Facsimile and static presentation processing – Natural color facsimile – Measuring – testing – and calibrating
Reexamination Certificate
2000-07-11
2001-08-07
Grant, II, Jerome (Department: 2624)
Facsimile and static presentation processing
Natural color facsimile
Measuring, testing, and calibrating
C358S520000
Reexamination Certificate
active
06271940
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for digitising colour negative pictures on an image carrier and for transforming the image signals in order to make them proportional to the spectral intensities of the photographed scene. Said method is intended for use in electronic colour scanners and pre-press systems.
DESCRIPTION OF THE PRIOR ART
It is known in conventional photography to make an imagewise reproduction of a scene, obtaining thereby a positive print. On the basis of the positive print, digital colour values can be generated that are suited for generating an image on a display unit or on a reproduction. Said digital colour values for the colour image on the print are obtained by the steps of:
separating the colour image in a number of colour components
converting every colour component in electrical colour values by means of a colour scanner;
transforming the electrical colour values in digital colour values by means of a transformation for obtaining a reproduction of true colour of the print; and
recording digital colour values on an image carrier suited for the reproduction process selected.
A digitiser is known to convert by means of a digitisation an optical image into electric signals giving each individual signal an indication of the optical density of a very limited part (pixel) of the carrier. On colour images, distinct spectral densities are distinguished that are defining the colour components in which the colour image is being analyzed. The expression “digitisation” in the present context refers to the conversion of an optical image into electric digital image signals. As a result, image data in electronic form is generated, e.g. in the way described herein below.
A colour image is exposed to any illuminant element emitting e.g. white light. The so generated light image resulting from the reflection or transmission of the emitted light is presented, possibly after directing it through a colour filter, pixel for pixel to a photosensitive member, which converts the pixel intensity of the light image in e.g. an analogue electric signal that is sampled and, possibly after amplification, presented to an ADC (analogue-to-digital converter) generating on the basis of the analogue electric signal a digital electric image signal, said image signal being representative of the light image presented. Said image signal in digital form is suited for being transferred by means of any transmission means, e.g. an electrical or optical cable, to a device arranged for storing the image signals in a volatile (random access memory) or fixed (disk) storage means. The image signals may be retrieved later on and utilised for controlling an image recording or reproduction system that converts the image in electronic form into optical density variations on an image carrier thus making it again available for visual perception.
An electrical colour value is each of the analogue or digital electric signals that constitutes, together with a number of similar signals, the representation of an image. Said similar signals may be applied successively and evenly spaced in time and vary continuously on an electric conductor, such as a coaxial cable in analogue video applications, alternatively they may be digital. The moment at which the image signal will be applied commonly determines the position of the pixel on the carrier said image signal is corresponding to. In many cases the voltage amplitude of said image signal is proportional to the optical density at the corresponding location on the carrier. The image signal may be stored in digital form in a memory unit. For an image signal in digital form, commonly eight storage elements are used, each of which may represent a zero-state and a one-state, so that to every digital image signal 256 discrete values may be assigned. For particular applications, especially if the dynamic range or optical density range of an image to be digitised is uncertain, it is advantageous to utilise twelve or thirteen bits per image signal, increasing thereby the number of discrete values up to 4096 and 8192 respectively. To each of these discrete values a certain optical density for image processing and recording may be assigned, allowing an optimum visual perceptibility and aesthetic appearance of the image on a carrier on which it is to be recorded. If the image involved is a colour image, several image signals will be generated for every location or pixel on the carrier. The colour signal from a pixel on the image is commonly separated in three colour components, i.e. red, green and blue (RGB), either by exposing the image successively to the above-mentioned colours and sampling the electric signals of a photosensitive member, on which the light impinges, or by exposing the image to white light with a broad spectrum and filtering the light reflected or transmitted by the image through a red, green or blue filter, before it impinges on a photosensitive member. Usually, for every pixel three colour signals are generated, i.e. red, green and blue. Conversion into electrical colour values is referring to the recording and short-term storage of, e.g., an analogue electric signal generated by a photosensitive member. If but a single photosensitive member is present (occasionally one for each of the colour components), as in the case of conventional drum scanners, said photosensitive member shifts its position with respect to the colour image, so that at a given moment every basic picture element, i.e. the pixel, is emitting light to said photosensitive member. At that instant, the electric signal generated by the member is amplified with a gain G and compared to a reference voltage V
0
. If need be, the amplification may proceed on a non-linear way for:
providing an analogue signal of which equal level differences correspond to non-equal optical density differences on the carrier, however, e.g. to equal perceptibility differences
purposefully performing a transformation on the analogue electrical colour values provided by the photosensitive member.
The so obtained new electric signal is sampled and made available in digital form to a storage means, where it will be stored for further processing. A colour scanner is a digitiser arranged for separating light into two or more spectral bands, allowing a colour image to be divided into as many colour components as there are spectral bands. Preferably, the spectral bands are selected in a way that the colour image may be reconstituted by combining the colour components. A colour component of the image is the full image obtained by making the light travel along the same path as on the colour scanner. If, e.g., the image is exposed to light passing through a red filter for obtaining the red image signals, the red colour component is produced. The term colour components in the present context refers to scanner-related colour components. As described above, the colour components or the spectral bands of a pixel are obtained by means of colour filters or different colour exposures. In U.S. Pat. No. 5,157,516 several systems are disclosed. Converting electrical colour values into digital colour values by means of a transformation may be effected:
by non-linear amplification of the analogue signals as described above;
by modification of the numerical value of the digitised image signals; or
by a combination of both processes.
The electrical colour values may be analogue or digital. The expression digital colour values as used in the present invention refers to the colour values corrected by a transformation.
Positive colour images are currently digitised by means of a colour scanner. A positive colour image is a reproduction of more or less true colour of any real scene that has been recorded, e.g. photographically, at an earlier time. A positive colour image may be provided on a transparent carrier such as in the case of transparencies. A colour photograph is an alternative form of a positive colour image, mainly on an opaque carrier, e.g. paper. For a colour photograph usually a col
Deschuytere Frank
Tuijn Chris
AGFA-GEVAERT
Grant II Jerome
Kelley Edward L.
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