Scribbling away that scratch on my car

I received an interesting comment/question on a LinkedIn thread about the Scribble pen. Answering this question provides me the opportunity to touch on a number of topics and color science, as well as the opportunity to show off just how incredibly smart I am. Since both of these are high on my list of things to do, I decided to dedicate a blog post to answering his question.

Malcolm

Malcolm Cutting

Paint & Development...looking for challenge to develop your business. Call me.

Just using imagination here and the touch up pen for the automotive smart repair scratches....now that could be quite possible.....or could it? Worth thinking about.

The short answer

No.

Requirements are tough

Malcolm is one of my oldest and dearest friends. I just met him on LinkedIn about 15 minutes ago. Feel free to draw your own conclusions about my track record with friends. From reading his LinkedIn profile, I know that he has been in the automotive coating business for multiple decades. He knows that matching paint on a car is one of the most demanding color applications, if not the most demanding.

The first issue is that on a scratch, you are trying to match two colors that are side by side. The human visual system (which includes those parts of the brain that are not devoted to memorizing every episode of Laverne and Shirley) is also very good at picking up on coherent anomalies, such as a line.

Juxtaposition accentuates color differences

The second issue is one that most of my readers are aware of. If you are successful enough in life to have access to the internet, then it is almost certain that your car is more expensive than mine. Naturally, we want our cars to look expensive to show that we are indeed more successful than some dufus who turns to blogging to feed his massive ego. Most of us express our identity through the cars we drive.

Would Guy Fieri still be cool in a Saturn?  I think not.

In short, people are picky about touching up scratches on their cars.

It has to be the same color

Naturally, when a scratch is retouched, it has to be the same color. In order to make it the same color, we need to measure the color accurately. Back in the days of Moses and Pythagoras it was done with the eyeball of someone who is both discerning and not colorblind. Ideally, this person would have been more discerning than the most demanding of customers. 

Nowadays, critical color matching is sometimes/usually/always done by expensive spectrophotometers, which are sometimes/usually/always more discerning than the most demanding of customers. I suspect if you go round and chat up the guy who does that painting at the local body shop, you'll find that he needed to go back to college to pick up a PhD in astrophysics (or some other branch of astrology) just to be able to run a spectro.

The kid who paints my bumpers 

The Scribble pen? It uses an RGBC sensor. Four channels. That's it. The standard ISO 13655, which provides the definition for a color measuring device in the graphic arts, requires that a spectrophotometer be used, and that the spectrophotometer must have at least 15 channels, and preferably 31. I ain't know arithmetic guy, buy I think 3 or 4 is less than 15.

In my own personal experience, I spent about two years working with a bunch of smart guys trying to teach an RGB camera to accurately measure color. Here is one of many papers I wrote about this sad experience:

Why do color transforms work? And here is another: Color measurement with an RGB camera. I have seen a lot of claims about this, and it burns my butt like a candle on a toilet seat. If you want discerning color measurement, you can't use RGB to measure the color.

The color must match under any lighting

In order to be happy about the matching of the color on a scratch, most car owners (I think) would want the match to be good wherever the car is. The illumination could be direct lighting from the Sun, diffuse bluer lighting from the clear sky, the incandescent lighting from one of the bulbs in my garage, the fluorescent lighting from the other bulb, and the sodium vapor lighting you see in parking structures. 

This may seem like no big deal, but those who have practiced saying the word metamerism will realize that this is not a given. It is possible for two objects to match under one lighting, but not under another. If you don't believe me, try to guess the color of a vehicle in a parking structure at night. If you want to match a color under all lighting, you will need to do spectral matching and you will need a lot of different pigments.

The RHEM indicator

The Scribble pen, as cool as it is, only has four pigments: cyan, magenta, yellow, black, and white. Or maybe that's five? I'm a math guy, not an accountant. Spectral matching is pretty much out of the question.

Hmmm... Metamerism might be a good topic for a future blog. Look for it at your favorite blog sites.

The color must match at all angles

If you were impressed by my use of the word "metamerism", I introduce another important-sounding word: goniophotometry. If you can slip those two words into casual conversation, you can pretty much bluff your way into any of those wild color science parties that you are always hearing about. Goniophotometry is the measurement of light when you change the lighting and viewing to different angles. When you reposition the lighting, or move your head around, there is a subtle or sometimes huge change in the color of an object.

The simplest example of this effect is something that is so ubiquitous that it probably goes unnoticed. In fact, I am going to guess that I might get some arguments about whether this is even a color change. I'm talking about gloss. When you view a glossy object at the gloss angle, the color of the object changes. 

This is a point where there might be some argument. One might argue that the color of the object didn't change just because of the angle of lighting and viewing. I would argue that the composition and intensity of the reflected light changes drastically at the gloss angle. 

