GB2060288A - Control circuit for flash discharge lamp - Google Patents

Abstract

A discharge lamp control system for use in a copier or camera includes a variable power supply (18) for providing energy to a discharge lamp (13). The power supply comprises a plurality of capacitors (C1-CN) connected to be charged in parallel and with their outputs selectively connectable through switches (S1-SN) to the discharge lamp. The appropriate capacitor(s) are discharged through the lamp in response to processor (20) which either predicts, or calculates in real time, the required exposure level at an object plane. The processor is responsive to a sensor (19) receiving light from a separate pre-flash lamp or from the main lamp (13) when supplied only from a single capacitor. <IMAGE>

Description

SPECIFICATION Discharge lamp control system The present invention relates to a discharge lamp control system and more particularly to a discharge lamp control circuit which provides energy to a lamp in the precise amount required to reach a desired exposure level.

The power to a flash unit is typically provided by charging a capacitor or series of capacitors to a desired reference voltage; the capacitor(s) is or are then discharged through the lamp and associated discharge circuitry creating the flash illumination.

The lamp is typically extinguished by quenching when a preset level is reached.

Since the density of the object(s) being reproduced may differ between exposures, some form of exposure control must be used. Some typical prior art efforts are disclosed in U S Patents 3 585 442, 3 871 761 and 4093 376. These systems are not completely efficient because some energy is wasted in the iamp-quenching circuit.

The present invention is directed towards a flash illumination system which energizes a discharge lamp with a precise amount of energy required for each exposure. The present invention provides a discharge lamp control system as claimed in the appended claims.

Specific embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a block diagram of the present invention in a copier flash illumination environment; Figure 2 is a schematic of a first embodiment of the circuitry of the invention; Figure 3 is a schematic of a second embodiment of the invention, and Figure 4 is a block diagram of the present invention in a flash fuser environment.

The invention as described below is directed to a variable output power supply which powers the flash illumination unit for exposing documents in a full frame copier. It should be understood, however, that the present invention is applicable to other types of devices which utilize flash illumination of an object with exposure of a lightor thermally-sensitive material. This would include other types of copying systems, microfilm and microfiche reprographic machines, and cameras which require automatic and continuous exposure control in response to variations in the reflectivity of objects being photographed. In addition, as described in further detail below, the invention can also be used to flash fuse the developed images formed on a photoconductive surface.

Referring to Figure 1, an object plane 10 is provided which supports a document 12. A discharge lamp 13, energized as hereinafter explained, is adapted to provide illumination of the document. The reflected light from the document is projected through lens 1 5 and exposes photoconductor image plane 1 6 producing a latent image pattern which can be subsequently developed. The energy which powers lamp 13 is supplied by variable output power supply 1 8.

In a first embodiment, an auxiliary low energy level pre-flash lamp 1 7 is caused to flash after the document 12 is placed on the platen but a few milliseconds before main lamp 1 3 flashes. The irradiance produced by this pre-flash is measured by photosensor 19 and converted from an analog to a digital value by processor 20. Processor 20 then predicts the exposure level required for document 12 and sends a signal to power supply 18 programming it to release only that amount of energy into lamp 13.

Upon initiation of a print command, a trigger pulse energizes lamp 13 causing it to flash. The energy supplied to the lamp is dependent on the calculated value determined by the preflash lamp 17 and processor 20. An image of document 12 is projected through lens 15 selectively discharging portions of the photoconductive image plane 1 6 and forming a latent image of the document thereon.

Referring now to Figure 2, the functional components of circuits 1 8 and 20 are shown in schematic form. Variable output power supply 1 8 comprises a series of capacitors C1-CN connected in parallel. The capacitance values are selected to provide energy in powers of XN" where N would ordinarily be a value of 2 for the system to operate at maximum efficiency. For example, if X = 1 joule, N = 2,n = 0,1,2,3 ...n then the capacitor energy levels for C1 C7 equals 1, 2, 4, 8, 16, 32, 64 joules and resolution accuracy would be within one joule. Of course, other values of X, N and n may be selected depending upon system requirements.

