JP2008081613A - Planographic ink printing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a planographic ink printing method by which the color reproduction region of RGB, especially B (blue violet), having been expressed by five to seven color printing methods using conventional yellow, red, indigo and red process four colors and further special colors such as violet can be reproduced by the overprinting of red and indigo. <P>SOLUTION: This planographic ink printing method excellent in the color reproductivity of high chromas, comprising overprinting a red ink having L*: 50 to 55, a*: 75 to 83, and b*: -14 to -20, and an indigo ink having L*: 52 to 58, a*: -40 to -45, and b*: -45 to -53 in chromaticity (JIS Z 8729) in a L*a*b* color system, when the red and the indigo are printed in concentrations of 1.52 to 1.56 and 1.63 to 1.67, respectively, is characterized in that the chromaticity of the overprinted portion is in ranges of L*: 23 to 29, a*: 28 to 33, b*: -63 to -68. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention is a lithographic ink printing method comprising four colors of yellow, red, indigo and black, and uses an ink system which is particularly excellent in color reproducibility of purple (blue violet) hues by overprinting red and indigo. Related to the printing method.

The IT revolution that began in the 1990s has led the environment surrounding the printing site to the direction of digitalization. By this digitization, workflow of conventional printing methods (photographing, positive, scanning, data, design, EPS)・ Imposition, film, printing plate, printing) was a multi-stage process, but it has succeeded in dramatically shortening the process with digital camera shooting, DTP, CTP, and printing. As a result, “RGB” conversion of the submitted data is being standardized, and the current situation is that the data to be handled is shifting to a wider color reproduction region.
However, in the lithographic offset printing consisting of the four main colors (CMYK) of yellow, red, indigo, and black, which are currently mainstream, because the hue is due to subtractive color mixing, the data submitted as “RGB” is narrower Color separation into CMYK in the color reproduction area, as well as turbidity in hue each time the color is superimposed, inevitably the color reproduction area becomes narrower than that of RGB, and color reproducibility between digital data and printed matter The difference became a problem. In particular, the purple (blue violet, equivalent to RGB “B”) hue expressed in two colors, red and indigo, is inferior in both saturation and lightness to the color reproduction region expressed by a single purple pigment. It was difficult to reproduce the “B” area of the “RGB” submission data with four colors (YMCK).
As means for solving these problems, in Patent Document 1, a printing method using an ink set of 5 to 7 colors is established as a high-saturation printing system. As a printing method using an ink set having each specified hue, process 4 colors are used. 6 colors (hexachrome printing) with orange and green added to 7 colors and 7 colors (high fidelity printing) with orange, green and purple added to 4 process colors have been established. In addition, as represented by hexachrome ink, methods such as adding fluorescent pigments to some colors as a means of suppressing secondary and tertiary turbidity and expanding the color reproduction range are also available. There are also disadvantages such as deterioration of the image quality (transfer defect, gloss reduction, etc.) and fading of printed matter due to insufficient light resistance. Furthermore, the number of ink colors used is 6 or 7, and an expensive multicolor printing machine with 6 or more printing cylinders is required. It is an indispensable condition, and it is limited to using this system due to huge capital investment and complicated color management to start a new one.

JP 2001-260516 A

The present invention has been made in order to solve such problems in the prior art, and the problem is that an RGB color reproduction region using a four-color printing machine that has been widely used in the past. In particular, it is to provide an ink for lithographic printing that can express the B (blue violet) region as much as possible.

In general, in order to expand the color reproduction region, it is necessary to approximate the ideal spectral reflectance curve of each color.
That is, the wavelength range in which a person recognizes color is 400 nm to 700 nm light (this wavelength is referred to as visible light). In yellow ink, the reflectance in the wavelength range of 500 nm to 700 nm is 100%, and the wavelength range of 400 nm to 500 nm. The red ink has a reflectance of 0%, and the red ink has a reflectance of 100% in the wavelength region of 400 nm to 500 nm and 600 nm to 700 nm, and a reflectance of 0% in the wavelength region of 500 nm to 600 nm. Therefore, it is said that it is ideal that the reflectance in the wavelength region of 400 nm to 600 nm is 100% and the reflectance in the wavelength region of 600 nm to 700 nm is 0% (the spectral reflectance of the ideal process ink) Curves are shown in Table 5).

