Process for recording images

Technical area

The invention relates to an image recording process for producing high resolution permanent electrostatic images on an electrostatic information recording medium.

In the prior art, a method for recording and reproducing electrostatic images is known, in which the "image exposure" is carried out by applying a voltage between the electrodes of a photosensitive section and an electrostatic information recording medium, which are arranged opposite to each other, wherein a durable electrostatic image having a high resolving power is provided on the electrostatic information recording medium (US-A-3 598 579, US-A-4 628 017).

Such an electrostatic image recording process is shown in FIG. 1 2a, an electrode of the photosensitive portion at 2b, a photoconductive layer at 2c, an insulating layer at 1a, an electrode of the electrostatic information recording medium at 1b, where an electrostatic information recording medium 1, a photosensitive portion 2, a support for a photoconductive layer , a support for the insulating layer at 1c and a current source at E are shown.

Referring to FIG. 1, the exposure is performed by means of the photosensitive section 2. The photosensitive portion 2 is constructed so as to support the translucent electrode 2b formed of a 0.1 μm-thick ITO on the support 2a made of 1 mm-thick glass. It further consists of the photoconductive layer 2c having a thickness of about 10 μm on the electrode 2b. The electrostatic information recording medium 1 is disposed opposite to the photosensitive portion 2 through a gap of about 10 μm. The electrostatic information recording medium is formed by vapor-depositing the Al electrode 1b in a thickness of 0.1 μm on the support of the insulating layer and through the insulating layer 1a in a thickness of 10 μm on the electrode 1b.

As shown in Fig. 1a, the electrostatic information recording medium 1 is first positioned so that there is a gap of about 10 μm with respect to the photosensitive portion 2.

Subsequently, a voltage is applied between the electrodes 2b and 1b by the voltage source E, as shown in FIG. In the dark, no change will take place between both electrodes, since the light guide 2c has a high resistance. However, when a voltage higher than Paschen's discharge voltage is applied to the gap, a discharge through the gap occurs depending on the magnitude of the applied voltage or leakage currents of the substrate electrodes, so that electrostatic charges corresponding to the discharge are generated on the electrostatic information recording medium. When the light-conducting layer 2c is irradiated with light incident from the support 2a of the photoconductive layer, it generates photocarriers (electrons, holes) in the irradiated area, and charges whose polarity is opposite to the electrode of the electrostatic information-recording medium migrate through the light-conducting layer 2c to the surface thereof. When the ratio of the voltage applied to the air gap exceeds the discharge voltage after Paschen's, a corona discharge or field emission occurs between the photoconductive layer 2c and the insulating layer 1a, so that the charges are pulled out from the photoconductive layer 2c and accelerated by the electric field can, resulting in a charge accumulation on the insulating layer 1a.

After the end of the exposure, the photosensitive portion and the electrostatic information recording medium are short-circuited as shown in Fig. 1c. Although the shutdown of the power supply is described by the open switch, it is noted that this can also be achieved by shorting the two electrodes. Subsequently, the electrostatic information recording medium 1 is removed, as shown in FIG. 1d to complete the fabrication of a durable electrostatic image. By switching off the applied voltage in this way or, in other words, by using a "voltage lock", it is possible to produce a durable electrostatic image; it is possible to dispense with a mechanical or optical shutter as with ordinary cameras.

The photoconductive layer 2c is an electroconductive layer which, when irradiated with light, generates light carriers (electrons, positive holes) in the irradiated region, whereby the carriers can move in the width direction. This layer can be formed from inorganic or organic photoconductive materials or their mixed forms.

The inorganic photoconductive materials used may include amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide, etc.

The organic photoconductive materials used are classified into single-layer types and function-separated types.

The single-layer type photosensitive material comprises a mixture of a charge-generating substance with a charge-transporting substance. Since the charge-generating type of the substances tends to absorb light and generate charges, the following substances can be used, for example: azo pigments, bis-azo pigments, trisazo pigments, phthalocyanine pigments, perylene pigments, pyrylium Dyes, cyanine dyes and methine dyes. For example, as charge transport type substances, the following substances are well suited for the transport of ionized charges: hydrazones, pyrazolines, polyvinyl carbazoles, carbazoles, stilbenes, anthracenes, naphthalenes, triphenylmethanes, azines, amines, and aromatic amines.

