Lighting Research Results

[matthewepler]

Hi all. Based on the lively and productive conversations on lighting design we’ve had here on the forums, I asked @johnarthurkelly to conduct some research to share back with the group. He has completed the research and I’m very excited to share it with you today.

The goal for the research was to determine our best options for a design that would maximize brightness, color, and eveness of field while also balancing cost and complexity. There were many variables to consider and I’m grateful we had such a great collaborator to sort through it for us.

The document below is not the definitive solution to Kinograph. It is a starting point for the next round of discussion. Please share your thoughts and what you think would be the most practical and useful next step for v2 of Kinograph. And thank you for all your contributions. Without you and your contributions to this forum, none of this would exist.

Document:
Kinograph v2 Illuminant Research

[cpixip]

@johnarthurkelly - what a fantastic report with very interesting results!

A question: normally, baffles are used in integrating spheres to make sure that no direct light from the inports reaches the outport. In theory, every beam of light needs to be reflected at least once from the surface of the sphere to create a homogenous field of light. I could imagine that the limited angle of illumination of the LEDs you used in the experiments kind of worked similar to a dedicated baffle between inports and outport. As you mention in your report, you specifically selected the RGB-LEDs for their narrow angle of illumination. How does the white-light Yuji-LED behaves in this respect?

Another question: did you research how even the aperature is illuminated when only a single LED is switched on? That point would become relevant if the inports do not use single unit RGB-LEDs, but instead each port would feature different LED-colors. Another case where such a question would become relevant if you use, say, three ports for white-light-LEDs, and the remaining fourth port for another LED filling in some gap in the white-light-spectrum.

As you pointed out in your paper, an integrating sphere for 35 mm is a large beast. One alternative option I want to suggest for discussion is to use your Direct Drive unit and correct the remaining (small) intensity variations in software. This could be done by taking an appropriate calibration image before the scan, without the film loaded. Given, you would loose a few bits of brightness resolution, but mechanical construction would be easier, I guess.

[junker]

I just found this paper in the Wikipedia entry for Integrating Sphere, ref #1, regarding “Low cost 3D-printing used in an undergraduate project: an integrating sphere for measurement of photoluminescence quantum yield” where they explore low cost coatings to approximate the more expensive Edmund and Avian coatings. May be useful?

[johnarthurkelly]

You are correct - normally that would be the case. I did not include them here for the simplicity of the model but it would be good to conduct additional tests with them. To optimize the LED selection for that portion I would select ones with slightly wider beams. Using such narrow beams and pointing them at an area of the sphere outside of the field of view of the naked aperture would result in at least two bounces to reach the aperture.

The white LED basically floods the inside of the sphere given its relatively wide beam angle. From my earlier testing though the beam is fairly uniform. I checked that the beam did not reach the aperture but adding a baffle would certainly ensure this in practice. I’d like to casually re-run these tests with a new sphere design when my printer isn’t under covid-19 lockdown in my office Additionally I’m gearing up to test a model of sphere made with white filament to see how it performs against the white coating.

Your point about using single ports/multiple whites with an augmenting wavelength is good and bears further testing - it should certainly be a consideration if the white LED has a less-desirable spectral distribution.

Correcting the direct drive approach is an option, but it would require some tweaking. I still think that there is more to gain from physical modification in that arena - i.e. I’d like to try adding some optics in the path that can potentially condense and collimate the light before hitting the diffuser (I know how wacky that sounds) but I think there might be gains to be made, especially with higher-power or a larger array of LEDs. Correcting the falloff digitally can be done, but even with a baseline characterization of the falloff it still sacrifices a lot of the range of the sensor. I think there’s middle-ground here though!

[johnarthurkelly]

Their approach to the coating process is a little bit different from the Utah State University lab - but it could provide an additional model to test. One thing of note that the Aberystwyth paper didn’t test was durability vs concentration of the coating or reflectance vs concentration of the coating - there’s definitely a point at which the BaSO4 (in powder form) will basically be destroyed by the slightest disturbance without some kind of protective medium. Even professional BaSO4 coatings deteriorate over time! But an almost pure coating would certainly be extremely fragile. I’d like to test a hybrid approach, since they did multiple wet coats and dry coats, wherein the sphere is coated with the BaSO4+paint multiple times and then “dusted” with BaSO4 like the Aberystwyth approach for the final coat while the paint is still wet.

[cpixip]

@johnarthurkelly, @matthewepler - just another note. I used to do color enlargements (in the 80s of the last century). My enlarger for 6x6 cm negatives had an interesting light box design. It kind of mixes the integrating sphere design with the Direct Drive design.

Basically, the light from the color-filter stack enters a light-mixing box through a hole sideways. There is no way this light reaches the negative directly, it needs to be reflected at least once. This is the integrating sphere part, simplified.

