1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a facsimile, which forms an image using electrophotography, and a program for controlling an image forming apparatus of this type.
2. Description of the Related Art
In an image forming apparatus of the electrophotographic type, the image density varies depending on temperature and humidity conditions of an environment under which the apparatus is used, as well as on the degree of usage of process stations (specifically, developing sections and electrostatic charging sections used for forming an image). The image forming apparatus carries out image density control to correct for such variations of the image density. For example, the image density control is carried out as follows. Density patches in respective colors are formed on photosensitive members, or an intermediate transfer belt (hereinafter referred to as the “ITB”) or an electrostatic (absorption) transfer belt (hereinafter referred to as the “ETB”), and then the density patches are read by density detecting sensors. The results of reading are fed back to different types of high voltage conditions and process forming conditions including laser power, thereby adjusting the maximum densities and halftone gradation characteristics of the respective colors to uniform levels. It should be noted that image density control that maintains the maximum densities of the respective colors constant is referred to as Dmax control, and image density control that maintains the halftone gradation characteristics linear with respect to an image signal obtained by reading an image on an original is referred to as D half control. The Dmax control serves to maintain the color balance between the respective colors constant, and further, the Dmax control also has such an important role as preventing scatter of a character formed by overlapped colors caused by excessive toner deposition and faulty fixing.
In general, the density detecting sensor illuminates a density patch using a light source, and detects the intensity of reflected light with a light receiving sensor. A signal representing the intensity of reflected light is subjected to analog-to-digital conversion and the analog-to-digital converted signal is subjected to predetermined processing by a CPU, and the signal after the predetermined processing is fed back to the process forming conditions. Specifically, in the Dmax control, a plurality of density patches formed under respective different image forming conditions are detected by optical sensors, a conditions which enable the desired maximum density to be obtained are calculated from the detected results, and the image forming conditions are changed based on the calculated conditions.
The density detecting sensor is roughly divided into two types, i.e. a type of detecting diffuse reflection (irregular reflection) components of the reflected light and a type of detecting specular reflection (regular reflection) components of the reflected light. First, a detailed description will now be given of the method of detecting the diffuse reflection components. The diffuse reflection components are components of reflection that are sensed as a color, and have such a characteristic that the quantity of the reflected light increases as the quantity of colorant, namely the quantity of a toner, of the density patch increases.
FIG. 12 is a graph showing the relationship between the quantity of the diffuse reflected light and the quantity of the toner, which is applicable to a conventional image forming apparatus. The reflected light also has such a characteristic that the light is diffused uniformly in all directions from the density patch. The type of the density detecting sensor for detecting diffuse reflection components is configured such that the illumination angle and the angle of incidence are different from each other to eliminate the influence of the specular reflection components, described later.
However, when the density detecting sensor for detecting diffuse reflection components is used to detect the density of a black toner, the black toner absorbs light, and therefore the sensor cannot detect light reflected from the black toner. Therefore, in this case, a method has been proposed in which a base in a chromatic color is used as the base of the density patch, and the density of the black toner is detected by measuring a quantity of reflected light from parts of the base other than those blocked by the black toner, for example.
When an image forming apparatus of an inline type which includes a plurality of photosensitive members is used, to reduce the number of the density sensors, it can be thought that a density patch is formed on an ETB or an ITB, and a single density sensor is used to detect the densities of the all colors, instead of forming and detecting density patches on the photosensitive members. In this case, it is necessary to adjust resistance generated between a sheet and the ETB or ITB to secure a sheet conveying force and image stability on the ITB, and therefore carbon black is scattered over the ETB or ITB. Consequently, the ETB or ITB often comes to present a black or dark gray color. Therefore, when the density of the black toner on the ETB or ITB is detected, light is not reflected from either the density patch or the base, and the type of the density sensor which detects the diffuse reflection light cannot detect the black toner. Thus, it is necessary to use the type of the density sensor for detecting the specular reflected light as described later.