However, the reflection spectrum of the ink composition for offset printing of yellow, red, indigo, and black, which is currently used in four process colors, is far from the ideal reflection spectrum. Since there is unnecessary absorption in the part that must be completely reflected, a turbid component of the ink exists, and the color reproducibility is narrowed.


That is, the present invention relates to the chromaticity according to the L * a * b * color system (JIS Z 8729) when printing is performed within the density range of 1.52 to 1.56 for red and 1.63 to 1.67 for indigo. )But,
Red ink, L *: 50 to 55, a *: 75 to 83, b *: -14 to -20
Indigo ink, L *: 52 to 58, a *: −40 to −45, b *: −45 to −53
A lithographic printing method characterized in that the above two colors of ink within the range are overprinted, wherein the ink containing CI Pigment Red 169 is used as the red ink, and the red ink is applied on the indigo ink. The chromaticity of the solid portion where the images are printed is L *: 23 to 29, a *: 28 to 33, b *: −63 to −68
The present invention relates to a lithographic printing method excellent in color reproduction with high saturation, which is characterized by being within the above range.

In addition, the present invention extends the color reproduction range of the L * a * b * color system with four process colors of yellow, red, indigo and black by combining the above-described ink with yellow and black ink. The present invention relates to a lithographic printing method as described above, wherein an ink containing CI Pigment Red 169 is used as a red ink.
Furthermore, these (a) red and (b) indigo relate to the above printing method having the following reflectance.
(A) In the wavelength region of 400 nm to 700 nm, when the maximum reflectance is 100%, the maximum reflectance in the wavelength region of 400 nm to 500 nm is 50% to 100%, and the reflectance in the wavelength region of 500 nm to 560 nm. A red ink containing 15 to 30% by weight of a red hue compound having a reflection spectrum of 1 to 20%, a reflectance of 630 nm to 700 nm and a reflectance spectrum of 90% to 100% based on the total weight of the ink.
(B) In the wavelength region of 400 nm to 700 nm, when the maximum reflectance is 100%, the reflectance in the wavelength region of 400 nm to 530 nm is 50 to 100%, and the reflectance of 600 nm to 700 nm is 1 to 30%. An indigo ink containing 10 to 25% by weight of an indigo hue compound having a spectrum with respect to the total weight of the ink.

By using the lithographic ink printing method provided by the present invention, in addition to the four colors of yellow, red, indigo, and black ink, the special colors such as purple have been used to express RGB, especially B (blue). Violet color reproduction area can be reproduced with red and indigo overprinting. Further, in the present invention, since a fluorescent pigment is not used as a means for improving the color reproduction region of the printed material, a highly saturated printed material can be obtained without deteriorating printability, fading with time of the printed material, and the like.

Next, the present invention will be described more specifically with reference to preferred embodiments.
The present invention comprises a pigment, a vehicle component obtained by heating and dissolving a synthetic resin, a vegetable oil, and a petroleum solvent together with a gelling agent such as aluminum stearate or aluminum chelate as necessary, and an auxiliary agent such as a friction-resistant agent. It is a lithographic ink consisting of four colors of yellow, red, indigo, and black. It is printed on ISO standard Japan color standard paper, for example, “Tokuhishi Art Double-Sided Plate / 110kg” manufactured by Mitsubishi Paper Industries, Inc., Gretag Macbeth When the density value measured with a D196 densitometer is in the range of 1.52 to 1.56 for red and 1.63 to 1.67 for indigo, L * a * b * Chromaticity according to the color system (JIS Z 8729) is red ink, L *: 50 to 55, preferably 51 to 54, a *: 75 to 83, preferably 76 to 81, b *: − 14 to -20, preferably -15 to -18, indigo ink, L *: 5 58, preferably 52 to 57, a *: −40 to −45, preferably −41 to −44, b *: −45 to −53, preferably −46 to −51. Furthermore, it is characterized in that it is in the range of L *: 23 to 29, a *: 28 to 33, and b *: −63 to −68 when the indigo ink × crimson ink is overprinted.

