Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-07-19
2003-11-04
Smith, Zandra V. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140RC, C313S1030CM
Reexamination Certificate
active
06642499
ABSTRACT:
DESCRIPTION
The present invention relates to measurement of spatial variations in the sensitivity of an optoelectronic imaging device, such as a streak camera, to enable calibration and correction for such spatial variations. The system utilizes uniform illumination of the entrance of the device during measurements. The entrance may be an input aperture, such as a slit, and is referred to as an input slit hereinafter, without limitation to the entrance having another shape. Particularly the invention provides for measurement of the flat-field response representing the variations in sensitivity across an output image of the device and provides a flat-field data set as the system response pixel-by-pixel to a constant in time and spatially uniform illumination source. The illumination source may include light pipes which provide spacial homogenization of the illumination of the input slit during measurements and obtaining the flat-field data set. Geometric distortions can be mapped in accordance with the invention by imposing a spatial modulation using a grid over the output face of the homogenizer. A temporal modulation of the illumination source may also be used.
Streak cameras are useful to make quantitative measurements of transient phenomena lasting typically less than a microsecond. Such streak cameras are commercially available and are described in the literature both as regards their optoelectronics and their sweep deflection circuitry. See, LLE Review, Volume 73, pages 6-73 and Chang, et al., U.S. Pat. No. 5,142,193, issued Aug. 25, 1992; and Kinoshita, U.S. Pat. No. 5,221,836, issued Jun. 22, 1993. Images from streak cameras may be obtained using CCD cameras as their recording medium thereby enabling quantitative measurements of the phenomena to be made with higher precision and higher signal-to-noise ratios than previously was the case with film as the recording medium. In accordance with invention, the image obtained by a CCD camera and particularly a cooled, scientific grade CCD, provides for accurate photometric calibration of the streak camera and when calibrated, accurate measurements of the phenomena can be made with the streak camera.
Various proposals have been made for calibration of streak cameras. Most require the use of calibrating illumination of high intensity and submicrosecond duration. See an article entitled “Flat-field Response and Geometric Distortion Measurements of Optical Streak Cameras,” by D. S. Montgomery et al. which appeared in SPIE, Vol. 832, High Speed Photography, Videography and Photonics V (1987, 283-288). Other proposal for calibration may be found in the following U.S. Patents, U.S. Pat. No. 4,628,352 issued to Boue, Dec. 9, 1986; Tsuchiya, et al., U.S. Pat. No. 5,043,568, Aug. 27, 1991; Koishi, et al., U.S. Pat. No. 4,945,224, Jul. 31, 1990; Oba, U.S. Pat. No. 4,714,825, Dec. 22, 1987; Schiller, et al., U.S. Pat. No. 4,435,727, Mar. 6, 1984; LeBars, et al., U.S. Pat. No. 5,118,943, Jun. 2, 1992; Arseneau, U.S. Pat. No. 4,323,977, Apr. 6, 1982; Stoub, et al., U.S. Pat. No. 4,298,944, Nov. 3, 1981; Malueg, U.S. Pat. No. 3,949,162, Apr. 6, 1976; Prager, et al. U.S. Pat. No. 5,726,915, Mar. 10, 1998; Therrien, U.S. Pat. No. 4,523,231, Jun. 11, 1985; Knoll, et al., U.S. Pat. No. 4,386,404, May 31, 1983; and Knoll, et al., U.S. Pat. No. 4,212,061, Jul. 8, 1980. The latter two patents disclose calibration of scintillation cameras which are examples of other optoelectronic imaging devices than streak cameras.
Proper calibration requires one to account for any localized differences in the recorded signal due to distortions, aberrations and defects in any of the streak camera components. To this end, one must measure very accurately the spatial variations in sensitivity across the output image, i.e., the flat-field response, and then perform a flat-field correction to the signal data set. The flat-field data set is the system response, pixel-by-pixel, to a constant in time and spatially uniform illumination source. The correction consists of dividing the signal data set by a normalized flat-field data set. Localized or small scale length variations in the system response could be due to photocathode or phosphor screen non-uniformities, differences in individual CCD pixel sensitivity, defects in the fiber optic window, etc. Long-scale length variations could be due to vignetting in the input optics or the electron optics, or to differences in the photocathode quantum efficiency along the input slit.
Calibrations must also include mapping any geometric distortions in the output image and correcting for them. Geometric distortions originating in the streak tube may be caused by the use of curved input or output surfaces, electron-optical spherical aberration or mechanical misalignment of the electrodes. Fiber optic components with twists or shears may also introduce geometric distortions. The calibrations discussed herein do not extend to establishing iso-temporal contours in the output image, or to corrections for nonlinear streak speeds. The techniques for calibrating streak speeds are well known.
Streak camera records in general have limited SNR due to the small number of photoelectrons per time and spatial resolution element that make up the signal. Excess current will introduce nonlinearities in the photoelectron beam that cannot be corrected. The photocurrent is restricted by space charge effects while the electrons are in transit from the photocathode to the screen and by charge depletion in the photocathode. In order to achieve the best SNR data, streak tubes are always operated close to their peak current handling capability. However, the system's flat-field response must be obtained with a SNR much greater than that of the signal data set so that the SNR of the corrected data set will not be significantly degraded. This is most important in those regions of the image where the sensitivity is poor to begin with. Since the SNR of a single flat-field data set can be no better than for a single signal data set, if they both are acquired with the same streak duration, multiple flat-field images must be collected and averaged to achieve the requisite SNR. This is regardless of the immense difficulty in producing a high brightness, constant amplitude light source with duration ranging from a few nanonseconds (ns) to a microsecond. Limited SNR also affects the precision with which the geometric distortions can be mapped and thereafter corrected.
In a streak camera that incorporates a fiber optically coupled, back-illuminated CCD camera (and no image intensifier), a streak tube photoelectron generates typically 25 CCD electrons. If the CCD pixel full well is 250,000 to 300,000 electrons, (typical of a 24 &mgr;m square pixel), the single pixel SNR is limited to about 100 by Poisson statistics. The regions of the image where the sensitivity is poor will have a lower SNR.
To record a flat-field image that has a SNR of 100 on a Megapixel CCD array requires that 1.6 nC of charge be extracted from the photocathode, (10
4
photoelectrons per pixel times 10
6
pixels). The time duration for extracting this amount of charge is limited by the current handling capability of the streak tube, the charge stored in the capacitor formed by the photocathode and accelerator electrodes and the charge replenishment rate from the power supply. Preferably the electrode potentials should not change by more than 1% during the flat-field image acquisition time period to avoid affecting the focusing of the electron optics. The peak photocurrent delivered to the screen is typically less than 1.6 mA for commercially available streak tubes. This is calculated as 1% of the Child-Langmuir space charge limited current density at the photocathode times the usable photocathode area times the fraction of photoelectrons emitted from the photocathode that contribute to the signal at the screen. A simple division of the charge required by the peak current gives a minimum flat-field acquisition times of 1 &mgr;s. The total charge stored in the photocathode is typically less tha
Boni Robert
Jaanimagi Paul
Lukacher Kenneth J.
Lukacher Martin
Smith Zandra V.
The University of Rochester
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