Relative illumination in the context of optical design is the phenomena of image roll-off (e.g. reduction) towards the edge of an eyepiece. This manifests in an image that is brighter at the center of eyepiece relative to the edge of the eyepiece.
Relative illumination is usually shown in a graph such as the one below
This particular graph is from an eyepiece with 60-degree horizontal field of view designed by Sensics. The graph shows how the illumination changes from the center of the lens, e.g. 0, to the edge of the lens, e.g. 30 degrees. The Y axis shows the relative illumination where the center illumination is defined as “1”. In this particular eyepiece, the illumination at the edge is just over 70% of the illumination at the center.
This effect can also be viewed in simulations. The first image below shows a simulated image through this eyepiece when ignoring the impact of relative illumination:
|Simulated image while ignoring the effect of relative illumination|
|Simulated image with relative illumination|
Relative illumination is perfectly normal and to be expected. It exists in practically every eyepiece and every sensor. It is often the result of vignetting – some light rays coming from the display through the eyepiece to the eye that are blocked by some mechanical feature of the eyepiece. This can be an internal mechanical structure or simply the edge of a particular lens. Light rays from the edge of the display are easier to block and thus typically suffer more vignetting.
When we look at an optical design, we look to see that the relative illumination graph is monotonic, e.g. always decreasing. A non-monotonic curve (e.g. a sudden increase followed by a decrease) would manifest itself as a bright ring in the image, and this is usually not desired.