For the purpose of our discussion, gloss is important. Would you say that a touch-up paint matches another if there is a difference in gloss? I think not! Once again, as cool as the Scribble pen is, it does not have a mechanism to adjust the gloss of the ink.

Metallic effects are another example of goniochromism (when color depends on angles of lighting and viewing). Cars often have a metallic paint. This is achieved by embedding flakes of metals within the paint, laying the paint down in thin layers to make sure that the flakes land flat, and polishing the heck out of it between layers. (This is my understanding... I am open to hearing an explanation from someone who has actually painted a car before.)

Painting a car is an exacting science

The Scribble pen, as cool as it is, does not include any metallic flakes. Nor does it include a polishing thingie. So, I am gonna guess that metallic effects might be a bit tough with this pen.

The creation of gloss and metallic effects are two areas where this pen, as cool as it is, is gonna fall short of the requirements for fixing my paint job when some mathaphobe keys my car. But I skipped over another missing feature. The Fix-The-Scratch-In-My-Car pen must also be able to measure the goniophotometric properties of the existing paint. Some cars are metallic and glossy. Some are just glossy. Other cars, particularly those left outside in Arizona, have lost some of that gloss. Some cars, such as mine, which reside in areas where they use salt on the streets to melt ice, have a glorious texture where a dull rust color is interspersed with the drab shade of pale lavender greenish orange. 

As I understand it, the big car companies use a goniospectrophotometer for quality control of their paint process. Such an instrument runs something over $100K and measures the spectra of reflected light at a zillion and a half different angles in order to make sure the color is correct from all angles. These are run by more astrophysics PhDs.

Goniospectrophotometer

Now, I could have that wrong. Maybe the big car companies use an abridged goniospectrophotometer, such as X-Rite's MA98, for quality control. Rather than a zillion and a half angles, these instruments make measurements at maybe 20 or 30 different angles. These instruments probably make more sense on the production line, since the measurement time is on the seconds, rather than minutes or hours.

The Scribble pen, as cool as it is, probably does not include even an abridged goniospectrophotometer. If it did, the price would be pretty cool, since the abridged gonios cost around $25K. 

Still more

There are a few more requirements for a Fix-The-Scratch-In-My-Car pen. Someone should probably mention that there is a difference between ink and paint. Ink is transparent. Paint is opaque. You want to paint your car with paint, since the bare metal is not all that nice looking.

Someone should also mention that the paint has to be durable. Durable enough to stay put under the effect or driving rain, beating sun, slush, and sandstorms. My wife also tells me that there are these places called car washes where these sudsy rollers come out to rub the grime off your car. The only place I am aware of like that has rollers who are wearing bikinis.

And then someone might wanna mention that it would be good if this pen could avoid scratching the car? I don't have all the details on the Scribble pen, but one possible implementation of the inking mechanism has an ink jet head spraying the back side of a ball point.

Conclusion

Malcolm, I think you got a cool idea there. I'm thinking the Scribble pen might not quite be up to the task, though. But, if we loosen up some of the constraints that came from making this device a handheld pen...

Flat paint is not flat

I needed to paint the inside of an ambient light exclusion device for an optical mensuration1 experiment. (To the layman, I was trying to paint the inside of a box black.) I had decided on flat black mainly because it would look cool. Flat black really gives A/B plywood an air of scientificalness.

The trouble was, this was autumn and the temperature in my garage2 was 45°. The warning label on the spray paint can advised me against painting at temperatures below 50°, but I threw caution to the wind. In single-minded devotion to the greater cause of Science, I forged ahead, hoping that I did not get caught by the Spray Paint Police.

I can look back on this today and laugh at my naiveté. I was quite frankly surprised by the results. The flat black paint came out glossy.

How can flat paint become glossy?

Naturally, my first reaction was to get out the microscope to investigate the microstructure of the surface. This is what I saw.

The image at the left is the surface that I had just painted. The one on the right was a surface that looked flat black because I had painted it at the regulation temperature. Clearly, the glossy surface on the left is smoother than the matte surface on the right. Ironically, the surface of “flat” black paint is not flat!

My “flat black” spray paint became glossy because the vehicle (the stuff that carries the pigment) evaporated much slower at 45°. This gave the surface of the paint time to smooth out.

Changing color with colorless water

Seeing these images, I immediately had a flashback to one day when I was five years old. I had found myself with a small plastic bucket filled with water and a paint brush. The young scientist in me bloomed when I painted the wall of our house with water, and found that it turned the house from a chalky light green to a richer green.

Some adult wandered by and asked me what I was doing. When I said that I was painting the house, they frantically went off to find my mom. My little ruse had fooled even them. I probably got spanked before the truth was realized. If only I had known Fresnel’s law at the time, I would have been able to explain the phenomenon to my mother before her hand came down on my precocious little butt.

