Measurement Standardization Process

The transformation from pixel distances to physical measurements is done by affixing a quarter to the forehead slightly above and right between the eyebrows. The points at the center of the eyebrow are used as the bottom of a square that is cropped out from the image.



Then a modified version of this sci-kit imaging code for circle detection is used to locate the quarter and determine its diameter. A quarter is 2.3cm in diameter, thus the scaling factor for pixels to cm is 2.3cm/(detected diameter in pixels).

A preliminary test of this calibration found resulted in a converted OCD of 9.8cm, as compared to 9.5cm when measured with a ruler. This translates to ~3% error.