Alternative Screens (thin-film, colloid, etc)
 

I decided to start this thread because the one about the slide viewer-based adapter has a different focus.

It was found that thin-film screens, while providing great light performance, had horrible diffusion of bright spots. I think I found the reason for this bad bokeh.

Some of the thin-film material (plastic bags or otherwise) shows that its diffusion is not rotationally invariant. This means that it diffuses light better in one direction than in others. While I may speculate that the phenomena has to do with the alignment of the polymer strands I think I also found that different materials have different invariances to rotation.

Below is a test using the LED display of a DVD player. The image on the left shows the screen vertically aligned, the middle image shows the screen horizontally aligned, and the final image shows the screen diagonally aligned.

Here you can see the variance very clearly, from one my "bag" screens:

http://mentemagica.com/35mmAdapter/RotVariance1.jpg

Here is another material with much less variance:

http://mentemagica.com/35mmAdapter/RotVariance2a.jpg

(the slight variations also have to do with me holding the screen by hand :) )


So the first material will produce bokeh that looks like streaks of light, while the second material will produce bokeh that looks nice and evenly diffused.

I will try the second material on my adapter (which is not very good but will do for the moment), to see if the second screen gives better bokeh (as I would expect).

Here are some tests:

The first group were shot with the original bag thin-film screen:

http://mentemagica.com/35mmAdapter/AdapterTest29.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29a.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29b.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29c.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29d.jpg

The second group were shot with the new rotational invariant material:

(excuse the bad framing, I forgot to check that on the monitor, and the LCD screen on my camera is overscanned)

http://mentemagica.com/35mmAdapter/AdapterTest36.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36a.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36b.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36c.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36d.jpg

The second group, while a lot more grainy (not an issue with a moving adapter), have less light loss and seem to look better than the first group.

What do you think?

Agreed. That's the kind of bokeh I can work with.

Where did you find this different, better bag material? Just another bag at the supermarket you picked up?

This material didn't come from a bag. It looks just the same but is thicker. For some reason it has even better light performance than the bag material, go figure.

I got it from a "pouch" that holds some medical information stuff (insurance). But I am trying to locate a source that other people can have access to. I am making a trip to Office Depot to check it out.

So the good news is, better bokeh, even less light loss. Bad news is, grain... so it will require a moving adapter.

This is consistent with what I'm seeing....more grain is required to get that "film" bokeh. I wouldn't call bokeh differeing from this to be "bad"...just different. If you were looking for a very unique look, this wouldn't be a bad think at all.

I think the bokeh difference in this case is related to the rotational invariance, not the grain per se. It just happens that the isotropic (rotational invariant) material I found also has more grain.

But I don't see anything preventing an isotropic material having smaller grain too.

Keep in mind that these are polymers, not ground glass, so they are quite different. Your ground glass has the advantage that it will be isotropic (unless you sand it in just one direction).

Also, as you say, the bokeh is just a purely aesthetic and subjective situation. It is like the discussion about 24p versus 30p or 60i.

Side by side comparison at almost the same angle (or pretty close)

On the leff is the original bag material, the out of focus areas look motion blured. On the right is the new material with an more evenly soft blur:

http://mentemagica.com/35mmAdapter/M...omparison1.jpg

I recently tried the following as a screen. It’s a HiMD case:

HiMD case

Here is what it looked like vibrated by a small disc motor. Now, the setup was not perfect as the screen was too rigidly mounted, but here is what it gives. A better motion would certainly help the grain disappear.

HiMD case (too rigidly) vibrated

That HiMD case is quite a reversion -- it looks exactly like the surface of the "frosted" CDR's Agus and everyone were scrambling for about two years ago :)

It also looks like, BTW, the frosted plastic you can get cut to spec from places like Canal Plastics in New York. At about $1 per 50mm circles, I thought I had the world figured out until I took it home and placed it next to my WAO5 ground glass -- there's a world of difference.

Has anyone looked into different types of frosted vellum? This was another thing I'd tried way back but, never stumbled across sheets with the right properties.

It's so odd how elusive "the perfect GG" seems to be. The Beattie people have a good design framework -- positive fresnel backed by a frosted, diffuse surface -- but despite how precise their manufacturing methods are, there's still evident grain. I wonder -- would a sub-1micron aluminum oxide grind taken to the frosted side of a Beattie get'r'done? Something makes me doubt it...

As a related aside, I spoke with one of the head optics people at 3M a while ago -- very eager to help and solve this riddle. But even he came up against a wall and couldn't suggest a (affordable) solution. He suggested contacting people at SPEI or OSA (major optics groups) and asking them questions along this line. I've started that up a bit but it's slow going...