As a method for expressing the color reproduction area, an XYZ color system (CIE 1931 color system), an X ₁₀ Y ₁₀ Z ₁₀ color system (CIE 1964 color system), an L * a * b * color system (CIE 1976), Hunter Lab color system, Munsell color system, L * u * v * color system (CIE1976), and the like.
In the L * a * b * color system, “brightness” is expressed by L * as the degree of brightness that can be compared regardless of hue, and indicates that the color becomes brighter as L * increases and becomes darker as it decreases. . In addition, “hue” that differs depending on each color is indicated by a * and b * values, a * indicates a red (+) to green (−) direction, and b * indicates a yellow (+) to blue (−) direction, In each direction, the color becomes brighter as the absolute value increases, and the color becomes dull as it approaches 0. This makes it possible to digitize one color using L *, a *, and b *.
In the L * a * b * color system, the color reproduction area of RGB “B (blue violet)” is L * of 50 or less, preferably 10 to 40, and a * of 20 to 80, preferably 30 to 70. , More preferably 30 to 60, b * is in the region represented by -20 to 80, preferably -30 to -70, and the point where both numerical values of a * and b * of the corresponding color intersect is outside the circle. The higher the hue is, the higher the hue is.

 In addition to “lightness” and “hue”, there is “saturation (C)” as a method for digitizing the degree of vividness, which can be obtained by the following calculation formula.

Ｃに関しても同様に、絶対値が大きくなるに従って色鮮やかになり、値が小さくなるにつれてくすんだ色になることを示している。


更に、Ｌ*ａ*ｂ*表色系で表された個々の色が持つ数値より、微妙な色の違い（色差）も数値で表すことが可能になる。2つの色の色差（「ΔＥ」と表現）は、以下の計算式にて求め
ることができる。

Similarly, C shows that the color becomes brighter as the absolute value increases, and the color becomes dull as the value decreases.


Furthermore, subtle color differences (color differences) can be represented by numerical values as compared with numerical values of individual colors represented by the L * a * b * color system. The color difference (expressed as “ΔE”) between the two colors can be obtained by the following calculation formula.

※ここで、ΔＬ*、Δａ*、Δb*は、2色のＬ*ａ*ｂ*数値の差を表す
。
ΔＥの絶対値が小さいほど２つの色が近似しており、大きいほど異なっていることを示してい
る。

一つの印刷物（印刷物以外のカラースペースも含む）で表現できる全ての色再現領域を演色領域（ガモット）と呼ぶが、ガモットを表す最も簡便な方法として、ａ*を横軸、ｂ*縦軸とした2次元空間に、単色ベタ部（黄、紅、藍）、及び、単色ベタ刷り重ね部（黄×紅、紅×藍、藍×黄）計6色のａ*対ｂ*の値を、プロットした六角形の面積で表現することが可能である。ガモットの面積が広い程、色再現領域が広いことを示している。

* Here, ΔL *, Δa *, and Δb * represent the difference between the L * a * b * values of the two colors.
The smaller the absolute value of ΔE is, the closer the two colors are, and the larger the difference, the different.

All color reproduction areas that can be represented by a single printed matter (including color spaces other than printed matter) are called color rendering regions (gamut). The simplest way to represent gamut is as follows: a * is the horizontal axis, b * vertical axis In the two-dimensional space, the solid color part (yellow, red, indigo) and the solid color overprinting part (yellow x red, red x indigo, indigo x yellow) total of 6 colors a * vs b *, It can be expressed by the plotted hexagonal area. The larger the gamut area, the wider the color reproduction area.

 With respect to the yellow and black inks used in the present invention, all conventionally known oxidation polymerization type lithographic printing inks can be used.

 Examples of red pigments used in the present invention include rhodamine dyes such as rhodamine B, rhodamine 3G, rhodamine 6G, molybdenum, tungsten metal lake compounds, or ferrocyanide copper lake compounds. The content is necessarily 10 to 30% by weight, preferably 10 to 20% by weight, more preferably 11 to 18% by weight based on the total weight of the ink.

 As the red pigment used in the present invention, the C.I. Pigment Red 169 may be used alone, or two or more of the above pigments may be used in combination.

 Examples of indigo pigments include copper phthalocyanine compounds such as CI Pigment Blue 15: 3 and CI Pigment Blue 15: 4, and 10 to 25% by weight, preferably 12 to 22% by weight, based on the total weight of the ink. More preferably, the content is 14 to 19% by weight. Furthermore, C.I. Pigment Green 7 may be used as a complementary color by adding 5 to 15% by weight, preferably 8 to 11% by weight, based on the total weight of the indigo pigment.

