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Photographing The Invisible

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Presentation on theme: "Photographing The Invisible"— Presentation transcript:

1 Photographing The Invisible
Using Invisible Light

2 Keene State College Rich Blatchly

3 Forming an Image Patterned Light Lenses Aperture Shutter Focal plane
Light-tight box

4 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.

5 Visible Light

6 Digital Infrared Photography
Note that silicon (basis for photosensors) is sensitive to IR.

7 What's different about IR
Visiting Professor Explores Light Perception in the Infrared - Daily Nexus <

8 More IR Differences

9 Diagram of Apparatus IR requires a source (sun?), a filter and an IR sensitive camera

10 Camera equipment Testing your camera

11 Filter Responses The common Wratten 89B is also called Hoya R72
Recommendation: I believe the most useful, general- purpose IR filter for digital photography is Hoya R72 (#89B). It blocks visible light well enough (if not entirely) to provide a well-pronounced IR effect, while still allowing for non-exotic exposure times. This filter should work fine with most of mid- to high-end amateur digital cameras (your mileage may vary, so check with someone who tried it on your camera). The small amount of visible (far red) light which this filter lets through does not affect pictures enough to spoil the IR effect, while coloring your images red (or purple), therefore they need to be converted to monochrome in postprocessing.

12 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.

13 What to shoot in IR Arial Photography in your backyard
Aerial Photography in Your Schoolyard [taken from APSnet ref above). Perspective can change how we view things. When we observe the world around us, changing our perspective can give us a more complete picture. Picture the playground at your school, or a nearby park. Perhaps there are areas of dead grass, or areas with lots of weeds. How could you measure how bad the problem is? Walking around the playground will give you an idea, but what if your playground is very large? That might require a lot of time. If we could go up in a helicopter or airplane, we could see the whole playground at once. If we took a camera with us, we could take a picture, and later measure the area showing the problem. Can you think of other ways of getting a camera above your schoolyard? Some people have tied cameras to balloons or kites. There are numerous web sites where people describe their experiences and how they set their cameras. Some of them are listed below. • < • < • < • < • < • < 40 Incredible Near-Infrared Photos « Smashing Magazine <

14 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 When setting the exposure compensation (SLR or not), you have to aim for a picture which will look like it is underexposed, too dark. This is because practically whole image information goes into just one of the RGB components: red, and you have to keep that component from saturation (i.e., running out of range). If your camera can display a brightness histogram for individual RGB components, make sure that the red one does not hit the upper limit. Otherwise use -1 EV or so of negative exposure compensation, adjusting this correction as you learn your camera/filter combination.Your exposures will be quite long: an IR filter combined with the camera's anti-IR one will let through less than 0.1% of the incoming light. A bright scene, requiring 1/500 s at F/8 in visible light will need about 1 s or longer at F/4 on most cameras. Not only this asks for using a tripod, but, if the air is not quite still, there will be a blur in the foliage, grass, water reflections, etc. This is not necessarily a bad thing, and it may add an extra feel to the imag

15 Processing
The lake at the front of my house is a nice IR subject, so here it is, on a late, August afternoon. The original image shot with the Hoya R72 filter is shown above at left; above right is the same file after desaturation, and at the right — after some histogram adjustment.The R72 exposure adjustment factor for the '5060, i.e., the exposure difference between this picture and the one below, is 2400, or 11.2 EV (the same as for the '5050).Note that the exposure program in this camers boosts up the CCD gain quite aggresively: ISO 250 in this case. This does not help the noise, usually prominent in infrared, but I don't mind it really

16 Mixed with Visible


18 How do leaves reflect IR?
The figure below shows how red (R), green (G), blue (B), and infrared (IR) light is reflected by a leaf. The palisade tissue containing chloroplasts absorbs the red and blue light, while the green light is slightly absorbed but mostly reflected by the same tissue. The infrared radiation passes through the cuticle and palisade tissue and comes into contact with the mesophyll tissue where it is scattered by the air spaces in between the mesophyll cells and then either reflected or transmitted. This produces the rise in reflectance seen in the following figure.

