1. Field of the Invention
This invention relates to a method of driving a light emitting element array such as an organic EL (electroluminescent) element array.
2. Description of the Related Art
Conventionally, there has been known an exposure system which comprises a light emitting element array comprising a plurality of two-dimensionally arranged light emitting elements, such as organic EL elements, and a drive circuit which controls the light emitting time (light emitting pulse width) of each of the light emitting elements on the basis of an image data carrying thereon a gradation image and exposes a photosensitive material to an image formed on the light emitting element array on the basis of the image data. An example of such an exposure system is disclosed in U.S. patent Laid-Open No. 20010052926.
As a representative of the light emitting element array, there have been known those of a system so-called a simple matrix system where a light emitting element is formed at each intersection of anodes and cathodes disposed in a two-dimensional matrix and is driven with the anodes and cathodes employed as the scanning electrodes and the signal electrodes.
In the light emitting element array of the simple matrix system, one of the plurality of cathodes which are employed, for instance, as the scanning electrodes is connected to a grounding terminal in sequence to be provided with a ground potential and the plurality of anodes as the signal electrodes are selectively connected to a power source on the basis of image data. With this arrangement, current supplies to the light emitting elements formed at the intersections of one cathode and a plurality of anodes are controlled independently of each other, and emission and non-emission of the light emitting elements are controlled. This state is created in sequence for each cathode with selection and scan of the cathodes, and a two-dimensional image is formed on the light emitting element array. The photosensitive material can be exposed to a two-dimensional image by projecting the image onto the photosensitive material through an imaging optical system.
As the driving systems of such light emitting element arrays, there have been known a constant-voltage drive system where each of the light emitting elements is applied with a constant voltage and a constant-current drive system where each of the light emitting elements is applied with a constant current. The former is excellent in response but is poor in stability due to drop and fluctuation of the forward voltage by change with time of the environment of use or each light emitting element. Whereas the latter is substantially linear in the light emitting intensity versus the drive current, and excellent in stability. Accordingly, recently, the organic EL element array often employs the constant-current drive system.
However, when the constant-current drive system is employed for the light emitting element array of the simple matrix system, there has been known a problem that the rise-up characteristics are bad. The problem will be described in detail, hereinbelow.
The light emitting elements formed at the intersections of the anode and the cathode can be considered as elements comprising a light emitting portion having diode characteristics and a parasitic capacity connected in parallel to the light emitting portion. When such a light emitting element array is driven in the constant-current drive, supposing that the cathodes function as the scanning electrodes, the current should be supplied only to a light emitting element (a selected light emitting element) at the intersection of the selected cathode and the anode out of a plurality of light emitting elements formed on the anode. However due to existence of the parasitic capacity described above, all capacities of the light emitting elements formed on the anode are charged with the constant current when the scanning electrodes are switched and accordingly, it requires a long time for the light emitting element to emit light after its capacity is charged, which deteriorates the rise-up characteristics.
In view of this problem, there has been proposed, in U.S. Pat. No. 5,844,368, a method of improving the rise-up characteristics by providing a period for which the anode and the cathodes are kept at the same potential upon switching of the scanning electrodes so that the parasitic capacity of the selected light emitting element is charged to the cathode off voltage (the anode drive voltage-light emission threshold voltage: generally the anode drive voltage) through the parasitic capacities of the non-selected light emitting elements upon initiation of drive.
Further, there has been proposed, in U.S. Pat. No. 6,201,520, a method of improving the rise-up characteristics by providing a period for which all the anodes and the cathodes are short-circuited to the cathode off voltage source and by switching only the selected scanning electrode to the GND after the period to avoid the charge of the parasitic capacity of the selected light emitting element upon initiation of drive.
In the two approaches, the rise-up time is minimized when the cathode off voltage is equal to the anode voltage of the selected element upon drive. However the anode voltage of each light emitting element upon the constant-current drive is not always constant and the anode voltages of the light emitting elements in one array fluctuate according to the initial difference and/or the change with time of the light emitting elements. Further, the anode voltage can fluctuate according to the temperature conditions. Such fluctuation and/or difference of the anode voltage lead to fluctuation of the rise-up time.
When the cathode off voltage is higher than the anode voltage of the selected element upon drive, the light emission of the light emitting element is increased above the normal value for a while after initiation of constant current drive, whereas when the cathode off voltage is lower than the anode voltage of the selected element upon drive, the light emission of the light emitting element is reduced below the normal value for a while after initiation of constant current drive. Though the fluctuation in the light emission does not give rise to a significant problem when the light emitting element array is used as a display means, the fluctuation in the light emission causes deterioration of the quality the exposed image when the light emitting element array is used as an exposure head. Especially, the difference in the light emission between the elements causes a scoring unevenness extending in a sub-scanning direction to greatly deteriorate the quality of the exposed image when the exposure head and the photosensitive material are moved in the sub-scanning direction (sub-scanning).