JP4794276B2 - Electrophotographic image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a composite machine using an electrophotographic method, and particularly uses a two-component developer that charges a nonmagnetic toner using a magnetic carrier. Hybrid development that develops an electrostatic latent image by forming a thin layer of only the charged toner uniformly on the developing roll and causing the toner on the developing roll to fly to the electrostatic latent image by the AC superimposed DC voltage of the developing roll. The present invention relates to an electrophotographic image forming apparatus using a container.

  In a conventional hybrid developer, a thin toner layer on a developing roll that is non-uniform under the influence of development history between papers (while moving from one printing process to the next printing process) is peeled off between papers. The developing roller is peeled off by the removing operation. When the toner thin layer formation is started after the peeling operation is completed, a uniform toner thin layer is formed on the developing roller, and prepares for the next printing process. However, even when the next printing process is completed, the toner thin layer after development becomes non-uniform due to the difference in the amount of toner consumed by the output image. Therefore, in the middle of printing from the start of the developing operation after the development roller 1 round, the toner thin layer non-uniformity, that is, the development history is erased at the nip portion between the magnetic roll and the developing roll, and a new toner thin layer is formed simultaneously. . Therefore, if priority is given to the stripping performance (thinner toner layer is made thinner), the development ghost is improved, but the formation of the thin layer becomes insufficient and the image density is lowered. Conversely, if priority is given to the formation of a thin layer (when the toner thin layer is made thicker), the development ghost deteriorates. Therefore, the DC / AC superimposed voltages applied to the developing roller and the magnetic roller are set so that the development ghost and the image density are satisfied at the same time.

As another technique for achieving both high image quality and development ghost suppression, in Patent Document 1, in the magnetic one-component development system, an auxiliary developer carrier is provided on the downstream side of the electrostatic latent image carrier of the developing roll. In the dummy image forming process between the sheets, a dummy image for at least one round of the developing roll is formed on the latent image carrier, and then the dummy image on the latent image carrier is transferred to the auxiliary developer carrier. Return to the developing roll.
Japanese Patent Laid-Open No. 9-26699 (FIG. 1, paragraph 0020)

  However, in the conventional hybrid developing device, the toner charge amount is reduced at the end of printing a large number of sheets continuously, or particularly in a high humidity environment, and the balance between the toner thin layer formation and the stripping during printing is balanced. It collapses to the side of insufficient peeling. As a result, the non-uniform toner thin layer on the developing roller is not sufficiently peeled off, so that the development ghost becomes noticeable.

  Accordingly, an object of the present invention is to detect development ghosts caused by a decrease in toner charge amount due to durability and environmental conditions in a hybrid developer, and to recover the toner charge amount by rotating the developing device. It is to prevent the occurrence of ghost.

  In the present invention, a ghost detection image set in advance for ghost detection is formed by density detecting means for detecting the image density of a toner image on a recording medium (paper, intermediate transfer member, photoconductor, etc.), and the ghost The developing roll not affected by the halftone image density A by the toner thin layer portion on the developing roll affected by the development history of the solid black patch portion (solid black portion) in the detected image and the developing history of the solid black patch portion. When the ghost occurrence is detected based on the measurement result of the halftone image density B by the toner thin layer on the upper side, the rotation of the developing device is driven for a predetermined time, and the toner deteriorated due to durability and environmental conditions (especially humidity conditions). Increase the amount of charge. In addition, when the toner charge amount is high, for example, ghost becomes conspicuous even when the toner charge amount is 20 μC / g or more. Under such a high toner charge amount, if the developing device is rotated, the toner charge amount further increases. However, since the image density is lowered and the solid image is uneven, the developing device is not rotated. A ghost having a toner charge amount of 10 μC / g or less and a ghost when the toner charge amount is high can be determined by detecting a solid black patch image density in the ghost detection image. That is, when the solid black patch image density at the time of occurrence of the ghost is 1.2 or more, it is determined that the toner charge amount is 10 μC / g or less. The reason is that if the charge amount is small, the toner thin layer becomes thick and the image density tends to be high. When the image density is 1.2 or less, the toner charge amount is determined to be 20 μC / g or more. The reason is that if the charge amount is large, the toner thin layer becomes thin and the image density tends to be low.