On the open side of the box, where the negative is, there is an additional diffusor. This is the Direct Drive part, so to speak.

From my lab work some 40 years ago I can tell you that this illumination construction had an excellent homogenity over the whole area of a 6x6 cm negative.

The best part of this approach is that it is extremly cheap to manufacture and quite compact. The box is made out of cheap styrofoam, no additional coating applied…

Because of covid-19, I can not make some images of my old unit, I however found the description of a similar enlarger on the web (japanese-build, mine is german-build).

Have a look at this page, Saunders/LPL 4500II 4x5 enlarger.

You have to scroll down about half of that web-page to see the inner workings of this design.

[johnarthurkelly]

I was actually hoping that the machine team at BK Research would eventually “fold” the direct drive model like that, essentially a hybrid of the two designs. When I’ve got printer access again it’d be interesting to mock up a build like that and see how it compares to the plain integrating sphere.

[junker]

Here’s a quick description of the diffuser vs condensor enlargers that cpixip was talking about, and a diagram of the light mixing box:

Choosing an enlarger.

[nachoseimanas]

We did several tests on our adaptation of the URSA telecine.
The one that gave us the best result was the adaptation of the diffuser enlarger.
We made a cardboard prototype that for now continues to work.
I am not going to the office due to the quarantine, when I return I send you photos and test results.
To evaluate the result we use imageJ https://imagej.net/, an excellent image analysis tool.

[johnarthurkelly]

I love imageJ, had to switch over to Fiji with my Macs though

[matthewepler]

@johnarthurkelly I’m interested in the improvements you mentioned for the direct drive approach. Did I get it right in this sketch? As you will see in the picture, a few questions popped up as I was making this. Does anyone know how we might answer them?

If you’re unable to enlarge this by clicking on it, you can download the original here.

[matthewepler]

Ah, looks like I should have read @junker’s post first. Great find with those diagrams.

Based on your research, @johnarthurkelly what advantages/disadvantages do you see in these approaches?

I have a 3D printer at home and can make some parts and send them to you for testing if you’d like.

Would love to see those photos @nachoseimanas. This is great. I think we’re narrowing in on a design to prototype. Very exciting.

This note about the effect of condensers in the link from @junker is interesting:

This crisp appearance to the photograph has the negative affect of bringing into sharp focus the grain, any scratches, flaws and dust from the negative, meaning more time spent on retouching. These enlargers are subject to the Callier affect this is where the highlights in the negative scatter the light more than the shadow areas creating the increase in contrast.

That makes me lean more towards the “Diffuser Enlarger” approach mentioned in the same article. Thoughts? Considerations?

[cpixip]

@matthewepler: the “Diffuser Enlarger” approach works differently from what you have drawn (at least different from what I had in mind when I wrote this post).

In an old enlarger, the bright and collimated light of a lamp was (or: of several lamps were) cast through a filter stack for tuning the color temperature directly onto a diffuse surface, made usually out of styrofoam.

From that bright spot of illumination, light rays were diffusively cast in all directions, usually reflected several times until they reached the exit pupil. So the whole setup was in a way very similar to the approach of an integrating sphere. Only that the “sphere” was a cube, and the perfectly diffusive surface of an integrated sphere was approximated by the properties of styrofoam.

There were constructions where the box was really just a box, so that the bright spot of light from the lamp was basically perpendicular to the exit pupil (where an additional diffusor was usually located).

I tried to sketch something like that to make the arrangement a little bit clearer:

The important point in this setup is that most of the light was reflected multiple times, randomly. This was required for mixing the colored light from the filter stack, as well as to ensure an even illumination over the whole area. In a way, this is just a poor man’s illuminating sphere.

The light box of the japanese enlarger I linked above has a side wall tilted by 45°. This improves the illumination level, but also reduces the light mixing ability of the whole box, because a lot of rays will be reflected only once before they reach the exit pupil.

Matthew, your drawing, especially with the mirror in the 45° position, is the opposite of the diffusive enlarger approach - with a good collimator in place (as you have drawn it), it would illuminate the film gate with parallel rays, leading to a high scratch visibility in turn.

[matthewepler]

Thanks for the drawing @cpixip. I’m much better with visuals

Unfortunately I drew and posted my diagram before reading the article on the enlargers. I see what you mean.

Do you think this approach shown in your diagram will work for the Kinograph? I imagine we’d probably just print with white plastic or make the box out of white acrylic plastic which is glossy and reflective.

It seems like a good approach and one that we could easily test. I will have access to a laser cutter in addition to the 3D printer within a week or so.

