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Optical Filters And Coatings

Apr. 01, 2024

Know About Optical Filters

Filters are optical devices used to select the desired band of radiation. Filter products are mainly categorized according to spectral bands, spectral characteristics film layer materials, application characteristics, and other ways.

Spectral bands: ultraviolet filters, visible filters, infrared filters;

Spectral characteristics: bandpass filters, cut-off filters, spectral filters, neutral density filters, reflection filters;

Membrane materials: soft film filters, hard film filters;

Hard film filters are widely used in laser systems not only in terms of film hardness but also in terms of laser damage threshold, while soft film filters are mainly used in biochemical analyzers.

-Bandpass type: 

Light in the selected wavelength band passes through and light outside the passband is cut off. Its optical index is mainly center wavelength (CWL), half bandwidth (FWHM). Divided into narrowband and broadband. For example, narrowband 808 filter NBF-808.

-Short-wavelength pass type (also called low-wavelength pass):

Light shorter than a selected wavelength passes through, and light longer than that wavelength cuts off. An example is the infrared cutoff filter, IBG-650.

-Long wavelength pass type (also called high wavelength pass):

Light longer than a selected wavelength passes through, and light shorter than that wavelength cuts off E.g. infrared transmission filter, IPG-800.

What is Anti-reflective

Reduced reflection (AR, anti-reflective), is to reduce the reflected light, less reflective, indicating that more light is through, so the reduction of reflection is also known as increased transparency.

Anti-reflective coating (AR) is a surface optical coating that increases transmittance by reducing the reflection of light. In complex optical systems, such as telescopes, it is possible to improve contrast by reducing scattered light in the system, which is important for astronomy. Transmission-enhancing coatings reduce the flash of binoculars in the dark.

Many coatings include transparent film structures with alternating layers of contrasting refractive indices and thin film structures with alternating layers of contrasting. The thickness of the film determines the wavelength of reflected light on which it acts. When light is secondarily reflected on a transmittance-enhancing film, it interferes with the original reflected light, thereby diminishing the reflected light. According to the conservation of energy, the energy of light remains constant. When the reflected light decreases, the transmitted light increases. This is the principle of permeation enhancement film. When selecting a transmittance enhancement film, it is necessary to determine the wavelengths, such as infrared, visible light, and ultraviolet light.

Filter Coatings

-Evaporation Deposition

In evaporative deposition, the source material in the vacuum chamber is heated or bombarded with an electron beam and vaporized. The vapor condenses on the optical surface. During evaporation, a uniform optical coating with a specific thickness can be created by controlling the heating, vacuum pressure, substrate positioning, and rotation. Evaporation is relatively mild and causes the coating to become loose or porous. This loose coating is hygroscopic, which alters the effective refractive index of the layer and will result in degraded performance. Evaporative coatings can be enhanced by ion beam-assisted deposition techniques in which the ion beam is aimed at the substrate surface. This increases the adhesion of the source material relative to the optical surface, generating more stress and making the coating denser and more durable.

-Ion Beam Sputtering

During ion beam sputtering (IBS), a high-energy electric field accelerates the ion beam. This acceleration gives the ions significant kinetic energy. Upon impact with the source material, the ion beam "sputters" atoms from the target material. These sputtered target material ions (atoms transformed into ions by the effects of the ionization zone) also have kinetic energy, which produces a dense film in contact with the optical surface. IBS is a highly repeatable technique.

-Atomic Layer Deposition

Unlike evaporative deposition, the source material used for atomic layer deposition (ALD) does not need to be evaporated from a solid but exists directly as a gas. Even though the technique uses a gas, high temperatures are still required in the vacuum chamber. In the ALD process, the gas-phase precursor is delivered in non-overlapping pulses that are self-limiting. The process has a unique chemical design, with only one layer adhering to each pulse and no special requirements on the surface geometry of the optics. This process therefore allows a high degree of control over the thickness and design of the coating but reduces the rate of deposition.

-AR Glass

(1) Visible light transmittance rate of the highest peak value of 99% average visible light transmittance rate of more than 95%

(2) The average reflectivity is less than 4%, the lowest value is less than 0.5%. 

(3) More colorful, stronger contrast so that the image color contrast is stronger, and the scene is clearer.

(4) Anti-ultraviolet, effective protection of the eyes ultraviolet spectral region of the transmittance rate is greatly reduced, can effectively block ultraviolet damage to the eyes. 

(5) High-temperature AR glass temperature > 500 degrees (general acrylic can only withstand temperatures of 80 degrees) (6) scratch-resistant wear resistance of the best AR glass film hardness is comparable to the glass, greater than 7H, (general PC board hardness of about 2H to 3H).

(7) Can be resistant to a variety of detergents cleaning acid, and alkali cleaning agent wipes, the film layer is not damaged.

(8) Strong impact resistance 3mm thickness of glass impact performance equivalent to 6mm acrylic.

(9) To maintain the angle of view of the general acrylic in the installation, the angle of view will become smaller; and with AR glass installed, the angle of view will not become smaller.

(10) Appearance AR glass surface flatness is far better than coated acrylic, and the larger the size, the more obvious the difference.

(11) AR glass's hot and cold deformation can be almost omitted, suitable for all kinds of environments; at the same time, AR glass has a glazed feeling, and the appearance is more beautiful.

Optical Filters And Coatings

CLZ Optical Co., Ltd. is a manufacturer of optical components, main products including spherical lenses, optical domes, windows, filters, mirrors, etc. The optical material we use are optical glass N-BK7, H-K9L, fused silica, sapphire, silicon and so on. Our product are widely used in various fields. We also could provide OEM service.

Please contact us free time if you have any needs.