photographing the invisible using invisible light
TRANSCRIPT
PhotographingThe
Invisible
PhotographingThe
InvisibleUsing Invisible LightUsing Invisible Light
Keene State
College
• Rich Blatchly
Digital Sensors• Sensors are
opaque, and are designed to detect only one color.
• Sensors are grouped (blue, red, and 2 greens).
• Each pixel yields a full spectrum, but two colors are interpolated.
Visible Light
Digital Infrared Photography
• Note that silicon (basis for photosensors) is sensitive to IR.
http://www.luminous-landscape.com/reviews/cameras/infrared%20dslr.shtml
What's different about IR
More IR Differences
Diagram of Apparatus
• IR requires a source (sun?), a filter and an IR sensitive camera
• Testing your camera
Camera equipment
Filter Responses
• The common Wratten 89B is also called Hoya R72
http://wrotniak.net/photo/infrared/index.html#FILTER
Aren’t Filters Expensive?
• Find a bottle cap that fits over your P&S camera lens
• A piece of unexposed, processed slide film can be a filter.
http://www.instructables.com/id/EMW6NFO0FPEQHO9ZGG/
• Exposure
• In many cases, built in is OK
• Try underexposing the photo to avoid red channel overload.
• With 0.1% of light, exposure changes by 10 “stops”. (Each stop is x2 in exposure; 210 = 1024).
• Focus
Taking the picture
Processing
http://wrotniak.net/photo/infrared/c5060.html
Mixed with Visible
http://www.rbfotografia.com.br/Bruna/natureza/content/B6_large.html
http://farm1.static.flickr.com/61/154130385_c0694b74f6_b.jpg
How do leaves reflect IR?
http://pirlwww.lpl.arizona.edu/research/biosphere/Lesson/
Young and Mature Leaves
Reflection depends on Health of Leaf
• Chlorophyll absorbs red and blue light and reflects green light.
• Near-infrared light is reflected by the spongy cell structure inside of leaves.
• Chlorotic (yellow) leaves have lower levels of chlorophyll
• Necrotic leaves do not have pigments or the spongy cell structure of living leaves.
Other structural color
• Leaves may appear lighter (gray, silver, white, blue, copper, or gold, due primarily to structures formed on the leaf surface that increase reflectance
Turtleback, Psathyrotes ramosissima (Family Asteraceae),
Desert Brittlebush
• These leaves reflect about 60% of solar radiation, thus reducing leaf heating and stress.
Encelia farinosa (Family Asteraceae)
Forensic Uses of IR
• Differences in ink can be detected in altered checks
http://www.neiai.org/index.php?option=com_docman&task=doc_download&gid=28&Itemid=54
Absorption Spectra of Inks
http://www.fbi.gov/hq/lab/fsc/backissu/oct1999/mokrzyck.htm
Forensic Uses of IR
• Writing on charred paper can be imaged
http://www.neiai.org/index.php?option=com_docman&task=doc_download&gid=28&Itemid=54
Bloodstains• Just as inks can be transparent in IR,
fabric dyes can reflect, revealing blood patterns.
http://www.neiai.org/index.php?option=com_docman&task=doc_download&gid=28&Itemid=54
More Bloodstains
• Where is the real crime?
Infrared Fluorescence
• Infrared Fluorescence is similar to UV/Vis fluorescence, but shifted in frequency/wavelength.
http://people.rit.edu/andpph/text-infrared-luminescence.html
The Photophysics
http://www.beyondvisible.com/BV0-Barebasics.html
What does IR Luminescence Show?
Wood in IR Fluorescence
• Wood is typically dark in IR, but pigments can absorb visible light and emit in the IR.
Camera Obscura
http://en.wikipedia.org/wiki/Camera_obscura
• First reported in the 11th century by Al-Hazen of Egypt.
• Arabic “quamera” or dark,gives us camera.
• Used by artists and scientists
• Some examples still survive (this is in San Francisco).
Lenses
• Simple lenses have problems
• Long working distances
• Color errors
• Weight
• Reflections (internal and external)
• Complex lenses with coatings usedhttp://micro.magnet.fsu.edu/primer/java/lenses/simplethinlens/index.html
http://micro.magnet.fsu.edu/primer/java/lenses/magnify/index.html
http://micro.magnet.fsu.edu/primer/java/microscopy/variablelens/index.html
Complex lenses
• Modern lenses use multiple elements with coating, different refractive indices and the ability to move as groups or alone while focussing and zooming.
• Phew!
http://www.opticalres.com/kidoptx.html#Lenses
Aperture and Shutter
• These control exposure
• Wider aperture increases light, decreases depth-of-field.
• Slower shutter increases light, increases potential blur.
Understanding f-stops
• Longer focal-length lenses (telephoto) collect less light than shorter lenses (wide-angle). f-stops help us correct for this.
• The aperture size is divided into the focal length to give the f-number
• For a 50 mm lens, a 25 mm aperture is half the focal length, therefore f/2.
• Apertures are arranged in factors of the square root of 2 (1.4, 2, 2.8, 4, 5.6, 8, etc.), yielding 1/2 the light for each stop.