  A first means for solving the above-described problem is to form a magnetic brush made of a two-component developer on a magnetic roll to which a DC / AC superimposed voltage is applied, and to remove only the toner in the magnetic brush from another DC. A hybrid developer that transfers an AC superimposed voltage to a developing roll and develops an electrostatic latent image with a thin toner layer on the developing roll, a density detection unit that detects the image density of the toner image, and a solid black patch unit And a ghost detection pattern consisting of a halftone portion, and the image density A of the ghost detection pattern by the toner thin layer portion on the developing roll affected by the development history of the test pattern, and the development history A ghost that determines the presence or absence of ghost generation based on the measurement result of the image density B of the ghost detection pattern by the toner thin layer on the developing roll that is not affected An occurrence determining means, and when it is determined that a ghost has occurred, the development drive of the hybrid developer is operated for a predetermined time according to the measurement result.

  By operating this development drive, the developer is agitated, whereby the lowered toner charge amount can be increased and the ghost can be reduced.

  The second means is that in the first means, the ghost generation determination means determines whether or not ghost is generated based on a ghost ratio A / B using the image density A and the image density B.

  Thereby, the degree of ghost can be quantified.

  The third means is to set a ghost ratio range of 1 or more in the second means and rotate the magnetic roll for a predetermined time determined according to the ghost ratio range.

  As a result, the toner charge amount can be controlled in accordance with the ghost ratio range to which the detected ghost ratio belongs.

  The fourth means is that in the third means, the predetermined fixed time is longer as the ghost ratio is smaller.

  Thereby, the toner charge amount can be significantly increased. The predetermined time is not necessarily proportional to the ghost ratio. When the ghost ratio is extremely small, the toner charge amount is significantly increased by significantly increasing the predetermined time based on an empirical rule.

  According to the present invention, the development ghost is generated by detecting the development ghost caused by the decrease in the toner charge amount due to the durability and environmental conditions in the hybrid developing device and recovering the toner charge amount by the rotation operation of the development device. Can be prevented.

  Embodiments of the present invention will be described below with reference to the drawings.

[Configuration of this embodiment]
FIG. 1 shows a front view of an example of a color copying machine (tandem machine) having a hybrid developing device as an example of an electrophotographic image forming apparatus. Note that the present invention can also be applied to a monochrome image forming apparatus.

  In the main body 1 of the tandem machine, for each color of yellow, magenta, and cyan black (in no particular order), an image forming unit (a charger 70, an exposure unit 20, a developing device 40, a transfer unit 90, a cleaning device around the photosensitive member 30, and a cleaning device). And a transfer belt 50 for conveying paper and a fixing device 120.

  A recording medium (such as paper) (not shown) is supplied onto the transfer belt 50 from the right in the drawing, the paper comes into contact with the photoconductor 30, and the toner image on the photoconductor is transferred onto the paper by the transfer means 9. After the increase of each color toner is transferred, the toner image is fixed on the paper by the fixing device 10, and the paper is discharged out of the main body 11 as a print. In a tandem type printer, it is important to design them compactly.

  Next, each part will be described with reference to FIG. FIG. 2 is a conceptual diagram of the developing device 40. In FIG. 2, 3 is a photoreceptor electrostatic latent image carrier, 2 is a developing roll, 1 is a magnetic roll, 5 is toner, 4 is a carrier, and the photosensitive body 3 is developed. A bias is applied to the roll 2. The power supplies 7a and 7b output a DC bias voltage Vdc2 and an AC voltage applied to the developing roll 2, respectively. The power supplies 8a and 8b output a DC bias voltage Vdc1 and an AC voltage applied to the magnetic roll 1, respectively. 6 is a thin toner layer on the developing roll 2, 10 is a magnetic brush, and 9 is a regulating blade for controlling the thickness of the magnetic brush.

  A semiconductor laser or LED can be used as a light source (not shown) of exposure light that exposes the electrostatic latent image carrier 3. A wavelength around 770 nm is effective for a positively charged organic photosensitive material (positive OPC), and a wavelength around 685 nm is effective for an a-Si photoreceptor.

