System usability investigation


Within medical device design, markings on instruments can be critical to achieving correct outcomes, but in many cases the only standard that exists to define them is that they should be ‘legible’. Contrast in particular can be a big challenge due to the bright lighting, reflective surfaces, and debris buildup common during surgery. As part of a range of solutions explored to improve and standardise the legibility of markings, this project explored understanding and creating usable levels of contrast on instruments.

The first stage of the project involved gaining an understanding of the different levels of contrast achieved with existing combinations of materials and markings. This was done by photographing existing instrument markings under a controlled light source, and analysing the images taken.

The results showed a wide range of variation throughout the instruments, in some cases markings for use having very low levels of contrast, whilst some product identifiers had much better markings.

The second part of the project involved gaining a further understanding of how contrast levels affect ability to read. Initial exploration involved getting surgeons to choose what they thought to be the minimum acceptable contrast when viewed at a set distance under surgical lighting conditions.

Further development involved the design and running of a digital test, to judge what the minimum threshold was users could reliably read at. This was inspired by a psychophysical test run by MIT AgeLab. Words and pseudowords were presented using Psychtoolbox at a set interval, with contrast ratio displayed to the user changing based on the participant’s correct reading of the words.

The results of both parts of this study have been used to inform guidelines for the engineering team, to help them to properly specify reliably legible markings, giving them a better understanding of what methods are suitable for each type of marking.