19 Young and Mature Leaves

20 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. Chlorophyll, the pigment used in photosynthesis, absorbs red and blue light and uses the energy to convert carbon dioxide into plant sugars. Since chlorophyll does not use green light, it is reflected and gives healthy leaves their characteristic green color. Near-infrared light is reflected by the spongy cell structure inside of leaves. Chlorotic (yellow) leaves have lower levels of chlorophyll. This may be due to disease or lack of nutrients. Since they do not have much chlorophyll to absorb red and blue light, these leaves reflect more red, blue, and green light than healthy leaves giving them a yellow appearance. Necrotic leaves are leaves that are dead and have turned brown. They do not have pigments or the spongy cell structure of living leaves. Notice how both healthy and chlorotic leaves reflect near-infrared light in Fig.3, but have lower reflection around 700 nanometers. This is known as the “Red Edge” and is characteristic of living leaf tissue

21 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 is a low herbaceous perennial of the California deserts having a hemispherical canopy. The canopy consists of small leaves that are silvery white and highly reflectant, due to the presence on the upper and lower leaf surfaces of special dry trichomes. Turtleback, Psathyrotes ramosissima (Family Asteraceae),

22 Desert Brittlebush These leaves reflect about 60% of solar radiation, thus reducing leaf heating and stress. The reflectant, silvery leaf type of California desert brittlebush, Encelia farinosa (Family Asteraceae) is formed when this desert shrub begins to experience water stress during the spring. This leaf is highly reflect, probably absorbing about 60% of solar radiation, but thereby reducing heating of leaf tissues by reflecting infrared radiation. The photosynthetic rate also is reduced, however, because some of the visible spectrum absorbed by chlorophyll, is also reflected Encelia farinosa (Family Asteraceae)

23 Forensic Uses of IR Differences in ink can be detected in altered checks

24 Absorption Spectra of Inks

25 Forensic Uses of IR Writing on charred paper can be imaged
The photograph at left depicts illegible charred documents. The photograph at right was taken using infrared reflected with Kodak High Speed Infrared Film with an 89B filter. Thetype is rendered legible using thistechnique.

26 Bloodstains Just as inks can be transparent in IR, fabric dyes can reflect, revealing blood patterns. The photograph at left depicts black cotton fabric with a bloodstain. The photograph at right was taken using infrared reflected with the Fuji S3 UV/IR digital camera with Peca 900 (18A) filter. The black cloth is rendered white and the bloodstain readily visible. ISO 400, F16, 1/750, Tungsten cross lighting

27 More Bloodstains Where is the real crime?

28 frogs with infrared reflective pigment
Some frogs have an infrared reflective pigment to reduce heating

29 How to do infrared photography-Sources
Wrotniak Apogee Photo Magazine: DIGITAL INFRARED PHOTOGRAPHY MADE EASY < shtml> Point and Shoot Digital Infrared Photography: Get Creative with Invisible Light | < nd_shoot_digital_infrared_photography> A Guide to Infrared Photography | teddy- risation™ < photography/> GentleIntro1 < 1.html> Infrared Photography with a Digital Camera <

30 Infrared Fluorescence
Infrared Fluorescence is similar to UV/Vis fluorescence, but shifted in frequency/wa velength. Join us to learn more about this project <

31 The Photophysics

32 What does IR Luminescence Show?
Various subjects have the property of changing incident visible wavelengths into longer invisible, reflected. ones. Inks of various kinds exhibit this property. This can be exploited as a means for detecting the presence of residual ink in documents that appear to be devoid of text or other data due to age, water damage, or other factors. It can also be used to possibly detect differences between to inks that look the same to the eye but fluoresce differently in the infrared. Sometimes a reflected infrared approach is sufficient to detect differences but the luminescence technique possibly yields additional corroborating information.

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


35 Capturing the image Chemical processes
Nie´pce (1827): Bitumen of Judea Daguerre (1839): Daguerreotype William Fox Talbot (1839): Calotype Frederick Archer (1851): Collodion Richard Maddox (1871): Geletin George Eastman (1884): Celluloid support

36 UV Photography

37 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).

38 Lenses Simple lenses have problems Long working distances Color errors
Weight Reflections (internal and external) Complex lenses with coatings used

39 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!

40 Autofocus--how does it work?

41 Aperture and Shutter These control exposure
Wider aperture increases light, decreases depth-of-field. Slower shutter increases light, increases potential blur.

42 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.

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