  As the synthetic resin used in the present invention, rosin-modified phenol resin, petroleum resin, alkyd resin, rosin-modified alkyd resin, petroleum resin-modified alkyd resin, rosin ester and the like can be considered. Preferably, a rosin modified phenolic resin is used. The rosin-modified phenol resin is not particularly limited, but it is preferable to use a resin having a weight average molecular weight of 10,000 to 300,000. When the molecular weight is 10,000 or less, the viscoelasticity of the ink is lowered, and when it is 400,000 or more, the fluidity as the ink becomes insufficient.

  Examples of vegetable oils include those derived from vegetable oils such as palm kernel oil, palm oil, cottonseed oil, peanut oil, palm oil, corn oil, olive oil, linseed oil, corn oil, soybean oil, safflower oil, and tung oil, Those thermal polymerized oils and oxygen-blown polymerized oils can also be used. Moreover, in this invention, these vegetable oils may be used independently and can also be used in combination of 2 or more type.

  The petroleum solvent used in the ink has an aromatic hydrocarbon content of 1% or less, an aniline point of 75 to 95 ° C, preferably 80 to 95 ° C, and a boiling point of 260 to 350 ° C, preferably 280 to 350. It is a petroleum solvent in the range of ° C. If the aniline point is less than 75%, the ability to dissolve the resin is too high, which is not preferable because the ink setting property is slow, and if it exceeds 95 ° C., the resin solubility is poor, so Meat etc. are bad and not preferable. When the boiling point is less than 260 ° C., the evaporation of the ink solvent on the printing press increases, and the transferability of the ink to a roller, a blanket, a plate, or the like deteriorates due to the deterioration of the fluidity of the ink. Moreover, when it exceeds 350 degreeC, since drying of heat set type ink is inferior, it is unpreferable.

Further, the lithographic ink composition of the present invention includes a gelling agent, a pigment dispersant, a metal dryer, a drying inhibitor, an antioxidant, an anti-friction agent, an anti-set-off agent, and a nonionic surfactant as required. Additives such as agents and polyhydric alcohols can be used as appropriate.

EXAMPLES Next, although an Example demonstrates this invention further in detail, the scope of the present invention is not limited to these description Example. In addition, the part of the following description represents a weight part and% represents weight%.

Example of production of rosin-modified phenolic resin To a four-necked flask equipped with a stirrer, a cooler and a thermometer, 1000 parts of P-octylphenol, 850 parts of 35% formalin, 60 parts of 93% sodium hydroxide, 1000 parts of toluene, The reaction was carried out at 90 ° C. for 6 hours. Then, 125 parts of 6N hydrochloric acid and 1000 parts of tap water were added, stirred and allowed to stand, and the upper layer part was taken out to obtain 2000 parts of a toluene solution of a resole type phenol resin having a nonvolatile content of 49%. It was.
A four-necked flask equipped with a stirrer, a condenser with a water separator, and a thermometer was charged with 1000 parts of gum rosin, dissolved at 200 ° C. while blowing nitrogen gas, and 1800 parts of the resole solution produced above was added. After reacting at 230 ° C. for 4 hours while removing toluene, 110 parts of glycerin was added, and reacted at 260 ° C. for 10 hours, with an acid value of 20 or less, a weight average molecular weight of 50000, Shin Nippon Petrochemical Co., Ltd. AF Solvent 6 A rosin-modified phenol resin having a cloudiness temperature of 90 ° C. was obtained.

The cloudiness temperature refers to 90% by weight of AF solvent solvent manufactured by Nippon Petrochemical Co., Ltd. and 10% by weight of resin in a test tube, dissolved at 200 ° C., and gradually lowered to a cloudy temperature. And The lower the cloudiness temperature, the better the solubility of the resin in the solvent.

Example of varnish production 40 parts of rosin-modified phenolic resin, 15 parts of tung oil, 30 parts of soybean oil, 14 parts of AF Solvent No. 5 (solvent manufactured by Shin Nippon Petrochemical Co., Ltd.), ALCH (gelling agent manufactured by Kawaken Fine Chemical Co., Ltd.) 1.0 part to 19
The mixture was heated and stirred at 0 ° C. for 1 hour to obtain a varnish.

