WO2014129305A1

WO2014129305A1 - Measurement apparatus and image forming apparatus

Info

Publication number: WO2014129305A1
Application number: PCT/JP2014/052592
Authority: WO
Inventors: Yasuo Kamei
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2013-02-20
Filing date: 2014-01-29
Publication date: 2014-08-28
Anticipated expiration: 2015-08-20
Also published as: JP5991775B2; JP2016513235A

Abstract

A measurement apparatus comprises: a light emitting means for emitting a light; a light receiving means for receiving a reflected light from a measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light-receiving elements; a determination means for determining correction information based on a first signal which is output by a predetermined light-receiving element of the plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the predetermined light-receiving element in a second state in which the light emitting means emits a light; and a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

Description

DESCRIPTION

TITLE OF INVENTION MEASUREMENT APPARATUS AND IMAGE FORMING APPARATUS

TECHNICAL FIELD

[0001] The present invention relates to a

measurement apparatus having a function of measuring a color, and an image forming apparatus.

BACKGROUND ART

[0002] The quality of an image (to be referred to as an image quality hereinafter) by a recently popular image forming apparatus (to be referred to as a printer hereinafter) is determined by various factors such as graininess, in-plane uniformity, character quality, and color reproducibility (including color stability) . A most important factor of them is color reproducibility.

[0003] As for color reproducibility, the color difference matters not only between devices of the same model, but also between devices of different models, between image forming apparatuses of different methods, and between an image forming apparatus and an image display apparatus. To perform color matching between such devices, software and a measurement device for generating a multidimensional LUT (Look Up Table) called an ICC (International Color Consortium) profile are commercially available.

[0004] As shown in Fig. 5, the contents of each ICC profile are calibrated in association with a device-independent color space based on color

measurement of a measurement image (patch) using the measurement device. An example of the color space is a CIE L*a*b* color space (CIE stands for Commission

International de 1¹ Eclairage) . By this processing, colors to be printed can match each other even between different devices. A CMM (Color Management Module) installed in an image forming apparatus or the like can generate print data by performing color conversion using these profiles.

[0005] Japanese Patent Laid-Open No. 2004-86013 proposes an inline measurement device arrangement in which a patch image formed on a sheet is detected by a color sensor constructed by a light source, diffraction grating, and position detection sensor, thereby

increasing the detection accuracy. A detection value from the color sensor is converted into a spectral reflectance, and the spectral reflectance can be converted into a CIE Lab value in consideration of the tristimulus values and the like. The color detection accuracy of the color sensor in Japanese Patent Laid- Open No. 2004-86013 degrades owing to fluctuation factors such as output fluctuations of the light source upon a change of the environmental temperature. To solve this, there is a method of performing calibration using a white reference plate arranged at a position facing the color sensor, and correcting the detection value of the color sensor. As the calibration method, light reflected by the white reference plate is

measured before or after executing color measurement of a patch image, and then calculation is performed based on the measurement value. Letting ίί(λ) be the quantity of light reflected by the white reference plate and Ρ(λ) be the quantity of light reflected by the patch, a spectral reflectance calculation method in calculation using the white reference plate is given by:

spectral reflectance R(X) of patch = ... (1)

^W ΐί(λ)

[0006] The white reference plate reflects light of a measurement wavelength region at almost the same reflectance regardless of the wavelength. Thus, the quantity W (λ) of light reflected by the white reference plate can be considered to be equal to the quantity of light incident on the patch image. By measuring both the quantity (that is, W( )) of light incident on the patch image and the reflected light quantity Ρ(λ), the spectral reflectance R( ) of the patch can be derived without the influence of output fluctuations of the light source.

[0007] As described above, the spectral

reflectance is derived from the light Ρ(λ) reflected by the patch and the light W (λ) reflected by the white reference plate. The output signal value from the position detection sensor which reads reflected light contains even a dark current value in addition to reflected light. There is a fear that fluctuations of the dark current value in accordance with the

temperature influence the color detection accuracy in measurement .

[0008] Since no dark current value can be read when the position detection sensor receives light, a method of arranging light-shielded light-receiving elements to read a dark current value has been used conventionally. However, this method requires the light-shielded light-receiving elements dedicated to correction. As a result, the space for the light- receiving elements dedicated to correction is required. Further, the light-receiving elements dedicated to correction are shielded from light and thus cannot be used as effective pixels.

SUMMARY OF INVENTION

[0009] According to one aspect of the present invention, there is provided a measurement apparatus characterized by comprising: a light emitting means for emitting a light; a light receiving means for receiving a reflected light from a measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light- receiving elements; a determination means for determining correction information based on a first signal which is output by a predetermined light- receiving element of the plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the predetermined light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

[0010] According to another aspect of the present invention, there is provided a measurement apparatus characterized by comprising: a light emitting means for emitting a light; a light receiving means for receiving a reflected light from a measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light- receiving elements; a detection means for detecting a temperature of the light emitting means; a

determination means for determining a target light- receiving element of the plurality of light-receiving elements based on the temperature detected by the detection means, and determining correction information based on a first signal which is output by the target light-receiving element of the plurality of light- receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the target light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

[0011] According to another aspect of the present invention, there is provided an image forming apparatus including a measurement apparatus, characterized in that the measurement apparatus comprises: a light emitting means for emitting a light; a light receiving means for receiving a reflected light from a

measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light-receiving elements; a determination means for determining correction information based on a first signal which is output by a predetermined light- receiving element of the plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the predetermined light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

[0012] According to another aspect of the present invention, there is provided an image forming apparatus including a measurement apparatus, characterized in that the measurement apparatus comprises: a light emitting means for emitting a light; a light receiving means for receiving a reflected light from a

measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light-receiving elements; a detection means for detecting a temperature of the light emitting means; a determination means for determining a target light-receiving element of the plurality of light- receiving elements based on the temperature detected by the detection means, and determining correction

information based on a first signal which is output by the target light-receiving element of the plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the target light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

[0013] According to the present invention, high- accuracy color measurement can be performed by

detecting fluctuations of a dark current value in real time without arranging light-receiving elements dedicated to correction.

