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Some Other Filter Types

filters color using status

MICHAEL R. PERES
Rochester Institute of Technology

Barrier Filter —Used in fluorescence photography to completely absorb all excitation energy in the system such that only wavelengths of energy longer than the excitation energy reach the imaging system.

Colloidal Silver Filter —The yellow filter that is integrated into a tripack color negative film. It is located below the blue emulsion and in front of the green-sensitive layer. This filter, which contains small particles of silver that are suspended in gelatin, strongly absorbs blue light. The filter is removed during C-41 processing and may also be referred to as the Carey-Lea layer.

Davis Gibson Filter —A filter made by using two separate liquid cells use solutions of copper and cobalt salts and other ingredients. These filters were adopted in 1931 to convert CIE illuminant A (2848 K) to an approximation of sunlight (illuminant B) or north skylight (illuminant C) and were first published by R. Davis and K. Gibson in the Transactions of the Society of Motion Picture Engineers in May 1928.

Dichroic Filter —A filter composed of two filters. One filter is seen using transmission and the other filter is observed using reflection. Its color is the complement of the first. The filter does not operate using absorption, but its action is a result of interference between the various filter layers made of thin refractive materials. Many layers may be used in the variety of dichroic filters producing very accurate and narrow spectral band transmission or reflections. This filter is also called an interference filter.

Exciter Filter —A filter used as the first filter found in a fluorescence imaging system. Its wavelength needs to be of the correct spectral wavelength required to stimulate a fluorescence emission from the sample in question.

Fog Filter —A colorless filter used at the camera with a treated surface which simulates the effect of foggy conditions. Filters such as a fog filter have seen fewer applications since image processing software all comes with numerous “special filters” in the various menu options such as Gaussian blur.

Graduated Filter —A filter that does not have a consistent color or density across its surface but exhibits a continuous transition across the density range from one of the elements to the other variant of the filter. The filter should be positioned in the system some distance in front of the lens to create a subtle transition. Similar to the fog filter, graduated filters have seen fewer applications since image-processing software all comes with numerous “special filters.” Image processing easily introduced post-image capture, which creates a multitude of effects rather than the single outcome each filter is capable of.

Haze Filters —Filters used to control tones and color of the sky especially in cloudy conditions. Haze filters might also be called ultraviolet or skylight filters. All of these filter types might often be used as a lens protection filter and more or less be left on the lens all the time.

Heat Filters —Filters used primarily in an illumination system to remove infrared energy or heat. Various forms of filters might be used including colorless glass. In various digital cameras, a hot mirror may be built into the camera to remove IR from the system before reaching the chip.

Infrared Filter —A visually opaque filter that transmits principally near-infrared energy by absorbing the visual range of the EMR. The designation of the filter such as Wratten 87 or 88 may indicate its peak wavelength transmitted, such as 870 or 880 nm. Some of these filters may be made of gelatin and used with IR film or a digital camera by placing the filter at the lens or over the light.

Neutral-Density Filter —A filter type that does not selectively absorb any specific wavelength in the visible spectrum but absorbs all wavelengths evenly providing a gray appearance. It is usually referred to by its optical density, e.g., ND.3. Such filters can be used with any type of imaging systems and work well in situations where there is too much brightness or where long exposure times are desirable.

Spatial Filter —A mechanical element found principally in laser beam applications. This filter consists of an opaque material with an aperture of suitable diameter upon which the laser beam can be focused using a short-focus lens.

Spatial Frequency Filters —Mechanical devices required in image formation when using coherent light, where the image is illuminated using a coherent laser and the far-field diffraction pattern produced is then imaged to form the picture. Filters in the shape of patch stops, annuli, or other shapes can be placed in this Fourier transform plane to selectively remove spatial frequencies by simply blocking their path to the imaging lens.

Star Filter —A clear glass or plastic filter with fine engraved lines in a regular pattern on its surface, which causes the directional spread of small high brightness from the scene to create a starburst effect to these regions without seriously compromising other areas of the scene.

Status Filters —Filters used in a densitometer that are designed to produce appropriate spectral responses in various applications.

Status A filters are used for color transparencies that will be viewed directly.

Status D filters are used for measuring color prints.

Status G filters are used with inks used in photomechanical reproduction.

Status M filters are used with color negatives and transparencies that would be printed.

Status V filters are used for visual densitometry.

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