Spectral Image Analysis of a natural color sample using Rewritable Transparent Broad-band Filters

Kanae Miyazawa(1), Markku Hauta-Kasari(2), and Satoru Toyooka(1)

(1) Graduate School of Science and Engineering, Saitama University255 Shimo-okubo, Urawa, Saitama 338-8570, JAPAN

Email: kanae@mech.saitama-u.ac.jp

(2) Department of Computer Science, University of Joensuu,P.O. Box 111, FIN-80101 Joensuu, FINLAND

Multispectral imaging computer vision, art, environmental monitoring…

The aim of this study

To acquire a spectral image of a sample from low-dimensional data & under arbitrary illumination

How to obtain the spectral images

ex.) by a CCD-camera with narrow-band interference filters

400nm410nm

700nm

420nm

Filter 1Filter 2

Filter 3Filter 4

step2) estimate the original spectral images from the compressed image data

step1) obtain the compressed image data by a CCD-camera with a low-dimensional broad-band filter set

a large amount of image data must be processed and stored

We presentWe present• Optical Implementation of transparent broad-band filter system which is

rewritable arbitrarily

• Application to two-dimensional spectral image ( Indoor / Outdoor )

•Spectral distribution of an object color can be represented by a set of inner products between the low-dimensional color filter set and the spectral distribution of the object.

•The color filter set we used are non-orthgonal.

•To estimate a spectrum s, a Pseudoinverse Matrix can be used.

s ’=W(WTW)-1 WTs ( W: color filter set, s : Spectral distribution of the object )

W(WTW)-1 ： known, (WTs) ： can be calculated optically

Spectral Distribution S’ can be estimated

Main idea of this study 　 ~Spectrum Estimation~

ACTIVE TYPE

Optimal Light Source

Sample Sample

CCDcamera

CCDcamera

OutdoorIndoor

Light Source

Optimal Filter

Inner product ＝Sample ＊ Optimal Light Source

Inner product ＝ Sample ＊ Optimal Filter

How to obtain the Inner product

PASSIVE TYPE

Monochrome

Image Board

Computer

Sample

Monochrome CCD camera

LVF LCSLM

White Light Source

Stage

Experimental Setup (indoor)

Lens

Characteristics of the Linear Variable Filter

Tra

nsm

ittin

g C

ente

r W

avel

engt

h[nm

]

Position of the LVF[mm]

0 10 20 30 40 50 60400

500

600

700

Size: 60×25×5t [mm]

The designed filter patterns corresponding to the color filters are written on the LC-panel.

LVF

LC-panel

100 200 300 400 500 600050

100150200250

Inpu

t Lev

el

400nm 700nm

Inpu

t Le

vel

Pixel Number

One of the Filter Function

Spatial Filter Pattern on the LC-panel

Spectral position of the transmitting center wavelength

400 500 600 700

0

10

20

30

40

Tra

nsm

ittan

ce[%

]

Wavelength[nm]0 10 20 30 40 50 60

400

450

500

550

600

650

700

Cente

r W

avele

ngt

h[n

m]

Position[mm]

Characteristics of the Linear Variable Filter

60×25×5t [mm]

(a) Spectral Position of The Central Wavelength(b) Transmittance of Linear Variable Filter

(a) (b)

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.1

Nor

mal

ized

Inte

nsity

Wavelength [nm]400 500 600 700

0.0

0.1

0.2

0.3

0.4

Wavelength [nm]

Filter No.2

Nor

mal

ized

Inte

nsity

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.3

Wavelength [nm]

Nor

mal

ized

Inte

nsity

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.4

Nor

mal

ized

Inte

nsity

Wavelength [nm]

Wanted FilterImplemented Filter

Max Error = 6.4%

Mean Error= 5.8%

Color Filter Set

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength (nm)

Inte

nsity

AFTER1

Light Source Spectrum

Wavelength [nm]

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.1

Nor

mal

ized

Inte

nsity

Wavelength [nm]400 500 600 700

0.0

0.1

0.2

0.3

0.4

Wavelength [nm]

Filter No.2

Nor

mal

ized

Inte

nsity

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.3

Wavelength [nm]

Nor

mal

ized

Inte

nsity

400 500 600 7000.0

0.1

0.2

0.3

0.4Filter No.

Norm

aliz

ed

In

ten

sity

Wavelength [nm]

Optically Implemented Filters

Norm

aliz

ed

In

ten

sity

4

400 500 600 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400 500 600 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400 500 600 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400 500 600 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Filter No.1

Filter No.2

Filter No.3

Filter No.4

Experiments with Real World Object (indoor)

Detected intensity images of the object through the 4 filters.

Estimated Spectra at the Spectral Images (indoor)

Blue sheet Green sheet Yellow sheet Red sheet

Strawberry Kamquat (Orange)

The spectra at different locations of the spectral images.Black lines 31 narrow band filtersRed lines 4 filters

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)

No

rmal

ize

d v

alue

Pixel (13,33)

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)N

orm

aliz

ed

val

ue

Pixel (5,77)

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)

No

rmal

ize

d v

alue

Pixel (7,175)

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)

No

rmal

ize

d v

alue

Pixel (7,261)

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)

No

rmal

ize

d v

alue

Pixel (183,31)

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Wavelength (nm)

No

rmal

ize

d v

alue

Pixel (183,201)

Spectral Images Converted to RGB-images (indoor)

4 Filters （ proposed system ）

31 Narrow Band Filters

(a) (b)

(a) RGB-image acquired by the proposed system using 4 filters.(b) RGB-image measured by the CCD-camera with 31 narrow-band filters.

Experimental Setup (outdoor)

Monochrome CCD camera

Sun

Object

Standard White (BaSO4)

Monochrome Image BoardComputer

LVF LCSLM

Stage

Spectroradiometer

controller

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength[nm]

No

rmaliz

ed

Inte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength[nm]

No

rmaliz

ed

Inte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength[nm]

No

rmaliz

ed

Inte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength[nm]

No

rmaliz

ed

Inte

nsity

Filter No.1

Filter No.2

Filter No.3

Filter No.4

Experiments with Real World Object (outdoor)

Detected intensity images of the object through the 4 filters.

Black lines 31 narrow band filtersRed lines 4 filters

Munsell, div by max of each RGB band

Red Blue Orange Yellow

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavelength[nm]

No

rmal

ize

d I

nte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavelength[nm]

No

rmal

ize

d I

nte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavelength[nm]

No

rmal

ize

d I

nte

nsity

400 450 500 550 600 650 7000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavelength[nm]

No

rmal

ize

d I

nte

nsity

Estimated Spectra at the Spectral Images (outdoor)

31 Narrow Band Filters

(a) (b)

4 Filters （ proposed system ）

Spectral Images Converted to RGB-images (indoor)

(a) RGB-image acquired by the proposed system using 4 filters.(b) RGB-image measured by the CCD-camera with 31 narrow-band filters.

1. We proposed the optical transparent broad-band filters, which is rewritable arbitrarily.

2. The spectral distribution of the intensity image through the proposed filter almost coincided with the expected filter functions.

3. Intensity images were detected through the proposed filters, and spectral images were acquired.

4. The estimated spectral images were compared to the spectral images measured by the use of CCD-camera with 31 narrow-band filters. The spectra obtained by the both method correlated well.

5. This system was applied to outdoor measurement under sunlight illumination and the spectral images were estimated.

6. The data obtained from the filtering process is only 4 monochrome images. It is convenient for storing and transmitting the spectral image.

Conclusions