Introduction: Compound Elliptical Reflector LED Task Light

Picture of Compound Elliptical Reflector LED Task Light

This light fixture uses an aluminum reflector shaped by plywood ribs, and a set of LED light strips, to illuminate a counter-top work area.


By the way... a Compound Elliptical Reflector (CER) is a easy-to-design optic which efficiently takes light from an area and puts it in a general direction. In this case, we light up the sink while essentially eliminating light shone directly on the users face (thus minimizing glare) and reducing the amount of light we waste out the window. For more information, see the Wikipedia articles on nonfocusing optics, especially the closely related Compound Parabolic Concentrator.

This is not a hard project: it requires moderate woodworking skills and a bit of electrical work, and probably takes 12 hours or so.
...And I suspect that the general design affords the use of laser-cut components, which would save several of those hours.

From an aesthetic standpoint, this piece hides behind a barrier... but with some slight modifications to the design, to include minimizing the vertical braces and perhaps skeletonizing the ribs,it could be quite aesthetic, if you like the skin-on-frame look of aircraft/airships. People who like steampunk, take note!

In this instructable, we'll:

  1. Design a custom reflector profile for our application. (Don't worry; it's not as hard as you think!)
  2. Use router templates to make a bunch of ribs.
  3. Assemble the wooden frame.
  4. Attach the reflector to the frame.
  5. Do the electrical work.
  6. Install the light fixture.

Step 1: Design

I recommend that you design of the optical parts of this project to your specific application.

In this task lighting situation, my main criteria were:

  • The light should not shine directly in the user's eyes while they are washing dishes. This specifies the maximum acceptance angle of the room-side (front) reflector and the maximum focusing angle of the window-side (rear) reflector. In other words, the front reflector must block the user's line-of-sight to the LED strips, and rear reflector must not reflect light at the user.
  • The height of the reflector should be about 3.4", so that it does not stick down below the board the fixture hides behind.
  • The fixture should shine the light onto the counter top.
  • The fixture should keep as much light as possible from escaping out the window.

So, we write up a little Excel spreadsheet (attached) and get to work. For this set of constraints, the workflow to determine reflector shape was as follows:

  1. Determine the width of the gap between reflectors required to accommodate the light source. Put it in D9. (In my case, this is five light strips @ 5/16" each = 1.56")
  2. Determine the width of the light source which actually emits light. Put it in D10. (In my case, 4*5/16" + the diameter of the LEDs themselves, at about 3/32" = 1.34")
  3. Determine the typical vertical location, and most forward location of the user's eyes during use, referenced to the center of the light source; this goes in C6, D6.
  4. Determine the vertical and horizontal distances from the light source to a point of interest on the target; goes in C7, D7. (in my case, the back edge of the counter top.)
  5. From (1,2,3)
  6. From (1,2,3), derive the maximum focusing angle (rear) and acceptance angle (front) for the reflectors. (see cells C10, C15)
  7. From (1,2,4), derive the preferred focusing angle for the front reflector. (cell C19)
  8. Adjust the acceptance angle of the rear reflector to reduce the vertical extent to 3.4" or less (see cell H13).
  9. Note that, unfortunately, for the front reflector, this combination of focus angle and acceptance angle isn't possible. I had to work out a compromise which made me happy...
    I settled on increasing the acceptance angle and focusing angle by 5.3 degrees, which means that a bit of light will hit the windowsill and the no-glare head position is moved back a couple inches.

A couple notes:

  1. No light will be reflected to a point further away from the reflector than the focus angle.
  2. No light will escape past the reflector at an angle greater than the acceptance angle.
  3. Some light will be shone in directions between the focus and acceptance angles.
  4. While maximizing the rear reflector's focus and acceptance angles maximizes the amount of light intersected by that reflector for a given reflector size, it may not optimize on-target light distribution... but I'm happy with my results - the actual artifact performs well.

