SFR 406 Spring 2015 Lecture 7 Notes Film Types and Filters

Transcription

1 SFR 406 Spring 2015 Lecture 7 Notes Film Types and Filters 1. Film Resolution Introduction Resolution relates to the smallest size features that can be detected on the film. The resolving power is a related term that is a function of the lens optical quality and film resolving properties. It is usually expressed as the smallest separation between two features and is expressed as line pairs per millimeter. By photographing a resolution test chart, the resolving power of the film can be determined. The target is a series of parallel black bars (3 per set) on a white, high contrast background. Grain Size The size of the silver halide granules vary between film types and even within the emulsion on a single piece of film. However, the average size of the grains largely defines the resolving power of the film. Larger grain film needs less light for proper exposure (faster film). Larger grains produce more metallic silver per area (therefore less number of grains in the film) and require less total energy to expose the film. The larger grain film can cut down on image motion as shutter speeds can be set faster. The following figure shows the resolution test chart with the black and white tones reversed. Film Speed Film speed relates to the sensitivity of the film to reflected light and is dependent on grain size, number of grains, and chemicals in the film. Film speed can be calibrated by different exposure indices. The index used for aerial films is the Aerial Film Speed System (AFS). The American Standard Association (ASA) or the International Standard Organization System (ISO) is the speed rating used for film in terrestrial cameras (e.g., 35 mm). Kodac 35 mm film with 64 ASA rating is a much slower film than with a 400 ASA. The larger grains in the faster 400 ASA film can be used in lower light conditions (e.g., indoors) whereby a flash would be necessary for proper exposure of the 64 ASA film. SFR 406 Film Type and Filters Notes 1

2 Resolution External Factors The resolution of the photo produced is a function of the resolving power of the camera/film system plus a number of external variables that can affect the sharpness of detail that can be resolved on the photo. The external factors originate from multiple sources such as atmospheric conditions (e.g., scattering), camera vibration, image blur, and film processing, among others. The dynamic resolution of the system can be measured by flying over and photographing a large bar target array placed on the ground. In this case, the external factors (discussed earlier) that affect resolution can come into play. This dynamic resolution of the system is perhaps a more operational or applied test of sharpness of detail that can be achieved on the image compared to the controlled or static laboratory test chart setting. Ground Resolved Distance Equation If resolution is expressed as ground distance, the term often used is ground resolution or ground resolved distance (GRD). Reciprocal of image scale/line pairs per mm x 1000 = GRD Example I = 20,000; lp/mm = /40 x 1000mm/m = 0.5m 2. Film Structure and Processing Introduction to Structure and Function of Film Film records reflected light energy. Film is both the recording and storage media. Terrestrial films (35mm; 70mm) are designed for indoor or outdoor use. Aerial films are designed specifically for aerial use in mapping cameras (240 mm or 70 mm). There are two major types of film: black and white and color. Black and white (BW) film has a more simple structure compared to color film. A black and white film negative is composed of 3 primary components: 1) emulsion layer, 2) base and 3) backing. The light sensitive emulsion layer is encrusted with silver halide crystals. The emulsion layer is coated on a transparent (plastic) base. The anti-halation backing prevents (absorbs) any light that may have transmitted through the layers, from reexposing the film. Incoming energy strikes the light sensitive silver grains. After exposure, a photo-chemical reaction develops the formation of the latent image. The silver salts are reduced to black grains. SFR 406 Film Type and Filters Notes 2

3 (sensitive to light) (80%-90% of film thickness) (antihalation: light absorbing layer) Negative and Positive Film The negative film has reversed brightness and geometry. A positive print represents the true brightness and geometry. Most aerial photographic prints are produced through a negative to positive processing called contact printing. The negative is positioned to the positive paper, emulsion to emulsion, and the image is created when white light is passed through the negative emulsion to expose the print emulsion. Positive images can be developed on plastic or glass-based emulsions known as diapositives or transparencies. Positive prints can also be produced directly from positive transparencies (color reversal film) through a method called Type R processing. Black and white film processing (negative or positive) generally involves five steps: developing, stop bath, fixing washing, and drying. Color Theory There are two color theories relevant to remote sensing. Additive color theory explains how humans perceive light in visible wavelengths and also how computer monitors work. Subtractive primary color theory explains how film works (and color photo copy machines). These two color theories should not be confused with primary colors used in paint pigments. Additive Color White light is made up of 3 primary colors: blue, green and red. Any combination of two primaries will produce a complementary color. For example, the combination of blue and green primaries produce the complementary cyan color. Blue and red produce the complementary magenta and green and red produce yellow. All three primaries produce white and the absence of all three, produce no color (black). Subtractive Primary Color The three subtractive primaries (pigments or dyes) in film are yellow, magenta and cyan. When a positive transparency or print is viewed under white light, the density of the dyes at any position of the photograph will subtract certain colors and determine what remaining colors will pass through. Yellow dye subtracts blue light, magenta subtracts green and cyan subtracts red. SFR 406 Film Type and Filters Notes 3

