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.