Photographing The Invisible Using Invisible Light.

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.