I must admit that I did not search back two years ago on this. :) I just saw this on my desk and thought it would be worth giving a try in a vibrating setup. I’ll still properly test it when I finish my vibrating GG adapter. It’s not fair to compare, but the light loss seemed to be much less than the GG provided in the DIY spinning GG adapter kit that I used.

BTW, what’s the WAO5 ground glass?

White Aluminum Oxide 5 microns. I eventually went to 3, then 1.4 micron. Each step better than the previous but none so good as "grainless".

Here are some Laser Tests of several materials I have tried (mostly bags :) )

This is the original sandwich bag material which seems to have the most extreme rotational variance:
http://mentemagica.com/35mmAdapter/S...Bag-Laser1.jpg

This is the thicker Pouch material with no variance (looks pretty circular):
http://mentemagica.com/35mmAdapter/M...uch-Laser1.jpg

Now, here is a test with two layers of the sandwich material. As you can see the difussion is now circular, but there is a higher light loss:
http://mentemagica.com/35mmAdapter/D...Bag-Laser1.jpg

Finally here are some other bags which are pretty much all similar:
http://mentemagica.com/35mmAdapter/BagA-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagB-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagC-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagD-Laser1.jpg

Of all materials tested, the sandwich bag is the most asymmetric of all. The pouch is almost circular, but provides the highest light transmission.

This is a simple and effective way of testing the materials. (even grain can be compared if some care is taken while taking the pictures.

So get a laser pointer and go test some bags ;)

More news...

Tested Magic tapes with the laser:

http://mentemagica.com/35mmAdapter/3...ape-Laser1.jpg

As you can see the 3M Magic Tape is not too good it has a very horizontal diffusion, plus it absorbs quite some light, has some striation (horizontal lines).

But...

http://mentemagica.com/35mmAdapter/O...ape-Laser1.jpg

This thinner (cheaper) tape from Office Depot brand is much better. It has almost the light transmission of the pouch material I am using, its perfectly round, plus is has very small grain!

So this is proof that there is a material that can do all: Little light loss, small grain and round diffusion. The trick now is to find it in a wider format.

Great work. I think the laser pointer is a great way of testing Alain! Wider is always going to be better, for covering a CD too!

guys,
make sure you dont point the laser directly onto the camera lense. i heard the CCD goes for a toss if we do that, i dont know how far its true, but its better to be safe.

krishna

Thank you Krishna, I am not doing that. I am pointing the laser at a white paper and taking pictures of that.

I would not think of pointing a laser into my camcorder! :)

I am doing some more objective tests of the screen materials I have available.

Here I posted some software I developed for testing light loss on screens or lenses:

http://mentemagica.com/35mmAdapter/L...sAnalysis1.zip

I found a bag material that has a transmission of around 96% which is fantastic (almost like regular glass), but has a rotationally variant diffusion, so two layers would be needed to create circular diffusion. This would bring the transmission down to 93%.

On the other hand a single layer of my thick pouch material offers circular diffusion with 95% transmission.

I still need to test the Office Depot tape which also offers circular diffusion... if we could only find it in wider format.

BTW, I would very much like to see anyone with a wax or a GG screen test it using my software test so we can compare results.

Alain,
I love the detailed work your doing in this area. Do your tests only apply to static glass solutions?

Thank you Wayne.

The tests methods I am developing apply to any system, both static or dynamic. Both types of systems use diffusion screens, so it is important that we can compare the different materials quantitatively. Only then can we objectively pick out the best.

The light loss computation is not perfect but will give us a good way to compare one screen to another. I will post the results of my tests of several materials, but I also want to encourage people with GG and wax screens to test them in the same way. That's why I am making the test software available to everyone.

Sounds good.

Im just wondering how changing more then 1 variable, say the device taking the image of the ground glass (scanner?) will effect the accuracy and consistency of the testing.

I am looking at that.

So far taking the image with a different camera and using different exposure settings doesn't affect the final transmission ratio.

But I will do a few more tests (scanner sounds good).

Well I tried the scanner and it doesn't work. The problem is that since the light has to come through the screen material, reflect on the background and come back, there is some reflection and that makes the screen area look brighter than the background alone.

Taping the screen materials to a computer monitor and taking a picture of that works best so far.

Ideally a transparency scanner would work best. But not many people have those. I will try with mine.

Yes true, light source needs to be behind the screen.