FIG. 13 is a diagram showing the relationship between the quantity of the specular reflected light and the quantity of the toner. A detailed description will now be given of the method of detecting specular reflection components of the reflected light. The sensor of the type that detects specular reflected light is disposed to detect light reflected in a direction symmetrical with the illumination angle with respect to a normal line to the base surface (the ETB or ITB surface). The quantity of the reflected light depends on the refractivity specific to the material of the base (namely the ETB or ITB) and the reflectivity determined by the surface condition of the base, and is sensed as gloss. When a density patch is formed on the base, a part of the base on which the toner is deposited blocks light and does not generate reflected light. Consequently, the quantity of the toner on the density patch and the quantity of the specular reflected light presents such a relationship that the reflected light quantity decreases as the toner quantity increases as shown in FIG. 13.
The density sensor of the type that detects specular reflected light is disposed to mainly detect not the reflected light from the toner, but the reflected light from the base, and therefore the sensor can detect the density of the density patch regardless of the colors of the toner and the base, and thus, is more advantageous in density detection than the density sensor of the type that detects diffuse reflected light. In addition, the quantity of the reflected light of the specular reflection components is generally larger than the quantity of the reflected light of the diffuse reflection components, and thus, the density sensor of the type that detects specular reflected light is advantageous also in the detection accuracy of the density sensor, and therefore, it is also desirable to use the density sensor of the type that detects specular reflected light when the density is detected on the photosensitive member.
However, there arises a problem when density sensor of the type that detects specular reflected light is used to detect a toner in a chromatic color. As described above, when light is irradiated on a density patch of a chromatic color toner, the diffuse reflected light increases as the toner quantity increases, and the reflected light scatters uniformly in all the directions. Thus, the light detected by the density sensor is the sum of the specular reflection components and the diffuse reflection components.
FIG. 14 shows the relationship between the toner quantity and the reflected light quantity when a chromatic color toner is detected by the density sensor of the type that detects specular reflected light. Namely the relationship between the toner quantity and the reflected light quantity is the sum of a thin solid line curve which represents the characteristic of the specular reflection, and a broken line curve which represents the characteristic of the diffuse reflection, and presents a negative characteristic shown as a thick solid line curve. Thus, to exhibit both the characteristics of the specular reflected light and the diffuse reflected light, there has been generally employed such a method in which radiated light from a single light emitting element 301 is detected by an optical sensor as shown in FIG. 3, which is comprised of two light receiving elements 302 and 303 for receiving specular reflected light and for diffuse reflected light, respectively, thereby detecting the density.
When the density sensor of the type mainly detecting reflected light from the base is used, if the surface state of the base changes with the use of the base, the reflected light quantity changes accordingly. Thus, it is effective for the density detection to apply correction such as normalizing the reflected light quantity of the density patch with the reflected light quantity of the base, and then, converting the normalized quantity into density information (hereinafter referred to as “base correction”). In this case, it is desirable that measurement of the reflected light quantity of the base for the base correction should be carried out in the same timing as the formation of the density patch and at the same part of the base on which the density patch is formed in consideration of material variation and aging change of the ETB or ITB. Thus, as a method of measuring the quantity of the light reflected by the base, there has been employed such a method as alternately measuring the density of the density patches and the quantity of the light reflected by the base as shown in FIG. 15, or successively measuring the density of the density patches and then measuring the quantity of the light reflected by the base for one turn of the ITB or the ETB as shown in FIG. 16.
However, when the base reflected light quantity is measured simultaneously with measuring the density patch in image density control, there is such a problem that the entire measurement takes time. For example, with the method shown in FIG. 15, if the measurement interval for the density patches and the measurement interval for the base reflected light quantity are the same, the entire measurement requires twice of the time period required in the case where only the density patches are measured. Also, with the method shown in FIG. 16, a time period for rotating the ITB or the ETB by one turn is additionally required compared with the case where only the density patches are measured.