Thursday, January 12, 2017

Comparison of inexpensive digital microscopes, part 2

In the previous blog post, I reviewed one popular USB microscope, an inexpensive one from Celestron, which can be had for about $40. Not a bad price, you say? Well, have I got a deal for you! How does $7.50 sound? Note the decimal point... Yes, I can get you behind the wheel of a digital microscope for less than the price of a hot date at Starbucks.  

I found this little baby at one of my favorite geek-stores, American Science and Surplus. I am lucky enough to live in Milwaukee, one of but a few cities that has a brick and mortar store. But this particular item I ordered online. Note the really alluring name: "95231P1 PHONE PHOTO LENS MICROSCOPE". 

A loupe for your cellphone

As you can see in the photo above, the device is not a USB camera at all, but a lens and illuminator that snaps onto your cell phone. Hence the really low price. It leverages Apple's team of 800 engineers who worked on the camera in the iPhone, and takes advantage the fact that you already spent a bunch of money cuz you just had to have the latest iPhone. Another secret to the low cost is (no doubt) their tight marketing budget for naming the gadget.

Seriously, the little gadget is convenient cuz it uses my cell phone's autofocus, the user interface is the same as my cell phone, and my granddaughter can teach me how to get the images I take onto my computer.

I took an image of a model's eye with the Celestron camera in the last blog post. Below is a quite serviceable image of those same halftone dots taken with my Phone Photo Lens Microscope. My granddaughter was able to help me out, of course.  

I am but a halftone dot in the model's eye

Just in case you were too lazy to read the previous post, I have assembled a side-by-side comparison of the images taken with the two microscopes. I should mention, I did not rely on the 800 engineers who designed the iPhone camera just to make my selfies look fabulous. I relied on the guy in his basement with a soldering iron who developed the camera in my Samsung. He did just as good a job. He just didn't have a good publicist.

My first impression is that the cell phone image is more better. The colors on the image from the Celestron (at left) are a bit washed out. This could be a veiling glare problem, but I did clean the Celestron lens as best as I could. Another thing of note -- the yellow halftone dots are far more discernible in the cell phone image (at right) than in the Celestron image. 

Below, we see a zoomed version of the white of the eye of the model - same digital images, but with a little digital jiggery-pokery. I also did a little enhancement of the brightness and contrast of the Celestron image in order to more fairly compare the resolution. Note the tiny little black halftone dot that can be seen in both images. If I had to make a call, I would say that the cellphone image gives a better rendition. In other words, the cell phone microscope has higher resolution than the Celestron.
At this magnification, the original sexiness of the model's eye has significantly diminished

What's the magnification?

I am gonna take a wild guess here. I am gonna guess that a fair share of the folks who read the first blog post in the series and are this far along in this blog post have been wondering about the magnifications of the two microscopes. I have intentionally left that little tidbit out, since it is not quite as relevant as one might think. Permit me a moment to enter the didactic mode of discourse.

Original definition of magnification

The image below illustrates the concept of magnification as it originally was applied to a system with a bunch of lenses. Only one lens is shown below (because I am lazy), but the concept applies equally well to a a combination of a bunch of simple lenses that constitute what we cleverly call a lens, as in camera lens. Yes, a lens (such as you buy at the camera shop) is made up of multiples lenses. I love the English language. This makes it great fun to write patents. 

Lenses can be used to focus arrows - important for deer hunters

In the depiction above, the object is the arrow at the far right of the picture. The lens images this arrow to the other arrow at the right right of the picture. This arrow at the far left is called the image. This is not to be confused with the digital file that comes out of your camera, which is called an image. This makes it great fun to write patents. Henceforth, I will try to remember to use the term digital image to refer to a digital file which is a representation of an object.

The magnification of this configuration is simply the ratio of the size of the two arrows. In this case, the magnification is 2X. The image of the arrow is twice the size of the original arrow.

A second definition

The previous definition of magnification works well for a microscope where you look through an eyepiece, or for those ancient pieces of technology that were called film cameras. Enter the digital camera. The object is now imaged on a CMOS sensor which might be a five millimeters across. If my microscope projects an image onto the sensor of an object that is four millimeters across (such as the first depiction of the model's eye), then the magnification of the lens is 0.80X. Wow. Doesn't sound like such good marketing to say that a microscope has a magnification of less than 1X! I think that even a company that can't afford to pay for a decent name would eschew publishing such a spec.

