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Different Types of Filters

Small and tiny though a filter is, it plays a vital role. As a part of the optical industry, filters play a key role. So what kinds of filters are there according to the spectral characteristics?


1. Long-wave pass filter


It is a filter that passes in the long-wave direction. It is mainly used in medical testing equipment, iris recognition, digital imaging, multi-band detection, stage lighting, beauty equipment, etc.


2. Shortwave pass filter


It is a filter that passes through the shortwave direction. It is mainly used in some areas such as laser protection, ultraviolet light sources, heat reflection lighting systems, digital imaging, stage lighting, fingerprint recognition, ink recognition systems.


3. Bandpass filter


It is mainly divided into narrow-band filters and band-pass filters. As for bandpass filter design, the half bandwidth of the narrow band filter is less than 20 nm, and the half bandwidth of the band pass filter is greater than about 20 nm. The half bandwidth is corresponding to the wavelength width between two wavelengths.


4. Anti-reflection coating (AR)


The transmittance of ordinary glass is usually around 92%. The glass surface has two sides, the reflection on one side is about 4%, and is 8% on both sides. Therefore, if you want to increase the transmittance of ordinary glass, you need to reduce the reflectance by optical coating. If the reflectance of the single-sided coating is below 1%, the transmittance will increase by more than 3%, and the double-sided coating will increase by more than 6%. In this case, the transmittance of ordinary glass double-sided coating can reach more than 98%, and if single-point AR coating is used, the double-sided can reach more than 99%. In simle terms, T (transmittance) plus R (reflectance) plus A (absorption) equals the filter.


5. Neutral density filter 


Neutral density filter is also called attenuation filter. There are two main types. One is the optical coating on ordinary glass to achieve the same ratio of attenuation of different wavelengths. The other is the glass firing process by adding A special metal element to achieve the attenuation of a specific light waveband, usually referred to as a colored glass attenuator. How is the attenuation ratio generally calculated? Let's take a look.


ND2=1/2*=50%

ND4=1/4*=25%

ND8=1/8*=12.5%

ND16=1/16*=6.25%

......


6. Spectroscopic film


The optical spectroscopic film is mainly an optical membrane filter that divides the light beam into two parts according to certain requirements and a certain way. It has several types, including wavelength splitting film, light intensity splitting film, and polarization splitting film.


7. High reflective film filter (HR)


It mainly includes dielectric reflective film and metal reflective film. As for dielectric reflective film, it can be used for high-power laser mirrors with the advantages of high reflectivity, and good film density. The metal reflective film has the advantages of relatively low cost and an average reflectivity of over 90%, but it has the demerits such as poor film density, more possibility of light leakage, and wide reflection range.

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