Jeff51's DYI Adjustable Lumen Tube

A Lumen Tube you can build that reads out in Lumens without needing adjustment tables.
“If you can’t make it Perfect – at least make it Adjustable” Said a wise man.

[EDIT – 8-20-21]
Part 2 – The Build and Tuning has been added to the next post below.

Some corrections / additions have been made to part one.

Addition to Part One
After fussing with this thing for some time now, I’ve believe I have some design philosophy that needs to be added.

The Spot-Flood Conundrum
One of the most vexing things about these designs is the difficulty of dealing with beam patterns. Seems if you get it calibrated for a flood pattern – it will be off for a tight beam.
This drove me nuts in the initial design where I was trying to duplicate the more compact versions using three 90 degree bend pipe sections.
I believe the solution lies in…

What Happens in the Tube -
Needs to Stay in the Tube
Take a look at these two pics.
Zoomy Pics

Both are a zoomy. See how much light is hitting and escaping with the flood pattern? You can see where the spill hits the side tube about 1/3 of the way down from the top.
Zoomed out the spill misses the pipe. Still the reflected and diffused light is lighting up the pipe.
Lining the Tube with reflective tape keeps all those photons inside (well most anyway).

In addition, the flashlight head must be mounted so that any light reflected back toward the top of the Tube is directed back into the Tube. Thus the cardboard rings fitted to each flashlight head and coated on the bottom with silver tape.
This makes a significant difference in the Lumen reading.

The elbow section, while made of thicker plastic, also absorbs/transmits light. This photo of a brighter flashlight illustrates that.
Elbow Pic 1

I tried different solutions.
Elbow Pic 2

The results were a bit of a surprise. Using the same light.
The naked elbow read: 157Lm
An elbow coated on the exterior with “chrome” paint: 154Lm
An elbow coated inside and out with “chrome” paint: 105Lm
The elbow with the exterior covered in aluminum foil: 165Lm
The “chrome” paint is more of a sliver. I tried two different spray cans and they both looked about the same.
Foil is the way to go thinks I.

What happens outside the Tube -
Needs to stay outside the Tube
The converse to the above is also true. Exposed PVC pipe sections allow exterior lighting conditions to influence the readings. Unless you are going to always make measurements in a darkened environment.
The Tube assembly needs to be insulated in something to prevent light pollution.
I found this particularly important where the lux meter attaches to the 3-2 reducer. Any additional exterior light in this area knocks the low Lumen readings off. A wrap of Aluminum foil does the trick.

Diffusion to the Max
The second part of solving the spot/flood conundrum is making sure that the light is fully diffused by the time it gets to 90 degree elbow. This prevents a spot beam from reflecting differently from a flood beam.

This, I think, is the problem with many of the twisty Lumen Tubes. It’s not that they don’t work or can’t be calibrated.
I just think it’s harder to calibrate if a not completely diffused beam is bouncing around inside the pipes. The bends are used to diffuse the beam, but the spot/flood reacts differently in the first bends (or so I surmise).

Take a look at the pic below.
How well does the Translum Film work?
Laser Pics

This image is taken at the bottom of the coupler that sits on top of the Light Valve.
That even green glow looks pretty consistent. The source is a laser. About as close to a point source as you can get. The ultimate tight beam. It is further diffused by additional film below the Light Valve.
My design has the beam very well diffused before it hits the elbow.

As long as the majority of the light makes out of the defused Tube area, I think most of the differences in beam patterns are minimized.

Taking this a step farther – I think a Lumen Tube that has no bends in it is the optimum design.
The straight sections can all be lined with reflective material and there will be no bends to react with different beam patterns.
If I decide to try to make a 6” version – It will be a straight Tube all the way.

[End Edit]

This is a work in progress. I was cruising along on the project pretty well till the bug hit.
I put this on hold while I social distanced, made masks, and started hording TP.
I’ve got that covered now and wanted to start my Lumen Tube project again.

I’m starting to have a few health issues and may not get it finished to my satisfaction in a timely fashion.
So - I decided to get this out so others could take a look at it. And I’ll keep moving toward the finished version as I can.
All the images link to a higher res if you want a closer look.

DYI Tube in progress pic.