  Hereinafter, an example in the case of using positive OPC will be described. The positive OPC 3 that is the electrostatic latent image carrier is charged to 400 V by a charger (not shown). Thereafter, when exposure is performed with an LED having a wavelength of 770 nm, the post-exposure potential becomes 70V. The positive OPC photosensitive member is disposed with a space of about 250 μm with respect to the developing roll 2. No wire electrode or the like is used in this space. When positive OPC is used as the photosensitive material for the photoconductor 3, the generation of ozone and the like is small and charging is stable. In particular, even when the positive OPC having a single layer structure is used over a long period of time and the film thickness changes It is ideal for long-life systems because it has little change in characteristics and stable image quality. In addition, it is also possible to use an a-Si photoconductor. When a long-life system is used, the film thickness of the positive OPC is set to about 20 μm to 40 μm. In the case of 20 μm or less, when the film decreases and reaches 10 μm, black spots are noticeably generated due to dielectric breakdown. On the other hand, if the film thickness is thicker than 40 μm, the sensitivity is lowered and the image quality is lowered.

  Next, the hybrid developer will be described. First, the developer composed of the toner 5 and the carrier 4 held on the surface of the magnetic roll 1 is held, and the toner 5 is charged to an appropriate level while stirring. The developer passes through the regulating blade 9 and contacts the developing roll 2 with a constant layer thickness. The gap between the regulating blade 9 and the magnetic roll 2 is 0.3 mm to 1.5 mm, and the gap between the magnetic roll 1 and the developing roll 2 is 0.3 mm to 0.5 mm, preferably about 0.2 mm to 0.4 mm. is there. The toner thin layer 6 on the developing roll 2 is set to a thickness of 6 μm to 50 μm, preferably 30 μm to 70 μm. This thickness is a value corresponding to about 5 to 10 layers of the toner 5 when the average particle size of the toner 5 is 7 μm. The gap between the developing roll 2 and the electrostatic latent image carrier 3 is 150 μm to 400 μm, preferably 200 μm to 300 μm. If it is smaller than 150 μm, it causes fogging. If it is larger than 400 μm, it becomes difficult to cause the toner 5 to fly to the photoreceptor 3, and a sufficient image density cannot be obtained. Further, it becomes a factor that causes selective development. By applying an alternating electric field to the conductive developing roll 2 and the magnetic roll 1 with the power supplies 7b and 8b, the development on the photosensitive member 3 can be performed accurately, and the development residual toner on the magnetic roll 1 can be easily collected. . The surface of the developing roll 2 is a rotating body (conductive sleeve) made of conductive aluminum. The material of the conductive sleeve may be a uniform conductor, and SUS, conductive resin coating, etc. can be applied. The DC voltage and AC voltage from the power supplies 7 a and 7 b are transmitted to the conductive sleeve via the shaft of the developing roll 2. The output of the power supply 7a is 100V, and the output of the power supply 7b is Vpp 1.6 kV, frequency 2.7 kHz, duty 27%. The magnetic roll rotating body receives the DC voltage and the AC voltage from the power supplies 8a and 8b on the shaft of the magnetic roll 1 and transmits them to the magnetic roll rotating body. The output of the power source 8a is 350 to 500v (variable by calibration), and the power source 8b has a Vpp of 300V, a frequency of 2.7 kHz, and a duty of 73%. The waveform of the AC component is preferably a rectangular wave. By applying these superimposed biases to the developing roll and the magnetic roll, it is possible to develop the electrostatic latent image on the electrostatic latent image carrier 3 with good developability and to form the toner thin layer 6 on the magnetic roll 1. The recoverability is improved and the stability of continuous printing is improved. In order to stabilize the image density in continuous printing, it is necessary to periodically remove the toner from the developing roll 2 according to the print data and refresh it. The toner is always refreshed if the toner is peeled off from the developing roll at the end of development, but it takes time to form a stable toner layer again, and a sufficient printing speed cannot be achieved. In order to supply sufficient toner to the latent image on the photosensitive member without increasing the sheet interval, the peripheral speed of the developing roll 2 is set to 1.5 times or more with respect to the electrostatic latent image carrier 3, and the time is shortened. The toner can be taken in and out. Further, when the magnetic roll 1 is set at a speed 1 to 2 times that of the developing roll 2, toner replacement is promoted. At this time, it is preferable that the rotating direction of the magnetic roll is opposite to the developing roll. In order to replace the toner thin layer 6 on the developing roll 2, the toner thin layer 6 of the developing sleeve is recovered by the magnetic brush 10 by changing the DC voltage in a state where AC is applied at the end of development.