[0014] Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings) .

BRIEF DESCRIPTION OF DRAWINGS

[0015] Fig. 1 is a sectional view showing an example of the arrangement of an image forming

apparatus ;

[0016] Fig. 2 is a view showing the arrangement of a color sensor;

[0017] Fig. 3 is a control block diagram;

[0018] Fig. 4 is a view for explaining an ICC profile ;

[0019] Fig. 5 is a schematic view showing a color management environment;

[0020] Figs. 6A and 6B are block diagrams each including a sensor and control unit according to an embodiment;

[0021] Fig. 7A is a flowchart in measurement of a white reference plate; [0022] Fig. 7B is a flowchart in patch image measurement (calculation) ;

[0023] Fig. 7C is a flowchart in patch image measurement (temperature detection) ;

[0024] Fig. 8 is a view of color measurement in the present invention;

[0025] Figs. 9A and 9B are graphs for explaining line sensor output fluctuation states before and after light irradiation;

[0026] Figs. 10A and 10B are graphs for explaining line sensor output fluctuation states before and after light irradiation; and

[0027] Figs. 11A and 11B are views for explaining a positional shift of a line sensor caused by

distortion of a housing upon a temperature change.

DESCRIPTION OF EMBODIMENTS

[0028] <First Embodiment>

The first embodiment according to the present invention will be described in detail with reference to the accompanying drawings .

[0029] (Image Forming Apparatus)

The first embodiment will be explained using an electrophotographic laser beam printer. Although the following description is based on the

electrophotographic method, the present invention is also applicable to an image forming apparatus which fixes an image by a thermal drying method, such as an inkjet printer or sublimation printer. The structure and operation of the image forming apparatus according to the present invention will be described.

[0030] Fig. 1 is a sectional view showing the structure of an image forming apparatus (to be referred to as a printer hereinafter) 100 in the embodiment. The printer 100 includes a housing 101. The housing 101 incorporates respective mechanisms for constructing an engine unit, and a control board storage unit (not shown) which stores an engine control unit 312 for performing control concerning respective print

processes (for example, sheet feed processing) by the respective mechanisms and a printer controller 103.

[0031] As the mechanisms for constructing the engine unit, an optical processing mechanism, fixing processing mechanism, sheet feed processing mechanism, conveyance processing mechanism, and the like are arranged. The optical processing mechanism performs formation of electrostatic latent images on

photosensitive drums 105 by scanning of laser beams, visualization of the electrostatic latent images, multiple transfer of the visual images to an

intermediate transfer member 106, transfer of the multiple-transferred color image to a sheet 110, and the like. The fixing processing mechanism fixes a toner image transferred to the sheet 110. The sheet feed processing mechanism performs sheet feed

processing of the sheet 110. The conveyance processing mechanism performs conveyance processing of the sheet 110.

[0032] The optical processing mechanism includes a laser driver in each laser scanner unit 107 to perform ON/OFF driving of a laser beam emitted by a

semiconductor laser (not shown) in accordance with image data supplied from the printer controller 103. The laser beam emitted by the semiconductor laser is oscillated in the scanning direction by a rotating polygon mirror. The laser beam oscillated in the main scanning direction is guided to the photosensitive drum 105 via a reflecting mirror 109 to expose the surface of the photosensitive drum 105 in the main scanning direction.

[0033] An electrostatic latent image which is charged by a primary charger 111 and formed on the photosensitive drum 105 by scan exposure with a laser beam is visualized into a toner image with toner supplied from a developing unit 112. The toner image visualized on the photosensitive drum 105 is

transferred onto the intermediate transfer member 106 to which a voltage opposite in characteristic to the toner image is applied (primary transfer) . In color image formation, images of respective colors are sequentially formed onto the intermediate transfer member 106 from a Y (Yellow) station 120, M (Magenta) station 121, C (Cyan) station 122, and K (blacK) station 123. As a result, a full-color visual image is formed on the intermediate transfer member 106.

[0034] Then, the sheet 110 fed from a storage 113 is conveyed. Simultaneously when transfer rollers 114 press the sheet 110 against the intermediate transfer member 106, a bias opposite in characteristic to the toner is applied to the transfer rollers 114. The visual image formed on the intermediate transfer member 106 is then transferred onto the sheet 110 which is synchronously fed in the sub-scanning direction by the sheet feed processing mechanism (secondary transfer) . Note that the photosensitive drum 105 and developing unit 112 are detachable.

