POTENTIAL OF MULTISPECTRAL TECHNIQUES FOR MEASURING COLOR IN THE AUTOMOTIVE SECTOR Meritxell Vilaseca, Francisco J. Burgos, Jaume Pujol 1
Technological innovation center established in 1997 with the aim of structuring the activities in the field of Optical Engineering (Terrassa Campus of the Technical University of Catalonia). http://www.cd6.upc.edu/
Manresa Terrassa Sant Cugat del Vallès Vilanova i la Geltrú Barcelona Castelldefels
Visual optics and color group: J. Pujol M. Vilaseca M. Arjona F. Díaz-Doutón S. Luque F. J. Burgos M. Aldaba J. A. Herrera H. Zuluaga F. Sanabria J. C. Ondategui E. Ferrer A. Giner 4
Outline Introduction What is a multispectral system? Components of multispectral systems Spectral sampling technique Calibrating a multispectral system Multispectral systems at the CD6 Applications in the automotive sector Hyperspectral systems Potential of multispectral and hyperspectral techniques for measuring color in the automotive sector (1st 5
Introduction How color is measured? Standard instruments for color measurement: spectroradiometers, spectrophotometers, colorimeters Cost (Diffraction grating) Integration of the viewing field Only one measurement simultaneously Slow speed 6
Introduction Is it possible to perform color measurements using a digital camera? High spatial resolution Linearity Rapid response Acceptable SNR High cuantum efficiency Wide spectral range Reduced size Scientific image 1. Control and correction of the noise sources Measurement instrument with high spatial resolution 2. Color characterization Instrument for color measurement Potential of multispectral and hyperspectral techniques for measuring color in the automotive sector (1st 7
Introduction Is it possible to perform color measurements using a digital camera? RGB OUTPUT CIE-XYZ CIE-Lab R G B R G B Potential of multispectral and hyperspectral techniques for measuring color in the automotive sector (1st 8
Introduction Is it possible to perform color measurements using a digital camera? Commercial digital color camera Color Matching Functions of the digital camera CIE Color Matching Functions xyz - Q10 CCD colour camera 1.0 0.8 0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 Wavelength (nm) RGB x - Q10 CCD Colour Camera y - Q10 CCD Colour Camera z - Q10 CCD Colour Camera Linear Transformation (Luther Condition) 400 450 500 550 600 650 700 Wavelength (nm) colorimetric applications are seriously limited xyz - CIE 1931 2.0 1.5 1.0 0.5 0.0 CIE-XYZ x - CIE 1931 y - CIE 1931 z - CIE 1931 9
Introduction Is it possible to perform color measurements using a digital camera? The solution: Multispectral Imaging Systems 10
What is a multispectral system? An imaging system with more than 3 acquisition channels Relative Spectral Sensitivity Q10 CCD colour camera 1.0 0.8 0.6 0.4 0.2 Transmittance 0.7 0.6 0.5 0.4 0.3 0.2 0.1 F400 F450 F500 F550 F600 F650 F700 0.0 400 450 500 550 600 650 700 0.0 400 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm) A multispectral system is able to provide spectral information instantaneously at each pixel of the captured image and therefore it allows performing more accurate colour measurements. 11
What is a multispectral system? An imaging system with more than 3 acquisition channels CIE-1931 XYZ Color Imaging Multispectral Imaging 12
What is a multispectral system? An imaging system with more than 3 acquisition channels Complete spectral information on each pixel of the image Multispectral Image 0.16 0.14 Color Image R,G,B Spectral reflectance 0.12 0.10 0.08 0.06 0.04 0.02 0.00 400 440 480 520 560 600 640 680 720 760 Wavelength (nm) 13
What is a multispectral system? Classification: They are classified depending on the number of spectral bands/acquisition channels: Number of channels Name 1 Monochromatic 3 RGB o trichromatic 4 to 9 Multispectral 10 to 100 Hyperspectral More than 100 Ultraspectral 14
Components of multispectral systems 15
Spectral sampling technique Building acquisition channels with different spectral features 1. Using a white light source and filters: + 2. Multiplexed illumination: Potential of multispectral and hyperspectral techniques for measuring color in the automotive sector (1st 16
