Longpass Filter

A longpass filter is a device that selectively transmits light of longer wavelengths and blocks or reflects light of shorter wavelengths. It can be used to isolate a specific spectral region or to enhance the contrast between different colors.

 

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Longpass Filter Catalog

Major Specifications of a Longpass Filter

  • Cut-on wavelength: This is the wavelength at which the transmission of the filter rises to 50% of its maximum value. It defines the boundary between the rejection and transmission ranges of the filter. The cut-on wavelength is usually given by a single number, such as 400 nm.
  • Rejection range: This is the range of wavelengths that are blocked or reflected by the filter with high efficiency. The rejection range is typically shorter than the cut-on wavelength, and depends on the design and quality of the filter. The rejection range is usually specified by the average or maximum OD, such as 2 or 6.
  • Transmission range: This is the range of wavelengths that are transmitted by the filter with high efficiency. The transmission range is typically longer than the cut-on wavelength, and depends on the design and quality of the filter. The transmission range is usually specified by the average or minimum transmission percentage, such as 95% or 65%.
  • FWHM: This is the measure of the bandwidth of the filter at 50% of its peak transmission. It is the difference between the upper and lower wavelengths where the filter transmits half of its maximum amount of light. A smaller FWHM means a narrower bandwidth and a more selective filter.
  • Surface quality: This is the measure of how smooth and defect-free the surface of the filter is. A higher surface quality indicates a lower amount of scratches, digs, bubbles, stains, or other imperfections that can affect the optical performance and durability of the filter. The surface quality is usually given by a standard notation, such as 80-50 scratch-dig or 13/1(0.25) wavefront distortion.

OD of a Longpass Filter

OD stands for optical density, and it is a way of measuring how much light a filter can block or reduce. You can think of OD as a scale that goes from 0 to 6, where 0 means no blocking and 6 means almost complete blocking. The higher the OD, the less light can pass through the filter. For example, an OD 0 filter lets 100% of the light through, while an OD 6 filter lets only 0.0001% of the light through. A higher OD can be useful for some applications that need to get rid of unwanted light, but it can also cause problems like heating, damage, or signal loss. Therefore, you need to choose the right OD for your purpose.

FWHM Explanation

FWMH of Longpass Fitler

FWHM stands for full width at half maximum, and it is a way of measuring how wide a filter is. You can think of FWHM as the distance between two points on a curve, where the curve shows how much light the filter can transmit. The two points are where the curve reaches half of its highest value. For example, if a filter has a peak transmission of 90% at 600 nm, and it reaches 45% at 580 nm and 620 nm, then its FWHM is 620 nm - 580 nm = 40 nm. A smaller FWHM means a narrower filter that can select a specific range of wavelengths, while a larger FWHM means a wider filter that can let more light through. Therefore, you need to choose the right FWHM for your purpose.

Coatings for longpass filter

The coating for longpass filters are thin layers of materials that are applied on the surface of a substrate to create a selective transmission and reflection of light. The coating can be either dyed in the bulk or have interference coatings. The dye or coating determines the cut-on wavelength, which is the wavelength at which the transmission reaches 50% of the maximum value. The cut-on wavelength is also called the edge wavelength or the transition wavelength.
The coating materials can be either dielectric or metallic, depending on the desired performance and application of the longpass filter. Dielectric coatings are made of non-conductive materials that have different refractive indices and thicknesses. They create interference effects that enhance or cancel out certain wavelengths of light. Dielectric coatings can provide high transmission, high blocking, and sharp cut-on slopes. They are also more durable and environmentally friendly than metallic coatings.
Metallic coatings are made of conductive materials that reflect light based on their electrical properties. They create absorption effects that attenuate certain wavelengths of light. Metallic coatings can provide high blocking, wide bandwidths, and low cost. They are also more flexible and compatible with different substrates than dielectric coatings. However, metallic coatings may have lower transmission, higher heat generation, and lower durability than dielectric coatings.

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