[0035] An image formation start position detection sensor 115, sheet feed timing sensor 116, and density sensor 117 are arranged around the intermediate

transfer member 106. The image formation start

position detection sensor 115 determines a printing start position when performing image formation. The sheet feed timing sensor 116 adjusts the sheet feed timing of the sheet 110. The density sensor 117 measures the density of a measurement image (patch) in density control. When density control is performed, the density sensor 117 measures the density of each patch . [0036] The fixing processing mechanism includes a first fixing unit 150 and second fixing unit 160 for fixing a toner image transferred to the sheet 110 by heat and pressure. The first fixing unit 150 includes a fixing roller 151 for applying heat to the sheet 110, a pressure belt 152 for pressing the sheet 110 against the fixing roller 151, and a post-fixing sensor 153 for detecting the completion of fixing. The fixing roller 151 is hollow, incorporates a heater (not shown) , and is configured to convey the sheet 110 simultaneously when the fixing roller 151 is driven to rotate. The second fixing unit 160 is positioned downstream of the first fixing unit 150 in the conveyance path of the sheet 110. The second fixing unit 160 is arranged to add gloss to a toner image on the sheet 110 that has been fixed by the first fixing unit 150, and ensure fixation. Similar to the first fixing unit 150, the second fixing unit 160 also includes a fixing roller 161, pressure roller 162, and post-fixing sensor 163.

[0037] Depending on the type of sheet 110, the sheet 110 need not pass through the second fixing unit 160. To reduce the energy consumption in this case, a conveyance path 130 is arranged to discharge the sheet 110 without passing through the second fixing unit 160. A conveyance path switching flapper 131 can guide the sheet 110 to the conveyance path 130.

[0038] A conveyance path switching flapper 132 guides the sheet 110 to a conveyance path 135. After a reversing sensor 137 detects the position of the sheet 110, a reversing unit 136 performs a switchback

operation to swap over the leading end of the sheet 110.

[0039] A color sensor 200 configured to detect a patch image on the sheet 110 is further arranged

downstream of the second fixing unit 160 in the

conveyance direction. An instruction about a color detection operation is issued in accordance with an instruction from an operation unit 180. Based on the detection result, the engine control unit 312 executes density adjustment, tone adjustment, and multicolor ad ustment .

[0040] (Color Sensor)

The structure and measurement operation of the color sensor 200 will be explained. Fig. 2 is a view showing the structure of the color sensor 200 in the embodiment. The color sensor 200 incorporates an LED light source 201, a diffraction grating 202, line sensors 203, that is, 203-1 to 203-s, a calculation unit 204, and a memory 205. The LED light source 201 irradiates, with white light, each toner patch (to be referred to as a patch hereinafter) 207 on a sheet 208. The diffraction grating 202 separates, for respective wavelengths, light which has been reflected by the patch 207 and passed through a window 206. The line sensors 203, that is, 203-1 to 203-s are constructed by s pixels each serving as a light-receiving element configured to detect light decomposed for respective wavelengths by the diffraction grating 202. The calculation unit 204 performs spectral calculation from the light intensity values of the respective pixels that are detected by the line sensors 203. The memory 205 saves various data.

[0041] The arrangement of the color sensor 200 may incorporate a lens which converges light emitted by the LED light source 201 to the patch 207 on the sheet 208 and converges light reflected by the patch 207 to the diffraction grating 202. The color sensor 200 measures light reflected by a white reference plate 210. The white reference plate 210 may include an

attaching/detaching mechanism. The white reference plate 210 may be moved to the vicinity of the position of the sheet 208 or moved to a position where the white reference plate 210 comes into contact with the reverse surface of the sheet 208 even in patch image

measurement .

[0042] Fig. 6A is a block diagram showing a sensor and control unit according to the embodiment. A CPU 3001 serving as a control unit performs light quantity setting of the LED light source 201 and data readout from the memory 205. The CPU 3001 issues an

instruction about the sheet conveyance timing to a sheet conveying unit 3002. The CPU 3001 controls an attaching/detaching motor 3003 to perform the attaching operation of the white reference plate 210 to a

predetermined position, and the attaching operation to the reverse surface of a sheet in patch image

measurement .

[0043] (White Reference Plate)

The white reference plate 210 is used to adjust the light quantity of the LED light source 201 and calculate the correction coefficient h{X) . The white reference plate 210 is desired to highly resist light, in order to suppress aged deterioration, and be strong for the attaching/detaching operation. To achieve this, for example, a white reference plate obtained by

performing ceramic processing for aluminum oxide is used.

[0044] The correction coefficient η(λ) is

calculated from a difference between a spectral

reflectance obtained when a standard measurement device

(not shown) serving as a criterion measured a white reference plate, and a spectral reflectance obtained when the color sensor 200 measured the same white reference plate. For example, assume that the spectral reflectance hs(500) corresponding to the wavelength 500 [nm] obtained when the white reference plate is measured by the standard measurement device. Similarly, assume that the spectral reflectance hu(500)

corresponding to the wavelength 500 [nm] obtained when the white reference plate is measured by the color sensor 200 is 85%. In this case, a correction

coefficient h(500) to correct the spectral reflectance from 85% to 91% is calculated by: h(500) =hs (500) /hu (500)

[0045] The value of the correction coefficient h (λ) of the white reference plate 210 is written in the memory 205 of the color sensor 200 in factory assembly or replacement of the color sensor. One white

reference plate is prepared for one color sensor. For example, when four color sensors are arranged in the image forming apparatus main body, four white reference plates are also arranged to be paired with the

respective color sensors.

[0046] [Basic Operation]

Next, an arrangement in which the result of detection by the color sensor 200 is fed back in the printer 100 will be described.

