What's with the new version of the X-Rite eXact???

I got a question the other day from my good buddy Steve (not to be confused with my mediocre friend Steve, or my sworn enemy Steve).  Steve was asking about the kerfuffle surrounding a new version of the X-Rite eXact. Something or other to do with polarization and clear films. What's up with that, John?

So, I did some research. And I have an answer to the question. I should mention that a lot what I have to say applies to the Techkon SpectroDENS as well.

First case, the easy case. If you are planning on using the M3 (AKA polarized mode) for measuring films, then I think your time will be more productively spent looking for Civil War medals with a metal detector at Malibu Beach. I hear that they are in desperate need of a good professional beachcomber.

Now, I am not a big fan of M3 in general. M3 only makes some limited sense when you are measuring ink that is still wet. I think if you are printing on clear films, you are probably not measuring wet ink, so why do you think you need M3? But more importantly, the use of polarized light to measure clear films will result in unexpected results that are unexpected. Just don't do it, ok? If you are skeptical, then read the rest of this blog post.

Now for the more complicated case. If you have an X-Rite eXact or a Techkon SpectroDENS, and are using it to measure some sort of films, then I suggest you read this whole blog post.

Here is the short answer. Measurement of films can be unreliable with the eXact in M0, M2, and M3 modes. There is no problem with the M1 mode. The Techkon SpectroDENS can be unreliable on films in any of the modes.

Both manufacturers offer a modification to their instruments to make them more reliable on films. Neither of the instruments will be able to measure M3 after they get back from surgery.


If you aren't sure about all this M0, M1, etc business, I know a guy who wrote an excellent blog post just for you: What measurement condition is your spectro wearing?  Fascinating post, really. And the guy who wrote it is so extinguished looking!

Films and polarized light

Measurement of films with polarized light is problematic. I put together a video showing the really awesome effects that you can get when you mix polarized light and clear films. I then blogged about this effect, giving my own explanation for what was happening. I am happy to say that the comments on the blog cleared up my wrong-headedness about the underlying physics.

I made some mistake about the physics, but the effect is real. As awesomely cool as the effects are to watch, the effects are ginormously terrifying for anyone who is trying to measure clear films with some kind of polarized light. Ok... maybe the superlative is a bit too ginormous?

The new JMG SuperSpec, available once I get FDA clearance

One of the important conclusions from the video had to do with which of the Roscolux filters exhibited this mind-boggling color shifting property. I quote myself from the video: "I dunno why that is. A lot of the filters I have don't do anything cool."

In writing this blog, I chatted with a number of folks who have seen this issue. In particular, I was trying to get a handle on when this Muenster can be expected to rear it's ugly head. Here are some quotes:

"The main culprits are films with some oriented grain structure, or anisotropy, a product of the extruding process. You might also say it occurs on all 'crinkly', cellophane-like films."

"I have seen the problem on matte clear films, so surface texture does not appear to reduce the effect, but the effect is not seen when measuring opaque white film."

"We have seen issues on matte, gloss, clear and 'opaque' films. What makes it all the more interesting is it isn't true on all of any of these (sort of demonstrated in your rosco experiment)!"

So, I think this just proves my point in the video: "I dunno why that is." I should say that a little less jocularly, since this is an important point. It is hard to predict when this really exciting property of films well come to poop on your parade.

I might add, measurement of polarized light sources, like computer displays, is also problematic. My good buddy Robin Myers has blogged about this.

M3 measurement condition (polarized)

Most spectros that are used in the graphic arts have a polarized or M3 mode. Why?  Read on...

When light bounces off the surface of something (specular reflection) the polarization isn't spoiled. When light enters something, and interacts, then it bounces around and quickly forgets the polarization it came in with. I'm sure you have been to parties like that as well.

This fact has been mercilessly exploited by manufacturers of spectrophotometers and (especially) by the users of those spectros. I have blogged about polarized spectrophotometers before. If you are reading this blog post because you are bored silly pretending to be paying attention to the opera with your spouse, you might want to go read that blog post. It will tell you why someone might want to use the polarized mode.

But for our purposes here, all we need to know is the concept of cross-polarized filters. (Get ready for the cool part...) If you illuminate the sample with light of one polarization, and then collect the light through a filter that only passes light of the other polarization, the only light you will measure is the light that has interacted with the sample. The specular light is extinguished. 

This has been implemented in spectros with a piece of glass that has one orientation of polarization on the outer ring, and a different orientation in the center, as shown in the really excellent artist's conceptual drawing below. 

The first spectrophotometer that I got to play with was the Gretag SPM 50, which had a thumbwheel that you had to rotate to put the unit into polarized mode. The polarized filter (no doubt) looked absolutely exactly like my drawing above.

