Photographing The Invisible Using Invisible Light Keene State

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%20 dslr. 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 89 B is also called Hoya R 72 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/EMW 6 NFO 0 FPEQHO 9 ZGG/

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 x 2 in exposure; 210 = 1024). • Focus

Processing http: //wrotniak. net/photo/infrared/c 5060. html

Mixed with Visible http: //www. rbfotografia. com. br/Bruna/natureza/content/B 6_large. html

http: //farm 1. static. flickr. com/61/154130385_c 0694 b 74 f 6_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/oct 1999/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/BV 0 -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 11 th 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). 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.