I’ve been using a home built light box to measure lumens based on a Styrofoam cooler (a Lum-a-box? Igloo-a-Lum?).
I use filters inside the box so that it reads out in Lumens on the Lux meter. It works OK for floods but it’s way off for throwers.

I could adjust the filters to make it work for different conditions, but that required opening up the box and messing with stuff.
A total pain in the butt and hard to reset after fussing with the darned thing.

I ruminated (always wanted to use that word) that a setup could be built that could read out in Lumens and be able to adjusted to correct for differences in - well - all the stuff that makes getting a good reading so hard to do.

I didn’t want to have to use any correction factors.
I also didn’t want to have to spend two lifetimes making little adjustments to make that happen.
Ergo - The Tube had to have external adjustment.

My solution is the Light Valve. A section of pipe that can change the amount of light that passes through it. Shown here in prototype.

High or Low K lights throw off the reading? – adjust the Tube.
Correct for differences in Lux meters? – adjust the Tube.
Phases of the moon throwing the readings off – adjust the Tube.
Want higher results so makers will send you more lights? – Adjust the… You get the idea…

As you see with the valve in this configuration, I can vary the output by about 20%.

Having watched other builds and most importantly the awesome work done by Texas Ace I decided I’d give it a go.

From TAs thread I perceive there were 3 main issues he had while making his tubes.

  1. The diffusion material was not consistent.
  2. Small changes to the tube assembly (how the Pipes/Diffusers Fit) would make changes to the readings.
  3. Each tube had to be carefully calibrated by hand – perhaps the biggest hurdle of all.

TA said it took literally 6 months of tinkering to get things sorted out. I believe him!
And TA, please feel free to correct anything I say regarding your epic build.

I set out to see if I could come up with a way to perhaps circumvent these issues.
Hopefully creating a design that any who wanted to could give it a shot could and build their own version.

So here it is so far.

If you want to skip the R&D part (and my long ramblings) and get to the end result, jump down to the next POST marked “DYI Build”.
There might be some useful build tidbits among all my ramblings….

Diffusion Confusion
The first issue was finding a diffusion material. Instead of the hardware store I turned to the photographic world.
I looked at various options (BHPhotovideo has lots of choices) and settled on something called Translum.

This is a diffusion film meant to be used over photo lights.
It is a thin plastic film. It comes in 3 strengths 0.75, 1.0, and 1.5 stops with correspondingly more diffusion for each one. Each also reduces the light transmission by the listed F-Stop (my grasp of the obvious is overwhelming).

Translum at BHPhoto

I bought a trial pack that had 2 sheets of each in it ($30).

Disks Video

Since it’s designed for Photo LEDs it should stand up to heat well. Being a photo product it is consistent across each sheet and from sheet to sheet.
The thin plastic film is sturdy enough to not warp under its own weight (at least in the 3” tubes). It is easy to cut with scissors.

Pipe Dreams
Next I turned to the tubes. I went to Home Depot (or Lowes) and picked up some 90 degree street elbows. These have a female on one side and a male on the other.

The brand of the fittings (at least the 90 degree thingy and the pipe) is Charlotte. Here’s a shot of the brand.

In addition I got a few in line couplers, a 3” to 2” reducer, and a short length of precut 3” pipe. All series 80.
I chose the 3” size because the fittings are MUCH cheaper than the 6” versions.

If it Fits well, Well it won’t – Fit well that is
Test fitting the pieces together is problematic if you expect to get any consistency. Some joints push up nearly flush while others require substantial force to get them close to seating.


Getting the tight joints apart is a royal pain. The parts need to come apart and be reassembled with consistency during the design and tuning phase. Even a 2mm difference in joint fit will change the measurement. And you will be shuffling parts around a lot.

Ring and Diffusion Video

Lord of the Rings
The joints need to be tight enough to stick, yet loose enough to make changes.
My solution was to make a guesstimate on how much space to leave between each joint. I then cut ring spacers from the pipe. I settled on a 1cm (10mm) thickness. Your mileage may vary depending on your parts.

The spacers and rings won’t fit all the way into the joint, so I made cut in the ring and opened it up until I got a good friction fit. These rings keep the joint fit consistent.

These rings also act as retainers for the diffusion film disks.
I found I wanted to hold film (in some instances) against the spacer rings. I cut some additional rings about 2mm thick and split them also. These are used to keep the film in place as needed.
These rings are also used as spacers in fine tuning the Tube.