Not only is it bad marketing, but it is a downright useless as a spec. I will never view the digital image at a size of a few millimeters. That digital image might be magnified to fit my cell phone, which is 110 mm across. In this case, the effective magnification is around 110/4 = 27.5X. Then again, I might view that digital image on my computer monitor which is 480 mm across. The magnification is now 120X. Or, of course, I could zoom in on the digital image as in the illustration where the field of view is big enough to capture five halftone dots across. I could make that into a billboard, and get a magnification of a gazillion X. Or, I could set up a high power projector to project the digital image onto the moon and be able to say that the magnification is a gazilliontillion.

Microscope with high mag wheels

But, the Celestron microscope mentioned in the previous blog has a spec of 150X. What does that mean? Their spec is base don blowing up the image to full screen size on some monitor. Whether the monitor is a 14" or a 36" monitor is unknown, at least from the Amazon website. This is a useless spec. (I should point out that the Celestron webpage for this model gives a complete (and hence useful) spec. They say the microscope is 150X on a 19" monitor. 

I think you see my point that magnification is kind of a meaningless spec without qualifying the spec by saying what size display is to be used and how the digital image was zoomed.

A third definition

There is a third definition of a quantity that relates to the power of a digital microscope which I kinda like. It's simple to calculate, even for the layman, and it is more useful than the X number: object pixel size. You simply determine the size of a pixel.

As an example, we look at the first digital image of halftones in this blog post, the one captioned "I am but a halftone dot in the model's eye". The field of view is about 4 mm horizontally. I look at the digital image of that on my computer, and find that my cell phone gave me an image that is about 4,000 pixels wide. The object pixel size is thus 4 mm / 4,000 = 1 micron. This is the highest magnification available with this lens and my cell phone. 

(I measured  the catalog page with a ruler to determine the 4 mm wide field of view. If your measurement is critical, I would suggest buying a thin ruler and placing that in the field of view for at least one of the pictures. Note that until you refocus the microscope, the pixel size of the object will be the same - if the object is still in focus.)

As a second example, we look at the Celestron microscope that was reviewed in that spectacular blog post that preceded this one. This is a 1.3 MP camera, which means that the entire field of view is 1.3 million pixels. So, the horizontal pixel count is around 1,000. When this microscope is adjusted so as to have a field of view that is 4 mm wide, then the object pixel size is around 4 microns. Of course the Celestron can go higher in magnification by moving the camera. I figger the tiniest object pixel size (highest magnification) of that Celestron microscope is about 1 micron.

This is image is four mega pickles

Based strictly on that number - minimum object pixel size, the effective magnification of the two microscopes is pretty similar.

A third definition

But that still doesn't tell the whole story. In the side-by-side comparison of the little black halftone dot in the white of the model's eye, I made the call that "the cell phone microscope has higher resolution than the Celestron". This is an incorrect statement, if we define resolution as the object pixel size. But resolution is a fuzzy thing. Or, to be literal, resolution is the opposite of fuzzy. Resolution means that the image is "sharp". And sharpness is a fuzzy thing to measure.

If I were a real scientist, and not just someone who plays a scientist in the blogosphere, I would start talking about using "resolution targets to determine the modulation transfer function". In simple terms, you point your microscope at a test target with finer and finer sets of lines. When you can no longer see (resolve) the lines, then your microscope has run out steam.

Everybody needs one of these in their wallet, by golly!

But alas... the test target sells for more money than the microscopes that I am looking at. My dear readers will have to settle for a picture that is worth 1K words.

The second test

I almost forgot about the second test that I did with the Celestron microscope, which was looking at the pixels of my Kindle display. Speaking of fuzziness and resolution...

Fuzzy Wiki was a bear...

This is an excellent example of an image with object pixel size that goes way beyond the inherent resolution in the image. Basically, no matter how hard I pleaded with my cell phone, I could not get it to focus on the Kindle pixels. There will be no discerning of red, green, and blue pixels in Mudville tonight.

Note, for this microscope to work, you need to set the lens atop of whatever you are looking at. On the plus side, this makes it easy to position the microscope. On the minus side, this makes it impossible to adjust to a different working distance. What you see is what you get.


The Phone Photo Lens Microscope is downright cheap. It's a bargain if you are in interested in a handy tool for looking at the quality of print, and documenting that. But it is a really a fixed focus device. Aside from the digital zoom of your cell phone, you have little control over the magnification of this puppy. But it's downright cheap. The image quality is good. And I should also mention that it is cheap.

I know there are other cell phone loupes on the market. I don't know if they work as well as this does. But I know you can get this one at American Science and Surplus. Buy one quick... I'm sure they will go outa stock as soon as this blog hits.