[0047] (Description of Basic Sequence of

Adjustment)

A basic sequence to generate a profile and output an image by using the profile in the printer 100 according to the embodiment will be explained. In the embodiment, an ICC (International Color Consortium) profile is used as a profile for implementing high color reproducibility. However, the present invention is not limited to this. For example, CRD (Color

Rendering Dictionary) employed from level 2 of

PostScript proposed by Adobe, a color separation table in Photoshop®, a CMYK simulation in ColorWise available from EFI to maintain black print information, or the like is also usable.

[0048] (Measurement of Spectral Reflectance and

Calculation of Color Value)

As shown in Fig. 1, the printer 100 according to the embodiment includes the color sensor 200 serving as a reading means on the upstream side of a post-fixing discharge tray (not shown) in the conveyance direction. The printer 100 can measure a spectral reflectance by using the color sensor 200. The printer 100 converts the measurement result into a color value, and

generates a color conversion profile by itself. The printer 100 performs color conversion processing by using the generated color conversion profile.

[0049] A calculation method of color value will be explained. A signal input by measuring a color by the color sensor 200 is detected on a CMOS sensor (line sensor 203) arranged in each wavelength region of 380 nm to 720 nm after light emitted by the LED light source 201 is reflected by a measurement target object and the reflected light is separated by the diffraction grating 202. The spectral reflectance is measured for the input signal. In the present invention, the spectral reflectance is converted into L*a*b* values via color matching functions or the like, as defined by CIE, in order to improve the detection calculation accuracy .

[0050] The color values (L*a*b*) of the patch image are determined based on the measurement result of the patch image. And then, CPU3001 sets the ICC profile based on the color values of the patch image and the signal values used by the engine unit to form the patch image.

[0051] (L*a*b* Calculation)

The following is a method (step) of calculating color values (L*a*b*) from the spectral reflectance. This method is defined by IS013655.

[0052] a. The spectral reflectance R( ) of a sample is obtained (380 nm to 780 nm) .

[0053] b. Color matching functions χ(λ), y {λ) , and ζ(λ) and a standard light spectral distribution SD50 (λ) are prepared. Note that the color matching functions are defined by JIS Z8701. 3Ο50(λ) is defined by JIS Z8720 and is also called a supplementary standard illuminant D50.

[0054] c. Wavelengths are obtained using the prepared functions:

R(X)x SD50( )X ζ(λ) [0055] d. Integration of the respective

wavelengths is performed:

[0056] e. The product of the color matching function y (X) and standard light spectral distribution

SD50 (λ) is integrated with respect to the respective wavelengths :

∑{SD50{X) x y{X)}

[0057] f . X, Y, and Z are calculated by:

[0058] g. L*a*b* values are calculated For Y/Y_n

> 0.008856, L*a*b* values are calculated by:

a* = 500

where X_n, Y_n, and Z_n are the tristimulus values of standard light:

[0059] Note that χ(λ), γ(λ), and z (λ) are also described as χ(λ) , γ(λ) , and ζ(λ) , respectively.

[0060] (Profile Generation Processing)

In part replacement by a customer engineer, before a job (a print processing) requiring color matching accuracy, or when the user wants to know the tint of a final output material at the design planning stage or the like, the user operates the operation unit 180 to perform color profile generation processing.

[0061] The profile generation processing is performed in the printer controller 103 shown in the control block diagram of Fig. 3. First, a profile generation instruction is input to a profile generation unit 301 via the operation unit 180. The profile generation unit 301 sends a signal to a engine unit (engine control unit 312) to output a CMYK color chart of the IS012642 test form (the patch images) without the mediacy of a profile. At the same time, the profile generation unit 301 sends a measurement

instruction to a color sensor control unit 302. In the printer 100, the IS012642 test form (the patch images) is transferred and fixed to the sheet 110 by processes such as charging, exposure, development, transfer, and fixing, and the color sensor 200 measures the color. Spectral reflectance data of the measured patch is input to the printer controller 103, and converted into L*a*b* data by a Lab calculation unit 303. The L*a*b* data are converted in accordance with a profile stored in an input ICC profile storage unit 304 for the color sensor, and are input to the profile generation unit 301.

[0062] Note that the conversion format is not limited to L*a*b*, and spectral reflectance data may be converted into a device-independent color space signal in the CIE1931XYZ colorimetric system.

[0063] The profile generation unit 301 generates an output ICC profile based on the relationship between the output CMYK patch signals and the input L*a*b* data (converted data) , and replaces, with it, an output ICC profile stored in an output ICC profile storage unit 305.

[0064] The IS012642 test form includes CMYK color signal patches which cover a color reproduction gamut outputtable by a general copying machine. A color conversion table is generated from the relationship between the respective color signal values and the measured L*a*b* values. That is, a CMYK → Lab

conversion table (A2B x tag) is generated. Based on the conversion table, an inverse conversion table (B2A x tag) is generated.

[0065] The ICC profile has a structure as shown in

Fig. 4, and is made up of a header, tags, and their data. In addition to the color conversion table, the tags describe even a tag (gamt) representing whether a given color expressed by a white point (Wtpt) or a Lab value defined in the profile falls inside or outside the reproducible range of the hard copy.

[0066] When a profile generation instruction is accepted from an external connection device (PC or the like) via an external I/F 308, a generated output ICC profile may be uploaded to the external device so that the user can perform color conversion by an application corresponding to the ICC profile.

