# getting red light by filtering white LED output... crazy??



## Steve K (Apr 5, 2013)

hi gang,

I just learned that a manufacturer of vehicle lights is now using white leds in their taillights and turn signals, and using the red or amber lenses to get the desired color. My initial reaction was predictable... "are they crazy??". The arguement is that white LEDs have gotten cheaper than other colors, and this is now the most cost effective method to make these lights.

Well, I don't know what LEDs they are using, so I don't know their actual costs. As one benchmark, I looked up the price of amber, red, and white Cree XP-E's at Digi-key. They aren't exactly equivalent in a variety of ways, but the rough numbers show that the amber LED costs $2.10 in quantity (p/n XPEAMB-L1-R250-00401TR-ND), the red LED is $1.39 in quantity (p/n XPERED-L1-0000-00401TR-ND), and the white is $1.68 in quantity (p/n XPEWHT-L1-0000-00F08-ND). 

Based on this, the red LED looks like a better buy than the white, but I can see an arguement for using white LEDs to make amber light. 

The engineer in me still says that it's crazy to use a blue LED die to illuminate phosphors that re-emit a fairly full spectrum of light, and then filter out a lot of that light when you can just buy a LED to produce the desired wavelength directly. It's harder to argue the economics, especially when you can design and build just one electronic circuit board and slap different colored lenses on the front. Crazy, or not??


----------



## AnAppleSnail (Apr 5, 2013)

It makes more sense to me than I'd like. Surely in theory it's a bad idea. But if the pigmentation in the colored red (For aesthetic reasons) enclosure filters out enough of the 20-nm-wide red or yellow LED output, you may well be in better shape with a broader spectrum.

Functionally, the white LED produces more yellow light than red. Also functionally speaking, I'm with you. But plastic tints (And filtering) wander around, especially after aging.


----------



## SemiMan (Apr 5, 2013)

Perhaps you are not looking deep enough into the problem .... from an engineering standpoint 

- A warm white LED shifts a lot of it's output into frequencies that would be compatible with an SAE red lens (or at least one specific to the requirement). Perhaps 4K is better. 100 lumens/watt in low CRI warm white is pretty easy and 120 lumens/watt in 4K low CRI is again easy today.

- RED LED is likely 60-70 lumens/watt ... out the gate

- White LED at 100C may still be at 90% of its 25C output

- RED LED at 100C may be only at 50% of its 25C output

- Brake function is short term, hundreds of hours perhaps even for a commercial vehicle. Hence, I could even drive that white LED hard enough to put it up to 135C without worrying about impacting the life of the LED. I could never push the RED that hard as the output would drop to nothing

- White LED output will be fairly consistent over environmental conditions ... much less brightness shift (feature, not specific need)


As opposed to a single high power device, they could be using a bunch of small cheap white SMT LEDs. The price of these has come down down because the volume is high.


I think when you look at the "system" as opposed to the LED, driving the crap out of a white LED on a smallish heat sink (or perhaps just a PCB), could quite possibly be cheaper than driving a red LED lightly and/or the added cost of getting rid of the heat.


Semiman


----------



## Steve K (Apr 5, 2013)

Well, I can't argue with the data, but there does seem to an intrisic "wrongness" about the whole situation!

Maybe a better question is why haven't red LEDs progressed like white LEDs have? Why can't a red LED be as, or more, efficient than a white? The use of phosphor in the white LED has got to be lossy, right? I'm going to guess that the answer is related to "bandgap", but I can't guess why.


----------



## AnAppleSnail (Apr 5, 2013)

One bulb vs two? I seem to recall my old Geo Metro (With easily replaceable bulbs) had an identical part behind amber, red, or white in the back, depending on its function. I also might be remembering wrongly.

Lumens may be the wrong metric for these lights, anyway. Equal watts of red, white, and yellow light will appear to be of different brightnesses. I believe the filament bulb is around 100 lumens, and is filtered down to about 20% of that in red lamp applications.

If we take this to Automotive, Scheinwerfermann may take a swing at it.


----------



## Steve K (Apr 5, 2013)

These lamp assemblies are not intended for automotive use. They are quite a bit larger, and intended, in this case, for earth moving equipment. I'm not clear on whether the red taillights will also function as brake lights, so they might run for many hours at low power, and briefly at high power. 

Speaking of automotive apps, I don't recall seeing cars with white LEDS under red lenses. I've seen some with clear lenses, so the LEDs are certainly red. There are others where I guess I don't know if the LEDs are red or white. Honestly.. this is such a shock to me.... finding out that people are using white LEDs in taillights is *almost* like finding out <spoiler alert> that there is no Easter Bunny!