So far, I have not received money from either Celestron or from American Science and Surplus in exchange for my reviews. Not that a little kickback wouldn't be appreciated. My granddaughters are starting to think about college. I'm just saying...

Tuesday, January 10, 2017

Comparison of inexpensive digital microscopes, part 1

I frequently find myself in need to a digital picture of something really tiny. Sometimes it's halftone printing dots or the pixels on a computer monitor. Sometimes, I just would like to have a closeup picture of an interesting specimen of hemiptera. After all, I'm a blogger who often writes about scientifical stuff. And since I make about $20 per year on this blog, my budget for toys to support this addiction of mine is ironically so tiny that it would require an expensive microscope just to see.

Me, looking for my next paycheck

Needless to say, I have long been on the lookout for an inexpensive microscope. In this blog post, I have a look at an inexpensive USB microscope from Celestron. In the next blog, I will look at a very inexpensive lens that snaps onto your cell phone. (Spoiler alert) In the third installment, I will look at my pick, a new inexpensive and very impressive USB microscope from Opti-Tekscope.

My first USB microscope

In 2010, I bought a Celestron 44302 1.3 MP for $56. I see that Amazon now lists it for $50, but that would not be a good bargain. The updated microscope, with a 2.0 MP camera, sells for $38. But if you happen to want the more expensive, lower quality one, I would suggest jumping right on it. When I checked, there were only seven left.

My old buddy, caught in a moment of leisure

This is a pretty cool toy. Let's see what it can do!

Low magnification image

I decided that I was going to get my wife something from the Fabulous Furs catalog. This first picture that I took with the Celestron microscope shows a woman wearing a parka thingie. This is what I got my wife from the catalog. I didn't get her the parka, of course. I got her a microscope image of a picture from the catalog. Her very own .bmp image. She will be very happy with me.

This picture is for my wife

I don't always know what my wife is gonna say when I get her a present, but I think in this case, she won't be all that excited. That's my guess, anyway. Since I have decided that I am gonna get her an image as a present, I better figger out what's wrong, and how to fix it.

The microscope has six LEDs for illumination, as shown in the image below. Since the lights are directly above the surface being viewed, the specular reflection (the light reflected directly from the surface) will bounce right back into the lens. Since specular light does not react with the sample, it provides us with little information about what the sample looks like. All we get are the delightful white splotches that hide the beautiful model.

The Celestron takes a selfie

So, in order to create an image of the catalog that is useful, I tilted the microscope with respect to the magazine. Thus, the annoying specular light bounced off the surface away from the camera, never to be seen again. 

This picture is for my wife

That's a pretty cool image. You can see the rosette patterns of halftone dots in this very sexy model's naked eyes, and this is definitely clearer than I can see the halftones with my own (equally sexy) naked eyes.

Higher resolution image

But there's more. The above image was taken at the lowest magnification available with the Celestron. The image below shows the other extreme. At full magnification, you can clearly see the individual black, magenta, and cyan halftone dots. You can even kinda make out the yellow halftone dots.

I suspect that my wife may not want this
high resolution image of a halftone representation
of a model's eye 

Another thing I like taking high resolution pictures of is computer displays. We all have our hobbies, right? Below are two Celestron images of the Wikipedia icon on my KindleFire.

Really taking a close look at Wikipedia

You can see bright spots in the four corners of the low magnification image. These are the reflections of the microscope's LEDs. There is no provision in this version of the microscope to turn off the illumination. As we have seen for flat surfaces this is a problem, and it is definitely a problem if we wish to use the microscope to look at emissive stuff like computer displays. (This has been fixed in the current version.)

I disabled the LED illumination of my older vintage Celestron with a delicate swing of a sledgehammer in order to capture the higher magnification image at the right. Here we can start to see what I was looking for - the individual pixels of the display. Below I show a cropped and magnified version of the image. In this image we can tell that the Kindle display has pixels that are rectangular, and are either red, green, or blue. Such an image is incredibly useful for someone who is trying to explain color, but the blue pixels are faint, rather hard to make out.

Kindle pixels

John the Color Science Guy needs to interject a point of interest here. Note that yellow halftone dots were a bit hard to see when the Celestron was pointed at print, and blue pixels were kinda hard to see with the image of the display. Both of these point to a deficiency in the blue channel of the camera. 


Speaking of kvetching about the microscope, I just complained about the blue channel and previously I complained about the inability to turn off the lights on the Celestron microscope.  As I said this has been fixed in the current version, but I'm still gonna kvetch about it cuz I like to kvetch.

