Mylar Light Diffuser for Recessed Ceiling Lights

Introduction

Recessed ceiling lights (a.k.a. pot lights) are commonly found in homes. However, the downlight they produce can be a bit harsh and glaring. To soften the light, try making this elegant and economical mylar light diffuser. All that's required are some fairly affordable materials, some fairly rudimentary geometry skill, and some basic paper-cutting skills.

Materials

drafting mylar
small magnets
scotch tape

Tools

X-acto or Olfa-type knife
cutting mat
steel ruler
computer printer (to print out paper template)

Design notes

This light diffuser is essentially a cone, rolled from a flat sheet of common drafting mylar. It's a cone that has a section removed from it, transforming it into a sort of 'light scoop'. The design inspiration comes from the skylit columns of architect Zaha Hadid's restaurant at the Serpentine Sackler Gallery extension in London.

Step 1: Measure Inside Diameter of Light Fixture's Trim Ring

Your recessed ceiling light has an outer trim ring, and the mylar light diffuser will be fitted into the inside of this ring, so we'll need to measure its inner diameter.

With a tape measure or ruler, measure the inside diameter.

There may be some variability in your measuring as the inner walls of your trim ring may be sloped, making it tough to make take a very precise measurement. But it's not crucial to get a super-precise number, so just determine a reasonably accurate number.

Step 2: Calculate the Cone Dimensions, Part 1

(Disclaimer: the next few steps will involve some math and geometry work, but it won't be overly hard)

Using the diameter you measured in Step 1, refer to the following diagram to calculate the length |AC| of the triangle ABC.

The length |AC| can be calculated from this formula below, with the following givens:

BC is the radius (equals the diameter you measured in Step 1, divided by 2)
AB is the desired height of the cone (choose a cone height you think you'll like)

|AC|² = |AB|² + |BC|²

Here's an example use of this formula. Assuming a ceiling light diameter of 5.5" and cone height of 13", the math would look like this:

|AC|² = |2.75|² + |13|²

|AC|² = 7.5625" + 169"

AC = 13.29"

Step 3: Calculate the Cone Dimensions, Part 2

Now that we know the three dimensions of the cone's triangle (from Step 2), we need to calculate the size and shape of the unwrapped cone. This is needed so that we can draw and cut the cone from a flat sheet of mylar. The drawing here shows the unwrapped cone – shaped like a pizza slice.

We already know the length of this pizza slice's sides (value AC from your Step 2 calculations), but now we need to calculate the following:

cone circumference (or arc length of the pizza slice's 'crust')
center angle

The formula for the circumference of a circle is

Circumference = 2 × π × r

...where π is pi (3.14159…), and r is the radius we measured from the pot light trim.

Our example calculation, using diameter of 5.5" and cone height of 13" would look like this:

Circumference = 2 × 3.14159 × 2.75"

Circumference = 17.28"

Step 4: Calculate the Cone Dimensions, Part 3

Next, let's calculate the center angle of our flattened cone. The formula is:

Angle = Arc Length / Radius

We know the arc length from the previous calculation (Step 3), and the radius is the AC dimension of the pizza slice – the radius of the unfolded cone, not the radius of the ceiling light.

Example calculation…

Angle = 17.28" / 13.29"

…which calculates to:

Angle = 1.3 Radians

To convert radians to an angle, multiply by 180 / π.

Angle = 1.3 * 180 / π

Finally:

Angle = 74.5º

Step 5: Draw the Flattened Cone

Next, let's take our flattened cone and draw it in a vector-based drawing program like Adobe Illustrator. (a CAD program would work too).

Draw your flattened cone using all the dimensions you calculated in the previous steps.

Step 6: Draw the 'cutout' Opening of the Cone

At the end of the last step, if we wrapped our drawing into a cone, we'd get a solid cone. In this step, we'll make cuts to the cone which will produce the cutout opening of the cone.

Using a bezier tool, draw an 's' curve that runs from one side edge of the flattened cone to the cone's (invisible) center line. Make sure this curve meets the side edge at a perpendicular angle.

Mirror this curve to the opposite side, then trim both side edges (eliminating the 'point' of the flattened cone), so that your drawing generally matches the second illustration here. (yes, it looks like mens' briefs)

Step 7: Add Mounting Tabs to the Cone

The cone will need some mounting tabs so that the magnets can attach the cone to the metal trim ring.

Offset the arc of the flattened cone (or 'pizza crust' edge) by 1/2", so that you have a new line that is 1/2" outside the original arc.
Draw some notches along the arc. These will be the mounting tabs. Hint: an easy way to position these notches is to draw them at either the left or right end of the arc, then copy-rotate them from the arc's center point by 1/10th of the degrees of the total arc sweep. eg. 74.5º / 10 = 7.45º.
Trim away the parts of the arc that are in between the mounting tabs.
Delete the original 'pizza crust' arc.

Now you have the final drawn shape of your cone.

Step 8: Print Out Your Drawn Scoop

Print out your drawn scoop, at full size / full scale. You'll probably have to tile several print sheets together to make the whole scoop.

Tape the tiled sheets together to make the entire scoop print.

**OPTION: if you're lucky enough to have access to a cutting machine (like a Kongsberg cutting machine http://www.nsksystem.com/product/kongsberg/), you can also 'print' your drawn scoop using it. A fully-cut flattened scoop will be your result.

Step 9: Cut the Flattened Scoop Out of Mylar

Place and tape your scoop print to a sheet of mylar.

Carefully cut the pattern with an Olfa-type knife.

Step 10: Roll the Mylar in to a 3D Scoop

Now take your mylar flattened scoop and roll it in to a 3D scoop. Connect the two flat edges with a piece of scotch tape on the inside of the mylar, to create the 3D scoop.