An optical filter rejects the extra amount of optical spectrum and transmits the specific amount required, commonly used in chemical analysis, spectroscopy, machine vision, and microscopy. Also, there are various optical filter fabrication techniques and filters like IR absorbtion filters and dichroic filters.
Optical Filter Fabrication Techniques
In general, filters play an essential role in reflecting unwanted light through the surface and absorbing unwanted light through colored glasses or dyes. This filter primarily helps you to achieve the desired transmission of shape and performance. The three filters include.
- Hard Coated Optical Filters have excellent optical performance and a single substrate with dense coatings.
- Traditional Coated Optical Filters are efficient filters laminated together to create a low-cost and typically a stack of absorbing materials, interference coatings, and metallic layers.
- Colored Glass Filters manipulate the filter’s spectral properties like other absorbing filters like Wratten Filters, introducing elements, Plastic Filters, compounds, dyes, or other colorants. The absorbtion filter integrates into illumination and sensing applications.
Types of Optical Filters
Some of the types of optical filters are:
- Bandpass filters have a narrow band or broadband transmittance across the substrate.
- Longpass filters convey all wavelengths longer than the specified cut-on wavelength.
- Short pass filters transfer all wavelengths more concisely than the specified cut off wavelength.
- Dichroic filters are coated with thin-films to attain the preferred diffusion and reflection percentage through a given spectrum.
Other optical filter fabrication techniques are absorptive and dichroic. Let’s look at some of the main differences between these two.
What is the difference between Absorbtive and Dichroic Filters?
The optical filters are divided into two main categories:
In an absorbtive filter, light is blocked, which is significantly based on absorption properties, and it doesn’t reflect off the filter light that is blocked. It absorbed the light only that it contains. It is ideal for the application where noise system from unwanted light is an issue.
Moreover, it is very angle sensitive, and here the light is incident from a wide range of angles and maintains the absorbtion properties. IR absorbtion filter generally have multiple uses and isolate a broad band of wavelengths by blocking the shorter ones and transmitting longer wavelengths. They are commonly available in glass, and Gelatin bases are coated, mixed, or impregnated with organic and inorganic dyes, Plastic-coated glass, and Acetate.
Further, the high-graded glass is used in high IR absorbing filters, which is beneficial for multiple applications like cameras, video cameras, surveillance cameras, and smartphones. The absorbtion filter has some unique features that make it the most effective optical filter.
- It has excellent surface excellence after the weather-resistance test.
- The cleanliness rate of this filter is High and standard.
- In a shorter lead time, it has a High-quality coating design.
- The absorptive filter has Low Light Reflectivity Absorbability (IR/Radiation).
- It has High Transmittance (Visible Light/UV) Shape.
- It has a plate-like shape.
Apart from these enhancing features, they are beneficial for automobiles, communication terminals, and medical care.
A dichroic filter mechanism reflects the unwanted wavelengths while conveying the anticipated portion of the spectrum. It is considered as the affective filter and suitable for many applications. The light is separated by wavelength into two sources, which happen when multiple layers are added, varying refraction indexes to exploit light waves’ interference nature.
Also, in interference filters, the light reflects a higher index material traveling from a lower index material. Consequently, only a preferable wavelength with a specific angle and wavelength will constructively interfere with the incoming beam and pass through the material. The other will reflect off the material.
Moreover, the dichroic filters cannot encounter the conduction and wavelength specifications initially indicated. According to the thumb rule, decreasing the angle will shift it towards longer wavelengths like redder. Cumulating the angle of incidence through a dichroic filter will change it to shorter wavelengths like bluer wavelengths.
Further, the Dichroic bandpass filters developed from two different techniques: traditional and hard coated. These methods have unique reflection properties and extraordinary transmission as they deposit numerous layers of alternating low index and high index of fraction materials onto glass substrates.