I currently own a Spectrolino. This has a cap that you have to snap into place with the polarizing filter. I wanted to check if it was anything like my drawing. The three pictures below are taken of the cap with light polarized in one direction (gibbous moon on the left), in the perpendicular direction (crescent moon on the right), and somewhere in between (in the middle). Yup. Pretty much what I thought.

One key point here: moving parts. I have described two possible designs for a polarized spectro. Both require a part to be physically moved into place. It would be advantageous for the user to not have to make this change. Enter a new design...

A new design

X-Rite filed for a patent for a new design in May of 2000: "Color measurement instrument capable of obtaining simultaneous polarized and nonpolarized data". It was a clever idea that obviated the need to slap a funky dual-polarizing filter into place. The idea was simple. A polarized spectro requires cross polarization; one polarizer on the incoming light and another one on the detector. How about just leaving one of them in place all the time? In that way, you don't need a funky two-element filter, you just need to swap a polarizing filter in and out of the light path going to the detector. Less moving parts, more reliability. 

I should explain that I still own a lot of tie-dyed shirts, so my concept of "new design" might be different from yours.

If you happen to have a design with a spinning filter wheel (like most of the spectros designed by X-Rite in Michigan), then the polarizing filters can "easily" be incorporated into the filters already present on the filter wheel.

Below is a diagram from US patent 6,597,454. Element 14 is the polarizing filter on the light source. Element 16 is the wheel with a zillion narrow bandpass filters. One filter might pass light between 400 nm and 420 nm. The next one might pass between 420 nm and 440 nm. That accounts for 15 of the filters. The other half have the same bandpass, but  are polarized. Thus, half of the measurements are cross polarized and the other half are only once polarized.

A new ride at Six Flags - the Polarizing Spectrophotometer

I don't know if I mentioned this before, but it's actually a pretty clever idea. If I was on the design team that came up with the idea, I would have immediately tried to take credit for it.

We all know that having exactly one polarizing filter in a spectro will always give exactly the same readings as a spectro without any polarizing filters. What could go wrong?

Oh. Wait. I think all the talk about the awesome video and the previous blog post about the funny things that polarized light can do to extruded thin films is enough foreshadowing to suggest that there might be an issue.

So, what about the eXact?

Does the eXact utilize this ultra-spiffy concept? 

I don't have an eXact, so I contacted my good buddy Mike Strickler. I can say that he is my "good buddy" because we once had dinner together. Neither one of us threw anything at the other, or stormed out of the restaurant, so it went better than most of the blind dates that I went on when I was single.

I had him put the spectro in each of the modes (see image below), and ask the instrument to take measurements. Unbeknownst to the instrument, he held it up in the air so that he could see the illumination on the table. (If the instrument knew that he did not have a sample at the aperture, it would have just folded its cute little arms and said "Foo on you! Ain't nothing there to measure!"

Stolen from the eXact manual, page 12

To make this interesting, I asked him to place a polarizing filter between the instrument and the table. He asked the instrument to take measurements as he rotated the polarizer. Lo and behold... in the M0, M1, and M2 modes, the illumination from the spectro was polarized. In the M1 mode, it was not. 

This suggests to me that the eXact makes use of the clever patent when it is in the M0, M2, and M3 modes, but not when it is in M1. The switch in the above picture slides a polarizing filter over the illuminant. But, that is just my guess. I'm still waiting for my mole at X-Rite to slip the mechanical design docs into an unmarked manila folder. I told him there was $10 in it for him. Dunno why he hasn't gotten back to me.

I don't know for sure what they have in their for the M1 mode. If any of you see me in your pressroom and have an eXact sitting out, I suggest that you make sure that there are no screwdrivers or hammers nearby. When I was five, my Dad learned a hard lesson about me that had to do with clocks.

I don't know why they decided not to incorporate the polarizing filter in the M1 mode. Polarizing filters do have a pesky tendency to mask UV light, and M1 requires a fair amount of UV. Maybe that's why?

Anyway, the Xp version of the eXact removes this polarizing filter completely. As a result, M3 measurements (polarized) are not available with the eXact Xp.

What if you already own the normal version of the eXact and are measuring thin films?  X-Rite does have a path to change your device to eliminate this problem.

Here is more information from X-Rite. They also recommend testing on clear films by rotating the instrument and seeing if the measurements change. They note changes as large as 2.5 DE.

What about other instruments?

Since X-Rite's patent was filed in 2000, it is likely that other X-Rite instruments have this issue. I don't know which ones.

Currently, I know of four manufacturers who make M-condition instruments for the graphic arts: Barbieri, Konica-Minolta, Techkon, and X-Rite. 

Barbieri (LFP) and Konica-Minolta FD-5 and FD-7 both have caps that snap on when you make polarized measurements. One can assume that these are cross-polarized filters, so it is unlikely that either have this issue. I have heard directly from Barbieri that the LFP does not have this problem. I have not heard back from Konica-Minolta.

