PFL Vent Score
When we first started comparing printed footbeds, we kept describing them with words like "airy" or "dense", which made our notes hard to compare from one month to the next. We wanted something that anyone could reproduce with a phone camera and free software, and that would put a number on how much of a footbed's top surface is actually open space. After trying a few approaches, we settled on the method described here and named it the PFL Vent Score.
To take the measurement, place the slide or the removable footbed flat on a sheet of white paper, top surface facing up, and photograph it from directly above with the camera parallel to the paper. Include a ruler in the frame so the scale is known. Crop the image to the outline of the footbed. Then convert the image to black and white with a threshold so that the material appears black and the openings, where the white paper shows through, appear white. Most free image editors can do this, and so can the open source program ImageJ.
Count the white pixels inside the outline and divide by all pixels inside the outline. Multiply by one hundred and round to the nearest whole number. That number is the PFL Vent Score. As an illustration only, if a footbed outline contains 200,000 pixels and 92,000 of them are white, the open share is 0.46 and the PFL Vent Score is 46. A solid foam footbed would score 0, because no paper shows through it.
In our notes we read scores below 20 as mostly closed, scores from 20 to 40 as moderately open and scores above 40 as very open. A higher score usually means more airflow and faster draining, but it can also mean less surface to support the foot, so it is a description and not a quality grade. The score only looks at the top surface; two footbeds with the same score can behave differently underfoot if their internal lattices differ. For lattice fineness we use the PFL Strut Density instead.
Lighting matters. Shadows inside deep cells can turn white areas grey, so we photograph under even, diffuse light and set the threshold by checking a few cells by eye. Repeating the photo three times and averaging the results usually keeps the score within two points.