Marking the cut is as easy as getting something the desired ring thickness and setting the pipe on end. Then spin it against the marker with a pencil on top to make a line around the pipe.

I used a band saw to section the pipe, but a scroll saw or fine tooth hand saw would work as well.

After cutting the pipe it needs to be de-burred by sanding the cut lines on the inside and out. It also helps if you taper the outer edge ever so slightly.

I used a cheapo Harbor Freight belt sander to smooth out the cooties left by the saw blades. Hand sanding or a file works too.

Flashlight Support
I cut a disk from 1/16” clear plastic to act as a support for the lights. This was just some scrap I had laying around. Anything strong enough to support the flashlight will do.

Use the rings mentioned above as templates for cutting the Translum and anything you want to fit inside a coupling or inside a pipe.

I used heavy paper to make the doughnuts that act as baffles to fill the space between the flashlight head and the Tube diameter. You will need one for each head size.
I coated the bottom of the doughnut with shiny metal tape.
Thin cardboard would work better as it keeps the light from moving about.

The Doctor is In

The meter is the ubiquitous Dr. Meter available on Amazon (about $45) and elsewhere. It fits nicely in the end of the 3”–2” reducer. I already had one on hand.
Interestingly the sensor of the Dr, Meter uses the same Osram photodiode that Terry Oregon and I tested for PWM measurements.

The diffusion film does a really good job of dispersing the beam.
Here is a shot of a zoomie through 1 layer. And again after a 10cm tube and a second layer of film.

The film works so well that multiple 90 degree turns in the Tube are not needed.
I wanted some space between layers so I chose a 10cm tube between the top layer and the next.
This will change in the final DYI design. Start with a longer tube and shorten it to tune the system.

Tube Lining
When looking at the Tube with a flashlight shining, I could see light passing through the straight pipe walls. I want to keep as much light inside the Tube as possible. Also I wanted to keep external light out of the system.
I took some heavy paper that is a little heavier than Nat. Geo. cover stock. (But thinner than Index card stock).
I cut rectangles that just fit inside the straight pipe. I covered one side with metal duct tape. Then I fitted it inside the pipe sections. Shiny side in. A little extra metal tape joins the ends together. It is just a slip fit.
You can see it here inside the Light Valve.

Valve Mounting Pic

Not much light was coming through the joint sections. It’s thicker and only a small part is exposed to the light.
I covered the outside center parts with the metal tape. Just to be sure.
The 90 degree elbow I covered on the outside with foil. I’ll try a more elegant solution later.
I also covered the outside of the reducer with metal tape. I left the inside alone. I think it would bounce some of the light back up the tube if treated with foil.

And yes, every little change made to the tubes makes a measurable difference in the readings.
Needing re-calibration each time. What a pain. I feel for you TA.

Mount The Tube
Using a single 90 elbow at the end made for easy mounting and parts swapping.

I cut a 4” hole in a hunk of 2x6 and stuck the reducer in that. I made a side cut and sqoosed the hole a little tighter using some hot glue (the DYI’ers best friend). It’s now a friction fit.
With the rest of the assembly vertical the joints need not be so tight to keep things together. That makes swapping out different pieces much easier. And you will be taking it apart often.

Trial and Error, and Error, and…
Now comes the fun(?) part. Slapping parts together and hopefully coming up with a combo that makes the LUX reading correlate to Lumens.

So after much (much) trial and error I came up with a design that was close to converting Lux to Lumens.
I filled many – many pages with failed attempts.

I don’t know about TAs adventures, but let me tell you when I finally started getting readings that were in the ballpark…
Well - The skies opened and the heavenly choir began to sing.

If you decide to build your own – Label everything – EVERYTHING!
Label Everything Pic

AND use something to record each change, no matter how small.
Ask me how I know. No don’t, it will take too long.

The Light Valve – Calabratable – Not Calibrated
The problem with Lumen Tubes is the final adjustment.
Let me tell you dialing in the last few percent is beyond a royal pain in the butt.
I didn’t want individual builders (or me) to spend way too much time adding or subtracting bits of film or be forced to use a correction factor.

The Light Valve
I was adjusting the tube by adding and subtracting filter sheets and parts of sheets.
Then I thought about a simple valve. It works for water and air, why not light?