[0067] (Color Conversion Processing)

In color conversion for a normal color output, an image signal input on the premise of R, G, and B signal values input via the external I/F 308 such as a scanner unit, or standard printing CMYK signal values such as Japan Color values are sent to an input ICC profile storage unit 307. The input ICC profile storage unit 307 performs RGB → L*a*b* conversion or CMYK —^■ L*a*b* conversion in accordance with the image signal input from the external I/F 308. The input ICC profile is constructed by a one-dimensional LUT which controls an input signal, a multicolor LUT called direct mapping, 4 052592

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and a one-dimensional LUT which controls the generated conversion data. The input image signal is converted from a device-dependent color space into device- independent L*a*b* data by using these tables.

[0068] The image signals converted into values in the L*a*b* color space coordinate system are input to a CMM (Color Management Module) 306. The image signals undergo gamut conversion, color conversion, black

character determination, and the like. In gamut

conversion, a mismatch in the output color reproduction range between the reading color space of the external I/F 308 such as a scanner unit serving as an input device, and the printer 100 serving as an output device is mapped. In color conversion, a mismatch (also

called a color temperature setting mismatch) between a light source type in input and a light source type in observing an output material is adjusted. Accordingly, L*a*b* data are converted into L*'a*'b*' data, which are input to the output ICC profile storage unit 305. As described above, the generated profile is stored in the output ICC profile storage unit 305. Color

conversion is performed based on the newly generated ICC profile to convert the L*'a*'b*' data into CMYK signals dependent on an output device. The CMYK

signals are then output.

[0069] In Fig. 3, the CMM 306 is described

separately for the input ICC profile storage unit 307 and output ICC profile storage unit 305 in terms of the block arrangement. However, the CMM 306 is a module which performs color management, as shown in Fig. 5, and is assumed to perform color conversion by using an input profile and output profile.

[0070] [Contents of Embodiment]

The basic operation in measurement of a spectral reflectance by the color sensor, calculation of a color value, Lab calculation, generation of an ICC profile, and color conversion profile processing has been explained. Details of a method of correcting

fluctuations of the dark current value of the line sensor 203 according to the present invention will be described below.

[0071] Fig. 7A shows a sequence by the CPU 3001 in the measurement operation of the white reference plate 210. At the timing when there is no sheet between the white reference plate 210 and the color sensor 200 after the end of a previous job before measuring a patch image, the measurement operation of the white reference plate 210 starts. In step S301, the CPU 3001 instructs the attaching/detaching mechanism of the white reference plate 210 to start the attaching operation. In step S302, the CPU 3001 determines whether the attaching operation of the white reference plate 210 has ended. As the attaching operation end determination method, for example, there are a method of waiting for the time taken for the attaching

operation, and a method of separately arranging a detection means for detecting whether the white

reference plate 210 has reached the attaching position, and confirming the attaching operation. Either method can be used. If the attaching operation is completed (YES in step S302), the process advances to step S303. In step S303, the CPU 3001 designates the ON operation of the LED light source 201. In step S304, the CPU 3001 instructs the color sensor 200 to start

measurement of the quantity ΐί(λ) of light reflected by the white reference plate 210 for each wavelength. In step S305, the CPU 3001 designates the start of the detaching operation of the white reference plate 210 upon completion of measuring the reflected light quantity ϊί(λ). After that, the measurement operation ends .

[0072] [Apparatus Arrangement]

Fig. 6A is a block diagram showing the sensor and control unit according to the first embodiment. The CPU 3001 performs light quantity setting of the LED light source 201, designation of the start of

measurement, write and readout of measurement data in and from the memory 205, and dark current correction with respect to the color sensor 200. Also, the CPU 3001 instructs the sheet conveying unit 3002 about the sheet conveyance timing in patch image measurement. The CPU 3001 controls the attaching/detaching motor 3003 to perform the attaching operation of the white reference plate 210 to a predetermined position in calibration and the attaching operation to the reverse surface of a sheet in patch image measurement. In the above-described attaching operation, the white

reference plate 210 is assumed to be moved to a

position spaced apart by a predetermined distance from the reverse surface of a sheet.

[0073] [Patch Image Measurement Processing]

A measurement operation of measuring fluctuations of the dark current value of the line sensor 203 and deriving the measurement value Ρ'(λ) of light reflected by a patch image will be explained with reference to Figs. 7B and 8.

[0074] Fig. 8 is a view showing an image of a color measurement chart according to the embodiment. In the color measurement chart, a plurality of (M) color patch images are arranged in the sheet conveyance direction at positions facing the color sensor 200. Ρ(λ) is measured at the timing when each color patch image passes through a position facing the color sensor 200. As a patch 207-1 to be measured first, a high- density patch is printed. The timing when the Ρ(λ) value when a patch passes on the measurement region of the color sensor 200 has changed from the value of the blank portion of a sheet is detected as a trigger to determine the timing of subsequent patch image

measurement .

[0075] Fig. 7B shows a sequence by the CPU 3001 in the Ρ(λ) measurement operation. The Ρ(λ) measurement operation starts after measurement of the white reference plate 210 which has described with reference to Fig. 7A. In step S401, the CPU 3001 instructs the sheet conveying unit to supply the color measurement chart. In step S402, the CPU 3001 measures the dark current value Ό(λ) of the line sensor 203. In step S403, the CPU 3001 saves the dark current measurement result in the memory 205. The Ό{λ) is corresponded to each of the plurality of light-receiving elements and stored. In step S404, the CPU 3001 designates the ON operation of the LED light source 201. In step S405, the CPU 3001 performs a wait operation for a

predetermined time. The wait operation is performed in order to wait until the line sensor 203 changes to the thermal equilibrium state by reflected light. In step S406, the CPU 3001 detects the non-light-receiving region of the first to nth (n is an arbitrary natural number) pixels in the line sensor 203. The detection method will be described later. The light-receiving region means pixels which receive light reflected by a measurement target (in this case, a patch or the like) among a plurality of pixels (detection region) included in the line sensor 203. To the contrary, the non- 2592

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light-receiving region means pixels which do not

receive the reflected light.