One thing that I did not kvetch about in the previous section is that it took me about ten minutes to capture the high magnification image of the Wikipedia icon. Partly this is due my mechanical ineptitude, but I am gonna blame it on the microscope anyway.

The image below shows the two parts of the focusing mechanism.

Focusing the Celestron
On the left, we see the mechanical part of the focus. There are two ball and socket joints held tight with a thumbscrew. Well, the joints have a ball, but the socket is more like two pieces of metal on either side of the ball. By articulating these two joints, we can adjust the working distance, which ultimately drives the magnification of the image. In the image at the left, we see the focus "knob". You use your thumb to rotate the light gray inner cylinder with respect the the darker outer cylinder.

This is a very simple, and one may say, clever, design. But, if I would apply the word clever to this design, I might be more apt to use it in the sense that the rack was a very clever design used during the Inquisition. It is torturous to get a high magnification image into focus. Kvetch kvetch kvetch... 

1) When you bring the scope closer (by bending both of the ball and socket joints, the scope will move to a different position on the sample, so you need to reposition ever time you adjust working distance. Note that I didn't mention the third ball and socket joint on the microscope itself that adjusts the orientation of the microscope itself. This joint also has to be adjusted when you zoom in, since the microscope will change orientation otherwise.

2) If the thumbscrew is tight enough to hold the microscope in place, it will necessarily take a bit of force to adjust, which means that the microscope will probably move while you are zooming in.

3) The focus knob takes a bit of torque to turn, certainly enough torque to move every other part of the microscope. At higher magnification, it doesn't take much motion to completely lose what one was looking at.

4) To top it all off... when the microscope is cantilevered out for a close working distance, the stand will tip forward.


When I bought this microscope several years ago, it was a great investment. Yes, it was tedious to get images, and the images weren't quite the quality that I would have liked, but any other microscope would have been well outside my price range. Just being able to collect images was a great thing.

We shall see that there are other options today.

Tuesday, December 27, 2016

Unambiguous regions in color space for the basic chromatic colors

I am going to start this blog post with the punchline. The image below shows the range in color space of the eight basic chromatic colors. I assert that any color that is within a given set of limits will be unambiguously identified by the corresponding color name by everyone except for people who are either Color Vision Deficient (CVD) or Color Naming Pretentious (CNP). 

If a color is in one of these regions, then it has an unambiguous name!

Note that this is an a*b* plot. Each color also has a viable range for L*. Stick around for the end of this post, and I will provide a simple mathematical description of these regions -- but that's a treat reserved for those people who read through this entire blog post!

Why is this important?

Before I go any further, I have a confession to make. I write this blog post (and put all that time into the data analysis) in hopes that I will someday win this running argument that I have with my wife. I know. Good luck, John. I can dream, can't I?

Here is how the argument typically plays out...

Math Guy: Did you see that woman with the gorgeous brown hair? She just winked at me and smiled. I'm gonna go ask her for her number.

Honey, she smiled at me!

Bride of Math Guy: Don't be such a dufus! Her hair is auburn, not brown! 

Math Guy: But, Honey! I know color. I am a color scientist!

Bride of Math Guy: That may be, but you're still wrong.

If only I could walk over to the brunette (who is obviously attracted to immensely intelligent guys like me), ask to measure her hair with the spectrophotometer that I always carry in my pocket, and then bring up a ColorNamer app to give me the unambiguous name for the color of her hair. If nothing else, you gotta admit that this is a novel pick-up line. And just maybe, it could be used to avoid marital strife.

If you have doubts about the importance of the question of color naming, witness the following. There is a prominent, well-respected, and humble color scientist/blogger who has devoted no less than six blog posts to the topic of assigning names to colors.

I dunno, maybe the name of a color is important for other reasons. I mean, there are a few odd cases where words are used to convey information, and the color associated might be important to somebody.

My data

I recently ran into a pile of papers written by Dimitris Mylonas. Unlike me, he has been doing a lot of real research. His research topic for his doctorate at University College London has been how people assign names to colors. He has been running an online experiment where he displays colors right on your own computer and then asks you to name the color. He has made the results available through an online color naming app where you can select from 30 color names and it will display the most common color associated with that name. Or, the other way around, you can click on a color from a palette, and you will get a word cloud with the most common names that your RGB combination has been given. Great entertainment for a rainy day. I gave up my subscription to Netflix when I found this.