I took a section of the straight pipe and drilled a hole suitable for poking a coat hanger through it.
By luck, I miss aligned each side just a bit. That made enough tension to hold the hanger without it moving.
I then added a disk of Translum to the inside of the pipe attached to the coat hanger/rod.
Scotch Magic tape – the frosty stuff works fine.

Shown is a 0.75 disk with a section of 1.0 material on top. Using mixed values you can adjust how much change the Light Valve makes to the readings.

I started with a 1.5 density and eventually got the rest of the tube close enough that a 0.75 disk would do.

Now I have a Tube that can be adjusted externally. I added a set of marking on a hunk of cardboard so I had a reference scale.

How repeatable is it?
Very. Using well regulated lights the reading is within a few Lumens (out of say 300lm) after moving the valve to other positions and bringing it back to the first setting.

A drop of hot glue (naturally) will fix the film to the rod after my experiments are complete.

By having an external adjustable Tube it is possible to dial in corrections for all the other variables.
All this, of course, is predicated on having lights of known values.
Another saga for another time.

So if you’ve lasted this long, and want to give it a try.

I’ll try to get the next section finished as quickly as I can.
See the Next POST For the Build.

Jeff51s Lumen Tube Part 2
The DYI Build.
You might want to look at the first POST to see some added details.
Links to the parts used will be found there. Details on making the various parts are there also.

This one is only concerned with building material, construction, and tuning.

Local store source.
PVC Pipe – all are 3” schedule 80.
1x 90 degree street elbow. The kind with different connectors on each end.
1x 3”to2” reducer.
2x In line connectors.
1x Pre-cut length of pipe. My local store had a 2’ length (which is plenty).

Metal Duct Tape. The stuff with a shiny finish. I used Nashua tape – because I already had some.
Tape Pic

Some tape that’s easy to remove. For making labels to mark parts. (And believe me YOU WANT TO MARK THE PARTS). I used yellow painters tape.
All the parts must be marked so that they index in the same orientation. Repeatability is key to not driving yourself nuts trying to get repeatable results.

Aluminum foil. To cover the joint where the Lux meter sensor attaches. It’s also used to cover the elbow. My version uses foil covering on the elbow. I tried “chrome” paint. That was a fail (see part 1).

Something to mount the tube on. This can be accomplished in various ways. I drilled a 4” hole in a 2x6 and used that to hold the 3” to 2” reducer. I attached that to a scrap big enough to keep the whole thing steady. I used an additional scrap under the elbow to support the weight of the Tube.
A sheetrock screw is driven in from the back side to lock the 3-2 reducer in place and prevent it from twisting in the 4” hole.
Wood Parts Pic

Clear Plastic
Something sturdy enough to hold a flashlight. 3” disk to fit inside one of the inline couplers.
I used 1/16” (~2mm) because I had some on hand.

Diffusion Film
Translum diffusion film.
This is sold in three grades from f0.7, f1.0, f1.5. I used 0.75 and 1.0 in my build. You could stick with only 0.75 disks if you take this into account while tuning the Tube.

This might be the only hard to source item. I purchased mine in a trial pack from bhphotovideo (see part 1).
I built the Lumen Tube several different ways using the different strength diffusion films. The film is very consistent. It’s the only true constant in the build.

My last version uses 6 disks of 0.75 TL Film, 1 disk of 1.0 TL Film, and a 0.75 TL Disk to fit inside the Light Valve. Cut an extra 0.75 disk. Keep this handy for tuning.
You may want to use a 1.0 TL Disk in the Light Valve for a larger range of adjustment.
Use the spacer rings as a template for cutting the TL Film Disks. Use the exterior for the diffusion disks. Use the interior for the valve film disk.

Lux meter
I used the Dr. Meter (from Amazon) – because I had one. A different meter (even a second Dr. Meter) will most likely need a different tuning.

Light(s) with known values.
Early on, you don’t need to be exact, since the Light Valve takes care of a lot of the grief. Something you are fairly confident in will do. And by confident, I mean pretty dang close; + - 10% (at most), + - 5% (or less) even better.

But let me tell you, dialing in the last few percent is like trying to rope the wind if you don’t have a good handle on your known light values,

Assorted tools and junk
A drill, a Drill bit to fit the coat hanger, scissors, a saw to cut the plastic pipe, coat hanger, something to snip the coat hanger, scotch magic tape (the frosty stuff), and some sandpaper.