Components of multispectral systems 1. Using a white light source and filters: Configurations based on monochrome cameras 0.7 0.6 QImaging QICAM Fast1394 12-bit cooled Motorized Filter Wheel Nikon AF Nikkor 28-105mm Transmittance 0.5 0.4 0.3 0.2 0.1 F400 F450 F500 F550 F600 F650 F700 0.0 400 450 500 550 600 650 700 750 Wavelength (nm) Narrowband filters: interference filters in a filter wheel 17
Spectral sampling techniques 1. Using a white light source and filters: Configurations based on monochrome cameras Narrowband filters: tunable filter (LCTF or AOTF) 18
Spectral sampling techniques 2. Multiplexed illumination: Configurations based on monochrome cameras and light-emmiting diodes (LEDs) http://www.cs.columbia.edu/cave/projects/multispectral_imaging/images/capturing_system.gif 19
Calibrating a multispectral system High resolution Linearity Rapid response Acceptable SNR High cuantum efficiency Wide spectral range Reduced size Scientific image 1. Control and correction of the noise sources Measurement instrument with high spatial resolution 2. Spectral reconstruction and color measurement algorithm Instrument for color and spectral measurements 20
Calibrating a multispectral system 1. Correction of the noise sources: Temporal noise sources: Image averaging. Determining the number of images to be averaged. Image averaging removes all noise sources except the spatial noise sources. 1.000 Relative SNUP 0.950 0.900 0.850 R Channel G Channel B Channel 0.800 0 10 20 30 40 50 60 70 80 # averaged images 21
Calibrating a multispectral system 1. Correction of the noise sources: Spatial noise sources: Spatial non-uniformity correction, flat-field correction or flat-fielding. 560 580 600 620 640 660 640 620 600 80 70 60 50 40 30 20 10 560 60 50 40 30 20 10 Row s lu m ns 580 Co Digital Value 660 0 0 560 580 600 620 640 660 640 620 600 580 80 70 60 50 40 30 20 10 560 60 50 40 30 20 10 Row Co lu m n Digital Level 660 0 0 22
Calibrating a multispectral system 2. Spectral reconstruction and color measurement How can we obtain spectral properties (reflectance, radiance ) from multispectral images? ND r(l) 1.2 1.0 Reflectance 0.8 0.6 0.4 0.2 400 500 600 700 Wavelength (nm) 23
Calibrating a multispectral system 2. Spectral reconstruction and color measurement Use of training sets: ND r(l) 24
Multispectral systems at the CD6 Multispectral system in the near-infrared (800 1000 nm) Expanding VIS techniques to NIR VIS NIR 380 780 800 1000 nm 25
Multispectral systems at the CD6 Multispectral system in the near-infrared (800 1000 nm) Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 1) Reflectance reconstruction system
Multispectral systems at the CD6 Multispectral system in the near-infrared (800 1000 nm) 80 samples (natural and manufactured) Plants Fabrics Food Plastics Marble Paper, cardboard Leather Wood 27
Multispectral systems at the CD6 Multispectral system in the near-infrared (800 1000 nm) P rec > 99.9 % & RMSE < 1 WIENER (Transformation 1) 2) WIENER (Transformation 2) 1) 1.0 1.0 Spectral reflectance 0.8 0.6 0.4 0.2 Fabric (original) Fabric (reconstructed) Spectral reflectance 0.8 0.6 0.4 0.2 Red pepper (original) Red pepper (reconstructed) 0.0 750 800 850 900 950 1000 1050 Wavelength (nm) 0.0 750 800 850 900 950 1000 1050 Wavelength (nm) 28
Multispectral systems at the CD6 Multispectral system in the visible range (380-780 nm) RGB camera QImaging QICAM Fast1394 12 bit cooled Nikon AF RGB Tunable Nikkor Filter 28 105mm 7-channel camera QImaging QICAM Fast1394 12 bit cooled Motorized Filter Wheel Nikon AF Nikkor 28 105mm Relative Spectral Sensitivity Spectral Transmittance 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400 450 500 550 600 650 700 Wavelength (nm) Wavelenght (nm) R Channel G Channel B Channel 400 450 500 550 600 650 700 750 F400 F450 F500 F550 F600 F650 F700 29
Multispectral systems at the CD6 Multispectral system in the visible range (380-780 nm) Mathematical method: PSE Number and features of the acquisition channels: 7 channels Color Rendition Chart (CCCR) DC Chart (CCDC) Training and Test Training and Test CCCR training & test CCDC training & test mean DE 3.91 3.02 min DE 0.46 0.47 max DE 14.91 9.70 30