The capacitors are initially fully charged from a conventional DC power supply source 22. They are connected, optionally, through an inductor 24, across the electrodes of conventional xenon discharge lamp 1 3 upon closure of associated normally-open switches S1-SN.

Circuit 20 comprises an integrator circuit 28, a conventional analog-to-digital convertor 30 whose output is connected to digital processor 32.

Processor 32 generates a series of outputs a, b, ...n which are connected by circuitry (not shown) to switches SI-SN causing these switches to be selectively activated (closed).

In operation, photosensor 1 9 generates an output signal I (t) whch has a magnitude directly related to the intensity of the impinging light from pre-flash lamp 1 7 on photoreceptor 16; i.e. to document exposure. This analog signal is integrated in integrator 28, and converted into a digital signal, in A/D converter 30. The digital signal is compared with previously-defined values stored within the processor housing. When the exact exposure value is reached, processor 32 correlates these values to one or more capacitors which will provide the required energy to achieve the values. As an example, if the required exposure level is 75 joules, processor 32 will generate signals a, b, d and g closing switches S1, S2, S4, S7.In real time sequencing, receipt of a triggering voltage across winding 36 of lamp 13 ionizes the gas within the lamp lowering its resistance.

Switches S1, S2, S4 and S9 were selected at presence and closed, allowing the energy stored in capacitors C1, C2, C4 and C7 to be discharged through the lamp producing a flash of light of the exact energy required to expose the particular document. It may be noted here that lamp efficiency will vary with respect to the instant power level. the processor can be programmed to factor in this variance.

A second way of determing the required energy levels is provided by the circuit shown in Figure 3.

In this embodiment, pre-flash lamp 1 7 is omitted and real time sensing is accomplished by processor 32'. Variable output power supply 22 again contains a series of capacitors C1-CN connected to lamp 13 in the above-described manner, except that switch S1 is replaced by diode Dl. The capacitance values are now arranged in a different manner according to the invention. Capacitor C1 is initially charged with energy approximately equal to or less than the minimum expected energy needed for exposure of the document, e.g. 70 joules. Capacitors C2 CN then assume the previously defined logical progression, i.e. C1 joule, C2 = 2 joules C4 = 4 joules, etc.

In operation, lamp 13 is triggered and capacitor C1 is discharged automatically via diode D-l by the processor 32'. Photodetector 19 generates an analog signal l(t) which is integrated by integrator 40, converted to a digital signal by A/D converter 42 and sent to processor 32'. Processor 32' compares this signal with reference sets of exposure values stored within a memory unit.

These values are correlated to increments of time elapsed since flash inception. As a result, within a fiew microseconds of flash initiation, processor 32' will relate the incoming integrated signals to a particular set of exposure values, predict the total exposure required for the document, and select those additional capacitors for actuation which will supply the energy required in excess of the original amount. As an example, if the processor determines an additional energy level of 1 0 joules will be required, actuating signals will be sent to switches S3 and S5 causing them to close and allow capacitors C3 and C5 to discharge through lamp 13.

Although the above embodiments have described applicability to a flash exposure system, the techniques are also appliable to flash fusing of an image pattern. Referring to Figure 4, a fuser station, generally designated as 50, is provided with a discharge lamp 52 mounted in the cavity formed by curved reflector 54. Passing beneath this flash arrangement in the indicated direction is a copy matrial support surface 56 having on a portion of its surface, a toner image 58. Reflector 54 is shaped so that each flash from lamp 52 will irradiate the entire toner image 58. Pre-flash lamp 60 and photodetector 62 operate in the same manner as lamp 17 and photodetector 19 in the Figure 1 exposure embodiment. Power supply 70 and processor 80 operate in the manner described in the Figure 2 embodiment.The values of the capacitors in power supply 70 will have higher values because of the higher energy range (500-800 joules) required for flash fusing. A typical capacitance sequence would therefore be C1 = 25, C2 = 50, C3 = 100, C4 = 200, C5 = 400 Joules. For a pre-detected energy level of 525 joules, capacitors C1, C3 and C5 would be activated in the pre-described manner.

