High-Quality Lenses are Crucial for Your Dome Projection Project
As the world opens back up after Covid closures, people are expected to flock to amusements and public attractions like beaches, parks, and theaters. Because of that, you should be sure that you’re offering the highest quality imagery in your production.
However, providing high-quality images isn’t as simple as just buying a better projection, as the global chip shortage is making it hard to buy new tech of any kind. Your options don’t end there though, as upgrading your lens can help improve your existing equipment.
Before discussing lenses, let’s first talk about digital projection and the importance of resolution, as this affects your decisions.
Why Do I Need a Digital Projection Instead of Analogue?
As new mediums for imaging technology emerge like augmented and virtual reality, the ability to produce immersive media improves exponentially. In a very short period, we’ve gone from simply accepting flat screens as a way of life to adapting our everyday smartphones to be VR-capable through the use of a headset and lenses.
Additionally, audiences have come to seek out higher quality images, not just in terms of resolution, but with high dynamic range and sometimes more rapid refresh rates. To this end, analog projection simply isn’t able to meet the desired standards for 4K resolution and contrast.
If you’re a planetarium operating on an old analog machine, it’s long past due for an upgrade.
How Do I Transition My Planetarium to Fulldome Video?
Fulldome video is what it sounds like, a video in which each image is warped into a ‘dome master’ so it can be projected onto the inside of a dome, like a planetarium.
These projections are a “fisheye image” that typically comes in a circular frame, and is also called a polar projection. The center of the circle is the zenith point, and the circle’s circumference is the horizon; if you’re thinking about your dome like half of a planet, the top is 0° North and the bottom is 180° at the equator.
For a better understanding of how this looks, Navitar has produced diagrams to help explain the projector placement for single and dual projector operations. With a single projector, a projector with a 180° lens can be placed at the exact center of the dome, precisely on the spring line, which is level with the bottom.
If your projector utilizes a 160° lens, it should be placed at a distance that is ~8.7% of the dome’s diameter beneath the base of the dome, to completely fill the dome. In either case, your image’s resolution is unaffected, but it’s important to note that the DMD (Digital Micromirror Device) for your projector should be 16:10 for a single-projector solution.
For those using dual projectors, they can be placed either at opposite ends of the dome facing toward one another (Cove mount) or both in the middle facing upward (Pit mount), which require slightly different image configurations and differently ratioed DMD chips.
This difference in image configurations is understood as the Image Circle Overage or the amount of the image that fits within the frame; the space that does not fit into the frame is the overage. With a Cove mount, the center of the frame is located at about ~120°, so your frame will be much higher in the image; for a Pit mount, it is ~180°.
This mounting is also described by projectionist Paul Bourke as ‘lens offset.’ In the case of a pit mount, you should be using a projector with a 16:10 DMD, and a 16:20 for cove mount. In either case, these will still produce images with a 100° horizontal angle in either 1080p or 2K.
Making this upgrade can appear to be an expensive proposition, as 4K projectors can be exceedingly expensive, with some starting at $5,000. But, there are technologies to help improve the perception of 1080p images, like Pixel Shifting.
What’s Pixel Shifting?
The way we perceive resolution in projectors differs slightly from typical display panels, as projectors are much more reliant on pixel density to produce clean images.
Projectors that are a lower resolution or even placed too far away can have a ‘screen-door effect as the pixels are overly dispersed in order to cover the surface they’re being projected on.
As a way of reducing this, some projectors implemented a technology called “pixel shifting” which, which is somewhat similar to interlacing except it targets resolution instead of frame rate.
Interlacing is a technique in which even and odd rows of pixels are rendered alternately to produce an effect that enhances the perceived motion by whoever is watching it, without consuming extra resources.
On the other hand, ProjectorPoint in the UK explains Pixel Shifting is when a projector “overlays two HD images on top of each other (depending on the projector’s panel/chip), shifted by a half a pixel up, down, left or right quickly so the naked eye can’t detect the shifting.”
This essentially refracts the pixels to help display them twice as often, increasing the perceived resolution without actually projecting extra pixels.
What Are The Benefits Of Pixel Shifting?
While there are some detractions to pixel shifting, it’s important to understand the benefits of the technology before writing it off as “FauxK” or a fake imitation of true 4K resolutions.
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First, and primarily, is the cost of your projection rig. As LifeWire explains: “With the cost difference between real 4K (where prices start at about $5,000) and pixel shifting (where prices start at less than $2,000), the cost is something to consider, especially if you find that the visual experience is comparable.” This is an exceptionally poignant point in the event that your facility requires the use of multiple projectors, as the price will multiply.
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Secondly, pixel shifting can actually display 1080p or 2K images better than 4K resolution can. In 4K, a 2K image’s pixels are expanded to fill the frame, and as such can sometimes appear blockier. Pixel shifting offers an alternative in which the image is presented in its original resolution, but looks higher fidelity due to the offset pixels. This is important for older content that was originally recorded in 2K, or for real-time rendering on devices that aren’t capable of outputting 4K.
However, these images will not look as good as a true 4K image will. But sometimes that is only noticeable in close, direct comparison. Projectionists should understand that resolution is not the only factor that determines image quality, and look a bit deeper. In evaluating projectors, it’s important to understand the color contrast for the display, brightness capabilities, and the amount of digital noise produced. A pixel-shifted image with a larger color range may appear better than true 4K and be cheaper, so it can be a more reliable option. However, all of these decisions can be for naught if a lens degrades the image.
How do Lenses Affect Dome Projection?
Your lens is a critical part of the projection operation, as it acts as a filter through which the entire image is seen. Having an incorrect lens can lead to an image being ill-proportioned or warped, and a low-quality lens can deteriorate your image’s contrast, clarity, or brightness.
With a high-quality fisheye lens, your image can look just as clear at its edge as it does the center, and in multi-projector setups can achieve a seamless overlap between the two images. More importantly, these fisheye lenses can help enable you to operate a single-projector solution, allowing you to decrease costs or increase resolutions.
The lens in your projector is just as important as any other step in your projection pipeline, and can easily diminish the results of the entire operation. All technologies are only as good as their weakest components, and that is especially true with optical devices like lenses.
Conclusion
Dome projection is a science unto its own and requires a lot of technical and optical know-how in order to do it right. Every piece of equipment needs to be carefully selected to maximize performance and value, which means understanding how they work together, and not just in isolation. When assembling a projection rig, it is imperative to evaluate every component involved, including the lens.
Sources:
Paul Bourke - FAQ: Spherical mirror projection for hemispherical dome projection | Paulbourke.net
Projector Point - 4K Resolution - Native Vs Upscaled Explained | Projectorpoint.co.uk