High Dome, Low Dome

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Jim_F

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Please help out a newbie. What are the differences between high and low dome. Is it brightness, is it design, etc. Inquiring minds want to know. Thanks.
 

Gransee

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Welcome to the CPF Jim. Good question!

Cut and Pasted from the Arc FAQ (which for many hours I slaved over a hot keyboard...):

How do White LEDs work?

White LEDs work differently than the other colors because current white LEDs actually use a blue LED with a phosphor cap to produce white light. The phospor converts some of the blue light into red/green which mixes with the blue to produce a white light. Getting this mix is a black art for the LED manufactures and there is currently quite a bit of variation from LED to LED. This also explains why a white LED has a blue cast to it.

LED Bin Codes Explained

In the manufacturing of semiconductor products, there is a variation of performance around the average values given in the technical data sheets. In LEDs, the variation is even greater than traditional CMOS found in your computer.

Like snowflakes, no two LEDs are alike. There are variations in color (tint), brightness (flux), forward voltage (Vf) and beam distribution/artifacts. Most LED manufacturers sort their LED by machine into bins. Each bin has a rank or window of values that all the parts in that bin fit within. In spite of the fact that only some of the variables are binned and the bins are fairly wide there can be quite a few bins. For the Luxeon Star, there are over 400 bin variations. A typical flux bin (Q for example) can have a 10 lumen window which means there will be an appreciable difference between parts even in the same bin.

From one extreme to another, a typical LED from the same production line, can have a 300% variation in one value alone.

Bin codes for 1W and 5W

A large percentage of the LEDs manufactured are unsuitable for flashlights in our opinion. Usually this is because the tint is too green for a white light. Arc hand sorts LEDs for each flashlight we make. With current yields, about 7 percent of the Luxeons received make it into a Arc-LS standard and about 1 percent make it into an Arc-LS Premium. This causes frequent shortages with the Arc-LS.

Even though Arc is so selective on the LEDs used, there will still be variations from unit to unit.

High Dome vs Low Dome

As explained above under how a white LED works, a phospor layer is used to re-emit some of the blue light. In most white LEDs, the phosphor is applied as a cap or fill around the LED chip ("die").

Lumileds started manufacturing their luxeon stars with a phosphor cap. This is called the "low dome" design because the dome over the LED is low and flat. In 2002, Lumileds improved the Luxeon Star by coming out with the EOS process. This EOS process changed how the phosphor is applied to the die. The die now has a even coating over the entire surface. This change was included with the new invertered die geometry ("Tip Chip, Lambertian") in their "high dome" design. The high dome product has a higher dome over the LED with a rounded tip.

Low dome LEDs, which are in the majority, have a tint gradient across the beam. Typically, the center of the beam is blueish while the edges are greenish.

High dome LEDs have a more consistant tint across the beam. They also have a smaller object size which makes for a more focused beam for a given optic. HDs (high domes) also show the die pattern and bond wires more clearly in the beam.

Here are some helpful links on this subject:
<ul type="square"> [*]Low Dome vs. High Dome
[*]LSH vs. LSL - which do you like better and why?
[*]Perceived Brightness, LD vs HD [/list]


Peter
 

Jim_F

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Dec 17, 2003
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Thanks for the info and the links, Peter. I guess I should be taking advantage of the search feature.
 
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