Screen capture from Mylonas' site

There was a similar color naming experiment that was conducted by Nathan Moroney of HP. You can get a free copy of his color thesaurus online.

Snippet from Moroney's book

For both of these sources (the online app from Mylonas and the book from Moroney), I harvested RGB values for each of the basic chromatic color names: red, orange, yellow, green, blue, purple, pink, and brown.

Why these colors? 

Why not beige, turquoise, plum, coral, lilac, etc?

I do have some logical foundation for the colors I chose. It is based on a seminal paper in the study of chromolinguists by Berlin and Kay in 1969. They did linguistic studies of color names in eleventy-seven zillion different languages and came to the following conclusion:

"... a total basic inventory of eleven basic color categories exists from which the eleven or fewer basic color terms of any given language are always drawn. The eleven basic color categories are white, black, red, green, yellow, blue, brown, purple, pink, orange, and grey."

The eleven basic color categories

I think that's pretty amazing. There are many independent roots of languages, and for some reason, they eventually all settle on eleven words for basic color names. The words are different, of course, but they all kinda translate. You don't run into a basic color word in Swahili that translates into "a sorta brownish shade of red, but not so dark". There must be something fundamental to the human eye or the neural pathway to the human brain that segregates color into these eleven groups.

I should mention here that the bulk of the Berlin and Kay paper dealt with a recurring pattern in the development of languages. They posited that nascent languages include white and black in their vocabulary, later adopt a name for red, then either yellow or green, followed by green or yellow, etc. The sequence up to these eleven colors is largely predictable.

There has been much research based on the work of Berlin and Kay, and it mostly supports the eleven-ness of color categories. Perhaps there are a few colors (such as beige, turquoise, plum, and coral) that belong on the next tier, but these are clearly the Magnificent Eleven.

In the old west, life was lived in black and white,
and there were only seven magnificent basic colors

I did a little tiny bit of research on this topic. Years ago, I taught algebra to people who hated math for University of Wisconsin Milwaukee. One semester I had about 50 students, half male and half female. I asked them to write down all of the single-word color names they could think of, and gave them two minutes. 

There were perhaps three or four lists that had only ten of the Magnificent Eleven, but almost every student had included the eleven basic color terms. The next two color names in terms of frequency, were silver and gold, which each appeared on about half of the lists. That in itself I found interesting, since as a color scientist, I know that silver and gold are gonio-apparent effects, and not actually colors.

My paltry little experiment demonstrated once again that there is something magical about these eleven colors.

So, for this experiment today, I decided to go with that set of eleven. But I left off the achromatic colors (white, black, and gray) due to some technical problems beyond my control. I didn't think that neutral colors were pretty enough.

A caveat

There is always a caveat, isn't there? These two online experiments are absolutely fabulous work. Incredible amounts of data. In one of his papers, Mylonas states that there had been over 1,400 participants in his experiment, Moroney claims over 5,000. 

Here's the caveat, though. You can bet that most of the computer displays were uncalibrated. There were certainly 6,400 different viewing conditions, if you consider intensity and white point of the display, ambient light, and background. So when Sidney from Sidney looked at a shade of lime green that was created by the RGB values 30, 230, 40, and Charlotte from Charlotte viewed that same combination on her monitor, there is apt to be a difference in what they are actually seeing. Anyone who has used a laptop with a second monitor can appreciate this issue. 

So, we identify that this is a source of experimental variation. We have literally scads of data, but are unsure about the quality of the data. But, I used it for my experiment anyway. I'm not proud.

Both researchers provided us only with RGB values associated with the color names. Before I go on, I should explain that "RGB" is not a standard. It could refer to any of the particular flavors of RGB values associated with whatever monitor or cellphone or camera you are using. But adding an s to the front of RGB wildly changes the connotation of the whole thing in much the same way that adding a little s does to your ex. sRGB is a unique standard that can be converted into the standard L*a*b* values. That would be a handy trick right now.

I asked my good buddy Dimitris Mylonas if it would be reasonable to assume that his RGB values were sRGB. I could almost hear him shrugging his shoulders through the email: "sRGB is safer than any other assumption." So, I used the standard computation to convert from sRGB to L*a*b*. Here is a website to do the conversion from sRGB to L*a*b*.

An obvious question here

I can see one of you bouncing up and down with your hand in the air... yes? You want to know whether the two data sets agree with each other? Good question! I wish I'da thought to do that! 

In the graph below, the circles represent the Mylonas data, and the squares with an outline represent the Moroney data.

