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CALIBRATING LIGHT EMITTING BASE SPECTROPHOTOMETERS AND COLORIMETERS

IP.com Disclosure Number: IPCOM000027773D
Original Publication Date: 1999-Aug-31
Included in the Prior Art Database: 2004-Apr-09
Document File: 4 page(s) / 160K

Publishing Venue

Xerox Disclosure Journal

Abstract

Disclosed is a means for calibrating a Light Emitting Diode (LED) based spectro-photometer that accurately and quickly measures spectral reflectivity of colored test patches printed on paper by electrophotographic printers.

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XEROX DISCLOSURE JOURNAL

CALIBRATING LIGHT EMITTING BASE SPECTROPHOTOMETERS AND COLORIMETERS Int. C1. GO3g 15/00 Fred F. Hubble, ID
Yao Rong Wang
Tonya L. Love

Proposed Classification
U. S. C1. 399/049

1

0.9

3

E3

1

0.8

   RELATIVE REFLECTIVITY

0.7

0.6

0.5

0.4

0.3.

" I 400

I I I I I I

450 500 550 600 650 700

WAVE LENGTH (NANOMETERS)

Disclosed is a means for calibrating a Light Emitting Diode (LED) based spectro- photometer that accurately and quickly measures spectral reflectivity of colored test patches printed on paper by electrophotographic printers.

Calibration of LED spectrophotometers and calorimeters for high measurement accuracy is required at the final assembly step of an electrophotographic printer. Each spectrophotometer is calibrated against a standard unit such as a S2000 spectro- photometer manufactured by Ocean Optics. The output spectrum of each LED'S in the

XEROX DISCLOSURE JOURNAL, - Vol. 24, No. 4 July/August 1999 163

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CALIBRATING LIGHT EMITTING BASE SPECTROPHOTOMETERS AND COLORIMETERS (Cont'd)

device under test is measured and recorded into a low cost Programmable Read Only Memory (PROM) and installed in the electrophotographic printer where a microprocessor or microcontroller reads the data to make accurate color measurements. By measuring the spectrum, die-to-die variations in output spectrum are accounted for in a deterministic fashion. The procedure takes approximately five seconds wherein; the PROM is shipped with the device so that both can be installed in the same printer.

Calibration information is used in the following way. A test patch is printed and the read by the spectrophotometer by sequencing through each of its LED'S. The detector is visualized to be a PIN photodiode whose response to any particular LED is given by

where

Rj is the response of the detector to the ith LED's flux reflected of the test patch

Zj is the radiant intensity distribution of the fh LED as a function of wavelength A

R(2) is the reflectivity of the test patch as a function of wavelength, and

r(A) is the response of the detector as a function of wavelength.

The integration is performed from 400 to 700 nanometers (nm), which conform to standard color measurement approximating the range of the human eye.

If the spectrophotometer has n LED'S, then n measurements are collected, R1, R2, R3,. . .R,. These are compared to the desired responses, which are calculated as follows

where

Dj is the response of the detector to the ith LED's flux reflected from the desired test patch,

D(A) is the reflectivity of the desired test patch as a function of wavelength.

164 XEROX DISCLOSURE JOURNAL - Vol. 24, No. 4 July/August 1999

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CALIBRATING LIGHT EMITTING BASE SPECTROPHOTOMETERS AND COLORIMETERS (Cont'd)

Since the desired response is known from the data the printer used to generate the test patch, the dete...