Dec 26 2007
Introduction
Uses of Optical Filters in Photography
Types of Optical Filters
Absorption Filters
Dichroic Filters
Interference Filters
Introduction
A large number of natural and man-made light sources give off a wide range of wavelengths which covers the entire spectrum of visible light; with certain extend, into the infrared and ultraviolet regions as well. For some lighting applications such as interior room lights and automobile headlights, it is acceptable and quite useful to have a wide wavelength spectrum.
On the other hand, it is advantageous to restrict or limit the range of wavelengths for applications which requires a selected region of frequency or color. This can be easily accomplished by the use of specialized filters, which selectively reflect, refract, diffract or absorb unwanted wavelengths and allow the remaining wavelengths to pass through the filter.
Uses of Optical Filters in Photography
Filters are manufactured into a number of shapes and sizes. Filters can be used to pass or eliminate bands of wavelength ranging from hundreds of nanometers in size down to a single wavelength.
Filters can also be used to fine-tune the color temperature of light to generate a light spectrum of having the feature of indoor tungsten illumination, bright daylight or evening sky. Filters are also useful for the adjustment in contrast of colored regions in black and white photography or adding special effects in color photography.
Types of Optical Filters
Absorption and interference filters are the two most common types of filters in use today. Special dichroic filters are used to polarize light. Heat-absorbing filters can reduce the heat and infrared wavelengths and permitting only visible light to pass through. Filters can also be used to remove harmful UV rays.
Absorption Filters
Absorption filters work by reducing the incident light through absorption of specific wavelengths. Absorption filters are commonly made from pigmented gelatin or dyed glass. The spectral performance of an absorption filter is a function of the quantity of the dye present in the glass or gelatin matrix and the physical thickness of the filter itself.
Absorption filters are used to generate special effects in numerous photography applications and are extensively used in the cinema industry. Absorption filters are also found in traffic signals and on boats, aircrafts and vehicles as directional signals.
Dichroic Filters
Dichroic filters are far more capable and precise in their ability to obstruct unwanted wavelengths when compared to glass and gel absorption filters. Multi-layered thin film coatings are used for the manufacture of dichroic filters. These coatings are built up onto optical-grade glass using vacuum deposition.
Dichroic filters are widely used in a number of applications such as specific filtration for photography and optical microscopy. Dichroic filters are used instead of absorption filters for high quality color enlarges to fine tune the light color transmitted through the color transparency or negative.
Interference Filters
Interference filters differs from absorption filters. Rather than absorbing, interference filters reflect and destructively interfere with unwanted wavelength.
Modern interference filters are formed after the Fabry-Perot interferometer designed in the late 1800s by Alfred Perot and Charles Fabry. Interference filters are manufactured with a number of layers of thin films applied to a flat optically transparent glass surface.
Successive layers of dielectric materials are used to produce modern interference filters. The thickness of the dielectric materials ranges from 1/4 to 1/2 of the targeted wavelength. The dielectric materials are coated onto a flat optical glass of a polymer surface under vacuum conditions.
Light which is incident on the multilayered dielectric surface is either passes through the filter with constructive reinforcement or reflected and decreases in magnitude by destructive interference.
Source: AZoOptics