4 Color Film Structure Color film is more complex in structure and function compared to BW film. Color film has 3 light sensitive emulsion layers and 3 pigmented (dye) layers, one associated with each of the 3 light sensitive layers. This film is sometimes called normal color, true color or natural color because it reproduces colors similar to the colors that humans perceive with normal color vision. A typical normal color film has blue, green and red wavelength sensitive emulsion layers. A yellow dye is associated with the blue layer, a magenta dye with the green, and a cyan dye with the red light sensitive layer. The dyes act as selective filters to block transmission of light at particular visible wavelengths. 3. Film Types and Filters Introduction to Filters Filters are an important component of any camera system. A filter can be made of glass, gelatin or plastic. The purpose of a filter over the lens of a camera is to select, amplify, or eliminate portions of the light spectrum from reaching the film. Filters are placed, in the optical path, over the lens of the camera. Aerial filters contain dyes and they range from completely clear to a variety of colors and opaqueness depending upon the wavelengths of light they are intended to absorb or pass. Filters are sometimes described by their color (qualitative) or by a Kodac Wratten number, which is referenced to published transmittance curves describing their quantitative properties. Categories of Filters Categories of filters can be categorized by their function (Paine and Kiser, 2003): 1. short-block or haze cutting 2. band pass 3. polarizing 4. anti-vignetting 5. color correction. A short-block filter absorbs short wavelengths and allows longer wavelengths to pass through to the film. These are also called haze-cutting filters. Many films are sensitive to ultraviolet (UV) light but these wavelengths would only degrade the image quality if they reached the film. A haze-cutting filter that only eliminates ultraviolet (0.3 to 0.4 microns) is called a UV or skylight filter. This is a perfectly clear filter to the naked eye. A UV filter is an optional choice for terrestrial photography but a necessity for aerial black and white panchromatic and normal color films, particularly the higher the altitude of the camera system (Figure ). SFR 406 Film Type and Filters Notes 4

5 Some haze-cutting filters will eliminate (absorb) ultraviolet and blue wavelengths (0.3 to 0.5 microns). This type of filter is always used with color infrared film and is very effective at higher altitude. This filter is sometimes referred to as minus blue thus describing its function. A Kodac Wratten 12 is a brand name for this particular filter. It is a yellow colored filter. Haze (rayleigh scatter) is more of a problem for aerial photography in the short visible (blue) and ultraviolet wavelengths. The higher the altitude, the more atmosphere there is between the aircraft and ground and hence, the more haze that will be in the field of view of the camera. A band pass filter will pass certain wavelengths and absorb others. Filters will pass wavelengths of their own color and absorb the others. For example, a high pass filter, like the Wratten 25A, is red in color and will absorb blue and green wavelengths but allow red and reflected IR to pass through. Other Kodac band pass filters select blue (Wratten 47), green (Wratten 58), and photographic infrared only (Wratten 89B). The following figure depicts black and white aerial photos taken with an Itex multi-lens camera using different band pass filters A polarization filter is used specifically to reduce reflection coming in at specific angles (approximately 35 degrees). These filters might be used at certain times of the day when SFR 406 Film Type and Filters Notes 5