Patience – large dose – medications help.

Cut the Pipe
Cut the 3” pipe so you end up with:
1x 140mm (this will end up being shortened during the tuning process).
On my Tube, this ended up being 100mm long. Yours may not.
Line the pipe with foil tape (see Valve instructions below).
Foil Lining Pic

1x 80cm (for the valve)

Initially I covered the outside of the pipe sections with foil tape. This proved less satisfactory in the spot /flood conundrum. Lining with foil tape proved superior.
I left the tape on the outside. It is not needed.

5 x10mm-12mm spacer rings. The thickness will be dependent on how your parts fit together.
1x ~ 3.5mm spacer ring. This is needed to in the top coupling tube to hold the film disk in the bottom

Make a set of spacer rings one each of about 2mm, 3mm, 4mm, 5mm. These might be needed for final tuning. They are also helpful to gauge how much change in the tuning tube is needed to get the lumen reading to align with the center area of the valve setting.

Now make a few test fits to see how tightly the pipe and fittings go together.
I found that a spacer of about 10mm or 11mm worked for me. This allows a full seat without too much binding. Make your accordingly.

Do a test fit of the 90 degree elbow into the 3” to 2” adaptor. Don’t try to set it all the way. You are looking for a firm press fit.
Measure the approximate distance from flush this joint is.
Spacer Measure Pic

Cut a spacer ring of this thickness. See Part 1 making spacer rings.
This is the only critical joint that needs a firm fit.
This joint supports the rest of the Tube. It should be firm enough to keep the rest of the Tube vertical while using the thing.
The vertical sections should be fitted so they are (fairly) easy to take apart. You will be doing this – a – lot! No need to cram these together
See part 1 about making the split spacer rings.
I included the exact size of my spacer rings. Most of these ended up sized this way because that’s how they came off the saw when I first made them.
I had no idea at the time how even small changes would affect the readings.
Still it matters not as long as the sections fit smoothly together and you keep everything in indexed and in the same stack order. The tuning process will account for any differences.

Build Note
The spacer rings are a critical part of my design. You must be able to repeat fitting the parts to within a fraction of a mm. Using the rings and keeping the parts indexed is the only way to assure you can take it apart and reassemble it and keep consistent dimensions and readings. Anything else leads to madness…

Shorten One Coupler to use as the light support.
Cut one end for use at the top about 20mm from the center stop. Shorter if you feel the need.

Take a look at the build diagram at the bottom. There is only a small portion of the coupler exposed to the interior light. Most is covered by the lined pipe sections.
Still I covered the couplers with foil tape to prevent possible light transmission.

Make a Scale
I used a protractor and a scrap of cardboard to make a scale to use with the light valve (as seen in the first part).
Scale Pic

I decided against sliding the coat hanger through the scale itself. I wanted to be able to make changes without messing with the valve.
I cut a slot in the card stock to let it slip onto the valve.
Later I made a nicer scale. A PDF copy can be downloaded from my Google Drive.

Make The Light Valve (see part 1 for more detail)
Cut the straight bottom part out of a coat hanger.

Drill the 80cm pipe in one side (I’ll leave it to you to find the middle side of a cylinder side). 35mm from the top – 45mm from the bottom. This leaves room to mount the scale.
Depending on the parts you end up with - you might have to trim the top and bottom of the respective couplers/elbow to make room for mounting the scale.

Use a drill bit that makes for a tight-ish fit for the coat hanger.
Push the coat hanger through till it reaches the other side. Make another hole there – But offset it from parallel ever so slightly so the coat hanger will bind a bit and not move freely.

Line the tube
Cut a heavy piece of paper so that it just fits the inside of the valve.
80mm x about 242mm worked for me.
Cover one side of the paper in metal tape. Slide the paper sleeve into the valve body. Secure it to itself with some more of the metal tape. Shiny side facing in (I am captain obvious).

Put a 90 degree bend in one end of the wire leave enough to use as scale marker. 4” should about do it.

