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.
The short answer
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.
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.
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.
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...