Fresnel’s Law

Fresnel’s Law3 describes what happens when light goes from one medium to another, in this case from air to the surface of the paint or water. When light goes from air to a surface that is optically “harder” (having a higher index of refraction), some light enters the surface and other light reflects directly from the surface, in billiard ball fashion – with angle of incidence equaling angle of reflectance.

Fresnel’s law predicts what percentage of the light is reflected and what percentage of light enters the second medium. The law takes into account the angle of incidence, the indices of refraction of the two media, and the polarization of the incoming light. All that is fabulously interesting, but all we need to know is that somewhere between two and five percent of the photons act like little billiard balls. We call this specular reflection.

I brought my good friend Smeldon in to demonstrate. He reads his magazine4 with the light behind him, and the magazine tilted slightly away from him. Without even thinking about it, he will orient the magazine so as to avoid the specular reflection. Smeldon could orient the magazine so he sees only the gloss of the magazine, as in the drawing on the right. If he were to do this, the print on the paper would be largely washed out by the specular light.

Smeldon’s magazine was printed on a coated stock. After the paper is formed, it goes through a second process where various additives, like kaolin (a white clay) and calcium carbonate, are affixed to the surface of the paper and then the paper is polished to a glossy finish. The coating makes the surface glossy. Most magazines are printed on coated stock. Newspapers and Sunday paper inserts are printed on uncoated stock.

If Smeldon were to try this same experiment with a newspaper, he would not see the same results. In fact, he would be hard pressed to find a way to orient the newspaper in such a way as to see gloss. (The interested reader will try this before going on. Go ahead. I’ll wait.)

Smeldon is not an accomplished color scientist and applied mathematician like me (and he is not nearly as good-looking as me) but he does have a bit of a scientific bent. His conclusion is that newsprint is an exception to Fresnel’s law. It would appear that newsprint has very little surface reflection.

Holding out on the dark side

Smeldon is inquisitive and intuitive, but in this case, he is wrong. A newspaper has plenty of specular reflection; it’s just hiding in plain sight. I’ll get back to that in a bit.

Anyone who has looked at a magazine and newspaper side by side will notice that the magazine has much richer color. A solid black on newsprint looks almost gray when sitting next to a solid black of a glossy glamour magazine.

Why is this? I have heard many otherwise intelligent people attribute this to “holdout”. As the explanation goes, glossy stock will “hold out” because of its nice smooth and hard surface. That is, it will impede the progress of ink that tries to seep in. Newsprint has no protective coating, so ink will seep into the paper, and some of the cute little pigment particles will hide behind paper fibers.

I am sure this is explanation is true, but hey, flies walk on the ceiling. I am not convinced that this is the predominant effect. I submit the following image as proof. The image is of newsprint. I borrowed my wife’s clear nail polish and painted over the word “FREE”. The black ink with the polish is noticeably darker. I didn’t add pigment. This was clear nail polish. This is not explained by the “ink hiding behind the paper fibers” theory – the cute little pigment particles are still in hiding. Why did it get darker?

Newsprint with splotch of nail polish

The next picture sheds some light on the quandary by shedding light from a different direction. To take the next picture, I left the camera and the newsprint where they were and moved the light over to the specular angle. The splotch of nail polish has suddenly gotten much brighter because the specular reflection has suddenly reappeared.

Same sample, with light at specular angle

Going back to the original pair of microscope images, we saw that the “flat” black ink on the right had a rough texture. Photons hit this rough surface and reflect specularly in all sorts of directions. Contrary to Smeldon’s conclusion, newsprint still follows the magic of Fresnel’s law, it’s just that the specular reflection goes in all directions.

In the first “free estimate” image, it’s not so much that the nail polish made the black ink darker. It’s more that the specular reflection in the other parts of the image made the black ink without the polish look lighter. The nail polish focused the specular reflection so that it all bounced away from the camera.

My conclusion is that smoothness will make a surface appear darker in color. This is not because there is less total light reflected. It is because the specular reflection from a smooth surface generally bounces off in a direction where we don’t normally notice it.

Notes 1)      No, I didn’t spell that word wrong. “Mensuration” means the act of measuring. All good mensurologists know this word and use it all the time. It’s like a secret handshake. 2)      Due to certain previous incidents, I wasn’t allowed to paint in the basement like other respectable mensurologists do. I guess we can see why I don’t have a Nobel Prize. Yet. 3)      In my humble opinion, Fresnel’s Law is way cooler than Snell’s Law. By the way, Snell’s Law was discovered by Ptolemy. I don’t know who discovered Fresnel’s law, but we know from a previous blogpost that it probably wasn’t Fresnel. As for the Fresnel lens? That idea came from Georges Louis Leclerc, and not Augustin-Jean Fresnel. 4)      Magazines were a technology common in the 20th century that was similar to the iPad. While articles could be read on these devices, you could only check the status of a few celebrities, and one magazine could barely hold a few dozen articles. To their credit, they were produced from a once common renewable resource called “trees”. Ask your grampa to tell you about trees.