When you hear the word “planetarium,” what do you imagine? Just a place where stars are shown on the ceiling? The latest planetariums are now “full-dome movie environments,” which show far more than just images of the night sky: 3D scenes are projected onto the entire dome, with a surround-sound system enabling visitors to enjoy a dynamic experience that brings the images amazingly to life.
The secret to this transformation is the digital planetarium. Conventional optical planetariums only project light onto a screen through tiny holes made in a “star field plate,” meaning that all they can show is a representation of the night sky. Digital planetariums can be described as high-performance projectors, capable of projecting a range of different images onto the entire 360° surface of the dome.
In 2001, with the idea of making planetariums more accessible, Konica Minolta developed the Media Globe, a single projector capable of projecting both the night sky and multimedia presentations. This was the world’s first full-color single-lens digital planetarium. As the entire dome-shaped ceiling constitutes the screen, projection in a planetarium requires a dramatically wider projection angle compared with that of a regular projector, and a large-aperture, ultra-wide-angle fisheye lens is therefore used.
The rise in the quality of films in recent years means that there is now increasing demand for digital planetariums to show high-resolution, high-definition video. To meet this need, digital planetariums were fitted with ultra-high-definition liquid crystal panels, but the development of lens units with unprecedented optical performance was essential in order to make the best use of their performance.
In the projector, a strong light is shone from behind and through an image shown in a small liquid crystal panel to project it onto the screen. Digital planetariums also use this mechanism, and the higher the resolution of the liquid crystal panel used, the higher the resolution of the projected image. Later digital planetariums incorporated liquid crystal panels with a resolution of 4,096 x 2,400 pixels, and a pixel pitch of 6.8 μm.
However, just as sunlight is split into seven colors by a prism, the rate at which light is refracted by the lens varies at different wavelengths, giving rise to something called “chromatic aberration.” If a lens with high chromatic aberration is used, the colors of the projected image will be misaligned, causing them to appear to run into each other and blur.
If chromatic aberration cannot be minimized so that it is smaller than the pixel pitch of the liquid crystal panel in a digital planetarium, then the resolution of the panel will not be reflected in the projected image.
The 172° fisheye lens unit developed by Konica Minolta for its digital planetariums has achieved exceptionally sophisticated correction, with a maximum chromatic aberration of magnification of only 1.5μm. This is far smaller than the 6.8-μm pixel pitch of the liquid crystal panel, meaning that high-definition images from the panel can be projected with their high quality preserved.
The first lens has also been made more compact with an effective diameter of 166 mm, making it around 26% smaller than conventional lenses.
The newly developed lens unit actually consists of two separate units: a projection unit with a short back focal length, and a relay unit to extend this to a long back focal length.
This use of two separate components means that the back focal length (the part of the projector that causes chromatic aberration of magnification) can be extremely short. The relay unit extends the back focal length to a length sufficient for color-combining prism installation and other purposes, and as this relay unit is bilaterally symmetrical, in principle it does not generate chromatic aberration of magnification.
Konica Minolta has developed first-rate technology and expert skills over more than eighty years of lens production. One area in which the company’s expertise is particularly outstanding is the high-precision polishing of large-aperture lenses, which are used not only for digital cinema and planetarium projectors but also in digital cameras with interchangeable lenses as well as measurement and evaluation devices.
Konica Minolta Planetarium “Tenku” (in TOKYO) uses a combination of three planetariums for projection: an optical planetarium that projects images of the night sky with realistic shining stars, and two digital planetariums to further improve the resolution of the dome images.
The optical planetarium (star ball) and the two digital planetariums that flank it on either side can be seen in the middle of the dome.
These two high-definition digital planetariums are utilized to their fullest extent, projecting images through the high-optical-performance lens to create unprecedentedly beautiful imagery and movies of overpowering realism and immersion.