Q. What are antireflection films and interference filters?
Ans. Anti reflecting Films: When a beam of light travels normally from a medium of refractive index µ2 to a medium of refractive index µ1, a part of light is refracted and a part of light is transmitted, then reflecting and transmitting coefficients can be given as
Such types of film are called Anti-Reflecting Film.
Consider Fig. given, an incident light reaches to an air-film interface. Some of it reflects back to the air and some of it transmits into the film. The transmitted part refracts in the films and reaches to a film substrate interface. Some of it reflects back to the air and some of it transmits into the substrate. The two reflected waves, which are generated at the film air interface and the substrate film interface, interact and produce a resultant reflected light waves.
The intensity of the net reflected light depends on the optical thickness of the film. Optical thickness of a film is given by,
When n is the refractive index and d is the physical thickness of the film. If the optical thickness of the film is a quarter waves, the phase difference of the two reflected waves is π. They become out of phase and interfere destructively. Then, the net intensity of the reflected wave is at minimum. If the optical thickness of the film is a half wave, the phase difference of the two reflected waves is 2π. They become in phase and interfere constructively. Then, the net intensity of the reflected wave is at maximum.
Interference Filter: An interference filter, which reflects one of more spectral lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. In an optical filter, a semitransparent metal film is deposited on a plate of glass. Then, it is coated with a thin layer of some dielectric material like MgF2, followed by another coating of a thin film of metal. Finally, another plate of glass is kept over the films for mechanical protection. Interference filters have a band pass of 100-150A and peak transmittance of 40-6-%. When a beam of light is incident normally on an interference filter, multiple reflections takes places between the two surface (Layers). If 't' is the separation between the two plates and 'µ' is the refractive index of the dielectric medium, then from the theory of interference in thin film the condition for maxima in transmitted light can be given as
2µt = n¥ .....(1)
If we use white light in this case, then different colors (wavelength) are found in transmitted light can be obtained using equation (1)
For example, if µ = 1.5, t = 6 x 10-5 meter, then only two wavelengths of visible region of value of 6000A for n = 4 gives us maxima. Since the wavelengths are widely separated, they can be filter by using a device called interference filter.
If a thin transparent spacer is placed between two semi reflective coatings, multiple reflections and interference can be used to select a narrow frequency band, producing an interference filter. It the spacer is half wavelength for the desired wavelength, then other will be eliminated by destructive interference. If the back layer is totally reflective, then the arrangement is called a dichotic mirror, reflecting only the selected wavelength.
Interference filers are widely used in instrumentation for clinical chemistry, environment testing, colorimetric, elemental and laser line separation, flame photometry, fluorescence, immunoassays, etc.
Designing and producing interference filter is a complex procedure requiring thin film expertise and sophisticated instruments.