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Rotating Prism Camera Minimizes Smear From CCD Sensors

Rodney M. Powell, Research Engineer at the Wright Laboratory Armament Directorate, Eglin Air Force Base, has invented the Rotating Prism Video Camera (RPVC), an optical device that minimizes "smear" in full frame and frame transfer CCD sensors. Smear is a reduction in image quality (reduced sharpness and contrast) due to the unintended addition of electric charge during the charge-shifting operation that moves the accumulated image off a CCD sensor. The VISIONEER 4050 TDI camera, 4MEG VIDEO Model 10 imaging board, and 4MIP image processing software, were instrumental to project development.

As a prerequisite to understanding the concept of smear, why it is a problem, and how the Rotating Prism Video Camera minimizes it, we must have a basic understanding of the methods used by the CCD sensor to transform an optical image, streaming through a lens, to an electronic representation, accumulated on a matrix of sensor elements. Each sensor element (also known as a photosite, and sometimes called a pixel) plays two roles in the formation of an accumulated image:

The Transducer Role
The simpler role to comprehend is that of "Transducer". The dictionary definition of transducer is: "A device for converting a non-electrical signal to an electrical signal." A sensor element, acting as a transducer, converts light energy (photons) into an electric charge. The amount of generated charge is proportional to the amount of light that strikes the sensor element.

The Charge Shifter Role
The second role is that of "Charge Shifter". In order to output the accumulated image, each of its thousands of charges must be transferred or shifted off the sensor. Under the orchestration of a Vertical Shift Clock, similar in principle to a bucket brigade, charges flow through a column of sensor elements which shift them toward their ultimate destination (monitor, image memory, or hard disk).

The Problem With Sensor Elements
In an ideal sensor element, there would be a very distinct separation between the role of Transducer and the role of Charge Shifter-the sensor element converts photons into an electric charge for a definite time period and then STOPs! Next, the sensor element does nothing but shift charge packets until the column is completely drained-but no sensor element is ideal. In full frame, and in frame transfer CCDs, even while shifting electric charge that originated elsewhere, a sensor element converts impinging photons and adds charge. A sensor element's propensity to add electric charge while in the shifter mode tends to corrupt the charge packets generated by the sensor elements above it. (For the sake of this discussion we'll assume that charges are shifted from the top of the sensor down.)

Definition of Smear
Smear is a reduction in image quality (reduced sharpness and contrast) due to the unintended addition of electric charge during the charge-shifting operation that moves the accumulated image off the sensor. The potential for corruption is greatest for the top rows of sensor elements since these charges must endure passage through a greater number of shifting operations, each sensor element converting more photons, adding electric charge whenever possible. The potential for corruption is greater for dark pixels-the ultimate example of a corrupted pixel being a black pixel (Grey Level 0) being converted to a white pixel (Grey Level 255). The degree of corruption is increased with the intensity of light, the sensitivity of the sensor, and with slower vertical shift rates-the greater the number of photons that can be converted to electric charge while the matrix is being drained, the greater the potential for corruption or smear.

CCD Sensors & Smear Control
Full frame and frame transfer CCD sensors are used in cameras providing high spatial resolution and/or exceptional light sensitivity. They provide maximum light-gathering capability by maximizing the number and size of sensor elements. Light sensitivity is maximized to the exclusion of in-sensor design attributes (such as buried channel or interline transfer construction) that could attenuate the smear problem. The most common method for minimizing smear in these CCDs is the use of a shutter, mechanical or electronic, to shield the sensor during charge transfer. Unfortunately, a mechanical shutter is subject to physical instability, especially at high frame rates, and an electronic shutter is bandwidth limited.

Minimizing the Problem
The RPVC minimizes smear by creating a vertical sweep of the optical image in synchronization with the draining of the accumulated image off of the sensor. Controlled by the same Vertical Shift Clock that controls the movement of charge off the array, a 4-faced prism spins in sync with the charge drain so that the photon stream that illuminates a sensor element in the Transducer mode, moves down the column in sync with the charge-shifting operation. With the optical image flowing down the matrix in sync with charge flow, the propensity for charge packet corruption, or smear, is minimized.

The Rotating Prism Video Camera
The prototype Rotating Prism Video Camera consists of a standard VISIONEER 4050 camera, by SIERRA SCIENTIFIC, modified with the addition of the rotating prism. The VISIONEER's TDI capabilities make it a natural for this application since the standard formation of an image on a TDI sensor is similar to image formation using the rotating prism. Image formation in standard TDI mode is caused by the physical movement of the subject past the sensor-image formation with the rotating prism is caused by the optical movement of the subject across the sensor.

The 4MEG VIDEO Model 10 & 4MIP Software
The primary reason for using the 4MEG VIDEO Model 10 is that it provides full support of the VISIONEER camera-no enhancement or modification is required. Four megabytes of image memory allow capture of a sequence of images at video rates. A continuous sequence of images is required to verify that the invention definitely reduces smear.

4MIP software provides all the software support required. Instead of spending time developing image examination software, Mr. Powell was able to concentrate on developing his invention. As he says, "Several operations were used. Video digitize/display was used to capture the images. Image zoom/scroll was used to look at captured images. The scroll feature was especially useful since the captured images have more lines than can typically be displayed on a video monitor. Of course, the image file load/save was used for image storage. The ability to store images in TIFF format was helpful for importing into various graphics software packages. Finally, the Export Video State's Structs To File made life easier by allowing the software configuration to be stored and conveniently recalled through use of batch files." No custom board control software was required.

Potential Uses
Mr. Powell is in the process of developing a Rotating Prism Video Camera capable of capturing up to a thousand frames per second, possibly more: "The main potential use of the RPVC is providing an alternate method of dealing with the inherent blurring encountered with frame transfer and full frame CCDs. Some cameras using these imagers get around the blurring by clocking out the charge very quickly. As these cameras are operated at higher frame rates, then the clocking speeds needed become very high, resulting in noise, signal generation, and heat dissipation problems. The RPVC reduces the clocking speeds needed because the rotating prism eliminates the blurring problem and so the fast read out is not necessary. So, if tests at higher speeds are good, then the RPVC will be useful in high speed video applications."

For additional information about the Rotating Prism Video Camera contact:

EPIX Vision - May 1995 Newsletter

1. New EPIX Software
2. Rotating Prism Camera Minimizes Smear From CCD Sensors