Monday, November 21, 2016

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.


  1. Hello John,
    Gee, I never thought this would be a problem with color spectrophotometers and densitometers. Since most of them have diffuse illumination, I would have thought this wouldn't be a problem. Ya learn something new every day!
    From the other end of the spectrophotometry universe (high end UV-Vis-NIR spectros), this is a constant problem. With monochromatic, directional illumination, you are at the mercy of the last grating the light hits before contacting your sample. All are slightly different and the optical design can give you all sorts of different polarization effects. The only solution is a not-all-that inexpensive common beam depolarizer, and even that isn't 100% effective. So we avoid these measurements like the plague, unless we have to, and then only in a full bio- hazmat suit :)

  2. Hello Art,

    These instruments are 45/0 and 0/45, so the lighting is directional.

    Thanks for crying in my beer with me.


  3. I'm confused. Is there an issue with the basic eXact if you use the M1 mode with thin films?

    1. Very reasonable question, Unknown. Sorry for not making it clear. Here is a quote from XRite: "With the X-Rite eXact, these orientation differences appear for the measurement modes M0, M2 and M3, but not for M1."

      So, apparently the standard version of the eXact works fine on films when in the M1 mode.