Step 2: Making the Ribs

Picture of Making the Ribs

The ribs are a bit boring to make but not all that hard:

  1. Somehow, get the shape of the reflector curves plotted, to scale, on paper. I don't have a printer I trust, so I just graphed them on grid paper. (check rows 47, 48; 54, and 55 for values)
  2. Transfer the shape to a piece of wood with a punch. (here I use 3/8" MDF)
  3. Cut the MDF so that it matches the shape of the curve you have transferred, making a template. (I only used the chisel to cut off the fibrous burr which formed while die-grinding)
  4. Cut a length of plywood so that it is the correct width. (for my reflectors, that was 3.4")
  5. Clamp the template to the plywood and use some sort of template-following router bit to copy the shape.
  6. Cut off the end of the length of plywood, so that you have a short section with a curved edge and a long section with a square end.
  7. Repeat step #5 and #6 until you have a nice stack of nearly-finished ribs.
  8. Some additional work may be required to finish the ribs; for instance, here, I taper the ribs towards the free ends so that they look less bulky.

Step 3: Woodwork

Picture of Woodwork

Assembly of the wooden parts is pretty simple:

  1. Glue the ribs, equally spaced, to an appropriately sized base layer by lining up their exterior edge - that's the edge away from their curved surface - with the edge of the base layer and using a staple (or similar) to keep them from moving around while the glue dries.
  2. Attach a perpendicular (vertical) reinforcing member to the exterior face of the ribs, again with glue and staples.
  3. Mark the end-caps to size by assembling the two halves of the fixture and tracing the shape of the ribs.
  4. Cut the end-caps to size, and glue them to the project.
  5. Wait for the glue to dry, then paint the project. There's no need to paint the surfaces of the ribs where they meet the reflector, and painting the mating surface between the end caps and the last rib could lead to an overly tight fit, so leave those surfaces unpainted.
  6. Drill mounting holes so that you can attach the project to whatever surface it will be installed on. I drilled four: one at each end of the half which will have the LED strips on it, and a pair, each 1/4 of the way from the end, on the other half.

Step 4: Attach the Reflector

Picture of Attach the Reflector

This part is tricky and, to be honest, the part of the project I am least happy with. Anyways:

  1. Obtain some thin mirror-finish aluminum sheet. I took aluminum flashing from fine sandpaper up through green (chrome oxide) polishing compound, but that's because I'm an idiot. You should probably just buy the stuff pre-polished.
    (If you do decide to use flashing, note that the coating on one side is probably harder to sand through than the other. Run an experiment and find out which - if it's not obvious, it doesn't matter.)
  2. Cut the aluminum sheet to size. Check cells H12 and H18 for values. I used scissors for the final to-size cut to minimize the amount of bending.
    (For the same reason, while it's tempting to try to bend the edge over for strength and smoothness... unless you are confident that you can do it without bending it at all, don't do it.)
  3. No need to measure: just line the aluminum sheet up with the wooden part and mark the location of the ribs on the sheet.
  4. Using a story stick or ruler, mark the location of the brad holes on the aluminum sheet. It'll work better the closer your edge-most holes are to the edge, within the limits of what is possible to attach to the ribs. (It'll be ugly if your brads poke out the exterior face of your ribs!)
  5. Drill the holes. I used a 1/16" drill bit. This is probably not a strictly necessary step, but it makes it much easier.
  6. Nail the aluminum sheets to the wooden parts. I used 5/8" 18 gauge galvanized wire nails.

With regards to order in which put the nails in, while it is fully possible that there's a better way, I got the best results as follows:

  1. Start with the hole at the bottom of the center rib.
  2. Work your way towards the ends of the aluminum strip, staying along the bottom row.
  3. Put a nail in the next hole up, again on the center rib.
  4. Again, work your way towards the ends of the aluminum strip, staying in that row.
  5. Repeat steps #3 and #4 until you are done.

I'm entertaining the idea that nails might not be the best way to do it. Perhaps something which allows more lateral shifting of the aluminum sheet, like stitching, would allow the aluminum sheet to relax, and so have fewer/less pronounced wrinkles.

You'll also note that the aluminum sheet tends to relax "outwards" - that is, towards being a flat sheet again - away from the ribs. Probably, a set of longitudinal supports, flush with the inside of the ribs, would be the best way to fix this... but would be hard to make without resorting to laser-cut components.

