Diffusing LEDs Right




Introduction: Diffusing LEDs Right

About: I am a hobby photographer specialized in lightpainting and building tools for this. If you want to see more of my art check my FB, Instagram or Flickr page! You can find me under the same name there.

LEDs are widely used these days even in everyday life and you can get a lot of information on how to use them. There are many tutorials on powering LEDs and including them in different light installations. But there is only very little information on how to control or shape the light that is emitted by LEDs.

LEDs are a light source that emits light from a very small point in every direction evenly. Depending on the type of LED and how it is built the light is often directed in a wide cone. High power LEDs or SMD LEDs like the WS2812b or APA102 usually have a beam angle of 120°-140°, 5mm LEDs can have a beam angle of up to 180°. This means that over the whole angle of this beam the same amount of light is emitted. But since we have a single point of origin if we shine the light of an LED on a flat surface we get a light spot that is brightest in the middle and looses brightness the further away you go from the center.

In light installations and in photography (a field I have much interest in) you strife for a more homogeneous distribution of light. The key to this is diffusion of the light. So in this Instructable I will share with you how you can diffuse LEDs the right way and what you have to pay attention to.

Step 1: The Math I

Lets assume we are using the normal LEDs you can find on a LED strip. These usually are 5050 LEDs, meaning that they are 5mm by 5mm square and if you have 60LEDs/m you usually have one LED each 17mm on the strip. These LEDs also usually have a beam angle of 120°, which result in a pattern like you can see in the sketch.

These strips are often used because only a short distance away from the strip the beams of each LED overlap so much that they merge into a bar of light. While this is true for most applications you still have hot spots and looking directly at the light you see each individual LED. For light installations or if you want to use the LED strip in long exposure photography this is not desired.

Step 2: What You Can Buy

There are several types of aluminium extrusions available that are meant to house LED strips and often also come with different kinds of diffusers. In the first photo you can see the most common ones.

The first one is barely deep enough to house the LEDs and is the least effective one, so lets put that one away and look at the others.

The second one has a deeper profile and a flat diffuser screen. The depth is around 11mm which places the diffuser around 10mm over the LEDs. Lets remember this values and take a look at the next.

The third one has a similar aluminium profile as the second one but uses a round profile for the diffuser. This places the highest point of the diffuser around 17mm away from the LEDs. The round profile also makes sure that the light is more even the further you get away from the middle of the bar (remember we have a single point of origin and the light has to travel further the more you get away from the middle).

Step 3: The Math II

Lets take a look at the two aluminium extrusions of the last step. We have 10mm and 17mm distance from the LEDs and a beam angle of 120°. This results in a pattern like you see in the sketches.

As you can see with the 10mm one the beam cones overlap for only around half the cones. The borders of a cone reaches nearly the middle of the next one. You might think this is enough to get an even distribution, but lets take a look at the other one.

With a distance of 17mm you get three cones to overlap quite strong which results in a better distribution of light. The cone of one LED nearly reaches the middle of the LED 2 places further down the strip. So its light is spread fully over the light of its neighbor.

Step 4: Testing the Extrusions

Lets see if the math we looked at in the last part adds up and we get a good distribution of light.

The first photo shows a LED strip put halfway into the extrusion with a depth of 10mm. As you can see you still get hot spots, but the space between the LEDs is lit quite bright too. If you use this in a long exposure and move it through the view of a camera as shown in the second photo you can see that there is a difference between bare LEDs and then one in the strip, but the spots where the LEDs are create bright lines.

The third photo shows a LED strip put halfway in the extrusion with a depth of 17mm. The light is distributes much better and you can barely see where the individual LEDs are any more. Again using this in a long exposure as shown in the fourth photo we see the difference between bare LEDs and this diffuser. The light is very homogeneous but if you look closely you can still see a variance in the brightness of the light, but it is much better then the previous one.

Step 5: The Math III

Lets get back to math and analyse what we have seen. With a distance of 17mm from the LEDs we already get a good result, but it can still be improved.

Lets remember that an LED is a single point shaped source of light that spreads its light evenly on every direction. The diffuser is a flat surface, so we have to look at the angle and intensity of the light. The further away we get from the light source the less bright the light will be.If you look at the first photo you can see that at a distance of 30mm a beam angle of 120° spreads the light over more then 100mm. But since the light has to travel much further at the border of this cone the light is a lot dimmer then in the middle.