The saturated toner amount of the toner thin layer 6 is determined by the difference between Vdc1 of the power source 8a and Vdc2 of the power source 7a. When Vdc2 is set to 150 V and the value of Vdc1 is set to 400 V, a toner layer of about 1.0 mg / cm ² is obtained on the second turn of the developing roll. The adjustment of the toner layer is basically obtained by a potential difference of (Vdc2−Vdc1), but factors such as the toner charge amount and the magnetic pole magnetic pole strength may also contribute.

If Vdc2 is controlled based on an actually obtained image in order to control the toner layer thickness, a uniform and good image can be obtained at a target density. When performing high density printing continuously, it is advantageous to set the value of (Vdc2−Vdc1) slightly higher. If the toner layer is too thin at 0.5 mg / cm ² or less, the followability of the density when a high density image is continuous is lowered and image unevenness is likely to occur. On the other hand, if the toner layer exceeds 1.5 mg / cm ² and is too thick, the development ghost tends to be noticeable and the toner scattering tends to be noticeable. The toner layer thickness also depends on the charge amount of the toner. When the toner charge amount is as low as 10 μC / g or less, particularly 5 μC / g or less, the toner layer thickness increases and scattering increases. Also, development ghosts become prominent. On the other hand, when the toner charge amount is 20 μC / g or more, the toner layer thickness becomes thin, the charge increases, and the developability of the toner decreases.

  The toner thin layer 6 of the developing roll 2 is collected by the magnetic brush 10 held on the magnetic roll 1, and new developer is carried to the developing roll 2 through the regulating blade 9.

  It is important to define the particle size distribution of the toner 5 in order to avoid selective developability. Generally, the particle size distribution of the toner is measured by a coal counter, and the spread of the particle size distribution is expressed by the ratio of the volume distribution average diameter to the number distribution average diameter. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size is deposited on the developing roll 2 in continuous printing, and developability is deteriorated.

The two-component developer forms a magnetic brush 10 composed of toner 5 and carrier 4 on the magnetic roll 1, and the toner 5 is charged by stirring. The magnetic brush 10 on the magnetic roll 1 is layer-regulated by a regulating blade 9, and a thin layer 6 containing only toner is formed on the developing roll 2 by a potential difference between the magnetic roll 1 and the developing roll 2. As the carrier 4, a ferrite carrier having a volume resistivity of 10 ⁸ Ωcm coated with a silicone resin, a saturation magnetization of 40 emu / g, and an average particle diameter of 35 μm may be used. When the average particle size exceeds 50 μm, the carrier stress increases, and the toner concentration cannot be increased and the amount of toner supplied to the developing roll 2 decreases. As the carrier, a magnetite carrier, Mn ferrite, Mn-Mg ferrite, or the like can be used. These carriers may be used as they are, but they may be used after being surface-treated within an appropriate resistance range. The mixing ratio of the toner is 5 to 20% by weight, preferably 5 to 15% by weight, based on the total amount of the carrier and the toner. When the mixing ratio of the toner is less than 5% by weight, the toner charge amount becomes high and a sufficient image density cannot be obtained. When the toner content exceeds 20% by weight, a sufficient charge amount cannot be obtained. The toner is scattered from the inside and contaminates the inside of the image forming apparatus, or toner fog occurs on the image.

FIG. 3 shows the bias between the developing roll 2 and the magnetic roll 1 between sheets (between images).
After developing the electrostatic latent image, the DC voltage Vdc2 of the developing roll 2 is set to zero, the toner thin layer is peeled off during one rotation of the developing roll, and then the development of the electrostatic latent image is started. Reform the toner thin layer. The offset voltage (DC) of the developing roll 2 is higher than the offset voltage (DC) of the magnetic roll 1, and the potential difference is preferably set to 50 to 200v. If it is 50 V or less, the peeling is insufficient, and if it is 200 V or more, the peeling is too strong, and it is difficult to form the next thin layer during continuous printing, and the second image becomes thin. . In addition, by setting the duty ratio to 50% or more at the time of peeling, there is an effect of preventing the toner from adhering to the surface of the magnetic roll, and the adhesion of the toner on the surface of the developing roll can be reduced more effectively.