I used a small piece of wood attached to the valve pipe body to attach the scale.
Scale Pic 2

Inside the tube attach a 0.75 or 1.0 Translum disk to the wire with some scotch tape. I used the frosty/magic tape kind. Make sure the disk is parallel to the part of the wire that is used as a scale.
Test the movement to be sure the disk doesn’t bind on the pipe section and stays aligned with the scale.

If you decide that 0.75 is too little and a full 1.0 is too much. Try adding a partial section of a second 0.75 disk on top of full 0.75 disk. This can be used to dial in the Light Valve adjustment to your liking.

My Design
Yours will most likely need a different adjustment because of the variables that can’t be controlled. Different lux meter, different reflective character of the metal tape, differences in the schedule 80 pipe. Phases of the moon.
Remember MARK EVERYTHING. And put alignment marks on the exterior so that parts can be reassembled in exact alighment.

Starting at the top. (Far right in the image)

Top Coupler:
1/16” (~2mm) clear disk to hold the light.
Internal Stop
0.75 TL Film Disk
10.6mm Spacer Ring
0.75 TL Film Disk
3.6mm Spacer Ring

Tuning Pipe – Start with a 140mm tube lined with foil tape.
Mine ended up at 100mm (dumb luck!).
That worked fine until I decided to make some cosmetic improvements to my build. After doing that I was about 10% brighter from where I started. The Valve could correct this, but I wanted to keep the calibration in the midpoint area of the dial. The added 3.4mm spacer at the top of the second coupler corrected this.

Second Coupler:
3.4mm Spacer disk – with some black sharpie marks to tune the Tube. This ring may not be needed if you are more careful than I when shortening the tuning tube.
1.0 TL Film Disk.
10.3mm Spacer Ring
0.75 TL Film Disk
Internal Stop
10mm Spacer Ring (no film here)

The Light Valve
80mm Pipe, Lined with metal tape. See part 1.
0.75 TL Film disk for the movable part of the valve. A 1.0 TL Disk can be used to give a larger range of adjustment.

90 degree Elbow - Cover in foil
0.75 TL Film disk
10.3mm Spacer Ring
0.75 TL Film Disk - This rests on an internal taper found in the elbow. There is no internal stop like in the coupler rings.

Mounting Stand
Here is what I came up with to hold the Tube. The block with the 4” hole is screwed n from the bottom. There is a screw hole in the middle of the left side that accommodates a sheetrock screw to keep the 3-2 reducer from twisting.
The angled support is not attached. It’s a slip fit with Tube and the two small rectangles mounted on the base. I did this so it is easy to dissemble the elbow/ reducer joint if needed.
Stand Pic

3-2 reducer - Covered in metal tape.
11mm Spacer Ring
0.75 TL Film Disk - This rests on an internal taper in the reducer.

Build note. All of my 10mm and thicker spacer rings are lined with metal tape. Once again - trying to keep as much light from being absorbed by tha plastic parts as possible. I don’t think this is significant, but the OCD in me had to do it.

Build note 2. These joints are fairly air tight. Don’t get too frisky pushing and pulling the joint apart. You can end up sucking the diffusion disks out of position.

Build note 3. Be sure to fully seat each joint every time you make a change. Failing here leads to madness…

Lux meter
Attach to the end of the 3-2 reducer and cover the joint area with foil.

The meter to reducer mounting must be very secure. Any small movement here will cause changes in the readings. Which leads to madness…

Now the fun begins. There are basically two methods to tuning the Tube.
The first is to start shortening the top pipe until you get close to a Lumen measurement for your known light values. Then proceed with caution until nirvana is obtained.

The second is to add and subtract full or partial Translum disks to the Tube to increase or decrease the readings.

Tuning pic.

Here is an image of some of the tricks to use. Partial Film disks are inserted in the light path. Do this on the top of the elbow section. By this point the light is diffused enough that any differences in beam pattern will be gone.
BUT always orient the partial disks in the exact same location. Failing this leads to madness…

Starting with the full 140mm tuning pipe, the reading will (or should be) way low.
Start shortening the tube in 5mm increments to approach the correct value for your known light.

As you shorten the pipe, keep flipping the valve from 0 to 90 to get a feel as to where you want to end up on the scale.

Hopefully this will end up in the middle of the Light Valve scale.

If you go too far and the reading is too high all is not lost. If you are just a smidge too bright, try adding one or more of the thin spacer rings to see if that does it.
Failing that try making a partial 0.75 Translum disk and insert that into the stack.