The fusing operation can also be achieved by using the real time sensing mode described in connection with Figure 3.

The digital processor described above may be tailored for the specific purposes set forth herein or designed into a system processor which can simultaneously perform other tasks. Design of these processors are well known in the art. The pre-detect embodiment may be accomplished using other means for providing pre-flash illumination without departing from the invention.

For example, a continuous lamp with a shutter arrangement may be used or the main flash lamp may be adapted to pre-flash at a lower energy level.

It is apparent from the above description that the flash illumination system of the present invention is highly efficient because the charge supplied to the flash lamp is precisely the amount required to expose the particular object or document to be reproduced.

Claims (10)

1. A discharge lamp control system comprising a discharge lamp for illuminating an object at an object plane; a variable output power supply connected to said lamp, said supply including a plurality of capacitors, each storing a discrete unit of electrical energy, and a plurality of switches connected between the capacitors and the lamp; and means for calculating the precise amount of energy required to obtain desired exposure of said object at an exposure plane and for closing only those switches associated with capacitors whose combined stored energy equals the calculated energy, whereby the lamp is supplied with precisely the amount of energy required to obtain the required exposure.

2. The system of claim 1, wherein said capacitors are adapted to provide energy in units which increase, according to the relationship XN", where X is a value in joules, N is a base unit and n is the power to which the base is raised.

3. The system of claim 1 , wherein one of said capacitors provides a first unit of energy equal to a pre-defined value and the remaining capacitors are adapted to provide energy in values which increase according to the relationship XN", where X is a value in joules, N is a base unit and n is the power to which the base unit is raised.

4. The system of claim 2 or 3, wherein N is equal to 2.

5. The system of any preceding claim, in which the calculating means includes; a second discharge lamp adapted to flash and illuminate said object at a time preceding illumination by said first lamp; a photosensor for detecting light reflected from said object as a result of the preliminary flash, said photosensor generating an analog signal having a magnitude directly related to the intensity of the detected light; an integrating circuit connected to said photosensor and adapted to generate a time integral output signal of said analog signal; an analog-to-digital converter connected to said integrating circuit and adapted to convert said integrated analog signal into a digital signal;; a digital processor connected between said variable power supply and said converter, said processor including means for comparing said digital integrated signal with previously stored signals, each stored signal corresponding to a specific energy value represented by a group of capacitors, and for selecting that value which corresponds to the integrated digital signal, and further including means for selectively activating switches associated with the selected capacitor group and means for triggering said first flash lamp into conduction whereby said pre-selected capacitor group is discharged through said activated switches through said first lamp providing the precise amount of energy required to expose said object at the required level.

6. The system of claim 5, wherein said processor further includes means for compensating for changes in lamp efficiency at instantaneous power loading levels.

7. The system of any of claims 1 to 4, wherein a first capacitor is connected so as to provide an initial discharge of energy into the lamp at time of triggering, said calculating means including: a photosensor for detecting light reflected from said object following said flash initiation, said photosensor generating an analog signal having a magnitude directly related to the intensity of the detected light; an integrating circuit connected to said photosensor and adapted to generate time integral output signals of said analog signal; an analog-to-digital converter connected to said integrating circuit and adapted to convert said integrated analog signals into digital signals;; a digital processor connected between said variable power supply and said converter, said processor including means for comparing said digital integrated signals with previously stored signals representing exposure values at differing post-flash time intervals and relating to a particular exposure sequence to the incoming integrated signals and further including means for selectively activating switches associated with those capacitors whose energy, when added to that supplied by said first capacitor/switch, provides the precise amount of energy required by the lamp to effect correct exposure.

8. A system according to claim 7, wherein said first capacitor stores a predetermined amount of energy approximately equal to the minimum expected energy required for exposure of the object.

9. A system according to claim 7 or 8, wherein the remaining capacitors are adapted to provide energy in values which increase according to the relationship XN", where X is a value in joules, N is a base unit and n is the power to which the base unit is raised.

10. A flash illumination system substantially as hereinbefore described with reference to Figures 1-4 of the accompanying drawings.