With respect to the function-separated type of the photosensitive material, the charge-generating substance tends to absorb light, but has the property of trapping photocarriers, whereas the charge-transporting substance is well suited for charge transport, but less capable of absorbing light. For this reason, both substances are separated from each other to make greater use of their individual properties. For use, the charge generating layers and the charge transport layers may be laminated, that is, laminated. As substances constituting the charge-generating layer, there may be used, for example, azo pigments, bis-azo pigments, trisazo pigments, phthalocyanine pigments, xanthic acid dyes, cyanine dyes, styryl dyes, pyrylium dyes, Perylene dyes, methine dyes, a-Se, a-si-, azulene-in-salt pigments, and squalene-salt pigments. Examples of suitable substances for the preparation of the charge transport layer are: hydrazones, pyrazolines, PVKs, carbazoles, oxazoles, triazoles, aromatic amines, amines, triphenylmethanes and polycyclic aromatic mixtures.

As for the nature of the carriers produced, it is known that in the case of the inorganic photosensitive material, the mobility μ is high but the lifetime -r is short, whereas in the case of the organic photosensitive material, the mobility μ is small but the lifetime is -r long , where the product Irr is approximately equal in both cases. The formation of a permanent electrostatic image by the "exposure by applying a voltage" can also be achieved by a mechanical exposure shutter or a voltage shutter alone. However, with the mechanical shutter alone, the voltage remains impressed between the photosensitive material and the electrostatic information recording medium. In turn, this leads to the problem that dark currents flow even when no exposure is performed, resulting in a dark potential.

On the other hand, if only the voltage shutter with the photosensitive material is used, there arises the problem that the size of the recording and the size of the charges change with the shutter speed of the shutter. This will be explained in detail with reference to FIG.

Fig. 2 is a graph showing the amount of charges on the electrostatic information recording medium at a constant light intensity but at different voltage lock times, namely, 0.01 second, 0.1 second, and 1 second. In the case of the inorganic photosensitive material having a high carrier mobility, the amount of charges corresponds to the amount of exposure even at different voltage-lock times, as shown by the characteristic curve A. On the other hand, from the use of the organic photosensitive material, the phenomenon that even in the same size of the exposure, there is a difference in the size of the charges between the voltage lock times of 0.01 second and 0.1 second, and 0.1 second and 1 second, as can be seen from the characteristic curve B. This is due to the lower carrier mobility of the organic photosensitive material. The carriers generated by the exposure disappear because the voltage is turned off before reaching the charge-carrying medium. Therefore, there is a problem that the image potential is different even with the same amount of exposure depending on the voltage-lock time.

When the photosensitive portion and the electrostatic information recording medium are short-circuited, as shown in Fig. 3, to turn off the power source, an increased inverse voltage is induced between the photosensitive portion and the charge carrying medium, resulting in discharge in the opposite direction. This will now be explained in detail with reference to FIGS. 4 and 5.

The photosensitive portion, the gap and the electrostatic information recording medium are all considered to be capacitors each having a certain capacity, and if the photosensitive portion and the electrostatic information recording medium have the same thickness, dielectric constant and area, they will have the same electrostatic capacity. With a gap of about 12-13μm between the photosensitive portion and the electrostatic recording medium, the discharge voltage in the gap will be on the order of about 400V. As an example, it is assumed that the exposure is performed by the application of voltage at an applied voltage of 2000V. Subsequently, the photosensitive portion is rendered electrically conductive in the light-exposed area. Consequently, the entire "image exposure system" can be considered as an equivalent circuit in which 400V and 1600V, respectively, are applied to the capacitors C2 and C3 of the gap and the electrostatic information recording medium, respectively, as shown in Fig. 4a. Similarly, the unexposed area can be taken as an equivalent circuit in which 800 V, 400 V and 800 V, respectively, to the capacitors C1, C2 and C3 of the photosensitive section, the gap and of the electrostatic information recording medium, as shown in Fig. 4b.

The potential distributions on the photosensitive portion and the electrostatic information recording medium are considered. For example, if the photosensitive section electrode is defined as a reference layer having a point P representing the end position of the gap, a point Q representing the end position of the gap, and a point R representing the end position of the charge-carrying medium, then the potential distributions are on the exposed and unexposed areas by PQR in Fig. 5a and PQR in Fig. 5b. This is because the photosensitive portion is an electrical conductor.

When the photosensitive portion and the charge-carrying medium are short-circuited in a state as shown in Fig. 5a, the point R is reduced to the potential 0 or to a point R ', and the point Q is the same potential difference to a point Q ', giving a potential distribution P-Q'-R'. Accordingly, there is a potential difference between P and Q ', that is, a voltage applied to the gap results in 1600 V.

This is also apparent from Fig. 5b; a potential difference between P and 0 ', that is, a voltage applied to the gap becomes 1600 V.

Consequently, the voltages applied to the respective capacitors change from the state of Figs. 4a and 4b to Figs. 4c and 4d, respectively, of the equivalent circuit shown in Fig. 4. This results in the problem that a counter voltage of 1600 V, which is much larger than the discharge voltage of 400 V, acts on the gap to immediately induce a discharge in the opposite direction, resulting in disordered recording of the recorded signals advised and make the picture cloudy.