Super fine tuning. I wanted to move my readings a tiny bit darker. Even the thinnest ring was too much.
I used a Sharpie to make some black marks on a thin ring covering about 40% of the inside surface.
This made a few percent difference and got me where I wanted to be.

If things get FUBAR, the pipe sections are the cheapest part. Buy a new one and start whittling it down again.

The Whole Thing

Wrap it up
Hopefully you are now have a Lumen Tube that you can calibrate without having to go through tedious internal changes.
And reads out in lumens without the need of correction tables.

Design Refinement
As I stated in the edit to part one - I now think the optimum design is a straight pipe with no bends.
All the pipe sections can be lined for better consistency of absorption.
Without a bend the spot/flood conundrum is minimized as far as possible.

Depending on interest (or inclination), I may make another version of the 3” Tube without twisties.
The 3” fits (barely) everything I own right now. I’ll save the adventure of making a 6” version for when I finally get my own portable Bat Signal.

For measuring the Big Guns, we really need a multi-thousand Lumen light with a known output.
I’ve always wondered how linear all these designs (mine included) are when scaled up to the portable suns now available.
All the Best,

Few more things.
Do not cover the outsides of the two pipe sections with anything (the tuning pipe and Light Valve).
That will prevent the pipe sections from seating fully into the joints on each end.
The foil lining takes care of light transmission problems.
Wait till the tuning is done to cover them for cosmetic reasons if you desire.

I believe I made the build sound more complected than it actually is. Think I was influenced by all the grief I went through during the initial build phase.

In reality, all the design grief is done.
The exact thickness of the spacer rings is unimportant as long as:
The elbow to reducer fit is firm enough to keep the rest of the build supported.
The other joints, as long as everything seats fully, are not important.

Basically build the bottom part – Lux meter through Light valve. Also the top light mount section.
Assemble and mount the bottom section. It won’t need to come apart again unless something goes terribly wrong.

Make sure the joints at each end of the tuning pipe are easy to dissemble.

Then start shortening the tuning pipe.
Once you get close, the final adjustment is made with the light valve.

The small tweak tricks I talk about are not normally needed. They can be used to get the reading closer to the center of the Valve adjustment dial.
But that’s not really important as far as using the Tube to make lumen measurements.
Dialed in is dialed in, no matter where on the scale that happens.
All the Best,

That is a really clever idea—i like the light valve and how well it works to adjust in single digit increments.

The overall “precision of the assembled joints” was quite impressive and in keeping with your sense of the obvious, which made this all fun to read and watch.

Now for a dumb question:

if a FL has a setting for 1k lumens, but is reading slightly higher at first, then you can use the light valve to adjust the output beam down to 1000. Now, how do you use that output beam, or what is it’s purpose?

Thanks for the kind words.
I’m not quite sure what you are asking.
The Light Valve is to adjust the light hitting the lux meter so it can read out directly a value in Lumens without needing to apply correction factors.
Basically so flashlight geeks like me can use it to measure FL outputs for review purposes. Then drone on and on about actual measured lumen values vs manufacturers ratings.

I plan to eventually add a photo diode to the bottom of the Tube and hook that into a simple circuit that I can use to generate run time charts using logging software. Which are a great tool so we can drone on and on about Turbo step step down values vs sustained Lumen values.

Naturally the flashlight itself could care less. They, as a rule, are heartless beasts. Who exist to tempt me to spend more money on new models that, well, shine light out the front. Just like all the others I have.

And of course it is an excellent device to confound my wife, who upon seeing it asks:
What the hell are you building this time? Something to meter toilet output?
All the Best,

I have the same Dr Meter —- For my tube I decided to glue a piece of 1 1/2” PVC pipe to the sensor housing — it’s about 1/4’ long (about the thickness of the fitting I stick it into —you have to chamfer the side that goes against the meter a little —- works great — no more of that taping in loose fit

Wow great job Jeff ! I have one of TA’s tubes . I did not realize how tedious dialing it in can be .

Oh yeah , hope you’re feeling better .

Nice work.

That’s a nifty / elegant solution.
Next time I’m at the hardware store I’ll pick up some pipe and give it a try.
I’ll have to re-calibrate I suspect. Hopefully a Valve adjustment will take care of it.
All the Best,