Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-08-04
2004-03-30
Font, Frank G. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
Reexamination Certificate
active
06713754
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a digital image generator and to a method for exposing light-sensitive material. The image generator comprises a light source that emits light in at least one color, a control unit connected to the light source for regulating the quantity of emitted light and a light measuring device, connected to the control unit. The control unit and the light measuring device form a control circuit for the image generator.
The published European patent application No. EP-OS 0 691 568 describes a photographic illuminator in which LEDs are provided as light sources. This reference explains that brightness differences of individual LEDs, leading to local inhomogeneities of the light source, are to be corrected. The way this is done is that point-by-point, in three colors (red, blue and green), the light that penetrates through a developed, non-illuminated test negative, is measured during photographic printing. The photometry data thus determined are used to adjust the individual LEDs so that the negative to be printed is then exposed uniformly.
It has been shown, however, that such a single-test procedure is insufficient. The LEDs are subjected to brightness fluctuations before each switch-on process. These fluctuations are dependent on the switch-on duration, and can lead to great quality impairment in the copy. The aforementioned EP-OS 0 691 568 proposes to remedy this problem by having LED characteristic parameters adopted and put into memory, conveying the connection between switch-on duration and brightness fluctuations for various LED brightnesses. These parameters must then additionally be allowed for in LED controls. One such parameter must be adopted and placed in memory for each LED initial brightness and switch-on duration that appears.
The published German patent application No. DE 197 33 370 describes a digital illumination system in which a color picture is generated by means of an exposure unit and a pixel-by-pixel controllable light modulator. The image is then exposed as an index print on photographic paper. LEDs are used here for exposing the light modulator, since they have a long service life and can be switched very quickly. This obviates the requirement of a locking device, which is necessary with other light sources.
The published German patent application No. DE 197 46 224 also uses LEDs of various colors as an illumination device for a light modulator, here a DMD (digital micro-mirror device). The DMD is used here to generate a blurred mask, which thus is a modified copy of the pattern. With the mask, the pattern for copying photographic films is superimposed, to reduce large-area contrast in the copy.
High quality is currently demanded of copied color pictures. This makes it necessary to measure and compensate for all brightness fluctuations of the light source that depend on switch-on duration. The correction proposed in the published European patent application No. EP-OS 0 631 568, using characteristic parameters, is too costly for this purpose. Additionally, it has been determined that clearly visible density fluctuations appear in spite of compensation according to the EP-OS 0 631 568.
SUMMARY OF THE INVENTION
Therefore, the principal object of the present invention is to provide a device and method for exposing an image onto light-sensitive material so that the brightness of copier light can be sufficiently controlled to result in uniform exposure. Regulation is to be implemented in a simple manner so that all fluctuations in copier light are counterbalanced so that they do not break down in the copied image as density fluctuations.
This object, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by providing a device and method for measuring at least two different spectral components of the same exposure color and for calculating the control parameters from the measured values.
It can be shown that in light emitters such as LEDs or laser diodes, during their switch-on period, not only brightness changes, but also their emission bands are subjected to a spectral shift. This means that during an LED switch-on period, a change occurs in the range in which its emission spectrum overlaps with the spectral range containing the corresponding sensitivity maximum of the photographic paper. Even if the operating current is post-adjusted during the switch-on period, so that the brightness remains constant, this spectral shift manifests itself as density fluctuations in reproducing an image.
In red LEDs, this spectral shift can amount to 6 nm in the direction of greater wavelengths during the switch-on period. In exposing AGFA CN photographic paper to a GaAlAs LED, the emission spectrum, for example, shifts in the direction of the red sensitivity maximum. At the same time, the LED intensity drops.
If a uniform density distribution is to be achieved on the photographic paper, the LED must be regulated while allowing for both effects.
Only when the brightness as well as the wavelength shift of emitted light can be determined, and the light emitter can be regulated while allowing for the spectral position of the sensitivity maxima, can the light-sensitive material be exposed correctly. In what follows, this regulation is designated as regulation of the “effective brightness.”
In invention-specific terms, the device is connected to a measuring instrument. This instrument uses at least two detectors for an exposure color (for example, red, green or blue) to record the brightness of the emitted light in at least two different spectral components of this color.
The shift of the red LED (the other colors can be treated the same way) can be determined, for example, as follows:
A first, non-filtered detector measures detector voltage U
1
, which in a first approximation, is directly proportional to the overall LED intensity emitted in red. A second detector is equipped with a bulk glass filter, translucent only for the longest wavelength portion of the red emission spectrum. Thus the detector voltage U
2
is proportional to the emitted intensity on the long wave edge of the red spectrum. If, during the LED's switch-on process, the spectrum of the red LED is shifted to longer wavelengths, then detector signal U
2
increases, while U
1
stays unchanged as the standard for the maximum intensity. Thus, the relationship U
1
/U
2
provides information about the spectral condition of the emission.
The brightness to be regulated as a function of the intensity U
1
and the spectral shift of the emission &dgr;&lgr; thus yields approximately
I
eff
(&dgr;&lgr;)=
U
1
&PHgr;(
U
2
/U
1
)
If the characteristic function &PHgr;is developed for the spectral sensitivity of the photographic paper according to powers from U
2
/U
1
, there is obtained:
I
eff
(&dgr;&lgr;)=
U
1
(
A+B
(
U
2
/U
1
) +
C
(
U
2
/U
1
)
2
+. . . )
with the paper-specific constants A, B, C.
In the simplest case it suffices to obtain
I
eff
(&dgr;&lgr;)≈
U
1
A+U
2
B
=constant,
thus to measure with two sensors U
1
and U
2
, to attain a satisfactorily uniform density rise in illuminating the photographic paper.
Constants A and B must be predetermined for each possible paper type and stored in a lookup table (LUT).
To attain a still more precise regulation of the exposure amount, more than two sensors per color can be used. By means of the additional measured quantities, detailed changes in the emission bands, such as their widening, can then be documented and allowed for.
This regulation in accordance with sensitivity always allows for the current conditions. Therefore, compensation is made for automatic aging effects of the exposure device as well as environmental influences that affect its radiation behavior. Also, the lighting unit can be replaced without necessitating recalibration of the exposure regulator.
In particular, shortly after switching the light source on, spectral change is evident. Therefore it is essential to correct
Agfa-Gevaert Aktiengesellschaft
Font Frank G.
Lee Andrew H.
Milde & Hoffberg LLP
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