Spray Pattern
I was curious about what the spray coming out of the brush looked like. So I set up an old slide projector as a light source and placed my 155 with a large siphon bottle filled with water in front of it. With the lights out, I took pictures of the spray as I worked the trigger. The air pressure was set to 15psi.
In this first picture, the trigger is barely pulled back. Notice that the spray pattern originates right at the point of the needle. The reason it doesn't start farther back is due to something called the Coanda effect. Moving air or liquid flowing next to a smooth surface, tends to cling to that surface. You can see how close you would have to get to produce a fine line.
In this picture, the trigger is pulled back a little more. Notice the hairy look around the spray. These are actually stray droplets being pulled back into the main pattern by venturi action.
As the trigger is pulled back even more, the spray originates farther back on the needle and the pattern gets wider.
Even though this is an internal mix airbrush, it's obvious that the paint and air don't really mix until the tip of the needle, which would seem external to me. However, the name is an industry standard for this type of brush. I would think that concentric mix would be a more accurate description as the air exits from a circular area around the needle. I guess it doesn't really matter what you call it, but it does mislead a lot of people. It's not uncommon to read that an internal mix airbrush mixes air and paint inside some chamber in the brush. But it just isn't true.