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Optical Filters

Introduction: What are Optical Filters

An Optical Filter is an optical component that selectively transmit a specific portion of wavelengths of light while rejecting or absorbing the remaining portion. Optical filters have extensive applications, such as shielding the eyes from dangerous exposure to certain wavelengths of radiation when integrated into laser safety glasses, separating the back ground noise from the fluorescence signal in fluorescence microscopes, etc.There are also a vast variety of optical filters which might be confusing. So it is important that one should learn about the characteristics of different types of optical filters, including their advantages, working principles, and their uses before making the final decision.

The principle of optical filter is the addition of various kinds of coatings or dyes onto the filter substrate, so that the optical media become absorptive or reflective to certain bands of wavelengths or a specific line of wavelength. The common characteristic of optical is that all the optical filters attenuates the intensity of light, because all of them absorbs or dumps portions of light, which results in the loss of light throughput, leading to attenuation in the brightness.

Types of Optical Filters:

Optical Filters can classified in different terms.

In terms of the transmission spectrum, optical filters could be divided into: UV Filters, VIS Filters, IR Filters, each allowing a designated portion of spectrum to pass.

Concerning the operating principles, optical filters could categorized into Absorptive Filters, Interference Filters, Polarization Filters, etc. The working principles of these types of filters are stated below:

Absorptive Filters harness the intrinsic absorption of the material, such as a semiconductor, a metal, or the extrinsic absorption resulting from ionic impurities in the material. Absorptive filters absorb specific wavelengths of electromagnetic radiation, However, because the unwanted portion of light is absorbed, the filters are susceptible to damage induced by heat and, therefore are not competent for high-power applications.

Interference Filters utilize the interference phenomenon to realize the screening of light. Dielectric coatings are tiled on the filters, when the incident light enters the the coating, interference takes place, and certain wavelengths of light will be eliminated in the interference phenomenon. Via thin-film design, it is possible to realize edge filters, low-pass, high-pass and band-pass filters, notch filters, etc. Because the unwanted part of the light is dumped in the interference process, interference filters tolerate high power densities.

Polarization Filters allow certain polarization to pass. The working principle is the same as that of the polarizers. Polarization filters convert non-polarized light into polarized light, and only the light in the identical polarization direction with the filter is allowed to pass.

According to the shape of the transmission curve, optical filters could be discriminated into Bandpass Filters, Notched Filters, Dichroic Filters, Neutral Density Filters, Laser Line Filters, etc. The characteristics and differences of the types of optical filters are explained below.

Bandpass Filter allows light in a specified band to pass through, while light outside the band is rejected. The major parameter of a bandpass filter is the central wavelength (CWL) and full-width half max (FWHM ). The light-transmitting band could be located in the UV, VIS, and IR spectral region according to requirements. Those with bandwidths less than 30nm are called narrowband filters, those with bandwidths greater than 60mm are called broadband filters.

Notched Filter is also known as a Band Stop filter or Band Reject Filter which separates the spectrum into sections. The light is rejected/blocked in a specific wavelength range named the stop band wavelength range, while passing above and below this band. Notched Filters can be viewed as an inverse of a bandpass filter. Notched filter can be viewed as an inverse of a bandpass filter. In addition, notched filters can be divided into short pass filters (which only permit the transmission of light in the direction of the short wavelengths), and long pass filters(which is the converse case of the shortpass filters. only permit the transmission of light in the direction of the short wavelengths).

optical filter transmission curve

Figure 1. and 2. The transmission curve of notched filters. the shape of the transmission curve could be viewed as the inverse of band pass filters.

Dichroic Filter transmits a narrow range of wavelengths while reflecting the other colors. Dichroic filters work based on the principle of thin-film interference, and produce colors in the same manner as oil films on water. Differences in the length of the light paths create Interference, resulting in the color seen. A longpass dichroic mirror exhibits high reflection below the cut-on wavelength and high transmission above it, while a shortpass dichroic mirror acts in converse manner. Heat / UV control, color separation, and LED color correction are a few of the most common uses of dichroic filters. Dichroic filters are excellent alternatives to absorptive filters since the rejected light was canceled out in the interference, giving them tolerances to high power. Other merits of Dichroic color filters include much better-filtering characteristics than conventional filters, much longer life than conventional filters, filter will not melt or be damaged except at very high temperatures (many hundreds of degrees Celsius).

A Neutral Density Filter is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light homogeneously, giving no changes in the hue of color rendition.

Laser Line Filters are optical filters designed to transmit a narrow band of wavelengths with the transmission peaking at the center laser wavelength.

The Applications of Optical Filters:

In photography:

In photography, optical filters could be used to increase the contrast of the scene. Enhance the image quality, making them more crisp by lowering the noise. Neutral Density (ND) filters can be utilized to reduce the amount of light entering the camera lenses, with which photographers are given more freedom to adjust the appropriate aperture size, shutter speed, and exposure time to obtain the desired depth of field which would be impossible to acquire without ND filters.

Medical and scientific Equipment:

Optical filters could be used in biochemistry analysis, for example, optical filters are crucial components in enzyme labeling instruments, optical filters are placed between the laser source and the sample, the sample, and the detector to select the appropriate wavelengths of light to pass.

optical filter

Figure 3. Narrowband Filters for Enzyme Labeling Instruments from Hangzhou Shalom EO

Illumination:

Dichroic Filters could be used in RGB light shows, LEDs, and CCD imaging with their excellent capability to separate color.

Laser:

Optical filters such as laser line filters could be used to separate wanted laser wavelengths from background noises, and intracavity filters in lasers can be used for wavelength tuning and suppressing lasing at unwanted wavelengths.

Conclusion:

The diverse selection of optical filter implies that we must evaluate various aspects when choosing the most suitable one. The functions, the production costs, and the robustness to stand high power or harsh application conditions should all be taken into considerations. Hangzhou Shalom EO is a professional supplier of optical filters, with superior expert knowledge, we offer a full portfolio of optical filters, including Bandpass Filters, Neutral Density Filters, Dichroic Filters, Notched Filters, Colored Glass Absorptive Filters, IR Filters, Laser Line Filters. The filters are available both in stocked and custom versions. We deliver optical filters with high cost-effectiveness and guaranteed qualities.

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