706.8.6pp Scientific Photography F21.pdf

Scientific Photography n Scientific Photography n n n Linda Rourke FOS 706 n n n 1 Color Temperature Lenses Photographic Film Digital Photography Camera Design and Function 2 Photography Purpose and Application photo = light n graphos = pertaining to drawing or writing 3 How is photogr...

Scientific Photography n Scientific Photography n n n Linda Rourke FOS 706 n n n 1 Color Temperature Lenses Photographic Film Digital Photography Camera Design and Function 2 Photography Purpose and Application photo = light n graphos = pertaining to drawing or writing 3 How is photography used in forensic science? – Visual documentation method Essential to crime scene documentation n Useful for documenting evidence details n 4 Purpose and Application n Light How is photography used in forensic science? – Analytical value n n n Visualize patterns and phenomena not detectable with the naked eye n Extraction of information! n 5 Purpose and Application Light, Illumination, Contrast, Filters n n Electromagnetic radiation Visible light Normal photographic films are sensitive to visible light Specialized film: infrared sensitive Images produced by gathering a portion of light emitted or reflected by an object & recording the light pattern 6 1 Illumination n n n n Filters & Contrast Available versus Artificial Angle of lighting Intensity Types of artificial illumination n – Use with black & white film – Emphasize/de-emphasize certain colored portions of a multi-colored subject – Use a filter of complementary color to darken a certain color in a photograph – Use filters to bleach out or to enhance contrast for a colored object – Unidirectional – Diffuse n Bounce – Multidirectional 7 8 Filters & Contrast Colors and Their Complements l (nm) Color 400-450 violet 450-480 blue 480-490 green-blue 490-500 blue-green 500-560 green 560-575 yellow-green 575-590 yellow 590-625 orange 625-750 red Filters & Contrast n Complement yellow-green yellow orange red purple violet blue green-blue blue-green 9 Color correction filters – Use to adjust color temperature of light source to match the film being used n Color films are designed for use with specific color temperature sources – Daylight film: 6000K – Electronic flash: ~6000K – Tungsten (photoflood lamp): ~3200K 10 Color Temperature Color Temperature n n The Visual Dictionary of Physics. (1995). London: Dorling Kindersley. 11 Color contrast filters Relates temperature (K) to the distribution of wavelengths given off by an incandescent source. The ideal source is a perfect absorber and emitter of radiation = black body radiator (idealized concept – not real!) – it will be used to develop the concept of color temperature 12 2 Color Temperature n n Color Temperature Real sources can approach the behavior of a black body radiator – referred to as grey bodies Consider a steel bar that is slowly heated from one end; it begins to glow; consider the color progression of the heated steel bar… The Visual Dictionary of Physics. (1995). London: Dorling Kindersley. 13 14 Color Temperature Color Temperature The Visual Dictionary of Physics. (1995). London: Dorling Kindersley. 15 The Visual Dictionary of Physics. (1995). London: Dorling Kindersley. 16 Color Temperature n Color Temperature Java Tutorial https://micro.magnet.fsu.edu/primer/j ava/colortemperature/index.html http://astronomy.nmsu.edu/tharriso/ast110/class16.html 17 18 3 Color Temperature n The color temperature of an incandescent continuous source is the temperature to which the theoretical blackbody must be heated in order to emit the same distribution of wavelengths. http://www.techmind.org/colour/coltemp.html 19 20 Forensics & Color Temp n n n n Interference Colors using PLM n n Color Photography Color Comparisons (paints, soils) Microscopy Photomicrography Comparison Microscopy n Color Balance Photography: Film Selection http://www.micro.magnet.fsu.edu/primer/lightandcolor/colortemperatureintro.html 21 22 Lenses Color Correction Filters n n n n 23 Glass or gelatin filters used to alter color temperature of a light source Done by preferential absorption of wavelengths from one end of the spectrum Use of filter è loss of energy Specific filters are designed for specific sources n n Image projected onto film by the lens in a camera is a REAL image. Latent image produced on film is due to spatial variation in intensity of light reaching it. 24 4 Lenses: Focusing n n n n n Lenses Alter the distance between the lens and the film Light from infinity (parallel light) comes into focus at the principal focus of the lens; for objects closer to the camera the lens must be moved farther away from the film. Normal helical lens mounts allow for focusing from a few feet to infinity. Extreme close-ups - greater extension is required. Macro lenses 25 n n – Normal lens, 50mm standard (40-58mm) – Macro lens, 50+mm, greater range of motion of focusing mount – Wide-angle lens, <35mm – Telephoto lens, >>45mm 26 Lens Speed n n n Lenses: f/# A measure of the light gathering power of a lens; faster lenses produce brighter images than slower lenses of same focal length. Related to maximum size of its opening (aperture): larger maximum aperture -> faster lens. Better quality lenses have adjustable apertures; largest usable aperture characterizes the lens speed. 