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Photographing The Invisible Using Invisible Light

Photographing The Invisible

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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. - PowerPoint PPT Presentation

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Page 1: Photographing The Invisible

PhotographingThe

Invisible

PhotographingThe

InvisibleUsing Invisible LightUsing Invisible Light

Page 2: Photographing The Invisible

Keene State

College

• Rich Blatchly

Page 3: Photographing The Invisible

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.

Page 4: Photographing The Invisible

Visible Light

Page 5: Photographing The Invisible

Digital Infrared Photography

• Note that silicon (basis for photosensors) is sensitive to IR.

http://www.luminous-landscape.com/reviews/cameras/infrared%20dslr.shtml

Page 6: Photographing The Invisible

What's different about IR

Page 7: Photographing The Invisible

More IR Differences

Page 8: Photographing The Invisible

Diagram of Apparatus

• IR requires a source (sun?), a filter and an IR sensitive camera

Page 9: Photographing The Invisible

• Testing your camera

Camera equipment

Page 10: Photographing The Invisible

Filter Responses

• The common Wratten 89B is also called Hoya R72

http://wrotniak.net/photo/infrared/index.html#FILTER

Page 11: Photographing The Invisible

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/

Page 12: Photographing The Invisible

• 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

Page 13: Photographing The Invisible

Processing

http://wrotniak.net/photo/infrared/c5060.html

Page 14: Photographing The Invisible

Mixed with Visible

http://www.rbfotografia.com.br/Bruna/natureza/content/B6_large.html

Page 15: Photographing The Invisible

http://farm1.static.flickr.com/61/154130385_c0694b74f6_b.jpg

Page 16: Photographing The Invisible

How do leaves reflect IR?

http://pirlwww.lpl.arizona.edu/research/biosphere/Lesson/

Page 17: Photographing The Invisible

Young and Mature Leaves

Page 18: Photographing The Invisible

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.

Page 19: Photographing The Invisible

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),

Page 20: Photographing The Invisible

Desert Brittlebush

• These leaves reflect about 60% of solar radiation, thus reducing leaf heating and stress.

Encelia farinosa (Family Asteraceae)

Page 21: Photographing The Invisible

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

Page 22: Photographing The Invisible

Absorption Spectra of Inks

http://www.fbi.gov/hq/lab/fsc/backissu/oct1999/mokrzyck.htm

Page 23: Photographing The Invisible

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

Page 24: Photographing The Invisible

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

Page 25: Photographing The Invisible

More Bloodstains

• Where is the real crime?

Page 26: Photographing The Invisible

Infrared Fluorescence

• Infrared Fluorescence is similar to UV/Vis fluorescence, but shifted in frequency/wavelength.

http://people.rit.edu/andpph/text-infrared-luminescence.html

Page 27: Photographing The Invisible

The Photophysics

http://www.beyondvisible.com/BV0-Barebasics.html

Page 28: Photographing The Invisible

What does IR Luminescence Show?

Page 29: Photographing The Invisible

Wood in IR Fluorescence

• Wood is typically dark in IR, but pigments can absorb visible light and emit in the IR.

Page 30: Photographing The Invisible
Page 31: Photographing The Invisible

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).

Page 32: Photographing The Invisible

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

Page 33: Photographing The Invisible

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

Page 34: Photographing The Invisible

Aperture and Shutter

• These control exposure

• Wider aperture increases light, decreases depth-of-field.

• Slower shutter increases light, increases potential blur.

Page 35: Photographing The Invisible

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.