[0076] If the CPU 3001 determines that the region of the first to nth pixels is the non-light-receiving region (YES in step S406) , it calculates a dark current fluctuation in step S407. If the CPU 3001 determines that the region of the first to nth pixels is not the non-light-receiving region (NO in step S406) , it

calculates a dark current fluctuation in the sth to (s- n)th pixels (s is the total number of pixels of the line sensor 203) in step S408. In step S409, the CPU 3001 saves the dark current fluctuation calculation result in the memory 205. In step S410, the CPU 3001 detects the timing when the first patch 207-1 passes through the measurement region of the color sensor 200. In step S411, the CPU 3001 sets an initial value of 1 in a variable m representing the number of measured patches. In step S413, the CPU 3001 designates

measurement of Ρ(λ) of the patch.

[0077] In step S413, the CPU 3001 determines

whether the measurement count has reached a

predetermined patch count M (m > M) . As shown in Fig. 8, the predetermined patch count M corresponds to the number of patches printed on a sheet. If the

measurement count has not reached the predetermined patch count M (NO in step S413) , the CPU 3001

increments the patch measurement count m (S414). In step S415, the CPU 3001 waits the measurement operation for the time based on a predetermined patch interval. The time based on the predetermined patch interval is determined by the interval between patches shown in Fig. 8 and the sheet conveyance speed. Then, the process shifts to step S412 to sequentially measure Ρ( λ ) of an unmeasured patch.

[0078] If the measurement count has reached the predetermined patch count M (YES in step S413), the CPU 3001 performs dark current correction for the

measurement value obtained by the line sensor 203 in step S416. The dark current correction method will be described later. In step S417, the CPU 3001 outputs the measurement result of the patch obtained by a calculation method (to be described later) . After that, the patch image color measurement operation ends.

[0079] In the flowchart shown in Fig. 7B, a dark current value before irradiation by the LED light source 201 is measured in step S402, and a dark current value after irradiation by the LED light source 201 (that is, a dark current value in the thermal

equilibrium state) is measured in step S407 or S408.

The measurement value is corrected based on the values before and after irradiation by the LED light source 201.

[0080] (Non-light-receiving Region Determination

Method) The method of determining the non-light-receiving region of the line sensor 203 after LED emission, and the method of calculating a dark current fluctuation in steps S406 to S408 of Fig. 7B will be described in detail .

[0081] Figs. 9A and 9B are graphs each showing the output fluctuation of the line sensor 203 in patch image measurement. In Figs. 9A and 9B, the ordinate represents the output value of the line sensor 203, and the abscissa represents each pixel forming the line sensor 203. In Figs. 9A and 9B, a dark current value indicated by a broken line is the dark current value of the line sensor 203 that is measured in step S402 of Fig. 7B. An output in measurement indicated by a solid line is an output from the line sensor 203 after emission of the LED light source 201. Fig. 9B is an enlarged view showing a portion of the 0th to 15th line sensor outputs among line sensor outputs shown in Fig. 9A. As shown in Fig. 9B, the end of the line sensor 203 has a non-light-receiving region where no light enters. Hence, a dark current output fluctuation a can be extracted from n pixels at the end. Calculation of a is given by: ar =— ...(2)

n

where P(i) is a line sensor output value in the ith pixel, and D(i) is a dark current value in the ith pixel. The D(i) means D (λ) corresponding to the ith light-receiving element.

[0082] The calculated a is compared with a preset threshold β to specify a non-light-receiving region. At this time, if α < β (that is, a is smaller than the threshold) , a is regarded as a dark current output fluctuation and is saved as a dark current correction value in the memory 205.

[0083] If a > β, it is determined that the

position of the line sensor 203 shifts and the first to nth pixels at the left end of the line sensor 203 receive light, as shown in Fig. 11B. Fig. 11A shows a state in which a plurality of pixels positioned at the left end of the line sensor 203 form a non-light- receiving region. In contrast, Fig. 11B shows a state in which a plurality of pixels positioned at the right end of the line sensor 203 form a non-light-receiving region. The state in Fig. 11A corresponds to the output state shown in Figs. 9A and 9B. The state in Fig. 11B corresponds to the output state shown in Figs. 10A and 10B. Fig. 10B is an enlarged view of part of Fig. 10A.

[0084] If it is determined that the first to nth pixels at the left end of the line sensor 203 receive light (NO in step S406) , a dark current output

fluctuation a' in the sth to (s-n)th pixels at the right end is calculated in step S408 of Fig. 7B. Calculation of a' is given by:

∑ (P(i) - D(i))

n

where s is the number of pixels included in the line sensor 203, and s-n is the nth pixel from the right end of the line sensor 203.

[0085] After obtaining a' by the above-described method, a' is compared with the threshold β, similar to step S407. If α' < β, α' is regarded as an output fluctuation and is saved as a dark current correction value in the memory 205.

