Full Color Electric Sensor
By Mac Gardiner
Photons come in colors; electrons don't. So, whenever we convert photons to electrons, we lose the color information. If we wish to perceive color in the final image, we have to sort the photons, by color, before the conversion process.
One way is to sort the photons with filters which block out all but the desired “color.” ”Red,” “Green” and “Blue” images can then be generated, whic h can then be combined to form the true color of the original. This process discards all photons of other colors through each filter, which is okay if you have a huge excess of photons.
One can either split the image into three and filter each, (Fig 1)
or simply take three photos, using each filter successively. (Fig 2).
The price paid is that of time, extra equipment and/or moving parts.
Astrophotography nearly always contends with a scarcity of photons. Time exposures ranging up into many minutes are required to get an acceptable number of photons. A color picture can multiply the time required by a typical factor of three. This is bad enough on earth, but in near orbit space, dusk to dawn is 45 minutes rather than 12 hours (average), so the process of getting beautiful color images of wispy far off and faint celestial phenomena is badly compromised.
One other feature of a typical photon is important. Photons hitting a silicon slab are absorbed and converted to electron-hole pairs at different depths, depending on their wavelength (frequency/color). This often establishes the color limits of CCD conversion, due to photons passing the “collection” region before interaction, or being absorbed before reaching the “collection” region.
On the other hand, this “depth sorting” can be used to effectively utilize all photons by establishing three collection volumes, stacked at appropriate depths corresponding to “Red,” “Green” and “Blue” energies of the photons. (Fig 3)
Neat, if you can build such a device.
Dr. Carver Mead claims that his company, Foveon Inc., has successfully done just that. On Feb. 24th, they demonstrated their Foveon X3 at PMA in Orlando. This system has a pixel matrix of 1280 × 960 (X3) with a pixel pitch of 5 microns. They claim extremely low power, high quality images, with “extremely low-noise readout and very high dynamic range.” Both Richard Berry and I tend to be suspicious of any such non-quantitative assessment, but I know Carver Meade personally and trust his genius and his veracity.
However, their system uses CMOS technology, rather than CCD, and their emphasis is on high quality “superior to 35 mm photography” systems for professional and consumer digital camera markets at the present time. They have had numerous inquires about astronomical use, and reply that it should work fine, but they plan to delay development of that sector until the big money market items are developed.
It’s obvious that an important use is for the ISSAT. We need the simplified system, like the pixel pitch, and are grateful for the low power of a CMOS device. However, the quantum efficiency, noise and reliability in space must be verified.
In addition, all terrestrial uses, particularly for portable systems, show real advantages. Low power drain, simple procedures, and minimum hardware sound good to all of us. Anyone planning to go into Astrophotography should maintain a file on Foveon (www.foveon.com). It could be the next real breakthrough.