Understanding Relative Illumination

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

The second image shows the impact of relative illumination which can be seen at the edges

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.

A Visit to the IMAX VR Center

A few weeks ago, I was in Los Angeles and decided to visit the newly-opened IMAX VR center. I went there as a regular paying customer - not some "behind the scene tour" - to see and learn. I've experienced Zero Latency, The Void and many others, so could not resist trying IMAX.

The lobby of the attraction is reminiscent of a small movie theater lobby. Vertically-oriented monitors on the walls announce the available VR experiences. A small reception area sells $10 tickets for the attractions. A display shows the available time slots for each 10-minute experience. After purchasing the tickets, a friend and I were asked to wait for our scheduled time. When the time came, an attendant escorted us to the VR area.

If I remember correctly, there were eight available experiences. Seven of them were based on the HTC VIVE. One - the John Wick Chronicles - was showing on Starbreeze headset. The HTC VIVE experiences did not appear to to be specially-made for this venue. For instance, one experience was Trials on Tatooine which can be freely downloaded from the Steam store. I think people come to movie theaters for an experience that they can't get at home. One would expect VR to be the same.

I have an HTC Vive at home (as well as many other headsets) at home. Using them is part of my job. However, most folks don't have easy access to PC-based VR equipment. For now, stock experiences might be just fine to get people exposed to VR.

Inside to the VR area, Each headset was in a space separated by low walls, a bit like an open space in an office. Headset cables were tied from the ceiling. HTC VIVE units had a leather face mask which is probably easier to clean. An operator administered each experience - one operator per headset. . Operators were friendly and enthusiastic about the VR equipment. I think their enthusiasm was contagious, which was nice.

Speaking of contagious, the operators told me that they wipe the face masks between users. Masks also get replaced every couple of weeks. I was told that visitors did not often complain about wearing a VR goggle that was used by many people before them.

I couldn't help but wonder about the economics. 15-minute timeslots: 10 minutes of usage plus some time to get people in and out of the experience. $40 an hour per station. One full-time operator per station. Now add rent, equipment, content fees, ticket sales, credit card fees, etc. Can you make money? Maybe making money is not the goal in this first location. Instead, the goal could be to have a "concept store" towards inclusion at the lobby of regular movie theaters.

Since I don't have a Starbreeze headset at home, I opted for the John Wick experience. It's a shooter game that encourages you to move in a space while holding a weapon. As expected, Virtual soldiers try to kill you. The headset was fairly light and the weapon comfortable. The experience was immersive though both the image and graphics quality could have been better. I can see why a person with little VR experience could enjoy these 10 minutes.

My friend did not have many VR experiences before this visit. He chose 'Trials on Tatooine" which he seemed to thoroughly like.

In all - a nice start to what can be the next big thing in entertainment.

Have you tried IMAX VR too? What did you think?