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.
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 xe2x80x9ceffective brightness.xe2x80x9d
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 U1, 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 U2 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 U2 increases, while U1 stays unchanged as the standard for the maximum intensity. Thus, the relationship U1/U2 provides information about the spectral condition of the emission.
The brightness to be regulated as a function of the intensity U1and the spectral shift of the emission xcex4xcex thus yields approximately
Ieff(xcex4xcex)=U1"PHgr"(U2/U1)
If the characteristic function "PHgr"is developed for the spectral sensitivity of the photographic paper according to powers from U2/U1, there is obtained:
Ieff(xcex4xcex)=U1(A+B(U2/U1) +C(U2/U1)2 +. . . )
with the paper-specific constants A, B, C.
In the simplest case it suffices to obtain
Ieff (xcex4xcex)≈U1 A+U2B=constant,
thus to measure with two sensors U1 and U2, 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 these temporal changes right upon being printed, obviating mechanical locks by constantly switching rapidly switchable light emitters on and off. Also in digital photographic mini-labs, as well as index print systems, printers in the medical field, etc., fast operational sequences and thus short switching times for image generator devices are becoming increasingly important, because corrections are becoming ever more necessary.
Generally it can be assumed that in all highly heated light sources, a spectral shift of emitted light takes place because of heating. If the heating of this device cannot be prevented by cooling or rapid heat removal, then correction of luminous intensity, based on measurement of spectral shift of the emitted light, is urgently needed.
In photographic printers, this can become a problem if various-color LEDs are used for printing negatives onto color photographic stock (paper). For one thing, these LEDs have an advantage in that they can be switched very rapidly. For another, they can be regulated directly in every color, so that color copier light generated by them can be adjusted directly to the spectral sensitivity of the photographic paper, without having to insert additional color filters into the ray path.
Fast-switch LEDs or laser diodes can also be used, such as the vacuum fluorescence printer head (VFPH) described in the European Patent No. EP 0 713 330 A1, in order to expose digital image data directly onto the photographic paper. In this case, the regulation according to the invention can be used to advantage.
Photographic paper, transparency screens or thermographic material are suitable as light-sensitive material for imaging digital pictures, along with all other materials which have light-sensitive color substances.
Light emitters, laser diodes, and all other normal light sources can also be used as exposure units in an image generator, whereby the light generated by the emitters or diodes is then modulated by means of a light modulator. Depending on the controls of the light modulator, an image is then generated, or the copier light for a negative to be copied is modified (for example, with fuzzy masking, color masking, or for compensation of color wobbles). In such image generators, the invention-specific exposure regulation can be done either through directly influencing the exposure unit orxe2x80x94if colors are to be exposed consecutivelyxe2x80x94by allowing for the spectral shift, and thus changing the effective brightness of the particular images to be exposed during light modulation with the light modulator.
Examples of light modulators such as LCDs, TFT-LCDs, PLZTs, FLCDs, or similar devices are used. All these light modulators have a disadvantage in that they use polarized light, thus leading to heavy light losses. Also, in part they switch too slowly for operation in today""s fast printers.
A DMD is superior because it switches very rapidly and does light transmission directly. It performs temporal modulation of the light by switching mirror units on and off pixel by pixel. The DMD""s electronic controls permit secure switching intervals to be pre-set. These can be added on in order to generate various density levels on the material to be exposed. To ensure uniform picture impression, it is necessary that at all equally long switching intervals, the same quantity of light be transferred to the material. Therefore, it is essential in such light modulators to keep effective brightness in the illuminator device constant. Thus, there is an urgent need to regulate the light source in accordance with the invention, based on the measurement of brightness fluctuations and spectral shift, if density fluctuations in the image are to be avoided.
The measuring device which determines the measured parameters for regulating brightness of an image generation device consists of several units. These units have sensors with varied spectral sensitivity. Photodiodes, transistors, CCDs, pyroelectric detectors or similar known detectors can be used as sensors.
In order to measure the different spectral components of a color, there must be at least two sensors present for each color. These sensors can either have varied spectral sensitivities; or measure varied spectral components of the light amount via color filters placed on the side closer to the radiation source; or be spatially separated, so that spatially spectrally separated light is measured at various locations. The light can be spectrally separated by prisms, partially translucent mirrors, grids or other optical components. It is also possible to measure the overall light of all colors to be regulated, using one sensor. With each second measuring unit per color, the long-wavelength or short-wavelength or some other spectral component of the light quantity of a color can be determined.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
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