[Operation of this embodiment]
FIG. 4 is an example of a ghost detection image. FIG. 4A is a solid black patch portion (solid black image), and a part of the printable area is a solid black image. The print density of the solid black patch image is measured by the density sensor 60 (FIG. 1). The hybrid developing device is rotated for a predetermined time after the measurement, the toner is charged by frictional charging, and the print density of the halftone image shown in FIG. 4B is measured after the predetermined time. The halftone image is, for example, a halftone image with a printing rate of 25%. If there is no development history, no ghost is generated and the print density of the halftone image is uniform. On the other hand, if there is a development history, a ghost is generated, and a solid black development history portion (ghost portion) appears in the halftone image depending on the degree of peeling of the thin toner layer after printing the solid black patch portion. As an index for determining the degree of ghost, for example, a ghost ratio (a value obtained by dividing the print density of the solid black image development history portion by the print density of the halftone image) may be used.

  The reason why such a ghost is generated is that the toner charge amount is insufficient or excessive due to a decrease in the durability of the toner or a humidity condition.

  Therefore, FIG. 5 shows the result of an experiment for examining the relationship between the toner charge amount and the ghost ratio using the black developing device of the color printer LS-C5016N manufactured by Kyocera Mita using the hybrid developing device. According to the graph shown in the figure, the ghost ratio reaches the maximum value (about 0.9) in the toner charge amount range of 0 to 25 μC / g when the toner charge amount is about 15 μC / g. Discrimination between a ghost with a toner charge amount of 10 μC / g or less and a ghost when the toner charge amount is high can be made by detecting solid black patch image density. That is, when the solid portion image density at the time of ghost generation is 1.2 or more, for example, it can be determined that the toner charge amount is 10 μC / g or less and the toner thin layer is thick and the printing density tends to be high. On the other hand, when the image density is 1.2 or less, the toner charge amount is 20 μC / g or more, and it can be determined that the toner thin layer is thin and the printing density tends to be low. In addition, when the toner charge amount is high, for example, ghost becomes conspicuous even when the toner charge amount is 20 μC / g or more. Under such a high toner charge amount, if the developing device is rotated, the toner charge amount further increases. However, since the image density is lowered and the solid image is uneven, the developing device is not rotated.

  FIG. 6 is a flowchart of a method for controlling the toner charge amount based on the ghost ratio. First, in S61, it is determined whether or not the ghost ratio is 0.8 or more, for example. The ghost ratio of 0.8 or more means that a ghost can be determined by visual inspection as having virtually not occurred, in other words, a level at which the generated ghost can be virtually ignored. Therefore, according to the graph shown in FIG. 5, the toner charge amount is an appropriate charge amount in the range of 10 μC / g to 20 μC / g. Therefore, when it is determined that the ghost ratio is 0.8 or more, the process proceeds to S62, and no dummy development (rotating drive of the developing unit without printing) is performed as no ghost is generated.

  On the other hand, when it is determined that the ghost ratio is not 0.8 or more, the process proceeds to S63, and it is determined whether or not the print density of the solid black patch portion is 1.2 or less, for example. If the print density is 1.2 or less, the charge amount is 20 μC / g or more, and the print density is low because the toner thin layer is thin. Therefore, when it is determined that the print density of the solid black patch portion is 1.2 or less, the process proceeds to S64, and since the toner charge amount is not increased any more, dummy development (the developer is driven to rotate without printing). ) Is not performed.

  On the other hand, when it is determined that the print density of the solid black patch portion is not 1.2 or less, the process proceeds to S65, and it is determined whether or not the ghost ratio is 0.7 or more, for example. If the ghost ratio is 0.7 or more, if the toner charge amount is slightly increased, the ghost ratio reaches 0.8 and becomes an appropriate charge amount. Therefore, when it is determined that the ghost ratio is 0.7 or more, the process proceeds to S66, and the developing device is driven to rotate, for example, for 2 minutes.