It is also known in the art to use a previously corona charged insulating film with an electrically conductive layer to provide a durable electrostatic image thereon. For this purpose, exposure may be performed while the voltage is applied between the electrically conductive layer of the insulating film and the electrode of the photosensitive portion connected thereto, or both may be electrically short-circuited.

However, a problem with the conventional "image exposure method by the application of voltage" is that an external power source is required to cause a discharge by applying voltage between the photosensitive portion and the electrostatic recording medium for exposure, thereby making the system large; In addition, there is a possibility that it is affected by variations in the voltage of the power source.

If the previous corona charged insulating film is used, there is a possibility to dispense with it by using an external power source for the exposure. So far, however, nothing was known about how durable images can be practically produced with it.

Fig. 6 is a schematic illustration showing a typical method previously proposed for recording electrostatic images by using a spacer.

Referring to FIG. 6 is a photosensitive portion 2 in which a transparent electrode layer 2b and a photoconductive layer 2c are laminated on each other on the entire surface of a transparent substrate 2a opposite to an electrostatic information recording medium 1 in which an electrode layer 1b and an insulating layer 1a are sequentially deposited on the substrate entire surface of a substrate 1c laminated with a spacer 3 interposed therebetween. When the voltage is applied between both electrode layers, the image exposure is carried out, for example, by the photosensitive section 2. Subsequently, the photoconductive layer 2c generates carriers in the exposed portion and is made electrically conductive, so that discharge can take place between the photosensitive portion and the electrostatic information recording medium, accumulating charges on and on the insulating layer 1a according to the amount of exposure Create a durable electrostatic image.

However, in the method of recording electrostatic images shown in Fig. 6, a change in the gap width between the photosensitive section and the electrostatic information recording medium produces changes in the field strength and, accordingly, in the discharge current. This results in a change in the amount of charges accumulated on the insulating layer even under the same exposure. Therefore, in order to make the amount of charges correspond to the exposure energy, it is necessary to keep the gap width constant. This is the reason why the insulating spacer 3 has been inserted between the photosensitive portion and the electrostatic information recording medium during the image exposure in order to keep the gap width constant. In order to increase the recording sensitivity, it is necessary to increase the amount of the charges formed on the insulating layer 1a and the voltage applied between the non-sensitive portion and the electrostatic information recording medium for this purpose. As the voltage increases, however, the problem arises that when dust and so on. is located between the spacer and the light-conducting layer, a discharge can take place in the region of the spacer, resulting in a costly failure of the light-conducting layer.

In addition, it is very difficult to set the spacer between the photosensitive material and the electrostatic information recording medium to keep the intermediate gap constant since the gap width is on the order of 10 μm. As a result, it is impossible to achieve a continuous removal of images at high speed. If the electrostatic information recording medium bearing the electrostatic charge information is stacked or rolled up for storage, which in this case should be flexible, there arises the problem that the insulating layers may come into contact with the associated substrates, causing them to be exposed stored information in disorder.

Usually, electrode layers are provided on all surfaces of the light-conductive material and the electrostatic information recording medium with a spacer made of an insulating PET film interposed therebetween to keep the discharge gap constant.

It is an object of the invention that the charge amount corresponds to the exposure energy regardless of the voltage-lock time, even when an organic photosensitive portion is used.

This object is achieved by the method described in the claim.

Short description of the drawing

Fig. 1 is a schematic illustration showing how electrostatic images are taken.

Fig. 2 is a graph showing the relationship between the exposure energy and the amount of charge in a conventional exposure method when a voltage is applied;

FIG. 3 is a schematic diagram showing how the voltage after the image exposure is turned off; FIG.

4 shows an equivalent circuit,

Fig. 5 is a diagram showing a mechanism for generating an inverse discharge;

Fig. 6 is a diagram showing a conventional image recording process in which a spacer is used;

Fig. 7 is a diagrammatic view showing the image recording process of the present invention in which a voltage is applied after the image exposure for a given time;

Fig. 8 is a schematic diagram showing an example of an electrostatic camera using the exposure method of the present invention by applying a voltage;

Fig. 9 is a graph showing the recorded potential versus the exposure energy when the optical shutter is synchronized with the voltage shutter, or when the voltage shutter is turned on at various times after the exposure.

Preferred embodiment of the invention

As already mentioned with reference to FIG. 2, the light-conducting layer formed by a photosensitive section, after exposure, generates carriers when the voltage is applied, but whose mobility is so small that they disappear when the voltage is switched off, before reaching the electrostatic information recording medium.

