# US 7,961,322 B2 · Method for conditional application of color measurement error compensation in spectral sensors

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- Number: US 7,961,322 B2
- Document ID: US7961322B2
- Status: Granted
- Kind: B2
- Application number: 12/203,520
- Filed: 2008-09-03
- Published: 2009-12-03
- Granted: 2011-06-14
- Earliest claimed: 2008-05-27
- Assignees: Xerox Corporation
- Inventors: Gary W. Skinner; Lalit Keshav Mestha; Paul S. Bonino
- Web page: https://patentgrep.com/patent/US7961322B2

## CPC

- G01J 3/52: using colour charts
- G01J 3/524: {Calibration of colorimeters}
- G01N 21/5907: {Densitometers}
- H04N 1/6033: {using test pattern analysis (H04N1/6055 takes precedence)}
- G01N 2021/5942: {for dot area ratio in printing applications}
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- H04N: PICTORIAL COMMUNICATION, e.g. TELEVISION

## Abstract

Color measurements of color samples are estimated by a system and method wherein an area coverage of a color sample is computed, the color sample is printed, a color of the printed color sample, such as a set of reflectance values, is measured at a first temperature, and a color of the printed color sample is estimated at a second temperature lower than the first temperature, the estimation being based on the area coverage and a thermochromatic model which represents relationships between measured colors of printed color samples at the first and second temperatures.

## Claims

### Claim 1

A method for estimating color measurements of color samples comprising:
computing an area coverage of a color sample;
printing the color sample;
measuring a color of the printed color sample at a first temperature;
estimating a color of the printed color sample at a second temperature lower than the first temperature, the estimation being based on the area coverage and a thermochromatic model which represents relationships between measured colors of printed color samples at the first and second temperatures.

### Claim 2 (depends on 1)

The method of claim 1, wherein the computed area coverage is based on the halftone dot density of the color sample.

### Claim 3 (depends on 1)

The method of claim 1, wherein the measuring of the color of the printed color sample at the first temperature includes measuring the color of the just-fused printed sample with an in-line spectral sensor.

### Claim 4 (depends on 1)

The method of claim 1, wherein the measuring of the color of the printed color sample at the first temperature includes obtaining a set of measurements at different wavelengths.

### Claim 5 (depends on 1)

The method of claim 1, further comprising, using the estimated color of the printed color sample at the second temperature as a basis for assessing operability of a color output device which printed the color sample.

### Claim 6 (depends on 1)

The method of claim 1, wherein the thermochromatic model comprises a matrix and the estimating of the color of the printed color sample includes applying the matrix only when the computed area coverage meets a predetermined threshold area coverage.

### Claim 7 (depends on 1)

The method of claim 1, wherein the thermochromatic model comprises a set of matrices, each matrix being assigned to an area coverage region and wherein the estimating of the color of the printed color sample includes applying a matrix from the set of matrices which is assigned to the area coverage region which includes the computed area coverage.

### Claim 8 (depends on 1)

The method of claim 1, wherein the thermochromatic model comprises a matrix, and the method includes computing a gain factor based on the area coverage and applying the gain factor to the matrix.

### Claim 9 (depends on 1)

The method of claim 1, wherein the thermochromatic model is generated by minimizing an objective function which is the sum, over a set of measured colors, of a function of the difference between measurements at the first and second temperatures.

### Claim 10 (depends on 9)

The method of claim 9, wherein the thermochromatic model is generated by minimizing an objective function of the general form:
J
=
arg
⁢
⁢
min
Q
⁢
∑
i
=
1
N
⁢
w
i
⁡
(
λ
)
⁢

S
i
⁡
(
λ
)
-
QR
i
⁡
(
λ
)

2
(
Eqn
.
⁢
3
)
where Q represents the matrix,
N represents a number of measured colors,
wi(λ) represents an optional wavelength dependent weight,
Si(λ) represents vectors of the N cool spectral samples, and
Ri(λ) represents corresponding vectors for the N hot spectral samples.

### Claim 11 (depends on 1)

The method of claim 1, wherein an area coverage is computed for each of a plurality of color separations.

### Claim 12 (depends on 1)

The method of claim 1, wherein the color sample comprises one of a test patch and a customer image.

### Claim 13 (depends on 1)

A computer program product comprising a non-transitory computer-readable recording medium encoding instructions, which when executed on a computer causes the computer to perform the method of claim 1.

### Claim 14

A thermochromaticity compensation system comprising:
a spectral sensor which measures colors of printed color samples at a first temperature;
memory which stores a thermochromatic model which represents relationships between measured colors of printed color samples at the first temperature and measured colors of printed color samples at a second temperature as a function of a computed area coverage; and
a processor which receives a measured color of a printed color sample from the spectral sensor at the first temperature, computes area coverage of the printed color sample, and accesses the model to estimate a color of the printed color sample at the second temperature based on the measured color and computed area coverage.

### Claim 15 (depends on 14)

A color output device comprising the thermochromaticity compensation system of claim 14 and a marking engine which prints the printed color sample.

### Claim 16 (depends on 15)

The color output device of claim 15, further comprising a source of area coverage information in communication with the processor, the processor computing the area coverage of the printed color sample based on the area coverage information.

### Claim 17

An algorithmic method to compensate for thermochromaticity errors of in situ spectral color measurements of a color printing device comprising:
obtaining spectral measurements at a plurality of computed area coverage values of printed colors generated by the color printing device measured at a first temperature by an in-line spectrophotometer and at a second temperature;
generating a model, based on the spectral measurements, which maps spectral measurements at the first temperature to spectral measurements at the second temperature, as a function of area coverage;
measuring a selected color corresponding to a color input signal representing a desired color at the second temperature, wherein the selected color is measured at the first temperature by the in-line spectrophotometer;
computing an area coverage of the selected color; and
applying the model to convert the measured color to a corresponding color when the measured color changes to the second temperature, based on the area coverage.

### Claim 18 (depends on 17)

The method of claim 17, wherein the measuring comprises spectrophotometric measuring of just-fused prints by the in-line spectrophotometer in the color output device.

### Claim 19 (depends on 17)

The method of claim 17, wherein the mapping comprises predetermining a thermochromaticity compensation matrix from empirical data including a set of sensor reflectance vectors.

### Claim 20 (depends on 17)

The method of claim 17, further comprising assessing if the color is different from an expected color intended by the color input signal.

### Claim 21 (depends on 20)

The method of claim 20, wherein the assessing comprises calibrating the color output device by adjusting an input signal to the color printing device corresponding to the measured in situ color in a manner wherein the in situ measuring of an output color corresponds to a desired output ambient color.