----------



## SemiMan (Apr 5, 2013)

Steve K said:


> These lamp assemblies are not intended for automotive use. They are quite a bit larger, and intended, in this case, for earth moving equipment. I'm not clear on whether the red taillights will also function as brake lights, so they might run for many hours at low power, and briefly at high power.
> 
> Speaking of automotive apps, I don't recall seeing cars with white LEDS under red lenses. I've seen some with clear lenses, so the LEDs are certainly red. There are others where I guess I don't know if the LEDs are red or white. Honestly.. this is such a shock to me.... finding out that people are using white LEDs in taillights is *almost* like finding out <spoiler alert> that there is no Easter Bunny!




I am not aware of a car that is doing this today, but I know it has been discussed. What they really want is a phoshor converted red similar to the PC amber, but I am not aware that somone has a good phosphor formulation as of yet.


For earth moving equipment, I could definitely see this. Is this exclusively "off road"? If so, they may not have to adhere as strictly to the SAE colors and could have a large pass band. It may be easier for them to buy big cheap white arrays as opposed to lots of smaller red LEDs too.

Semiman


----------



## Steve K (Apr 5, 2013)

Some earthmoving equipment is roadable, such as motorgraders, wheel loaders, backhoe loaders, etc. I don't know enough the optical requirements to know if they have to meet SAE colors or ISO or whatever (although ISO does seem to try to be compatible with a lot of SAE stuff).

The lights use circuit boards of their own design, so they are buying reels of LEDs as opposed to an LED array. I'll have to see if I can figure out what LEDs they are using, assuming I can find an excuse to pop off the lens and get a look at the board.


----------



## Anders Hoveland (Apr 5, 2013)

There is good reason for us to think that using red filters over a white LED is crazy, besides just the fact that red LEDs exist. 
It seems many people do not understand spectrum differences of artificial light sources. In the old days, you would just take a light bulb and put a color filter over it to get the desired color. Incandescent bulbs emit fairly evenly accross all frequencies of the spectrum (although less into the blue part). In fact, incandescent bulbs give off more red light than any other frequency, so using a red filter works just great. White LEDs, on the other hand, only emit low levels of red frequency light relative the other frequencies. The light from a white LED may _look_ white, but is is essentially just a mix a mix of blue and mostly orangish frequency light. 

For this reason, trying to make red light by using a white LED covered with a red filter drastically reduces the efficiency. Nevertheless, it is often still used to cut costs in LCD screens with LED backlighting. Using an RGB LED backlight doubles efficiency over using a normal white LED backlight because the wavelengths are better matched to the green and red pixel filters in the LCD screen, even though RGB LEDs are otherwise only half as efficient at producing light.

One of the great advantages of LEDs are that they are great at producing colored light, since they typically emit at just one specific frequency. This also typically gives a purer color than that which would be obtained using a color filter.


----------



## ianfield (Apr 6, 2013)

Steve K said:


> These lamp assemblies are not intended for automotive use. They are quite a bit larger, and intended, in this case, for earth moving equipment. I'm not clear on whether the red taillights will also function as brake lights, so they might run for many hours at low power, and briefly at high power.
> 
> Speaking of automotive apps, I don't recall seeing cars with white LEDS under red lenses. I've seen some with clear lenses, so the LEDs are certainly red. There are others where I guess I don't know if the LEDs are red or white. Honestly.. this is such a shock to me.... finding out that people are using white LEDs in taillights is *almost* like finding out <spoiler alert> that there is no Easter Bunny!



There is one potential advantage of white LEDs behind a red rear lens, on many vehicles the rear lens has a clear window on the underside to illuminate the number plate.


----------



## Ken_McE (Apr 6, 2013)

In terms of pure efficiency, it's crazy. Maybe Rube Goldberg is alive and well and working in the automotive industry?  Maybe they have a warehouse full of generic white LED units that are already paid for, and they just slap them into everything?


----------



## SemiMan (Apr 6, 2013)

Ken_McE said:


> In terms of pure efficiency, it's crazy. Maybe Rube Goldberg is alive and well and working in the automotive industry?  Maybe they have a warehouse full of generic white LED units that are already paid for, and they just slap them into everything?




Actually it is not as crazy as you think if you look at what a real world design may look like.

Let's say with a white LED you target a 125C max junction temp. The white LED will be putting out say 85% of its 25C light output (assume Lumileds REBEL).

Now let's say you have a red led. At 125C max junction temp, you may be at 35% of the initial output.


The RED led may start out at 60 lumens/watt, the white at 100 lumens/watt (assume warmish for most red). However, at temp, the difference may be 20 lumens/watt versus 80.

The question is, can 25% of the output be used for the tail light? For a warm white LED, I am not sure you will get 25%, but you may get in the ballpark.


You can make the argument that you would never let the red led get to 125C, but then maybe you have a bigger heatsink, or more leds, and suddenly your cost goes up compared to the white LED.

Semiman


----------