[johnarthurkelly]

@cpixip @matthewepler - to the point about collimated light, it would definitely not be desirable at the image plane, but using a collimated beam to illuminate a diffuser at (or very near to) the image plane might be slightly advantageous in that we could afford to move the light source a little further away (potentially folding the light path with a mirror). This is why the bottom drawing would be better than the top drawing since you gain the ability to move most of the illuminant “stuff” away from the film path with roughly the same penalty in light loss.

Condensing the light and collimating it are not necessarily the same, and we would not necessarily have to collimate the beam to make it useful in a compact arrangement. Collimating the light (having the light rays all propagating in parallel) is most useful if we have to send the beam a long distance, but in our case always with a diffuser at the end

Re: the questions on the drawing-
A - The shape of the tube might not matter that much pre-condenser as the goal of the condenser would be to collect as much of the beam as possible and so it would likely be pretty close to the LED itself.
B - No, a mirror could be used in collimating a beam of light but in this case would be more complicated.
C - We can chose our distance without having to calculate it, to a degree, we can pick optics based on their focal length to suit the position of the light source and the diffuser. Basically we need to know what the shortest distance the beam would need to travel is, then we can pick condensing optics to suit. Of course based on what optics are available we may need to adjust that distance slightly. The second critical distance to consider is the distance between the diffuser and the film plane - we definitely don’t want to risk having the diffuser able to be in focus at the same time as the film. Since we’re mostly working under macro conditions I wouldn’t worry about this too much, as long as the film isn’t sitting directly against the diffuser.
D - No other special advantage!

[cpixip]

Ok, this starts to get interesting. Here are some additional, random thoughts I want to throw into the discussion:

• It is probably a different challenge if you work with white-light LEDs compared to a setup with separate LEDs for red, green and blue. In the former case, you primary task is to evenly spread a single light source over the area of a 35 mm frame. In the later case, you also have to assure that the light of every single LED (placed at different positions in the setup) is evenly distributed independently.

• LEDs differ somewhat in their radiation pattern. A narrow-beam LED can be moved much farther away from the scan area than a wide-angle LED. Provided an appropriate diffusor is available.

• Any optical element in the illumination path, like a collimator or condensor for example, will make the unit more expensive. And potentially more difficult to setup and source.

• From my experience it is difficult to get an even illumination with using a diffusor only, in a light path with more or less collimated rays. In case of light radiating from a single LED, one usually “sees” a blurred version of the LED if the diffusor not sufficiently strong (a “hotspot”). You need a thick enough diffusor (or several diffusors after each other) to get a sufficiently diffusing action. But a thick diffusor reduces the amount of light reaching the scan area.

• From a theoretical point of view, an integrating sphere with the appropriate surface coating is the perfect thing to create an evenly illuminated aperature - without any diffusor (such a thing could actually have some negative effects here). Well, as I am a theoretical physicist by training, this was what I opted for in my design. Only to find out that it a) works, b) is not so easy to manufacture and c) turns out to be a little bit to bulky to integrate it into the rest of the machine.

• From an engineering point of view, it seems that the design efforts of nearly a century of photographic enlargers could be an interesting resource for the Kinograph project. Some enlargers work with a condensor lens system, but try to find some replacement parts for them nowadays! Many enlargers have used diffusing lightboxes (mixing boxes). This is the cheapest solution I know of. An appropriately designed rectangular lightbox mimics the behaviour of an integrated sphere, ie multiple random reflections between source and exit port (= appropriate dull inner surface + large enough room/surface area for mixing).

@matthewepler: - yes, I certainly think that an approach patterned after some old enlargers for 35 mm stock could work for the Kinograph. The important thing is that the inner surfaces of the mixing box need to be as dull as possible ( Lambertian reflectance) - which rules out raw 3D printed surfaces; you have to coat such 3d-printed surfaces if they need to work properly. But what about making a box directly out of Styrofoam as “in the old days”? Styrofoam can be cut by hot-wire cutters and it is easily available in most places.

[junker]

I was just going to suggest what @cpixip mentioned that you would want a matte surface and not a glossy surface for the light mixing (whether it would be a box or sphere).

@johnarthurkelly, looking at the images of your Direct Drive tests I noted that there was a hotspot because the camera was looking directly at the LED. By putting a 45° bend at the end of the tube and mounting the LED from the side, we might be able to get better results by essentially making a light mixing box. And it wouldn’t need a 3D printer to make anything, you could cut a shape out of foamcore (or maybe even poster board), score the bends on the backside, and then “snap” the folds to make a box. The foamcore/poster board would be matte enough to run some simple tests for comparison and could be made in a matter of minutes.