27 Lens aperture size is numerically expressed as the focal ratio or f-number, calculated by dividing focal length by aperture diameter: n f - number = f focal _ length f = = # diameter d f-number is inversely proportional to the aperture diameter Adjustable-iris diaphragms used to vary aperture opening f-stop: each incremental stop changes the amount of light passing through the lens by a factor of two n n n 28 Lenses: f/# n n n n n Photographic Film Amount of light reaching film µ area of aperture opening; doubling area opening reduces f/# by one stop doubling area ¹ doubling diameter: n 29 Multi-element lenses used to approach ideal lens behavior Lens types differ in focal length Film = recording medium in traditional photography Photographic emulsion n n – A = p (d2/4) \ Doubling area requires increasing diameter by a factor of 2 Adjacent f-stops differ by 2 Stopping down = decreasing lens aperture Light sensitive silver halide grains suspended in a uniform gelatin layer on a cellulose acetate support 30 5 Fast Film vs. Slow Film Photographic Film Film types n – Based on illumination source n n – Daylight film Tungsten film (indoor illumination) Based on film speed n n Fast films (higher ISO or ASA #) Slow film (lower ISO or ASA #) 31 32 Digital Photography n Digital Photography Instead of film, image is captured electronically using one of the following n – CCD (charged couple device) – CMOS (complementary metal-oxide semiconductor) sensor. 33 The CCD is an array of light sensitive diodes which convert photons into electrons. – the charge is transported across the chip and read at one corner of the array – an analog-to-digital converter translates each pixel's value into a digital value 34 Digital Photography n CMOS is an array of transistors that amplify and move the charge using traditional wires. – Each charge is read individually 35 https://www.cei.se/download-ccd-vs-cmos 36 6 Color Determination Digital Photography n n n 37 The analog charge is “read” using an analog-to-digital converter (ADC) which measures each pixel charge 38 Digital Photography n n n Digital Photography Resolution based on the number of pixels (picture elements); somewhat analogous to film speed ISO is a measure of the sensitivity of the image sensor With rapid technological advances in the last decade, digital resolution is slowly approaching that of a high resolution film 39 n White balance is the digital analog to color temperature choices for film http://equipped.outdoors.org/2011/08/take-better-pictures-part-1.htm l 40 Camera: Image Formation The Camera n n 41 Each diode = pixel The light captured by the diode is converted into an electrical charge The simple camera is a light tight enclosure with lens and a light sensitive detector mounted in proper position. Modern cameras have conveniences which make fine tuning easier, but lens and digital resolution capabilities are the most important parameters of the camera. 42 7 The Camera Other Camera Accessories Shutters provide automatic control of exposure times; mechanically or electronically timed. n n n n – Leaf shutter – in the lens – Focal plane shutter – in front of the film plane, most commonly used 43 n n 44 The Camera Control of Exposure The Camera Control of Exposure Illumination level Reflectance of objects in the field of view n n n ISO setting (film speed) Exposure settings on the camera n n – – n Aperture setting (f/#) Shutter speed 45 Primary control of image density is provided by exposure settings on the camera: the f/# and the shutter speed. Shutter speed scale designed to be compatible with f/# and ISO (film speed) scales 46 The Camera Control of Exposure Stops n n 47 Focusing mechanism Viewing aid Built in meter for exposure determinations Flash illumination Interchangeable lens provision Use of lens opening, shutter speed and ISO (film speed) scales all based on one-stop increments. In photography, a “stop” refers to the incremental parameters for aperture size, shutter speed and ISO (film speed). n Equivalent exposures – Obtaining the same exposure by simultaneously increasing the aperture one stop while decreasing the shutter speed one stop and vice versa. – Also possible to obtain equivalent exposures by using different ISO (film speed). 48 8 The Camera Equivalent Exposures The Camera Control of Exposure ISO 32 f/16 @ 1/15 f/11 @ 1/30 f/8 @ 1/60 f/5.6 @ 1/125 f/4 @ 1/250 f/2.8 @ 1/500 f/2 @ 1/1000 f/1.4 @ 1/2000 n ISO 64 f/16 @ 1/30 f/11 @ 1/60 f/8 @ 1/125 f/5.6 @ 1/250 f/4 @ 1/500 f/2.8 @ 1/1000 f/2 @ 1/2000 ISO 125 f/16 @ 1/60 f/11 @ 1/125 f/8 @ 1/250 f/5.6 @ 1/500 f/4 @ 1/1000 f/2.8 @ 1/2000 49 Choice of camera setting depends on: – relative movement of camera and subjectèrapid shutter speed desirable – depth of field = range of acceptable focus in directions closer to and further away from the camera n better depth of field with wide angle lenses or with small aperture openings. 50 Exposure Determination Methods Exposure Determination Methods Light level meters: – Reflected light meters - built in meters measure light reflected off subject Light level meters: – Incident light meters - measure level/intensity of light falling on subject Different reflectance of black and white subjects - even though incident light reading is same, reflected light reading is different; black objects usually overexposed and white objects are underexposed. n Requires use of 18% gray card n Best for determining illumination available for image capture. n Must be placed in the middle of subject area. n Not always practical n 51 52 Exposure Determination Methods Exposure Determination Methods Use of an 18% gray card n – Take nominal exposure as determined by the camera’s reflected light meter. – Stop down the exposure by closing aperture or increasing shutter speed; take (-) exposures in half stop increments – Increase the exposure by opening aperture or decreasing shutter speed; take (+) exposures in half stop increments v18% = average reflectance of light, reflectance for which meter is designed vUse as a standard substitute scene. vIt is a neutral card in that reflectance is same for all wavelengths. vProvides an accurate assessment of the illumination available for image capture. 53 Bracketing with (+) and (-) exposures 54 9 The Camera Control of Exposure n Reciprocity failure – Failure of the film to respond to equivalent exposure settings at extremes of exposure (very short/long shutter speeds or very large/small apertures). 55 10