© 2006 Hewlett-Packard Development Company, L.P.The information contained herein is subject to change without notice

Efficient Color Printer CharacterizationBased on Extended Neugebauer Spectral Models

Pau Soler and Jordi ArnabatHewlett-Packard Large Format Printing Division -

Barcelona, Spain

2 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

The need for color profilingStrong demand of media flexibility in photographic and fine arts markets•+100 paper types•If same color profile is used, differences can be up to 30dE

canvas

lithocoated

glossysatin

Color profiles determine the amount of ink needed for each media, ensuring accurate colors.

Using color profile

3 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Building color profilesTypically, to create a color profile a big target (~1000 patches) needs to printed, measured and processed.And it takes time… (~2 hour)

Printers with embedded color sensors enable users to easily create color profiles.But it takes time… (~20 minutes)

Ink space (e.g. CMYK) CIE L*a*b*

4 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Goal: reduce the number of patches

color prediction

model

small target(~50 patches)

Standard IT8.7/3large target (~700 patches)

The goal is to use a color model with less number of sampling points, reducing:• Time of operation• Media usage• Ink usage

5 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Neugebauer Spectral Model

The human visual system ‘averages’ halftone pattern colors

where:ai: area covered by each colorantRi(λ): spectral reflectance of Neugebauer colorantsŘ (λ): estimated spectral reflectance of halftone patternNeugebauer model predicts such ‘average’.

6 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Yule-Nielsen Correction

where:n : Yule-Nielsen factor

Actually, light propagates within the media. This phenomena is known as optical dot gain.Yule-Nielsen proposed a parametric non-linear correction to compensate this effect.

7 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

The valid range for n Originally linked to dot gain, n is supposed to be positive.Experimental fit to data improves as n ∞.Which is the valid range of values for n ?

Data fitting improves as n ∞Viggiano [ICIS ‘06] andLewandowski et al. [J. Opt. Soc. 06] suggest the use of negative values.

where:n : Yule-Nielsen factor

8 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

The valid range for n (2)

As internal scattering is higher, light interacts more with pigments, decreasing the reflectance of the patch.The actual reflectance spectra lays between Neugebauer estimation (no optical dot gain) and the minimum of the colorant’s spectra (infinite scattering).n positive sets an arbitrary limit to this range, which might not suffice to match the actual color.

colorant 1

colorant 2

Neugebauer estimation

Actual colorvalid

ran

ge

9 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Yule-Nielsen as generalized averaging

Yule-Nielsen is actually a generalized averaging of the colorant spectra, for the particular case of and .

As function of n varies from min to max:

General form of generalized averaging

Particular case for n = 1/α

∞1

1/20+

-10-

valid

ran

ge

n

neugebauer (n=1)

min

10 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Practical considerations:Physical dot gain correction

Extra primary patches are printed and measured to correct area coverage:

No area coverage correction With area coverage correction

Area coverage computed with DeMichel’s equation:

11 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Practical considerations:Ink limitsFully saturated colors can not be printed due to ink limiting.Neugebauer colorants are estimated using the closest printable color and the other colorants, as:

Maximum values of CY… …corresponding CIEL*a*b* values

12 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Results

color prediction

model

Printer:HP designjet 2500CP(inkjet CMYK)Spectrophotometer:Minolta-CM508c(45/0, D50, 8mm)n value optimized for each case

Better results with dye inks than pigment inks

44 patches

13 Jan 31, 2007 Electronic Imaging '07 Extended Neugebauer Models

Conclusions•We gave a numerical explanation for negative n values.

•Yule-Nielsen is a form of generalized averaging•n values with physical meaning are (−∞, 1] U [1,∞, 1).•Probably optimize 1/α makes more sense than n.

•Extended Neugebauer model performs better with dye inks than pigmented inks.

•Feasible approach to reduce number of patches to create a color profile

•Future work includes:• extend to multiple ink system•improve colorant estimation•many other Neugebauer extensions (e.g. cellular).