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I have a white LED light source, the spectrum of which looks as follows ---

White LED Spectrum

want to filter out red light from this white LED source. But as you can see the spectrum dwindles in the red zone.

And therefore, when I put red filter (absorptive filter) in front of my white LED light source, the brightness suddenly drops down to abysmal values.

Is there a way in which I could use this spectrum and get red lights without compromising on the brightness? I thought about interference and then I came across dichroic filters. But I'm not sure if that'll do it.

I also came across these filters called LEE LED filters which are filters specifically meant for LEDs. But I do not understand the scientific principle they work on. I would be grateful if you can follow the link and explain to me the scientific principle. From the spectrum shown in the link, these filters can give you more than 85% red. But the overall transmission is just 10.1% of the original intensity. So it's kinda bleh. Or maybe I don't understand properly.

A red LED light source would be just perfect for me, but a high wattage (50W-100W) red LED light source is hard to come across in the market.

Let me know if there are other options you guys can think of, for producing cool red LED light. Right now I use a tungsten lamp with RED absorptive filter, but there is so much heat (infrared) in it that my houseflies get fried up. [ I work in an insect flight lab and we use red light for high speed videography, since houseflies dont see well in the red zone]. So I want to switch to LEDs.

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  • \$\begingroup\$ Why not use several red LEDs \$\endgroup\$ – Andy aka Aug 5 '14 at 10:09
  • \$\begingroup\$ @Andyaka I dont want the LED setup to become too big. And how to resolve the heat sink issues? \$\endgroup\$ – Black Dagger Aug 5 '14 at 10:13
  • \$\begingroup\$ "A red LED light source would be just perfect for me, but a high wattage (50W-100W) red LED light source is hard to come across in the market." \$\endgroup\$ – EM Fields Aug 5 '14 at 14:19
  • \$\begingroup\$ @All What do you think of using RED filters on a warm white LED. They have a good amount of red compared to the cool white LED. Here's a comparison --- sitelighting.com/techtalk/…. Against Incandescent Lights --- physics.sjsu.edu/tomley/ObjectSpectra/tungstenbulb.html \$\endgroup\$ – Black Dagger Aug 7 '14 at 12:04
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As poster states,

A very bright RED LED lamp (~50W) with a lamp tripod or something would be perfect for me.

Given the fact that one can easily buy off-the-shelf, 50 to 150w, LED flood lamp, with heat sink and drive electronics, but they typically comes with white LED.

enter image description here

One may consider simply replacing the central coin sized LED module and make corresponding small change in drive V and A.

Either do it yourself or if, in (small) batch quantity, manufacturer can do it for you.

For example, I am using this 10W LED, which is 9 LED die, 3 series in one set and 3 set in parallel. It is 350mA, 10V. The constant current driver board is adjustable up to max. 5A.

The RED LED module shown (which I do not have on hands, but, same principle as the 10W I have) is 10 LED by 10 LED, each die is one 1W LED, module is about 32V and 3.5A.

The example spec. is white LED (which is actually blue then changed to white) and voltage is slightly different from red but is same range 3.x to 4.x V

enter image description here

Commercial 100W driver with CE, 85 to 250V AC in, out DC 20 to 38V, 3A

enter image description here

enter image description here enter image description here 10 to 100 W Red multiple chip LED module

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  • \$\begingroup\$ I suspect you underestimate the difficulty of modifying such a fixture. \$\endgroup\$ – WhatRoughBeast Aug 5 '14 at 19:09
  • \$\begingroup\$ I edited the answer, based on 10W module I am using now. 100W module is same principle, it is just adding more LED die, each 1W, in 10 by 10 format while 10W is 3 by 3. Mechanical modification is screw in. The constant current board sets current via the blue trim pot. \$\endgroup\$ – EEd Aug 5 '14 at 19:52
  • \$\begingroup\$ @WhatRoughBeast. Like your comment. Appreciate it. Sorry for not writing my answer clear enough one first version, to style 'norm' of this community. I am new to stackexchange. \$\endgroup\$ – EEd Aug 6 '14 at 4:57
  • \$\begingroup\$ Thank you for the answer. I also came across remote controlled RGB LED lights. These are 50 W RGB lights --- youtube.com/watch?v=rKRx_hqlfmg . They have a strip each of Red, green, and blue LEDs. And you have the freedom of switching each strip individually. What I dont understand is whether the net wattage is 50 W or the wattage of each strip is 50 W. If the net wattage is 50 W then each will have 50/3 W. And thats too low, given the large size of the floodlights. \$\endgroup\$ – Black Dagger Aug 7 '14 at 11:52
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    \$\begingroup\$ May need time to find and buy. Actual assembly 30 minute. Similar mechanical housings with build-in heat sink for 50, 100 (bigger, same shape as 50w) and 200 (two LED module) watts. Install LED module is just 4 simple screw and solder two wires. Power unit (as above orange unit, size of a soap) is ready made. You can search on line shop to buy these. As housing and power supply comes with different shape (thin, fat, tall, short). You need to find a seller that per-check these and sell you a kit that is mechanically fit. Red-only LED is not as common as white but available. \$\endgroup\$ – EEd Aug 9 '14 at 16:19
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Your "white" LED is actually a very bright blue LED combined with a phosphor that absorbs some of the blue light and re-emits it as the broad yellow peak you see in your spectrum.