Ink Example (Red ink)
In the formulation shown in Table 1, C.I. I. Pigment Red 81 (Fanarose RNN-P, manufactured by Fuji Kasei Co., Ltd.) was subjected to primary dehydration by gradually adding and kneading varnish in a kneader at a temperature of 75 ° C. Next, vacuum dehydration was performed for 1 hour under conditions of a kneader temperature of 100 ° C. to 120 ° C. and a degree of vacuum of 76 mmHg, and secondary dehydration was performed so that the water content in the base ink was 0.5% or less. After dehydration, the remaining varnish and petroleum solvent were added, kneaded and diluted, and the undispersed base ink was taken out from the kneader. The extracted base ink was kneaded using a three roll with a roll temperature of 60 ° C. until the particle size became 7.5 microns or less with a dispersed particle system measuring machine (grind meter) to obtain red base ink 1. Subsequently, varnish, vegetable oil, a compound, a metal dryer, and a drying inhibitor were added to the base ink 1 according to the formulation shown in Table 2 to obtain a red ink 1.

Ink Example (Red ink)
In the formulation shown in Table 1, C.I. I. Pigment Red 169 (FanalPink D4810 manufactured by BASF) was mixed with varnish, and kneaded meat and red base ink 2 were obtained until the particle size became 7.5 microns or less with a dispersed particle measuring instrument (grindometer). Subsequently, vegetable oil, a compound, a metal dryer, and a drying inhibitor were added to the base ink 2 according to the formulation shown in Table 2 to obtain a red ink 2.

Ink Example (Red ink)
After mixing the base ink 1 and the base ink 2 prepared according to the formulation shown in Table 1 according to the formulation shown in Table 2, vegetable oil, petroleum solvent, compound, metal dryer, and drying inhibitor were further added to obtain a red ink 3.

Ink Example (Indigo Ink)
In the formulation of Table 2, C.I. I. Pigment Blue 15: 3 (LIONOL BLUE GLA-SD manufactured by Toyo Ink Manufacturing Co., Ltd.), C.I. I. Pigment Green 7 (LIONOL GREEN YS-2A manufactured by Toyo Ink Manufacturing Co., Ltd.) was mixed with varnish, and after milling with a dispersed particle system measuring machine (grind meter) to 7.5 microns or less, vegetable oil, compound, Indigo ink 1 was obtained by adding a metal dryer and drying inhibitor.

Ink comparison example (purple ink)
In the formulation of Table 2, C.I. I. Pigment Violet 23 (Toyo Ink Mfg. Co., Ltd. LIONOGEN VIOLET R6200) was mixed with varnish, and after milling with a dispersion particle measuring machine (grind meter) to 7.5 microns or less, vegetable oil, compound, metal dryer A purple ink was obtained by adding a drying inhibitor.

Printing Evaluation Test For the inks of the above examples and comparative examples, the solid density values of red and indigo are in the range of red: 1.52 to 1.56 and indigo: 1.63 to 1.67 under the following printing conditions. The printed matter was evaluated. The yellow and black inks used general oxidation polymerization type lithographic printing inks were printed within the density ranges of yellow: 1.40 to 1.44 and black: 1.85 to 1.90. For purple ink, since the density control is generally impossible, printing was performed with the ink transfer amount varied within an arbitrary hue range.

Example 1: Printing was performed using a combination of red ink 1, indigo ink 1, and known yellow and black inks.
Example 2: Printing with combination of red ink 2, indigo ink 1, and known yellow and black inks.
Example 3: Printing was performed using a combination of red ink 3, indigo ink 1, and known yellow and black inks.
Comparative Example 1: Printing with a combination of known yellow, red, indigo and black ink.
Comparative Example 2: Purple ink was added under the same conditions as in Comparative Example 1, and purple ink was used for the red and indigo solid part overprinted (blue violet hue), and only the purple ink was transferred in three stages. (Comparative Examples 2 to 4) were printed.

Printing conditions Printing machine: Heidelberg Speedmaster Kikuzen 5-color machine (Heidelberg Japan Ltd.)
Paper: Tokishi Art Double Sided 110Kg (Mitsubishi Paper Co., Ltd.)
Dampening water: Astro Mark 3 (Niken Chemical Laboratory, Inc.) 2.0% tap water printing speed: 10,000 sheets / hour

Measurement conditions for printed matter Density: Measure the density value of the single-color solid portion of the printed matter with Gretag Macbeth D196 Colorimetric: Single-color solid portion (yellow, red, indigo, purple) of the printed matter with X-Rite 938 and single-color solid overprint L *, a *, b * values of the parts (yellow x red, red x indigo, indigo x yellow).

