Visual Judgement of Parallel and Perpendicular

[FLisa]

I am looking for information on the accuracy of visual assessment of angles, specifically how many degrees off of parallel or perpendicular do two objects (medium size 1 foot to 4 foot range) have to be for someone to notice?

 

[jagr]

The human eye can perceive at a minimum a visual angle of approximately 1 minute of arc (1 degree /3600).

However, there is a lot of environmental factors such as adequate lighting, the vision of the inspector, whether the person is searching for the error (which leads into training issues and fatigue), confusing or misleading background cues (are there other lines in the background that mislead the inspector. A rule of thumb is to not expect better than 4 minutes of arc difference from the top and bottom of the object (assuming that the top and bottom are relatively close to each other in the field of view). If you don't have control over the lighting, contrast between objects and background, etc. you may not want to expect better than 40 minutes of arc.

Again, this is a multi-variable problem, but these guidelines should put you in the ball park.

These numbers of course are assuming someone is actively looking. You could run an experiment in your particular case, but from my experience if detection is important, don't depend on anyone just "noticing"... even if the two objects were at a 45 degree angle to each other.

 

[FLisa]

Thank you for a clearly written, succinct and complete answer.