What we are looking for is the same altitude to area covered ration. If we shine the light on a flat surface and the distance from the light to the surface is more or less equal we have a more even distribution of light. This can either be achieved by making the diffuser a sphere with the light source at its center or we can look for another angle to to the math with.

If you calculate it you will get an angle of about 53,13° at which the altitude of a triangle equals the length of the segment opposite of the angle. To make it a little bit easier lets take an angle of 60°. In the second sketch you can see the result if we apply the 60° angle. The spot of a 60° cone has roughly the same brightness if you look at it or capture it with a camera. Applying this to the diffuser with 17mm depth we can see that this was designed quite well.

What all this tells us is that if you want to create your own diffuser place it at the same distance away from the LEDs as the LEDs are away from each other. That way you will get quite good results already.

Step 6: Improving the Results - Double Diffusion!

Since I was not happy with the results so far I was looking for a way to get an even better spread of light.

So lets think about the difference between directed light and diffused light. The main difference that is of importance here is that with a directed light we have straight lines of light going away from a single spot. So the further away we go from this one spot the less light we will get. Projecting it on a flat surface we will always get a brightness falloff. Diffused light means that we don't have a single source of light, but a large one. And also that the light spreads from each point of this large light source in every direction. A diffuser is a device that transforms a direct light into a diffused light, so the diffuser essentially becomes a new light source that this time is not just a single spot.

Now if we take this light source, that still has some hot spots and diffuse it a second time, we will get a completely homogeneous distribution. The first layer of diffusion has hot spots, that is true, but only a little distance away from these the light from all the points in this hot spot overlap so much that it is not visible any more. The only downside is that to diffuse a light we have to use a material that is a little bit opaque so it reduces the intensity of the light. With a double diffusion we reduce the intensity even more, but in the applications where this is important this is not that important.

A very simple and effective way to create a double diffusion is to place some wadding between the LED strip and the diffuser. In the photos you can see the result of some wadding put into the 10mm deep and the 17mm deep aluminium extrusion. As you can see the 10mm one improves and the 17mm one gets nearly perfect to work with.

Step 7: Another Solution: Increase the Distance to the Diffuser

Another solution is also to increase the distance from the LED to the diffuser. If you think back a few steps with an altitude of the distance between each LED you will get an area covered that is equal to the distance between the LEDs. But if you increase the distance these light cones overlap even more and will result in the hot spots overlapping so much that the merge into each other. In this tool I designed for lightpainting the distance between LEDs and diffuser is roughly double the distance between each LED. And as you can see the resulting light is well distributed. The last photo is a long exposure where I used these tools to draw some streaks with its light.

Step 8: Conclusion

If you want to build a nice looking light installation with LEDs take care to diffuse the light right. In some cases a single point of light is desired, but most of the time you want a more pleasing look, and a diffused light source will get you this. If you work in cinematography or photography you should already know a lot about direct vs. diffused light and here you get some insight on how to turn one into another.

If you want to do a more professional double diffusion you can use sheets of acrylic. There are acrylic sheets with a light transmission of 79%, these are usually used in bathroom installation as a privacy protection. These have a good opacity to be used as a diffuser if you double it. For a double diffusion the full distance between each LED is not needed. Place the first layer of diffusion around 1/3 of the distance between the LED and the second layer at 2/3 of the distance. This way you will get a very even light distribution on the second layer. But you can also simply use distance between LEDs and place first level in the middle of it.

There are many more ways to achieve this like using acrylic light channeling but those are more complex and it is usually easier to use either single diffusion with enough distance or double diffusion.

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    13 Discussions


    9 days ago

    I tried layering extra diffusers on top to see if that helped, but I still see the stripe effect pretty clearly, and I don't have a great way to mount diffusers with spacing. Is there a particularly good type of wadding/filling that can be used to increase scattering without too much absorption?


    Reply 8 days ago

    Second diffuser with no spacing does only reduce light, not much more.
    If you have enough room to add some wadding try the stuff you get in cosmetic shops that is used to wipe of cosmetics. It either comes in pads (useless) or a full bag of loose wadding. The stuff in the full bag is pretty good if you fluff it up. Take a stand about the thickness of one LEDs width and fluff it with you fingers or maybe tweezers to the thickness of the casing. Should work quite well. If scattering is not enough just add a little bit more.


