Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1993-04-16
1994-03-15
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
2504831, H01J 3150
Patent
active
052947882
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a low light level, high resolution imager.
A large amount of work in physics (nuclear physics, astrophysics, biophysics) requires it to be possible to locate with great accuracy light sources that are of very low level in terms of photon count.
For example, much work performed in molecular biology derives its information by studying the three-dimensional positioning of chemical species (strands of DNA or of RNA). To do this, the commonly-used technique consists in marking the species being studied with a specific radioactive probe. An experimental result is thus expressed in the form of a map of .beta.-emitting populations, the map being optionally quantitative and of greater or lesser resolution.
For several years, proposals have been made to pick up an image of light sources using charge coupled devices (CCD).
For example, in the article by Y. Charon et al. entitled "A high resolution .beta.-detector" published in the document Nuclear Instruments and Methods in Physics Research, A 273 (1988), 748-753, a system is described as shown diagrammatically in accompanying FIG. 1, which system is particularly adapted to experiments in molecular biology, and comprises: and computer 50.
The scintillator 20 generates photons when it detects an electron coming from the sample or from an equivalent source. The light is amplified in the tube 30 and is then applied to the electron camera.
The module 42 controls this camera in one-shot mode, not in video mode. In video control mode, frame-cycles follow one another at a fixed rate (where each cycle is made up of a stage during which the charge coupled device is reset to zero, followed by an image integration stage, and then by a read stage). In contrast, in one-shot control mode, each frame-cycle is controlled independently of the preceding cycle.
More precisely, according to the above-mentioned document, the camera 40 is controlled in repetitive one-shot mode by the external trigger generator 60, i.e. the camera 40 is controlled to have short and repetitive integration cycles as opposed to a simple one-shot mode which would consist in integrating the image of the light source over a long period and then in reading it solely at the end of acquisition.
The control of the camera 40 is shown diagrammatically in accompanying FIGS. 2, 3, and 4.
FIG. 2 shows the time distribution of a light source or sample.
FIG. 3 shows the corresponding response of a light-amplifying tube 30. Noise pulses can be seen in FIG. 3.
Finally, FIG. 4 shows the response of the tube 30 superposed firstly on the cycles of the charge coupled device 41, each of which includes a stage during which the CCD is reset to zero, an image integration stage, and a CCD reading stage, and secondly the signal which triggers the cycles.
The technique of controlling the camera 30 in repetitive one-shot mode makes it possible to escape in part from the major cooling required in repetitive one-shot mode because of the contribution of thermal noise in the light-amplifying tube and in the camera, which noise is proportional to the integration time.
However, controlling the camera 30 in competitive one-shot mode does not give complete satisfaction. It suffers from the following drawbacks:
events are lost during dead time, thereby loosing efficiency;
given the integration time, the contribution of thermal noise from the amplifier tube remains large; and
pixel brightness information cannot be used quantitatively since events occur randomly within the integration window.
Attempts have been made to eliminate these drawbacks by triggering the integration cycle of the charge coupled device only in the presence of a light event.
The system proposed to do this is described in the article by Y. Charon e1. entitled "H.R.R.I., a high resolution .beta.- imager for biological applications", published in Nuclear Instruments and Methods in Physics Research A 292 (1990), 179-186. That system is also shown diagrammatically in accompanying FIG. 5.
There can be seen again in FIG. 5:
However, in the system shown in FI
REFERENCES:
patent: 3742224 (1973-06-01), Einstein
patent: 3777201 (1973-12-01), Einstein
patent: 4893020 (1990-01-01), Ono
patent: 4900930 (1990-02-01), Takiguchi et al.
patent: 5235191 (1993-10-01), Miller
"High Spectral Resolution, Photon Counting Detector for Doppler Temperature Measurements in Magnetically Confined Plasmas"; R. D. Benjamin et al.; Review of Scientific Instruments; 58(v) Apr. 1987; pp. 520-529.
"A High Resolution Beta-Imager for Biological Applications"; Y. Charon et al.; Institute de Physique Nucleaire; Sep. 1989.
Charon Yves
Gaillard Jean-Marc
Laniece Philippe
Leblanc Michel
Mastrippolito Roland
Allen Stephone B.
Centre National de la Recherche Scientifique "CNRS"
Nelms David C.
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