The Techkon SpectroDENS has the same issue with films. As with the X-Rite, Techkon has a modification for folks who measure films. They call it the SpectroDENS Flex. As with the eXact, the modification means that the M3 mode is no longer available.

As for inline (on-press) instruments, I have been told that the spectrophotometers from QuadTech, AVT, and BST do not utilize polarized light.

Explanation of "Cool tricks with polarized light" video

I saw a cool YouTube video yesterday. Let me warn you, if you don't like being barefoot, then I suggest you invest in a pair of sock garters before you watch this next video, cuz without them, this video will knock your socks off.

Wow. The guy in the video repeatedly says he doesn't know what's going on. I suspect he knows a bit more than what he let on. But just in case he didn't know the physics behind this, I decided to write this blog. 

Polarized light

Let's start with a little bit on polarized light. Light can be thought of as a rope that wiggles to and fro. It might be wiggling up and down, or right and left, or maybe at angle. The orientation becomes important if the light goes through a picket fence, as shown in the diagram below. If the wiggle direction aligns with the direction of the pickets (as shown on the left), the wiggle is not obstructed by the pickets. If the wiggle runs perpendicular to the  pickets, the energy doesn't go through. if the wiggle angle is somewhere in between, then some energy passes through.

Dreaming of a place with a white picket fence and a wiggly rope

That's all we need to know about picket fences and polarized light to explain the video. Well, maybe something else will come up. I dunno.

Extruded films

A lot of people I know would say that their favorite line from The Graduate is "Mrs. Robinson, you are trying to seduce me... Aren't you?" That is a great line, but for me, it's when Mr. Braddock whispers the key of the future into Dustin Hoffman's ear: "Plastics." This line, by the way, is listed as number 42 in the American Film Institutes 100 best movie quotes. "Plastics" became a movie symbol of the era, a symbol of fake eyelashes, fake relationships at cocktail parties, and aspirations to the fake nirvana promised by consumerism. At least that's what my seventh grade teacher (Mr. Mattke) told us about The Graduate.

Plastics are made up of molecules (monomers) that have chained themselves up into a conga line known as a polymer. These very long lines of connected molecules are what give plastics their unique combination of strength and flexibility.

Monomers polymerizing into a conga line

Here is where I come up with an extremely clever analogy that brings these two thoughts together. Picket fences are a social symbol of the aspirations of one era, and plastics are a social symbol of that of another. But they are connected in another way... a way that involves polarization of light, especially when it comes to thin films of extruded plastics.

Imagine if you will, a dance floor with multiple conga lines kinda winding around at random. Imagine further that the head person in each conga line suddenly gets mesmerized by the evil villain The Extruder. The Extruder induces these charismatic people to suddenly start running due north. What happens to the conga lines? It is easy to visualize that those conga-regants who hang on will one by one start heading north. In the end, each conga line will be running basically north to south, and the lines will be close to parallel. 

I haven't spent much time actually in an extruder nozzle, but this is what happens when molecules in the chamber of an extruder accelerate as they head out the nozzle. The homogeneity of orientation depends (I would guess) on the amount of acceleration that the material experiences when it exits the nozzle, the viscosity of the polymer in it's liquid phase, and the amount of time it takes for it to harden. But I'm just guessing. I am not a polymeric chemist. Nor do I play one on TV.

Here comes the important part where I draw this all together. For a wave of light, the two symbols of the aspirations of the generations (picket fences and plastics) look kinda the same. So, any extruded polymer film is inherently polarized, at least to some extent.

Example - cellophane

Cellophane is a polymer of good ol' glucose. The polymer is formed into sheets by extruding it through a slit. For whatever reason, cellophane seems to have a lot of this polarizing effect that I just predicted. I investigated a bunch of thin films for this blog post, and cellophane seems to be the most pronounced. 

(Interesting trivia: If you remove the letter a from cellophane, and move the o to where the a was, you get the word "cellphone". If that's not proof of a conspiracy involving Reagan and ray guns, then I don't know what is!)

For the next three pictures, I used my KindleFire as a backlight. The KindFire, like many displays, emits polarized light. The first picture is just a piece of cellophane that I tore from a box of Lipton tea. Nothing exciting here. Move on to the next image, please.

Boring image of cellophane on my tablet

For the next picture, I put a polarizing filter between the camera and the cellophane. Wow. There are two interesting things happening here. First, the black area shows that the light emitted from the Kindle display is indeed polarized, just like I said in the last paragraph. (All this time, the news articles you have been reading on your Kindle were polarized, and you didn't even realize it! Yet another conspiracy? I dunno. You be the judge.)