My frugal nature (my wife says “cheap”) leads me to look for the most simple, inexpensive solution. I don’t always get there (I really love my 3D printer) but it keeps me thinking. I’m also guided by the engineering adage, “Never make what you can buy (or reuse/repurpose).”

• I’ve been looking on Craigslist for broken monitors/flat screen TV’s that people are giving away to harvest the diffuser panels out of (a 42" TV would be a lifetime supply for our purposes)
• Foamcore would be a quick way to mock up sample light mixing boxes
• I’ve looked at hollow balls to make simple integrating spheres from (maybe just cut a racquetball in half?).
• From the AV Home Theater world, it seems that Sherwin Williams Duration Extra White household matte paint could be a good substitute for Barium Sulfate for coating integrating spheres/mixing boxes.

[matthewepler]

Foam core is a genius idea @junker. I like how you think The first Kinograph used the diffusion from a computer monitor and it worked pretty darn well.

@johnarthurkelly thanks for addressing my original questions. Much appreciated.

Firstly, for myself and anyone following I’d just like to make sure we’re all using the same terms in the same way.

• condenser - converts a divergent beam from a point source into a parallel or converging beam.
• collimator - converts a divergent beam into a parallel beam, a type of condenser.

In other words, it’s a rectangle/square thing. All collimators are condensers, but not all condensers are collimators. Some focus the beam into a small point, etc.

Do I have that right? Not trying to split hairs, just making sure we don’t confuse one another.

I’m trying to condense (pen intended) what we have into a way forward. Here are some thoughts…

1. Sphere vs. direct.
   There actually isn’t a huge difference for me in terms of easy of build. they both require printing/making parts and assembling them with electronics and mounting them to Kinograph. I’d have to attempt to do both to see the real trade-offs. Which brings me to #2…

2. Design factors for each:
   a) direct

• light should bounce multiple times before hitting diffusion
• if we need to move the light source away from the diffusion plane, a collimator could help cut down light loss in the straight-aways.
• what type of condensor we choose (if any) is largely based on the beam spread of the LED and how far the light source is from the diffusion plane.

b) sphere

• maybe some a few tweaks for ease of assembly, mounting, and efficient use of material

c) both direct and sphere

• inner surface should be matte white
• both should be easy to assemble, and relatively easy to build/source in small batches

3. Price - as far as I can tell, there still isn’t a huge difference here either. Again, it would take more tests and hands-on experimenting before we lock down the design and therefore the costs.

In sum, the sphere is actually pretty straight-forward from what we’ve discussed. We agree it’s a good light mixer. It just remains to be seen how it would integrate with the current design. I can help there. I have John’s CAD files which I can toss into Fusion360 tonight and see how it looks.

In the direct version, it seems like taking the simplest approach and building in complexity as needed is a good approach.

SO NOW WHAT?!

I say we pool our resources and start prototyping. If we want to have John test it with the same rig for comparison, we can ship what we make to him.

If anyone wants to source the LED and driver for their own experiments, I can front the cost of a few.

I can 3D print stuff and so can @junker. I can make runs to hardware stores for other supplies as well.

What say you, friends? What do you want to build and what do you need to do it?

[matthewepler]

I know that was a lot of words, so I’m summarizing here for my own sanity. The answers I hope to answer are:

1. can the integrating sphere fit, and if so what other considerations are there in terms of physical design

2. what dimensions, shape, and components are required for a “pretty darned good” direct illumination approach?

3. which of the two options makes the most sense for v2 Kinograph (with cost, ease of assembly, and ease of production as key factors)

[matthewepler]

Well, good news on the CAD front. Both options fit just fine without any need for adjustments to the existing design. The sphere is just the right size and the tube is small enough to be flexible on positioning.

I sketched some quick thoughts here. A couple of notes:

1. @johnarthurkelly how big of a deal would it be to move the LED from the top of the sphere to its center? I’m pretty sure that is less than ideal but just checking. I ask because of the next note:

2. We would want to consolidate connections as much as possible. Ideally we can integrate the LED driver into the Arduino shield and then just solder some wires to the LED, glue it in place, and then connect it to the shield. Alternatively (and more modular) the LED is part of a standalone PCB that connects to the shield only for PWM control. Lastly and perhaps most annoying is the LED is separate from its driver which is in turn connected to the shield. .
   What do you all think? Original pic here for download if needed.

3. The tube design might be easier to repair/replace for people without easy access to a 3D printer. Even if they do, printing itself can become a bottleneck if they’re troubleshooting failed prints, sourcing filament, etc. In a pinch, the tube could be created from just about any material. Advantage tube. Agree?

I’m tempted to just move forward with both and see if we can’t figure out the LED/PCB/Shield connection so that it could support either direction. Then it’s as easy as choosing your preference and printing/building it.

Okay, enough out of me for one night.