Here are some results from the Light Loss test:

http://mentemagica.com/35mmAdapter/S...%20bag%201.bmp
http://mentemagica.com/35mmAdapter/V...%20Bag%201.bmp
http://mentemagica.com/35mmAdapter/T...0Pouch%201.bmp
http://mentemagica.com/35mmAdapter/S...0layer%201.bmp
http://mentemagica.com/35mmAdapter/C...tector%201.bmp
http://mentemagica.com/35mmAdapter/O...e%20w-glue.bmp
http://mentemagica.com/35mmAdapter/O...0NO%20glue.bmp

The Veggies bag that I tested (your local veggie bag will differ) has the highest transmission of all materials. But, as I have noted, it does not give round diffusion. A double layered set with perpendicular layers would produce round diffusion but will drop the transmission to around 93% (0.966 x 0.966 = 0.933).

The thicker material is still the best with round diffusion and a high transmission of 95%. The Office Depot generic magic tape is next at 93.5%, but not wide enough to make a screen from it (much smaller grain than the thicker material).

For those who do not have a thicker material like the one I have, a double layer veggies bag aligned to give round diffussion is a good bet, with a 93% transmission (around the same as the Office Depot tape).

I am still looking for a bag with round diffusion and a transmission of 96% or more. If I find one I will try to get as much of that material as possible :)

PS. The images were generated with the updated version of the Light Loss Analysis software I posted earlier:
http://mentemagica.com/35mmAdapter/L...sAnalysis1.zip

Hey, I really have no idea what you are talking about in most of this stuff. But I did see that you haven't or anyone for that matter mention the material to laminate things. I have some of this and was going to try it on a "static gg" and haven't been able to yet, because I haven't got all the parts in. Seeing how you have this stuff down to a science you might want to look into it. If you can't find it in your country, I'd more than glad to ship some samples out to you.

Rh

But, you probably live in America, so yeah....

Thank you for your suggestion Richard. I will take a look. From what I remember this is a frosted type of material, so it really isn't the same type of polymer diffuser (or maybe it is) At any rate, I will test it.

I just did some tests to calculate the light loss of some of my lenses.

Canon coated wide andle lens (triplet): 97.5% transmission
Cheap plastic Fresnel lens: 97%
Sheet of common glass: 96%
Magnifier DCX: 95%
Slide viewer PCX: 92%

Ideally, we could possibly use a good lens system:

0.975 x 0.95 = 0.926 -> 92.6% transmission

(lens system X polymer screen= total transmission)

I assume that reducing the whole system to a single condenser (PCX or aspherical) would rate even better. Also note how a fresnel lens can be a good idea in terms of light loss (but getting a high resolution, distortion free Fresnel will be expensive).

So, in terms of light loss, the screen is not the only important factor.

(Note.- Intuitively I am guessing that rating a lens this way may not be correct since one would have to actually measure the whole amount of light that goes through the lens, but in my tests there wasn't much of a variation from testing a small sample area than measuring the whole lens area.)

I forgot to post the diffusion chart:

http://mentemagica.com/35mmAdapter/D...ngleChart1.pdf

Print the chart, fold it at the dotted line (90degrees) and place your screen material at the line. Shoot a laser pointer and see the diffused image on the chart. From it you can approximately gauge the diffusion angle of your screen.

Interestingly enough, more diffusion does not mean more light loss. One of my best screens (rated at 95% transmission) has the widest diffusion angle of all the materials I have tested.

Also I will soon post some promising tests using coated screens that have higher transmission.

Alain, did you receive and test your POC material?

I received three samples from them this week:

LSD5PC10
LSD20PC10
LSD30PC10

According to their documentation, this is 0.010" thick
polycarbonate with a 5°, 20° and 30° angle, respectively.

I also received my Beattie on Friday.

I’m not finished building my vibrating adapter, but at least I can compare the respective "brightness" of these screens with the setup I have, along with the Redrock M1 GG on my DIY spinning adapter.

I’ll be doing that in the next few days, before I install the Beattie more "permanently" in my vibrating adapter.

My test will be quite simple, and certainly not as scientific as the methods described here. I’ll put a chart on the wall and shoot the same frame in the same lighting conditions, from the same distance. I will let my GS400 set the aperture automatically for each screen, and then I will manually select aperture increments above and below what the GS400 has selected (for comparisons in my NLE afterwards).

Edit: I must add that my vibrating adapter does not have a condenser lens, so the POC would somewhat start with a disadvantage compared to the Beattie, which has a fresnel.

If anyone has suggestions or advice for this test, I’m all ears.