While it would be theoretically possible to substitute a different phosphor that emits the red light you seek, I don't know of any commercial products that do this.

However, very bright red LEDs are readily available. They're used as the strobe lights on emergency vehicles, such as ambulances, fire engines, police cars (depending on your jurisdiction), etc.

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  • \$\begingroup\$ A very bright RED LED lamp (~50W) with a lamp tripod or something would be perfect for me. \$\endgroup\$ – Black Dagger Aug 5 '14 at 11:59
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You can't add light with a filter that isn't there in your spectrum to start with.

Operating theatre lights often use pure red LEDs to augment the spectrum of 'white' LEDs. I would imagine that you need good colour rendering at the blood end of the spectrum in that environment.

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I'm afraid you understand it perfectly, although you don't much like it.

Once your light is generated, there really isn't anything you can do with the unwanted frequencies except filter them out. There just aren't any good ways to convert one frequency to another. (DPSS lasers are one exception, but not applicable here). Dichroic filters are used because they have relatively sharp cutoff characteristics and so can produce narrow passbands which provided "purer" colors, but they don't actually convert light from one frequency to another.

As a general rule, incandescents produce about 10% of their power as visible light, and virtually all the rest as IR, which is why your red-filtered light is very hot - the red filter is not eliminating the IR. LEDs provide considerably better efficiency, but as you've noticed, not much of it is red. With not much there to begin with, any red-filtered version will be, in your words, abysmal.

What you need, I think, are IR blocking filters, or cold filters, for your incandescents. There are two types. The most common is used in cameras and other sensors, and has the problem that they won't handle a lot of heat. The other variety is used in things like LCD projectors, and they are designed to take heat, although cooling design is also important. One source is http://www.mecanusa.com/polarizer/ColdFilter.htm , but you'll notice that these puppies can be expensive.

As to LEE LED filters, a general, non-technical outline is here http://www.leefilters.com/lighting/led-02.html#led-explain but it's actually pretty simple. Regular filters work on incandescent light, with a standard black-body intensity curve. As you may have noticed, your LED intensity has this big fat spike in the blue. The LEE LED filters just have what it takes to compensate for this spike.

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  • \$\begingroup\$ These IR blocking filters --- Do they absorb the IR or they reflect it backwards? If they absorb it, I'll need something really strong because I will be using 1000 W incandescent lights. If they reflect it backwards, wont it damage the light source itself. On the LEE LED filters --- These filters would give me a good color match on red, but I dont think the intensity will be any better. If you click on the red filter you'll notice that transmission is only 10.1%. \$\endgroup\$ – Black Dagger Aug 5 '14 at 11:08
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    \$\begingroup\$ @BlackDagger - The quick answer is "either". The reflective versions are called, for obvious reasons, cold mirrors. And while they don't have the power dissipation problems of absorption filters, they really only postpone the day of reckoning. Once you've reflected your 900 watts of IR, what do you do with it? It's got to be absorbed somewhere. So you need to make an IR trap for the reflected beam, and deal with the heat there. Then you can filter the red out of the "cool" visible. As you can tell, this is going to take some work on your part. \$\endgroup\$ – WhatRoughBeast Aug 5 '14 at 11:19
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    \$\begingroup\$ Actually, I sort of misspoke. Depending on which is more convenient, you can use hot mirrors or cold mirrors. It depends on which component you want to deal with, transmitted or reflected. If you want the IR trap next to the source and in an isolated area near it, a hot mirror might be easier to design. God (or the Devil, depending on your source of folk wisdom) is in the details. \$\endgroup\$ – WhatRoughBeast Aug 5 '14 at 11:25
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    \$\begingroup\$ And about the Lee filters and intensity: my lead sentence in my answer remains unchanged, merely expanded in scope. \$\endgroup\$ – WhatRoughBeast Aug 5 '14 at 11:26
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An longpass optical filter with a cutoff at 480nm would do the job.

I mean a "real" optical filter, like interferometric filters with very sharp transition.

They might cost 10th of $

For the brightness, this is the best solution. Because the filter is quasi ideal, almost all the energy in the red part would pass through it. If this is not bright enough, that means there is not enough power into the emitted light at the first place. No way to boost this. Except adding LEDs or focusing the output light to the ROI.

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