6 sun angle reflection might be directed right into the camera optics. A polarizing filter will only transmit light coming in at a certain plane and absorb light rays at different polarizations. Anyone who has owned a pair of polarized eyeglasses can relate to the function of this type of filter on an aerial camera. Anti-vignetting filters compensate for reduced light that can occur around the edges of a lens such as a wide angle lens. The filter is denser near the middle and progressively less dense toward the edge creating a better balanced lighting across the photograph. A color correction filter is usually not used on the camera but in the photo lab during film processing. For example, when printing color infrared photographs, a color correction filter can adjust the color contrast to improve the interpretability of the prints. An aerial photo contractor, processing color infrared film, often has a standard charge for color balance and the color correction filter is used in the process. In special cases, a color correction filter might be used with CIR film during the photo mission, therefore shifting the color balance to improve the interpretability of certain earth features on the photography (Paine and Kiser, 2003). Filter Factor A filter factor is the ratio of normal exposure to increase exposure needed to compensate for varying opaqueness of filters used. Most filters (except UV) absorb part of the visible light spectrum thus reducing the total light energy that reaches the film (Paine and Kiser, 2003). If lower quantities and qualities of light are reaching the film then the film will be under-exposed if a filter factor is not adjusted to crank up the exposure. Most UV filters are clear and thus require no additional exposure compensation, hence the filter factor is 1. A filter factor of >1 means that the f/stop needs to be opened up or the shutter speed needs to be reduced accordingly to increase the light coming through the lens. Film types have different spectral characteristics therefore the filter factor will vary among films even using the same filter. Each film-filter combination has known properties so that a filter factor can be determined. Filter factors are keyed to film speed or to the Aerial Exposure Index (AEI). An effective exposure rating can be determined by dividing the AEI by the recommended filter factor. For example, for an AEI of 200 and a filter factor of 2, the effective film speed rating would be 100. Black and White Photography Black and white (BW) panchromatic film records wavelengths from 0.4 to 0.7 microns. BW panchromatic film is known for its good contrast and high resolution. BW Pan was once a staple film for the USDA-Soil Conservation Service (now Natural Resource Conservation Service) for soil type mapping. BW Pan is sometimes preferred for land surveying and boundary applications that prefer hardwood leaf off conditions or any other related application that use the ground conditions more than the vegetation types on the ground. BW infrared film is rarely used anymore; color infrared has replaced it in most applications that use IR film. BWIR film records from approx. 0.6 to 0.9 microns with the W25 filter (red color). Modified IR photography uses the W89B filter and records only 0.7 to 0.9 microns (the photographic infrared region). SFR 406 Film Type and Filters Notes 6

7 BLACK AND WHITE PANCHROMATIC BLACK AND WHITE INFRARED Color Photography With a UV filter over the lens, normal color film records visible wavelengths from approx. 0.4 to 0.7 microns (same as BW Pan) and the colors resemble the natural colors that human perceive at the same wavelengths. Normal color films have largely replaced BW Pan where it was formerly used for most applications in vegetation and soil mapping. The following figure is a fall normal color aerial photo (original scale 1:2,000). SFR 406 Film Type and Filters Notes 7

8 Color Reversal Film - Positive Transparencies Color reversal film produces positive geometry and color on the original transparency film in the camera. Kodac film 2448 (normal color) and 2443 (color IR) are color reversal films. 35 mm slide film is an example of a color reversal film used for terrestrial photography and small format aerial applications. Positive transparencies produce better resolution products for photo interpretation than positive prints made from a negative. This is because the interpreter is viewing the original film rather than a 2 nd generation product as in the case of negative to positive contact print. Color Infrared Photography Color infrared (CIR) film records approximately 0.5 to 0.9 microns with the minus blue filter blocking out the blue wavelengths. To expose CIR film properly and produce the correct color balance intended for the film, the blue wavelengths must be eliminated. The photography has superior haze penetration. Color infrared film is extremely popular with foresters in distinguishing among vegetation types and for stand mapping. It is not used in height measurements, however, because the elimination of blue wavelengths causes shadows to be completely black thus making it difficult to locate the ground to make a measurement with a parallax wedge or bar. The following figure is a fall CIR aerial photo (original scale 1:2000) of a mixed softwoodhardwood forest stand. SFR 406 Film Type and Filters Notes 8

9 Advantages/Disadvantages of BW and Color Aerial Photography Advantage Disadvantage BW Pan Best resolution No color Least expensive processing Veg. discrimination more difficult Familiar gray tones Detail in shadows Shallow water penetration Normal Natural color for humans Not as good as IR for vegetation Color Detail in shadows Shallow water penetration BWIR Good distinction of hardwood No color and softwood No information in shadows Cuts through haze Good land/water definition Color IR Good distinction of vegetation types No information in shadows Good land/water definition More expensive processing Cuts through haze Not natural colors SFR 406 Film Type and Filters Notes 9