Multispectral imaging - Paul Bourkepaulbourke.net/papers/multispectral/multispectral.pdf · •...
Transcript of Multispectral imaging - Paul Bourkepaulbourke.net/papers/multispectral/multispectral.pdf · •...
Multispectral imaging
Rock art is often very obvious and interesting Other times less so
Concept
• A normal photograph is throwing away a huge amount of information.
• The energy across a range of wavelengths is being (weighted summed) into just 3 numbers, single R,G,B values.
• Can imagine materials that reflect strongly in different wavelengths but appear to be the same colour.
wavelength
intensity
BG
R
350nm 400nm 450nm 500nm 550nm 600nm 650nm
Human visual system,response of cones
CMOS sensor with standardRGB Bayer filters
• Most SLR cameras use a Bayer filter, each RGB filter has it’s own response curve.
• Also applies to the human eye which has cones that respond to different wavelength bands.
Image cube (x,y, )
wavelengthimage plane
R
G
B
x
y
Hand print, West Angeles rock shelter.
x
y
wavelength
image plane
x
y
“Continuous” wavelength multispectral cameras
• Generally a line scan camera + a diffraction grating, or similar wavelength splitting device.
• See also “pushbroom” multispectral cameras.
• Increasingly being used in the mining industry with mineral signature databases.
300 400 500 600 7006505504503500.0
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1.0Power
Low cost alternative
• Capture narrow wavelength ranges.
• For this initial experiment used 8 interference bandpass filters across the visible range.350nm to 700nm.
• Filter banks 50nm apart and 20nm wide.
wavelength
intensity
350nm 400nm 450nm 500nm 550nm 600nm 650nm
~20nm
700nm
50nm
8 interference bandpass filters
20nm
Image cube (x,y, )
wavelengthimage plane
x
y
Example
• Might imagine multiplying 500nm and 550nm and subtracting 650nm.
• Note that here we are interested in identification, much of multispectral imaging is more about quantitative analysis.
400nm 450nm 500nm
550nm 600nm 650nm
Future work
• Demonstrated the potential with two of three images from the West Angeles rock shelters.
• Next test will be 16 filters and a more convenient means of changing filters.
• Develop algorithms to optimally combine slices to identify features.
• Employ higher dynamic range B/W camera. Avoid multiplicative response curves of Bayer filter.