    Reply 8 days ago

    I'm trying to see if I can order something online, just because I'm not familiar enough with cosmetics shops to know what I should ask for. Would it be something like unwoven cotton or rayon balls? Either of those seem opaque. Would be cool to find something transparent. But trying to figure out what keywords I can search online to buy something.


    Question 9 months ago

    Hello, i am currently trying to install led strips on ceiling cove (see 1st attached image).

    Does it matter if the light is smooth/diffused or is with hot spots since it's hidden anyway?

    Also, as i cannot find any aluminium extrusion that allows for double diffusion, i am going with step 7 which is to increase the distance.

    I am using the same type of led strip which has about 17mm of distance between LEDs so i am assuming i need 35mm of distance between the top and the bottom.

    However, i cannot find any profile with that depth without getting one which is very wide. Does it matter if the profile is wide though? Does it affect the angle the light is reflected?

    I have attached a 2nd image, can you advise which profile is the best for my use? Had no choice but to include flat cover as well since there is no round cover with high enough depth.

    Also, from your statement on step 5, you seem to advocate against moving the diffuser further away to the LED than the distance between LEDs since it will be dimmer on the side than the middle.

    Doesn't this contradict with step 7 which advises on doubling the distance?



    Answer 9 months ago

    You experienced a problem that is quite common. There are no stock extrusions that do it quite right. All the normal ones are usually not deep enough and there are none that use a double diffusion. So either you can use the wide one (it doesn't matter, just gives more diffuser area) or build something yourself.

    It looks like you attach the LEDs so that they shine upwards against the ceiling and then let the light bounce out of the cove. In this case that cove and the walls sort of become your diffuser and the best way here would be to use no diffuser on the LEDs at all. Just put them in a profile either without any diffuser or with a completely clear one to protect the LEDs from dust.

    What I meant in step 5 is that for normal lighting applications the 17mm ones are ok. If you look directly at them the diffuser is lit everywhere but still shows hot spots. But it is not as prominent anyways so you can get away with it. But for my application I need something that is lit uniformly, so those profiles don't work at all. Thats what I talk about in step 7. To get an uniformly lit diffuser you either need to move the diffuser away further or apply double diffusion.

    But all this is really only important if you care about how the diffuser is lit. In a lighting situation where you don't see the diffuser there is no need to go to this length. Or if you shine the light on a wall for indirect lighting and the LED strip is hidden then there is no need for a diffuser at all, it will only reduce the brightness.


    10 months ago

    Do you mind describing the process for getting the shapes in step 7?


    Reply 10 months ago

    Those are designed to be 3d printed. They are just some geometric shapes with a max dimension that I could still print on my printer. So the dimensions are not really connected to the light.
    The only thing that matters here is how far the diffuser is away from the LEDs. It was designed so the strip is double the distance away from the diffuser then the distance between each LED. That way the light from the LEDs intersect enough so that you don't see individual LEDs.
    It could have been smaller if I used double diffusion there like described in the other steps, but out of simplicity I designed these tools this way.


    1 year ago

    Thanks very much for those valuable informations.
    I'm planning to make room lighting pendant with 5050 stripes and acrylic. Should I use clear acrylic w/sandblasted surfaces or milky acrylic directly?


    Reply 1 year ago

    Both should work just fine, so it probably is more a question of aesthetics.
    There is special milky acrylic with the right amount of diffusion, usually called opal. These are made for such an application. Sandblasting diffusion can vary depending on grain size and time you blast it. So it can be more or less diffusion then the milky acrylic. But if you blast the inside the outer side will look more shiny like clear acrylic. On the other hand if you blast the outside you get extra diffusion because the clear surface reflects some light back and lets it bounce around inside. The outside will look rough.
    So all in all I guess it depends on what you want here.


    Reply 1 year ago

    Thanks for your answer.
    So, I'm going to try both with smaller
    pieces. My feeling is sandblasting transparent arylic is better.
    Planning to try soda blasting to outer surface manually after final
    shape of it....


    2 years ago

    love this lesson .

    what type of wadding do you use and is it fire safe?

    do have a place where you go to get the extruded enclosures ?

    i have been playing a lot with led adhesive strip lights (kitchen and shed )


    Reply 2 years ago

    It is normal wadding used in teddy bears and such stuff. Don't think it is fire safe but if you put an led strip in an aluminium enclosure they will never get warm enough to char the wadding.
    Usually getting those enclosures on ebay, many sell them here in different shapes and colors. Depends on where you live I guess.