Multispectral systems at the CD6 Multispectral system in the visible range (380-780 nm) 31
Multispectral systems at the CD6 Multispectral system in the visible range (380-780 nm) Pladellorens et al. Skin Res Appl 2008 32
Multispectral systems at the CD6 Multispectral system in the visible range (380-780 nm) 0.16 0.14 Spectral reflectance 0.12 0.10 0.08 0.06 0.04 0.02 0.00 400 440 480 520 560 600 640 680 720 760 Wavelength (nm) Vilaseca et al. Appl Opt 2008 Herrera et al. Col Res Appl 2011 33
Arbitrary Units Arbitrary Units Multispectral systems at the CD6 Multispectral system in the UV-VIS-IR range composed of light-emitting diodes (LEDs) Spectral emission Near UV-Vis-NIR (350-900nm) 1 0.9 0.8 Emission spectra for module 1 1 0.9 0.8 IR (900-1700nm) Emission spectra for module 2 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 300 400 500 600 700 800 900 1000 Wavelength (nm) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 900 1000 1100 1200 1300 1400 1500 1600 1700 Wavelength (nm) Martínez et al. J Imaging Sci Technol (accepted 2011) 34
Spectral Sensitivity Spectral Sensitivity Multispectral systems at the CD6 Multispectral system in the UV-VIS-IR range composed of light-emitting diodes (LEDs) Near UV-Vis-NIR (350-900nm) IR (900-1700nm) Imaging Sensors QICAM CCD HAMAMATSU InGaAs Spectral sensitivity 1 0.8 1 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 300 400 500 600 700 800 900 1000 Wavelength (nm) 0 800 1000 1200 1400 1600 1800 Wavelength (nm) 35
Multispectral systems at the CD6 Multispectral system in the UV-VIS-IR range composed of light-emitting diodes (LEDs) 36
Multispectral systems at the CD6 Multispectral system in the UV-VIS-IR range composed of light-emitting diodes (LEDs) Senserrich & Font 2009 37
Multispectral systems at the CD6 Multispectral system in the UV-VIS-IR range composed of light-emitting diodes (LEDs) 38
Applications in the automotive sector Benefits of multispectral systems: Spectral and color information pixel by pixel (Multispectral image) Low cost: digital camera, filters, LEDs etc. High speed: many spectra from one single measurement (or few) CIE-Lab XYZ 39
Applications in the automotive sector Potential applications: Conventional color (CIELAB) characterization of materials used in the automotive sector: paints, plastics etc. E.g.: Quality control. But also SPECTRAL! (Avoiding metamerism, color fakes etc.) 40
Applications in the automotive sector Potential applications: Multi-angle spectral and color characterization of goniochromatic materials: metallic and pearlescent colors, interference pigments etc. E.g.: Analysis of CIELAB at different angles (15 / 25 / 45 / 75 / 110 / -15º) Analysis of texture (sparkle, graininess and mottling effects). 41
Applications in the automotive sector Potential applications: Characterization of three-dimensional (3D) samples E.g.: Planar and curved samples To expand the spectral information beyond the visible (VIS) range: UV (Ultraviolet) and IR (Infrared). E.g.: Fluorescent pigments, detection of color replicates etc. 42
Applications in the automotive sector Our current goal (UPC-UA): Development of a gonio-multispectral system in the UV-VIS-IR range (250-1700 nm) based on light-emitting diodes (LEDs) 43
Applications in the automotive sector Our current goal (UPC-UA): Development of a gonio-multispectral system in the UV-VIS-IR range (250-1700 nm) based on light-emitting diodes (LEDs) Automated system with diffuse & directional illumination geometries: - 6 aspecular viewing angles (15 / 25 / 45 / 75 / 110 / -15º) - 3-angle circumferential illumination, 1-angle viewing geometry http://www.byk.com http://www.konicaminolta.com 44
Hyperspectral systems Camera with a diffractive element: Imaging spectrograph + monochrome CCD camera 45
Hyperspectral systems Camera with a diffractive element: Image λ Scene Spatial axis Scanning of the scene 46
Hyperspectral systems Camera with a diffractive element: Spectral analysis of samples with complex spatial patterns: 47
Centre for Sensors, Instruments and Systems Development (CD6) Dept. of Optics and Optometry TECHNICAL UNIVERSITY OF CATALONIA (UPC) CD6 GRACIAS! e-mail: mvilasec@oo.upc.edu TERRASSA 48