Photographing
The
Invisible
Using 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
Camera equipment
• Testing your camera
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/
Taking the picture
• 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
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/wa
velength.
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
• First reported in the
11th century by AlHazen of Egypt.
• Arabic “quamera” or
dark,gives us
camera.
• Used by artists and
scientists
• Some examples still
survive (this is in
San Francisco).
http://en.wikipedia.org/wiki/Came
Lenses
• Simple lenses have problems
• Long working distances
• Color errors
• Weight
• Reflections (internal and external)
http://micro.magnet.fsu.edu/primer/java/lenses/simplethinlen
• Complex lenses with coatings used
s/index.html
http://micro.magnet.fsu.edu/primer/java/lenses/magnify/inde
x.html
Complex lenses
• Modern lenses use multiple elements with
coating, different refractive indices and the
ability to move as groups
or alone while
http://www.opticalres.com/kid
focussing and zooming.
optx.html#Lenses
• Phew!
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). fstops 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.