For purposes of illustration, it will now be assumed that the exposure and the voltage closure are at a time t & sub1; and the exposure shutter is turned on at a time t & sub2; is switched off. According to the invention, the voltage closure is then applied at a predetermined time t & sub3; turned off, so that a sufficiently long period .DELTA.t results, so that all the generated carriers are able to reach the electrostatic information recording medium, as shown in FIG. 7 is shown. This makes it possible to form an image with the amount of charge corresponding to the exposure energy. Since the period .DELTA.t, starting at t.sub.2, when the exposure shutter is turned off, until the time t.sub.3 when the voltage shutter is turned off may vary depending on the type, thickness, and other factors of the photosensitive portion, it is desirable that the summarized under varying conditions periods .DELTA.t in advance in a table. Once the associated conditions have been determined, the desired time period Δt can be found in the table to set the time when the voltage lock should be turned off.

Fig. 8 is a schematic drawing showing an example of an electrostatic camera in which the exposure is applied by the application of a voltage, the same components as in Fig. 1 being represented by the same reference numerals. Further reference numerals represent the following components: 11 an image-sensing lens, 12 a mirror, 13 a shutter, 14 a focusing screen, 15 a pentagon prism, 16 an eyepiece, 17 a negative image and E a voltage source.

In this electrostatic camera, the photosensitive portion 2 and the electrostatic information recording medium shown in Fig. 1 are used in place of a single-lens reflex camera film. A voltage is applied to the photosensitive portion and the electrostatic information recording medium by a switch, not shown, for turning on the power source E, and the shutter 13 is triggered at a predetermined time to turn the mirror 12 to the position shown by the broken line permanent electrostatic image of an object on the electrostatic information recording medium 1 is formed. After a predetermined time has elapsed after closing the shutter, the voltage applied between the photosensitive section and the electrostatic information recording medium is turned off. If necessary, the electrostatic information recording medium is then developed with a toner to obtain a negative image 17. It may also be arranged to generate electrical signals by reading out the electrostatic potential for a screen display or the transmission to another recording means such as a magnetic tape.

example 1

With respect to the photosensitive portion and the electrostatic information recording medium, they were made of an organic photosensitive film having a thickness of 10 μm and a fluoropolymer film having a thickness of 3 μm, respectively, respectively, and arranged opposite each other with a gap of 10 μm. The photosensitive portion was used as a positive pole and a voltage of 750 V was applied to the electrodes. As a light source, a tungsten lamp with a color temperature of 3000 K was used.

Fig. 9a shows a characteristic diagram wherein the amount of light with which the photosensitive portion was exposed is used as the abscissa, and the potential taken by the electrostatic information recording medium serves as the ordinate. The graph was obtained after an exposure of 0.1 second duration was performed by applying the voltage with the voltage shutter synchronized with the optical shutter, and the voltage was turned off simultaneously with the exposure (Δt) off.

Fig. 9b shows the results of an experiment in which the same sample used in Fig. 9a was exposed at the same exposure intensity for 0.1 second, the voltage application was continued for a further period of 0.1 second (Δt = 0.1 seconds).

A comparison of Fig. 9a with Fig. 9b shows that although the photosensitive portion thereof was exposed to the same light energy, the potential stored in the electrostatic information recording medium in Fig. 9b is much larger than in Fig. 9a, where the voltage pulse synchronizes with the optical shutter was. This shows that Fig. 9b, wherein the supply of voltage is continued even after the closure of the optical shutter, is much more effective than Fig. 9a.

Example 2

Under similar conditions as mentioned in Example 1, the voltage supply was continued for another 0.2 second (Δt = 0.2 second) after the exposure. The results shown in Figure 9c were much more improved than those shown in Figure 9a, where the optical shutter was synchronized with the tension lock.

Example 3

Under similar conditions as mentioned in Example 1, the voltage supply was continued for another 0.3 second following the exposure (Δt = 0.3 second). The results shown in Fig. 9d were significantly better than those shown in Fig. 9a with the optical shutter synchronized with the tension lock.

Example 4

Under similar conditions as mentioned in Example 1, the voltage supply was continued for another 0.4 second following the exposure (Δt = 0.4 second). The results shown in Figure 9e were significantly better than those shown in Figure 9a, with the optical shutter synchronized with the tension lock.

Example 5

Under similar conditions as mentioned in Example 1, the voltage supply was continued for a further 0.5 second following the exposure (Δt = 0.5 second). The results shown in Fig. 9f were significantly better than those shown in Fig. 9a with the optical shutter synchronized with the tension lock.

It is therefore possible to accumulate all generated carriers as charges on the electrostatic information recording medium in an amount corresponding to the amount of exposure irrespective of the voltage-closing time.