Photography – Sensitometry
Reexamination Certificate
2003-10-21
2004-10-26
Perkey, W. B. (Department: 2851)
Photography
Sensitometry
C355S077000, C356S404000, C430S030000, C430S359000
Reexamination Certificate
active
06810209
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method to establish a sensitometry curve for a photographic medium such as film. A sensitometry curve means a curve, a characteristic table, or a set of density values and exposure energies, which enable a medium exposure value to be linked to its optical density. The sensitometry curve is still called the Hurter-Driffield curve. Optical media, especially films, generally have a known sensitometry response. The film's response is an important datum for adjusting a number of cameras or devices taking the film. Among these, for example one can mention still cameras, development equipment, and film digitizing systems. The exact adjustment of these devices, according to the film's response, enables the restoration, at the output of a processing chain, of images reproducing, as faithfully as possible, the scenes taken.
The sensitometry response of a photographic medium is sensitive to parameters like the manufacturing processes, the conditions, and storage duration of the medium. It can also vary in time and its prior knowledge can turn out to be inaccurate at the time the medium is processed. This difficulty can be overcome by establishing for each photographic medium a specific sensitometry curve that allows for its aging. The aging is allowed for both before and after development.
The invention has applications for all types of photographic media and, especially, photographic papers and films. While not being reserved solely for the field of the professional image, the invention mainly aims to establish sensitometry curves for films used in motion picture cameras.
BACKGROUND OF THE INVENTION
To establish the sensitometry curve of a photographic medium, a sensitometry control is formed on a reserved part of the medium. Sensitometry controls generally comprise one or more ranges that are exposed with various exposure energies. These energies are known and carefully calibrated. Sensitometry controls comprise, for example, 21 ranges, subject respectively to various energy exposures, but uniform for each range.
In a motion picture camera, a series of 21 consecutive views can be exposed, taken in a leader part of a film, with increasing calibrated energies.
The sensitometry curve can easily be established by measuring the optical density in each range of the sensitometry control and by associating to these measurements the values of the exposure energies. The establishment of the sensitometry curve can be limited to the simple collection of the measurements, associated with their exposure values, or possibly be represented in graph form. The representation is generally produced as a logarithmic scale.
The accuracy of the sensitometry curve depends on the quality of the density measurements and the accuracy of the exposure of the various ranges of the sensitometry controls. In so far as the equipment used to form the controls and their reading is perfectly calibrated, the establishment of the sensitometry curve is not especially difficult.
Devices for forming the sensitometry controls with perfectly known exposure energies are however costly. Moreover, when many different cameras are liable to be used to produce the shots, it is necessary to make uniform the sensitometry controls produced by the exposure equipment of the various cameras. Thus the cameras must be provided with calibrated standardized exposure means.
These difficulties are obstacles to establishing and automatically allowing for a film's sensitometry response.
SUMMARY OF THE INVENTION
The goal of the invention is to propose a method for establishing the sensitometry curve of a medium that enables the difficulties mentioned above to be obviated.
One goal in particular is to propose such a method that does not require an accurately calibrated exposure means for forming sensitometry controls.
One goal is also to propose a method enabling a reliable sensitometry curve to be obtained despite having especially rudimentary equipment on board the camera.
One goal is finally to propose such a method that enables the area of the sensitometry control to be limited to a smaller area of the photographic medium.
To achieve these goals the object of the invention is more precisely a method for establishing the sensitometry curve for a photographic medium, the method comprising: the formation on the medium of at least one sensitometry control by exposing many ranges of the medium with various exposure energies, the exposure energy of each range being modulated according to a spatial modulation profile (P(x)) identical for all the ranges; the capture of optical density values of the sensitometry control in each range and in regions corresponding to various values of the modulation profile; the formation of sensitometry curve sections, each section being formed from density values captured in various ranges of the sensitometry controls, but in regions corresponding to the same value of the modulation profile of the exposure energies; and the energy offset of the curve sections to obtain partial section overlapping corresponding to neighboring exposure energies.
In the sense of the invention, the sensitometry curve is considered, independently from its graphic representation, as a means enabling the optical densities to be linked to the exposure energies of a medium. It may be summarized as a table or a simple collection of numerical values linking the optical density of the medium to the exposure energy received by the latter.
At the time of the exposure of the sensitometry control, the value of the exposure energy supplied in each range is not known with any great accuracy. The uncertainty about the exposure energies originates essentially from the uniformity defects of the exposure light sources liable to equip the cameras, and in the inaccuracy of their calibration. When the exposure means are rudimentary, the uncertainty about the exposure energies can be significant.
In a preferred implementation of the invention method, the exposure energies of the various ranges can follow a regular or not determined progression. In addition, the progression can take place with reference to a known or not energy value. While the regularity or exact knowledge of the progression of the energies is an advantage, it is not essential. This aspect will be re-examined in the description that follows. The progression of the exposure energies can be increasing or decreasing.
The lack of sure information as to the real value of the exposure energies received by the photographic medium is somewhat compensated for by the sure information according to which the modulation profiles of the various ranges are identical. In this way, by energy offsetting the curve sections, according to the invention, one can combine the information coming from the various regions of the exposure ranges, for the various modulation values of the exposure energy. This combination enables a continuous sensitometry curve to be obtained.
It should be noted that after the energy offset of the curve sections, and obtaining a continuous sensitometry curve, this curve can again be assigned with a global energy error. This global error results from the absence of an absolute energy reference for at least one of the sections. The global energy error of the sensitometry curve is not however prejudicial to its use. In fact it does not affect the essential characteristics of the curve, such as its slope and inflexions.
The formation of sections under step c) may be done in graph form. However, it preferably comprises the association with each density value, of an exposure energy value estimated according to the range of the sensitometry control in which the density value is captured, and in addition, the formation of density value sets, each set containing respectively the optical density values captured in the various ranges of the sensitometry control but in regions corresponding to the same value (P) of the modulation profile. Thus, step d) of the method can comprise simply the uniform offset of all the energy values of
Close Thomas H.
Eastman Kodak Company
Perkey W. B.
LandOfFree
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