[0086] (Dark Current Correction Method)

Details of dark current correction calculation in step S416 of Fig. 7B will be described. The

measurement value Ρ'(λ) after correction of light reflected by a patch image is calculated by:

P" (λ) = Ρ(λ) - ϋ(λ) - a (or α') ...(4) α and α' are the same value for all light-receiving pixels .

[0087] Based on Ρ'(λ) obtained by equation (4) and ( ) measured in advance, the calculation unit 204 in the color sensor 200 calculates the spectral

reflectance R{X) of the patch image according to:

spectral reflectance R( ) of patch image = —

W( )

• · . (5)

[0088] Based on a dark current-corrected value, the spectral reflectance is obtained.

[0089] (Effects)

According to the present invention, fluctuations of the dark current value of the line sensor 203 are detected in real time without arranging light-shielded light-receiving elements dedicated to correction. Thus, there can be provided an image forming apparatus which performs high-accuracy color measurement by correcting a dark current, as needed, and performs color

adjustment in accordance with the detection result.

[0090] Note that the embodiment has explained an arrangement in which a dark current fluctuation is calculated only once. However, a dark current

fluctuation may be measured at every interval between patches to correct the measurement result of every patch in order to improve the accuracy. Also, while the color sensor 200 receives the reflected light from the patch image, CPU 3001 may calculate the output fluctuation a (or a') based on a output value (a dark current value) of a non-light-receiving pixel

corresponding to a non-light-receiving region of the line sensor 203. By this construction, the apparatus can detect the dark current value on a real-time basis and measure a color with higher accuracy. Also, the embodiment has explained an arrangement in which one color sensor measures the colors of patch images

arranged in line on a sheet. However, even when the colors of patch images of a plurality of lines are measured using a plurality of color sensors, the measurement results can be corrected similarly.

[0091] <Second Embodiment>

As another embodiment of the present invention, an arrangement will be described, in which fluctuations in a non-light-receiving region are estimated based on a temperature change to correct an output value.

[0092] [Apparatus Arrangement]

Fig. 6B is a block diagram showing a sensor and control unit according to the second embodiment. In addition to the arrangement shown in Fig. 6A in the first embodiment, a color sensor 200 further includes a thermistor 211 serving as a temperature detection unit. A CPU 3001 further instructs the color sensor 200 to detect a temperature by the thermistor 211.

[0093] [Patch Image Measurement Processing]

A measurement operation of measuring fluctuations of the dark current value of a line sensor 203 based on a temperature detection result and deriving the

measurement value P' (λ) of light reflected by a patch image will be explained with reference to Figs. 7C and 8. A color measurement chart has the same layout as that in Fig. 8 described in the first embodiment.

[0094] Fig. 7C shows a sequence by the CPU 3001 in the Ρ(λ) measurement operation. The Ρ(λ) measurement operation starts after measurement of a white reference plate 210 which has described with reference to Fig. 7A. In step S501, the CPU 3001 instructs a sheet conveying unit to supply the color measurement chart. In step S502, the CPU 3001 measures the dark current value of the line sensor 203. In step S503, the CPU 3001 saves the dark current measurement result in a memory 205.

In step S504, the CPU 3001 designates the ON operation of an LED light source 201. In step S505, the CPU 3001 performs a wait operation for a predetermined time.

The wait operation is performed to wait until the line sensor 203 changes to the thermal equilibrium state by reflected light.

[0095] In step S506, the CPU 3001 instructs the thermistor 211 to detect a temperature. Based on the result of the temperature detected in step S507, the CPU 3001 compares the temperature detection value with a threshold T (threshold for estimating a preset non- light-receiving region) by a method (to be described later) . If the temperature detection value is smaller than the threshold T, the CPU 3001 estimates that the first to nth pixels among pixels included in the line sensor 203 form a non-light-receiving region. Note that pixels forming a non-light-receiving region or the fluctuation amount of a non-light-receiving region may be defined in advance in a table or the like in

accordance with a detected temperature. In this case, the determination is made using the table representing the relationship between a detected temperature and the pixels of a non-light-receiving region.

[0096] If the detected temperature is lower than the threshold T (YES in step S507), the CPU 3001 calculates a dark current fluctuation from the first to nth pixels of the line sensor 203 (step S508). If the detected temperature is equal to or higher than the threshold T (NO in step S507), the CPU 3001 calculates a dark current fluctuation from the sth to (s-n)th pixels of the line sensor 203 (step S509) . In step S510, the CPU 3001 saves the dark current fluctuation calculation result in the memory 205. In step S511, the CPU 3001 detects the timing when a first patch 207- 1 passes through the measurement region of the color sensor 200. In step S512, the CPU 3001 sets an initial value of 1 in a variable m representing the number of measured patches. In step S513, the CPU 3001

designates measurement of Ρ(λ) of the patch.

[0097] In step S514, the CPU 3001 determines whether the measurement count has reached a

predetermined patch count M (m > M) . If the

measurement count has not reached the predetermined patch count M (NO in step S514), the CPU 3001

increments the patch measurement count m (S515) . In step S516, the CPU 3001 waits the measurement operation for the time based on a predetermined patch interval. The time based on the predetermined patch interval is determined by the interval between patches shown in Fig. 8 and the sheet conveyance speed. Then, the process shifts to step S513 to sequentially measure P (λ) of an unmeasured patch. If the measurement count has reached the predetermined patch count M (YES in step S514), the CPU 3001 performs dark current correction for the measurement value obtained by the line sensor 203 in step S517. In step S518, the CPU 3001 outputs the measurement result of the patch obtained by a

calculation method (to be described later) . After that, the patch image color measurement operation ends.

