Grey value contrast sensitivity of dental practitioners in function of luminance and age

Grey value perception is important in viewing and interpreting X-ray images. It is possible that ageing decreases the number of gray values that a person can distinguish. This hypothesis was tested in a group of dental practitioners from students to experienced dentists using a 12-bit grey scale.

A custom-made computer software was used based on the principle of the "just noticeable difference" (JND). Observers were shown a picture of an outer square with increasing grey value in which a smaller square of different grey value popped up in changing positions. As soon as the observers could see the difference, they clicked the inner square. The grey values were shown with a 12-bit depth in batches of 400 grey value steps. 59 dentists of 3 age groups (20-24, 25-45 and 46-70 years) participated in the study. A subgroup of 20 practitioners performed a validation test with test-retest and test in reversed grey value sequence (white to black). JND was calculated and plotted against grey value using a third-order polynomial function. These curves were compared statistically.

Test-retest, also in reversed order did not yield different curves (p values between 0.79 and 0.97). Curves between different age groups showed significant differences, with older practitioners needing more contrast to accomplish the task. Contrast sensitivity showed an optimum in the darker third of the grey scale (values around 1200).

Age and grey value played a significant role in grey value perception by dental practitioners.