Step 5: Eletrical Work

Picture of Eletrical Work

Again, simple:

  1. Drill a hole for your power connection to pass through. Back the plywood up with a block of wood so that you don't get horrible splintering.
  2. File the bottom of the hole flat, so that the cord can sit flat against the bottom of the wooden part.
  3. Attach a pair of fully-stripped wires to the ends of your cord, to serve as busbars.
  4. Insert the cord through the hole, making sure the busbars are aligned such that they miss the mounting hole, and glue in place. (I used hot-melt adhesive, but epoxy would work swell too.)
  5. Trim the busbars flush with the edge of the base.
  6. Cut the LED light strip to length, bisecting the copper contacts so that you have a pair on each end of the strips.
  7. Strip the rubber top from the LED strips, to expose the contacts. I had nothing but success carefully cutting through it with a box cutter - it'll make a slight "tearing" sound as it punches through the bottom of the rubber - and pealing it off.
  8. Attach a pair of leads to the end of the LED strips. (or, in my case, both ends - I have a cord coming out both sides of the fixture so that I bridge power over the sink).
  9. Stick the LED strips to the base.
  10. Solder the leads from the LED strips to their busbar.
  11. Pot the whole mess to protect it from mechanical forces, dirt, etc. I used hot glue because I don't know any better and it's convenient.

Step 6: Installation.

Picture of Installation.

To install:

  1. Drill holes in adjacent cabinets - or the ceiling, or whatever - for your cord access. It'll be best, if the ends of the light fixture are close to these surfaces, if you drill the holes substantially oversized.
  2. Mark and drill holes to mount the half of the light fixture with electrical components on it.
  3. Mount said half of the light fixture, including routing the cords through the holes.
  4. Assembling the other half of the light fixture with the half already attached to the surface, mark the surface and drill mounting holes.
  5. Mount the second half of the light fixture.

Comments

BoilingLeadBath (author)2015-01-22

If anyone has trouble understanding how to design their application-specific CER, feel free to ask for help.

BoilingLeadBath (author)2015-08-19

A few months after installation, the LED light strips began to separate from the plywood backing. This is likely a feature of the particular brand of LED light strip I used, rather than surface prep, as I had this problem with the application of these strips in another location as well.

A generous application of about 3$ worth of epoxy seems to have solved the problem: several months later, there's absolutely no sign of any problems...

(The (clear) epoxy, JB Weld PlasticWeld, was applied over the top of the LED strips and down into grooves ground with a die-grinder cuttoff disk, forming a mechanical (dovetail-like) and adhesive joint.)

shibuza (author)2015-02-15

A well thought out and comprehensive instructable. But ... if you took plastic vertical venetian blind slats (which, to my eye, come quite close to the curvature of your reflectors) and used the plain white plastic slats as the reflectors attached to the LED strip base at appropriate angles, you'd get much the same effect and lumen output without all the complex wood cutting, painting and assembly. (And if you HAD to have a mirror like reflective surface ... apply shiny aluminum tape to the slats.)

I think it would be an inexpensive way to construct energy efficent LED grow lights of any length. A 2x2 with 2 properly spaced, angled, and deep lengthwise slot cuts as the LED tape base to insert the venetian slats into would minimize construction time and materials. Assuming that LED strips with that weird purple-pink color could be acquired.

The regular white LED's would work for cheap shop lights (I just acquired a badly underlit garage I want to make into a shop.) THANKS for the idea! No more expensive florescent shop lights!!!

BoilingLeadBath (author)shibuza2015-02-15

I measured some aluminum blinds - which, IIRC are flatter than the plastic kind - and their radius of curvature was 1.45".

This is substantially smaller than the smallest instantaneous curvature in my reflector, but if you rescaled the emitter (ie, used two side-by-side strips rather than 5) you could make it work, admittedly with some loss of function. (The blinds are circular in shape, but the ideal shape is elliptical)

The more important point is that, yes, you really do need shiny reflectors for this design. If you are using diffuse reflectors, you want an entirely different reflector shape.

...but that's not entirely bad thing; for instance, my intuition says that it starts mattering a lot less if the reflectors are curved (I'd have to do a raytracing calculation to find out how much, to be sure that I am correct about that - and I don't have such a calculation coded up)

...if you don't want to fuss around with specular (shiny) reflectors (and I can't blame you), and have an undemanding lighting situation, it might be best to just use something like a board with a flat-bottomed V-Groove, painted white, with a LED strip in the bottom of the groove.

peppypickle (author)2015-01-22

awesome, thanks for sharing this Instructable!