A polarizing element has interjected itself into the picture

But the odd part is what happens to the light that was emitted from the Kindle through the cellophane and then though the polarizing filter. That light does not seem to be polarized. Did the cellophane scatter the light so that it was now randomly polarized??? (Note the use of multiple question marks. This could be a sign that this "answer" to the befuddlement is tentative, and may not actually be correct.)

Just based on the picture above, I am thinking that perhaps the cellophane is not truly scattering the polarization of the light. You see, polarizing filters will attenuate randomly polarized light. Some of the light doesn't pass through. But the gosh darn cellophane appears almost as bright as the Kindle light that does not pass through the cellophane or the polarizing filter.

The following picture proves that the cellophane does not randomize the polarization. For this picture, the polarizing filter was rotated by around 70 degrees. Heavens to Betsy! The cellophane has rotated the polarization by about 70 degrees! So, the conga lines in the extruded glucose polymer (the cellophane) are not acting like a typical bouncer at a nightclub, only allowing photons with certain orientations to pass. The bouncer at the Cellophane nightclub is actually twisting the orientations of the photons as they come in the door! I will refrain from political commentary about whether this is appropriate behavior for a bouncer.

A simple twist of the filter, and Holy Buckets!

Also note that the light that passes through both the cellophane and the polarizing filter is not neutral gray or black; it's brown. This shows that the reorienting of the the polarization is not the same at all wavelengths. Here I am telling you just a bit more than what I know, but it seems to me that the width of the conga lines must be on the order of the size of the wavelength of light, so that (for example) larger photons (longer wavelength photons, like red) tend to be too big to experience this effect. Smaller photons are a bit more likely to be intimidated by the bouncer. 

The video demonstrates that we can get some bizarre effects when we start mixing things up a bit.

But I see there are some questions in the audience... 

Why did the color change as we rotated the Roscolux filters?

The distance between the conga lines depends on the orientation of the polarized light, as shown below. Since we have decided that the distance between polymer strands is in the neighborhood of the wavelength of visible light, we would expect to see different wavelengths effected differently as this distance changes.

Effective distance between conga lines depends on the angle you hit them at

Why does an individual filter show changes in color along its length?

Here is my conjecture: the thicker the film, the more rotation of the orientation of polarization. In the image at the left (below), there is a small difference in the effective thickness of the film, probably due to either the film not being perfectly flat, or due to manufacturing tolerances in the thickness. At the left, I have pinched the ends of the film together so that the effective thickness changes dramatically along the filter. As can be seen, the rainbow has been squashed together.

When films turn into inchworms

Why does stacking the filters cause such cool stuff to happen?

I'm gonna say "read the previous explanation". If you put one filter overneath another, you have something like twice the thickness, so there is more opportunity to rotate.

Why do some of the filters not show this effect?

I dunno, but I have an educated guess. According to the Roscolux website: the filters are "comprised of two types of body-colored plastic filters; extruded polycarbonate and deep dyed polyester". The fact that they use two different materials for the filters might explain why some exhibit this bizarre behavior, and some do not? 

But I dunno, there is also (perhaps) an effect caused by the colorant. It seemed to me that higher amounts of colorant tend to suppress anything fun and interesting.

An introspective comment on the nature of science

When my kids would ask me science questions, I would usually have a pat answer. "Why is the sky blue?" "It's not blue, it's cyan, and it's because of Rayleigh scattering." "What is electricity?" "It is the flow of electrons through a wire."

Sometimes, but not always, they would ask a follow up question like "what's Rayleigh scattering?" or "What are electrons, and why do they flow in wire but not in wood?" Eventually, they just stopped asking questions. Maybe they lost interest, or maybe they realized that I was just kicking the can further down the road.

I have given what I think is a reasonable explanation of why polymeric chains have a tendency to align during the extrusion process. I dunno if this is truly the case. Maybe if I finally get that electron microscope I have been begging Santa for, I will be able to find out?

I have given my supposition that these aligned polymeric chains can cause polarization effects and that they can further effect the orientation of the polarized light. I realize that I neglected to explain just why that might happen. 

I have noted that some films show this effect, and some do not. I didn't follow up on this or suggest anything substantive on why cellophane and some of the Roscolux filters do this.

Another mystery to me... I know that most display devices emit polarized light because they use liquid crystals to modulate the light. A weak electric field causes the liquid crystals to align one way or the other, so that the polarization changes. What I can't figger is why my KindleFire emits polarized light. I have been told that it has a layer of quantum dots that selectively absorb light from below, and then re-emit it (fluoresce) in a fairly narrow wavelength range. In this way, the light emitted can be closer to monochromatic. This gives the device a wider color gamut without the normal loss of power from a purely absorptive filter. But... I would think that the re-emission would be randomly polarized.

I was gonna pat myself on the back for a great explanation of a really cool effect, but all I have done is kick the can a bit further down the road.