        The C value and ΔE were obtained from L *, a *, and b * by the following formula.

光沢 ：村上色彩技術研究所製、デジタル光沢計にて６０°−６０°反射光沢を測定。

結果を表３に示す。比較例と比べて実施例の紅×藍のＣ値が大きく、つまり印刷物の彩度が高く、紫インキを使用した場合に近いことがわかる。
また、実施例の紅×藍と比較例の紫インキとのΔＥが小さく、それぞれの色相が近似している
ことがわかる。更に、ａ*を横軸、ｂ*縦軸とした2次元空間に、各ａ*、ｂ*値をプロットし、２次元のガモットで比較した結果、ブルーバイオレット色相に関して実施例の色再現領域が広いことがわかる(表４)。

また、得られた分光反射率曲線を表６に示す。比較例の従来インキに比べ、実施例のインキの方が理想の分光反射率曲線に近くなっており、完全反射しなければならない部分の不必要吸収が少なくなっている。そのため、インキの濁り成分が減少し、色再現領域が広がっている。

Gloss: Measured 60 ° -60 ° reflection gloss with a digital gloss meter manufactured by Murakami Color Research Laboratory.

The results are shown in Table 3. It can be seen that the C value of red x indigo of the example is larger than that of the comparative example, that is, the chroma of the printed matter is high, and is close to the case where purple ink is used.
Further, it can be seen that ΔE of the red x indigo of the example and the purple ink of the comparative example are small, and the respective hues are approximate. Furthermore, as a result of plotting each a * and b * value in a two-dimensional space with a * as the horizontal axis and b * vertical axis and comparing them with a two-dimensional gamut, the color reproduction region of the example with respect to the blue violet hue is It can be seen that it is wide (Table 4).

Further, Table 6 shows the obtained spectral reflectance curve. Compared to the conventional ink of the comparative example, the ink of the example is closer to the ideal spectral reflectance curve, and unnecessary absorption of the portion that must be completely reflected is reduced. For this reason, the turbidity component of the ink is reduced and the color reproduction region is widened.

Claims (3)

Chromaticity according to L * a * b * color system (JIS Z 8729) when red is printed within the density range of 1.52-1.56 and indigo is 1.63-1.67.
Red ink, L *: 50 to 55, a *: 75 to 83, b *: -14 to -20
Indigo ink, L *: 52 to 58, a *: −40 to −45, b *: −45 to −53
A lithographic printing method characterized in that the above two colors of ink within the range are overprinted, wherein the ink containing CI Pigment Red 169 is used as the red ink, and the red ink is applied on the indigo ink. Excellent in color reproduction with high saturation, characterized in that the chromaticity of the solid portion on which is printed is L *: 23 to 29, a *: 28 to 33, and b *: −63 to −68. Lithographic printing method. The high saturation that can expand the color reproduction range of the L * a * b * color system with four colors of yellow, red, indigo and black by combining the ink of claim 1 with yellow and black ink 2. The lithographic printing method according to claim 1, wherein an ink containing CI Pigment Red 169 is used as the red ink.
2. The planographic printing method according to claim 1, wherein the reflectance of (a) red and (b) indigo is as follows.
(A) In the wavelength region of 400 nm to 700 nm, when the maximum reflectance is 100%, the maximum reflectance in the wavelength region of 400 nm to 500 nm is 50% to 100%, and the reflectance in the wavelength region of 500 nm to 560 nm. A red ink containing 15 to 30% by weight of a red hue compound having a reflection spectrum of 1 to 20%, a reflectance of 630 nm to 700 nm and a reflectance spectrum of 90% to 100% based on the total weight of the ink.
(B) In the wavelength region of 400 nm to 700 nm, when the maximum reflectance is 100%, the reflectance in the wavelength region of 400 nm to 530 nm is 50 to 100%, and the reflectance of 600 nm to 700 nm is 1 to 30%. An indigo ink containing 10 to 25% by weight of an indigo hue compound having a spectrum with respect to the total weight of the ink.