of somewhere around 80 lumens/watt by adding green-fluorescing yellow plastic to appropriate white or blue LEDs!

UPDATE 5/11/2009: The most efficient modern blue LEDs good for this may achieve 125 lumens/watt with this trick, maybe with modifications. The most efficient low power white LED that I am aware of achieves 140-150 lumens/watt at 20 mA (Nichia NSPWR70CS-K1, A.K.A. Nichia NSPWR70CSS-K1), with prospect for slight improvement from there by converting most of its blue spectral content to green light. Overall color with that 5000 Kelvin white LED would probably be changed to a yellowish-chartreuse "Gatorade" color. Higher color temperature white LEDs will be made a bit more greenish, but still almost as yellow as green. Blue LEDs will be changed into something much more of a green, as in a "slightly whitish yellowish lime green" shade of green, maybe close to that of the green phosphor in CRT monitors and CRT color TVs.

You will need some of this plastic. There are various kinds that are yellow and glow yellow-green in a manner resembling that of alkaline solutions of fluorescein (or of salts thereof). You can get acrylic with such a tint at some of those plastics shops that artists and signmakers go to.

(Stuff on using other plastics deleted on 9/23/2001 due to repeated unsuccessful results along with high risk of fire/burns/fumes.)

UPDATE 9/16/2001: I have found acrylic sheet and tubing to be better than fluorescent beer cups.

UPDATE 5/11/2009: Beware that I now know of two visibly discernable (at least to me) variants of green-fluorescing yellow acrylic. One has fluorescence being "lime green", while the other has its fluorescence being a more yellowish shade of green like the color of most "old tech"/"cheap" green LEDs. I recommend the one with the more greenish fluorescence.

My best results so far were achieved by cutting a piece of 3/8 inch or 1/2 inch acrylic rod into a piece maybe 3/4 inch long and drilling a hole into one end of this piece and gluing the LED into the hole. You may want to use a smaller size LED such as T1 (3 mm) and/or sand/grind down the LED to a smaller size to enable use of a smaller hole. Acrylic cracks easily when drilled. Drill gently with a higher speed drill to minimize the chance of breaking the rod.

THERE IS ANOTHER STEP

Smooth transparent fluorescent plastic will trap much of the fluorescence by total internal reflection. Outgoing photons that hit the surface at an angle more parallel to the surface than some critical angle around 45 degrees have a 100 percent rate of being reflected back in and have a high risk of getting absorbed before finding a way out. So you need to roughen up the outer surface with sandpaper to allow more of the fluorescence to escape. Also, the light from the fluorescence and the light from the LED chip will take different paths and cause an uneven color unless you make the LED lamp diffused enough to make the light from both mechanisms go everywhere.

I would like to add/clarify on 5/11/2009 that I recommend filling the airspace between the LED and the drilled-out acrylic rod with epoxy or with acrylic casting resin. This reduces internal reflections of kinds that tend to favor losses.

UPDATE 9/16/2001 - Actually made an "LED fluorescent lamp" by drilling a 3/8 inch acrylic rod and gluing a small LED into it! Reddish magenta plastic was disappointing for ability to utilize blue light for purposes of creating a "purple LED lamp". But the yellow stuff that fluoresces green looks especially promising.

Note that fluorescent acrylic ("Plexiglass" and the like) is generally not good at efficiently fluorescing from UV the way it sometimes is good at fluorescing from some visible wavelengths!

RELATED MAD IDEAS:

1. Use blue LEDs instead of white ones to get more of a lime-yellow-green instead of a chartreuse-greenish-yellow.

2. Use white or blue LEDs with shorter peak wavelength around 450-460 nm as opposed to longer around 470 nm.

How an LED Lamp Manufacturer Would Do This:

I think the real way to do this is to tint the epoxy used to make the LED lamp body with a suitable fluorescent dye as well as a diffusing agent to make the LED lamp translucent ("milky") instead of transparent.

Another method would be putting a small blob of *translucent* (as opposed to transparent) fluorescent plastic (or powdered fluorescent plastic or a mixture of powdered fluorescent plastic and epoxy) into the die cup of the LED lamp and over the semiconductor die before molding the main epoxy body over all of this. Just don't cook the semiconductor die molding on molten fluorescent plastic! WARNING - the fluorescent dye may degrade in unacceptably short time if it is close to the LED die and is illuminated very intensely.

Written by Don Klipstein.

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