Comparison of two versions of the basic colors

So, the answer is no. They don't match. The color difference ranges from 12.5 DEab to 46.4 DEab. Not good matches by any stretch of the imagination. There is a consistent pattern, however. The Moroney data is always more saturated. And the two data sets have very similar hue angles. With the exception of yellow and blue, the hue angles are all within  of each other. 

I don't have a ready explanation for why the two experiments differed so much. Given the size of the data sets, the difference is due to some sort of bias between the two experiments, and is not a statistical anomaly. If nothing else, this is a caution for this endeavor: If we try to precisely define colors, there be dragons.

The eight color map problem

So here I am: I have L*a*b* values for the eight basic chromatic colors, but, truth be told, these numbers have somewhat of a checkered past.

Enter Sturges and Whitfield

If only I had some data that was taken under standardized conditions. Even if it were to be done with less than a cast of thousands, if it corroborated the online studies, then the online studies would be corroborated.  The good news is that such a study was done in 1995 by Sturges and Whitfield. They elicited the help of twenty students at their university in England. Half were male and half female. None had any specialized training in color, err, excuse me, in colour.

The experimenters selected 446 color chips from the Munsell Book, and asked each subject to give a monolexic name for the color of the chip. (From mono, meaning "the kissing disease", and lexic meaning "someone who can't remember the first three letters of their learning disorder", monolexic means "single word". Hence pink, aubergine, and sploofrinde are all monolexic. Burnt sienna and reddish-green, are not monolexic. Owyell, by the way, is dyslexic, and there are no English words that rhyme with orange. Except for sporange, which means "word that rhymes with orange".

Did I mention? Twenty subjects named 446 randomly ordered patches, and I forgot to mention that they were given each patch twice.

S&W thus had a lot of data to distill down -- about 18,000 words. Among other things, they tried using consensus to decide when a given name was proper for a given patch. If all twenty trials on a given patch yielded the same name, then there was consensus. I was a bit surprised but even with this stringent test, there were 102 patches where there was a consensus as to the name. For about one-quarter of the patches, twenty people independently came to the same conclusion about the monolexic name.

So, my third data set was this set of 102 colors and their associated names. Since the colors were reported in Munsell notation, I used the Munsell renotation data to convert to L*a*b*.

Am I done yet?

Just to be safe, I wanted to throw in a few more data sets. I happened to have measurements from a Macbeth Color Checker chart. (This shows my age. People who don't immediately recognize the names "John, Paul, George, and Ringo" will know the Color Checker by the name X-Rite Color Checker.) This chart unfortunately does not include pink or brown.

And I did wy own color naming experiment. I tossed a Pantone book at my wife and asked her to find the best representation of each of the basic color names in the book. Her brother, who is also color-savvy, was given the same test, and I recorded my answers as well. (In case you were wondering, we disagreed on pretty much all of them.)

Here are the results for the color purple. Each dot represents a "sample". Thus, the 5,000 people who took the Moroney test get one circle. The twenty college students who spent the better part of a weekend staring crossed-eyed at Munsell chips instead of going out to a proper pub got a total of fourteen circles -- one for each Munsell patch that they all agreed was pruple purple. And I got one circle all to myself. And, begrudgingly, I gave one circle to my wife and brother-in-law as well. Life isn't always fair. If one of the people who took the Mylonas test wants more than 1/1,400th of a circle, they can get write their own darn blog post.

The X in the middle is the average of all the data.

The range of the color purple

Looking at the scatter of points of purple and of other colors, I saw a shape that was bounded on two sides by two hue angles, on two other sides by two chroma values, and (not shown) bounded on top and bottom by two L* values. Since I had some clear outliers, I let Excel tell me the 10th percentile and 90th percentile of  L*, C*, and h.

Note: I had originally called this last one H*. Thank you Tammo for the correction!

Results, in graphical form and numeric form

The graph below may look familiar to those who bothered to read the first part of this blog, and who were also paying attention. It is the same graph as above, meticulously duplicated for the benefit of my dear readers.

Partitioning of color space into base color names

Low L* High L*  Low C* High C* Low h High h
Red    41    49     59    86     27    37
Orange       62    72    67    96    57    67
Yellow    81       90    68   109    86    86
Green    31    72      29    80   122   168
Blue    31    71    24    58  -112   -71
Purple    25    52    26    81   -56   -35
Pink     62    81    25    54   -23    21
Brown    29    41    26    43    55    76

Let me know if you find some use for this. I found some use... I wrote a blog post, and looking at this graph gives me a bunch of ideas for future blogposts.

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.