I once contemplated finding the source for raw motor vehicle windscreen licence stickers, which seemed to work but were damaged when I tried to get the print off them. I found an AO5 dressed groundglass was better and did away with careful jointing of the gg material when sticking it onto the disk, so I didn't follow up on the stickers. The sticker material might be too coarse in texture.

Francois, actually the POC people didn't even reply to me at all.

If you don't want to perform a lightloss test using my software, I can do it for you. Hold the screen material against the computer monitor and take a picture with a digital camera as follows:

-Display a 180RGB gray image on your computer monitor.
-Hold or tape the screen material such that its lower right corner is at the center of the monitor.
-Place a digital camera on a tripod around 50-70 cm away from the monitor, disable the flash and turn off all lights (needs to be done at night).
-Take the picture.

From those images I can compare the brightness coming out unrestricted from the monitor with that coming through the screen material.

It should only take a few minutes, and if you send me the pictures I can analyse them and post the results back for everyone to compare.

I would really appreciate it :)

I will look into the material that you mention. So far I have found two types of plastic diffusers, one that looks like frosted plastic with a texture you can actually feel by touch, and the "holographic" type materials which diffuse based on a much smaller scale granularity and feel perfectly smooth.

The frosted type has been explored and has properties similar to that of ground glass. The smooth type is the one I am currently experimenting with.

Alain, OK I’ll try that tonight, and I downloaded your program, so I should be able to do the analysis myself.

Thank you Francois.

BTW, I just noticed that one of the links to my software got messed up by the "..." and I can't edit the post now. So here is the link again:

http://mentemagica.com/35mmAdapter/L...sAnalysis1.zip

and here is an image of a sample setup:

http://mentemagica.com/35mmAdapter/SampleSetup.jpg

Alain, just a note on the light loss experiments. I'm thinking that at 0 distance (against your LCD), light shining through the diffuser is mostly at 90 degrees to it on entry. I suspect that as you move the diffuser away, the difference between say a 5 degree diffusser and 30 degree diffuser would logically become greater in terms of transmission loss as incident angles increase. I wonder if the test would have more relevance if the flange to GG distance (average 44ish mm) was maintained.

Francois and I have pretty much done all the testing on the camera, same lighting, framing lens etc. but switching the diffuser material only...and then checking to see what the camera has chosen for exposure. To me this provides a better real world comparitive analysis. Just food for thought. I like your approach.

Since the diffusion angle does not vary, the amount of light that goes through should be the same. I will try changing the distance from the monitor and see what that changes. At least if you test all your screens using the same procedure the test should be reliable.

Your idea allows us to see the quality of the whole setup, not just the screen, so as an overall measure of the adapter it is a good one. We may systematize such test by doing a measurement by pixel brightness not just by eye. But we will need very controlled conditions (no sunlight, which varies within the minute, for example). The other thing is to use always the same exposure settings (as we have no way of knowing how the camera is choosing the exposure) and then compare the different brightness levels at different stops for plain camcorder and adapter shots.

What you propose is a great idea if properly handled for the adapter shootout that was proposed.

My basic technique so far is this:

1. In a dark room, I light up an ISO 122333 chart.
2. The camera and adapter are set up to frame the chart.
3. I switch the camera to manual, then toggle throught shutter speed and aperture, noting what the camera has decided to use. Therefore the camera (and not my eye) decides on exposure level
4. I then swap discs, or in the case of POC, swap samples, leaving the camera on, and on the tripod. Light, framing, distance, position are left exactly the same.
5. With the new sample/disc in place I toggle through the manual settings again, and note the new aperture settings chosen by the camera.

This is the best "relative" test that I could devise. It only applies to one system, but it evens out all the other variables. I like your light transmission concept, but there are so many differences in cameras, CCDs, gamma curves, panels, ambient light etc. I'm not sure how valid different results will be.

Perhaps the better question is this. So far, do your transmission results (from your software) correlate to actual light loss observed with the films in the adapter? If you can demonstrate this, you have a convert.

Beattie Intenscreen
Lo: 153 Hi: 157,6 Tx: 97%

POC LSD5PC10 (5°)
Lo: 144,7 Hi: 152,9 Tx: 94,7%

POC LSD20PC10 (20°)
Lo: 138,9 Hi: 149,3 Tx: 93%

POC LSD30PC10 (30°)
Lo: 140,9 Hi: 154,8 Tx: 91%

Redrock M1 GG
Lo: 123,3 Hi: 151,6 Tx: 81,3%

Impressive. Thanks Francois!