FILTERS 

The filters exploit the interference effect to transmit or reflect desired wavelength regions. Compared with absorptive colored glass filters and gelatin filters, interference filters have extremely good contrast, with very narrow or sharply defined transmission bands. Although most interference filters have transmission side bands on both sides of the center wavelength, these bands can be blocked by combination with a multi-layer blocking filter or glass filter. Due to the above properties, the filters are being applied to the optical instruments such as calorimeters, flame spectrometers, monochromators, and optical systems such as laser systems, space systems, optical communication systems, and optical data storage systems in the fields of physics, chemistry, biology, medicine, pharmacology, and astronomy

Characterization of Interference Filters

When a filter is tilted with respect to the incident light, its transmission band shifts toward the shorter wavelength. At angles up to about 20°, the shape of the spectral characteristic remains approximately the same, but at larger angles of incidence, the transmission band splits into two separate peaks, because the P-component (parallel to the plane of incidence) and the S-component (perpendicular to the plane of incidence) are shifted by different amounts. In an M I F-S filter, the peak of the S-component is shifted more than that of the P-component as shown in Fig. 3. The shifts also depend slightly on the wavelength. Since the sharpness of the transmittance curve of P-component is virtually independent of the inclination, this angular dependence can be turned to advantage: a single interference filter can be tilted at various angles to obtain a variety of wavelengths of monochromatic light. However, in this case, a polarizer should be used at the same time so as to utilize only P-polarization. In normal usage, angles of incidence greater than 20° are not recommended.

Except for special types, interference filters can be used for long periods at temperatures up to 50°C and for short  periods at temperatures up to about 80°C.

A metal interference filter has a triple-layer structure consisting of two partially transmitting metal layers separated by a transparent dielectric layer. These layers are deposited on a glass or quartz substrate. For protection, a glass, quartz, or colored glass filter is cemented onto it. The optical path length between the two metal layers is 1/ 2 of the transmitted wavelength, or an integer multiple of 1/2 of the wavelength. Other wavelengths are almost entirely blocked. The filter is called a first-order, second-order, or third-order filter depending on whether the optical path length is equal to the half-wavelength, twice the half-wavelength, or three times the half-wavelength. For filters of equal peak transmittance, the higher the order, the narrower the half-width is and the better the characteristic is. High-order filters have the disadvantage, however, that the lower-order transmission side band on the long-wavelength side and the high-order transmission side band on the short-wavelength side approach the main transmission band. Except for special requirement, first- and second-order filters are available. For instance, a 656nm second-order interference filter has a third-order transmission band in the vicinity of 437nm (2/ 3 X 656), and a first-order transmission band in the vicinity of 1.3µm (2 X 656) in the infrared. While for interference f-filters of below 959nm, the second- order interference is utilized, for those of above 960nm, the first-order interference is utilized, and the higher-order side bands are completely blocked by glass filters for both cases.

Typical spectral characteristics

A W-type filter is narrowband filter with a sharply sloped transmittance curve. It has a five-layer configuration equivalent to two S-type filters, stacked one atop the other. Although it has a wider half-width than that of an S-type filter, its contrast Is also better by a factor of 20, due to the steep slopes of its transmittance curve. When a flame spectrometer is used in atomic spectroscopy of sodium (589nm), calcium (622nm) and potassium (768nm), for example, if an S-type interference filter is employed, the closeness of the sodium and calcium lines complicates the measurement, and background effects appear. With a W-type interference filter, there are almost no interfering effects among the Na, Ca, and K lines, so sufficient sensitivity and accuracy can be obtained. W-type filters are also used to separate the 546nm and 578nm lines, or the 405nm and 436nm lines, in mercury spectral lines.

Typical spectral characteristics

This filter consists of dielectric multilayer films instead of metal films of MI F-S interference filter. This filter is also called single-cavity filter, and has a narrow half-width and high maximum transmittance. All-dielectric filters are classified' as type A, B, or C depending on the ratio of the half-width to the center wavelength. Narrowband interference filters of these types show different measured values of maximum transmittance and half-width depending on the wavelength purity of the spectrophotometer. If the effective bandwidth of the spectrophotometer is 1/5 the half-width of the interference filter or less, however, the measured values are constant.

Typical spectral characteristics

This filter has the following features: minimization PDL for transmission and reflection; high isolation by a steep slope, high durability by OCJ's proprietary Plasma Ion Deposition process. Custom design is available.

Typical characteristics:

T 1.48µm </=0.3dB    R 1.55µm >/=30dB

Typical spectral characteristics

This filter features high durability by OCJ's proprietary Plasma Ion Deposition process. Custom design is available.

Typical characteristics:

T 1.55µm </=0.3dB    R 1.31µm >/=30dB

Typical spectral characteristics

A band-pass filter with a rectangular band characteristics consists of dielectric multilayer films instead of metal films of MIF-W interference filter. The number of cavities can also be increased to create a multi-cavity filter. Band- pass filters are classified as type BPF-1 to BPF-5 according to the ratio of the half bandwidth to the center wavelength.

Typical spectral characteristics

An infrared band-pass filter is fabricated by depositing multilayer films on a suitable substrate, such as quartz, sapphire or germanium. These filters are classified as IR-BPF-1 to IR-BPF-4 according to the ratio of the half-width to the center wavelength. This type of filter has a number of transmission side bands on both sides of the center wavelength, but they can be removed by various blocking filters, or by exploiting the absorption of the substrate.

Typical spectral characteristics

This interference filter is a long-wave-pass filter with a sharp cutoff at a specific wavelength.

Typical spectral characteristics

The cold filter has the opposite spectral characteristics of a cold mirror: it transmits visible light and reflects infrared light. Two types are available, with different substrate materials: CF-A and CF-B. 

CF-A type :      This type has an average transmittance of 85% or above in the visible region and a reflectance of 85% or above at 1 um, so it is useful with halogen and xenon lamps which have high spectral energy in that vicinity. The coating can also be applied directly to the condenser lens to make it function as a cold filter. 

CF-B type :      This filter utilizes a beat absorption glass plate as a substrate. Combining spectral characteristics of the substrate with that of A type, this filter has an beat proof effect in a longer wavelength range. 

Typical spectral characteristics

A dichroic filter has spectral characteristics similar to a dichroic mirror, but is used for normal incidence, whereas a dichroic mirror is used for non-normal incidence. Standard dichroic filters are available as Yellow, Magenta, Cyan, Blue, Green and Red filter types.

Typical spectral characteristics

These filters have nearly constant spectral transmittance in a specified wavelength range. Neutrality is excellent, but a constant absorption is present across the entire bandwidth, due to the absorbing material deposited. These filters are used mainly as attenuation filters. 

These filters which reflect NIR light and correct colours of the visible light are essential optical components for colour TV camera tubes. The filter coating can be applied directly to the lens of the window of the camera tube.

Typical spectral characteristics

The filters are for the conversion of the colour temperature of light sources.

Special characteristics of this filter closely follow the relative luminosity curve.