[0098] (Non-light-receiving Region Determination

Method)

The method of determining the non-light-receiving region of the line sensor 203 after LED emission based on the temperature detection result in step S507 of Fig. 7C, and the method of calculating a dark current

fluctuation will be described in detail.

[0099] In the second embodiment, Figs. 9A and 9B each show the output fluctuation of the line sensor 203 in patch image measurement when the temperature

detection result is smaller than T. In Figs. 9A and 9B, a dark current value indicated by a broken line is the dark current value of the line sensor 203 that is measured in step S502 of Fig. 7C. An output in

measurement indicated by a solid line is an output from the line sensor 203 after emission of the LED light source 201. The remaining part is the same as that in the first embodiment, and a description thereof will not be repeated.

[0100] If the temperature detection result becomes equal to or higher than T, it is estimated that the position of the line sensor 203 shifts upon a

temperature change and the first to nth pixels at the left end of the line sensor 203 receive light, as shown in Fig. 11B. Hence, the dark current output

fluctuation a' in the sth to (s-n)th pixels at the right end is calculated (step S509) . The calculation method is the same as that in the first embodiment, and a description thereof will not be repeated. The dark current correction method in step S517 is also the same as that in the first embodiment, and a description thereof will not be repeated.

[0101] (Effects)

As described above, the second embodiment can obtain the same effects as those of the first

embodiment .

[0102] Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable

instructions recorded on a storage medium (e.g., non- transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment (s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above- described embodiment (s) . The computer may comprise one or more of a central processing unit (CPU) , micro processing unit (MPU) , or other circuitry, and may include a network, of separate computers or separate computer processors. The computer executable

instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM) , a read only memory (ROM) , a storage of distributed computing

systems, an optical disk (such as a compact disc (CD) , digital versatile disc (DVD), or Blu-ray Disc (BD)™) , a flash memory device, a memory card, and the like.

[0103] While the present invention has been

described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such

modifications and equivalent structures and functions.

[0104] This application claims the benefit of

Japanese Patent Application No. 2013-031424, filed

February 20, 2013, which is hereby incorporated by reference herein in its entirety.

Claims

1. A measurement apparatus characterized by

comprising :

a light emitting means for emitting a light;

a light receiving means for receiving a reflected light from a measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light-receiving elements;

a determination means for determining correction information based on a first signal which is output by a predetermined light-receiving element of the

plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the

predetermined light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and

a correction means for correcting the spectral reflectance information output by the light receiving means based on the correction information determined by the determination means.

2. The measurement apparatus according to claim 1, characterized in that the predetermined light-receiving element is, of the plurality of light-receiving

elements, a light-receiving element which is located outside of a region that receives the reflected light from the measurement target.

3. The measurement apparatus according to claim 2, characterized in that the plurality of light-receiving elements includes a first light-receiving element which is located outside of the region and a second light- receiving element which is located outside of the region and is different from the first light-receiving element, and

wherein the determination means determines, based on a output result by the first light-receiving element in the second state, whether the first light-receiving element should be set as the predetermined light- receiving element or the second light-receiving element should be set as the predetermined light-receiving element .

4. The measurement apparatus according to claim 3, characterized in that,

if the output result by the first light-receiving element in the second state is lower than a predefined value, the determination means determines the first light-receiving element as the predetermined light- receiving element, and

if the output result by the first light-receiving element in the second state is not lower than the predefined value, the determination means determines the second light-receiving element as the predetermined light-receiving element.

5. The measurement apparatus according to any one of preceding claims, the determination means determines the correction information based on the first signal, the second signal, and other signal which is output, in the second state, by other light-receiving element which is different from the predetermined light- receiving element of the plurality of light-receiving elements .

6. The measurement apparatus according to any one of preceding claims, characterized in that the light receiving means is a line sensor in which the plurality of the light-receiving elements are arranged in line, and

wherein the light receiving means further

comprises a diffraction grating which diffracts the reflected light from the measurement target.

7. The measurement apparatus according to any one of preceding claims, characterized in that the light receiving means outputs the spectral reflectance information based on a light quantity of each

wavelength of the reflected light from the measurement target .

8. The measurement apparatus according to any one of preceding claims, characterized in that the spectral reflectance information is information indicating a spectral reflectance of the measurement target.

9. A measurement apparatus characterized by

comprising :

a light emitting means for emitting a light;

a light receiving means for receiving a reflected light from a measurement target, and outputting

spectral reflectance information, the light receiving means comprises a plurality of light-receiving

elements;

a detection means for detecting a temperature of the light emitting means;

a determination means for determining a target light-receiving element of the plurality of light- receiving elements based on the temperature detected by the detection means, and determining correction

information based on a first signal which is output by the target light-receiving element of the plurality of light-receiving elements in a first state in which the light emitting means does not emit a light and a second signal which is output by the target light-receiving element of the plurality of light-receiving elements in a second state in which the light emitting means emits a light; and

10. An image forming apparatus including a

measurement apparatus, characterized in that

the measurement apparatus comprises:

a light emitting means for emitting a light;

elements ;

11. An image forming apparatus including a

measurement apparatus, characterized in that

the measurement apparatus comprises:

a light emitting means for emitting a light;

a light receiving means for receiving a reflected light from a measurement target, and outputting spectral reflectance information, the light receiving means comprises a plurality of light-receiving

elements;

a detection means for detecting a temperature of the light emitting means;