  On the other hand, when it is determined that the ghost ratio is not 0.7 or more, the process proceeds to S67, and it is determined whether or not the ghost ratio is, for example, 0.6 or more. If the ghost ratio is 0.7 or more, the toner charge amount may be increased and the ghost ratio may be increased to 0.8. Therefore, when it is determined that the ghost ratio is 0.6 or more, the process proceeds to S68, and the developing device is driven to rotate, for example, for 3 minutes.

  On the other hand, when it is determined that the ghost ratio is not 0.6 or more, the process proceeds to S69, and the developing device is driven to rotate, for example, for 4 minutes.

  The above flow chart is merely an example. Generally, it is only necessary to define one or more ghost ratio ranges to which the measured ghost ratio belongs, and to operate the hybrid developer for a predetermined time determined according to the ghost ratio range. . The predetermined time may be set longer as the ghost ratio is smaller.

  FIG. 7 is a table and a graph showing the relationship between the developer rotation driving time (dummy developing time, in other words, toner stirring time) and the ghost ratio when the magnetic roll bias is OFF. In the table shown in FIG. 7A, the ghost ratio change in the range of the stirring time from 0 minute to 5 minutes is shown. The stirring time of 0 minute is a state where the toner charge amount is 5.1 μC / g and the ghost ratio is 0.61. The stirring time of 0 minute or 1 minute corresponds to S62 or 64 in the flowchart shown in FIG. 7, respectively, and the stirring time of 2 minutes, 3 minutes or 4 minutes corresponds to S66, S68 and S69, respectively. In the example shown in FIG. 7, the ghost ratio increases and the toner charge amount increases to an appropriate value as the stirring time increases from 0 minutes to 4 minutes. However, when the stirring time is 5 minutes, the toner charging amount becomes excessive because the stirring time is too long.

  INDUSTRIAL APPLICABILITY The present invention can be used in a developing method used in an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a composite machine using an electrophotographic method. In particular, a non-magnetic toner is charged using a magnetic carrier. The two-component developer is used, and only the charged toner is uniformly formed into a thin layer on the developing roll, and the toner on the developing roll is caused to fly to the electrostatic latent image by the AC superimposed DC voltage of the developing roll. It can be used for hybrid development for developing a latent image.

1 is an overall view of an example of a color copying machine. It is a conceptual diagram of a hybrid developing device. 4 is a timing chart of toner thin layer peeling and thin layer formation on the developing roller. It is an example of a ghost detection image. 6 is a graph showing a relationship between a toner charge amount and a ghost ratio. It is a flowchart of toner charging control. It is a table | surface and a graph which show the improvement of the ghost ratio by toner stirring.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic roll 2 Developing roll 3 Electrostatic latent image carrier (photoreceptor)
4 Carrier 5 Toner 6 Toner thin layer 9 Regulating blade 10 Magnetic brush 11 Main body 20 of tandem type color image forming apparatus Exposure unit 30 Photoconductor 40 Developer 50 Transfer belt 60 Density sensor 70 Charger 80 Cleaning device 90 Transfer means 120 Fixing device

Claims (4)

A magnetic brush composed of a two-component developer is formed on a magnetic roll to which a DC / AC superimposed voltage is applied, and only the toner in the magnetic brush is transferred to a developing roll to which another DC / AC superimposed voltage is applied, A hybrid developer for developing an electrostatic latent image with a thin toner layer on the developing roll;
Density detecting means for detecting the image density of the toner image;
The image density A of the ghost detection pattern by the toner thin layer portion on the developing roll affected by the development history of the test pattern using the test pattern having the solid black patch portion and the ghost detection pattern comprising the halftone portion. And a ghost occurrence determination means for determining the presence or absence of ghost generation based on the measurement result of the image density B of the ghost detection pattern by the toner thin layer on the developing roll not affected by the development history,
An image forming apparatus, wherein when it is determined that the ghost has occurred, the development driving of the hybrid developing device is operated for a predetermined time determined in advance according to the measurement result.   The image forming apparatus according to claim 1, wherein the ghost generation determination unit determines whether or not ghost is generated based on a ghost ratio A / B using the image density A and the image density B.   3. The image forming apparatus according to claim 2, wherein a ghost ratio range of 1 or more is determined, and the magnetic roll is rotated for a predetermined time determined according to the ghost ratio range. The image forming apparatus according to claim 3, wherein the predetermined time is longer as the ghost ratio is smaller.

Images