Television – Special applications – Film – disc or card scanning
Reexamination Certificate
1998-07-15
2001-05-29
Le, Vu (Department: 2613)
Television
Special applications
Film, disc or card scanning
C348S097000
Reexamination Certificate
active
06239832
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the conversion of colour film images into corresponding electrical signals, and in particular to the conversion of cinematographic film into video signals, for example using a telecine machine.
BACKGROUND TO THE INVENTION
Telecine machines are well known. Examples of such machines include the “URSA Diamond” manufactured by Cintel International Limited in Ware, England and the FDL 90 manufactured by Broadcast Television Systems (BTS) in Darmstadt, Germany. The URSA Diamond is the latest in the URSA series of telecines, which represent the most commonly used “flying spot” telecine machines. In a flying spot telecine, a spot of light (the “flying spot”) is produced on the face of a cathode ray tube (CRT) by bombarding the phosphor on the inside of the CRT face with a beam of electrons. The beam of electrons is controlled so that the flying spot traces a raster pattern across the face of the CRT. The light from the CRT is directed through a film frame and is detected after it has passed through the film frame by a photodetector. The photodetector produces an electrical signal corresponding to the intensity of the received light. In a colour telecine, photodetectors are provided corresponding to each of the primary colours of light, i.e. red, green and blue, to produce electrical signals corresponding to the detected intensity of each of these components. As the flying spot moves across the face of the CRT, and thus across the film frame, the electric signals from the photodetector(s) are sampled at each position of the spot to build up a set of signals corresponding to the colour transmission of the film frame, and thus the image on the film.
Known flying spot telecine machines use a single phosphor which has a relatively narrow spectral output across the wavelengths of the visible spectrum. The light from the CRTs of such telecines is generally greenish yellow and has little blue or red content. This means that the average level of detected red or blue light transmitted by the film will be lower than the average level of the detected green light, resulting in a low signal-to-noise ratio (SNR) in the red and blue channels.
SUMMARY OF THE INVENTION
In an attempt to overcome the disadvantages of a single phosphor described above, we have investigated the possibility of using a mixed phosphor to increase the red and blue content of the light from the CRT. A mixed phosphor contains a mixture of more than one phosphor, each phosphor capable of emitting light over a different region of the visible spectrum. However, when a phosphor is deposited on the inside of the face of a CRT, it forms a granular structure. Different phosphors exhibit structures of different granularity. We believe that unfavourable results would be achieved with a mixed phosphor CRT, due to the differences in the respective grain sizes of the phosphors in the mixed phosphor. In particular there could be a problem with the grain size of the blue phosphor. This would provide unacceptable variations in luminance within the blue range across the face of the CRT.
According to a first aspect of the present invention, there is provided, apparatus for the conversion of colour film images to corresponding electrical signals comprising:
a cathode ray tube provided with a phosphor composed of at least two phosphor components each having a distinct spectral output;
primary photodetector means arranged to detect light produced by the cathode ray tube, which light has been modulated by a film, and to produce primary electrical signals including information about the colour content of the detected light;
secondary photodetector means arranged to detect light produced by the cathode ray tube, which light has not been modulated by the film, and to produce secondary electrical signals including information about the colour content of the detected light corresponding to the emission from each phosphor component of the mixed phosphor; and
processing means for adjusting the primary electrical signals with reference to the secondary electrical signals.
Mixed phosphors have not generally been used in telecine applications as each of the two phosphors will have a different granularity, such that the light from the face of the CRT will be uneven not only in intensity but also in colour composition. The apparatus according to the invention utilises secondary photodetector means to produce modulating signals responsive to the colour of the light emitted by the CRT, and this can be used to compensate for the effects of differential grain size in the mixed phosphor. Because the grain size generally is related to the colour of the phosphor, modulation of the primary signal on the basis of the colour of the unmodulated light from the CRT is an effective method of compensating for the effect of differential grain size in the mixed phosphor. In preferred arrangements, the light output in the red and blue regions is greatly improved compared to the single phosphor. As more light is produced by the mixed phosphor in these regions, more light passes through the scanned film and thus the SNR of the red and blue video signals is higher compared to that for a single phosphor.
According to a second aspect of the present invention, thee is provided a method of converting colour film images to corresponding electrical signals, wherein light from a mixed phosphor cathode ray tube, which light has been modulated by a film, is converted to primary electrical signals including information about the colour of the modulated light; light form the cathode ray tube which has not been modulated by the film is converted to secondary electrical signals including information about the colour of the unmodulated light; and the primary signals are modulated by the secondary signals.
The secondary electrical signals may be produced by one or more photodetectors, for example photomultipliers, which may be arranged to detect the intensity of a particular spectral region of light. In particular, it has been found that the blue phosphor component of a mixed phosphor has a considerably higher granularity than, for example green or red phosphors. Thus, at least a photodetector for detecting the intensity of the blue light from the CRT may be provided.
The photodetectors may be arranged to produce respective electrical signals corresponding to the intensity of the light emitted by respective phosphor components of the mixed phosphor. Conveniently, however, respective photodetectors may be provided corresponding to each of the conventional components of an RGB video signal, i.e. red, green and blue. In this way the processing of the secondary electrical signals with the primary electrical signals is simplified, as the primary electrical signal swill also generally be in the form of RGB components and thus a corresponding secondary signal is provided for each primary signal.
A convenient processing algorithm for the secondary and primary signals is to divide the primary signal by the magnitude of the secondary signal. This can be effected in the analogue or digital domain with suitable conversion of the primary and/or secondary signals as necessary.
In general, the primary and secondary electrical signals will be synchronised so that continuous processing of the signals can be achieved. Thus, corresponding primary and secondary electrical signals will be produced for each position of the flying spot on the CRT face and the primary signals will be modified according to the corresponding secondary signals.
The photodetectors for producing the secondary electrical signals may comprise a plurality of photodetectors for each colour component.
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Antonuk et al, “Thin-Film
Innovation TK Limited
Le Vu
McDonnell & Boehnen Hulbert & Berghoff
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