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Longpass Filter

Longpass Filter

Regular price $100.00 USD
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What is a Longpass Filter?

A longpass filter is an optical filter that allows light with a wavelength longer than the cutoff wavelength to pass through, while blocking shorter wavelengths. Longpass filters are used in a variety of applications, such as fluorescence microscopy, colorimetry, and machine vision. Some features of longpass filters are:

  • They have a steep cut-on slope that defines the transition between high rejection and high transmission regions.
  • They have a high optical density (OD) that measures the light blocking ability of the filter.
  • They have a high transmission in the passband region that is close to 100%.
  • They can be combined with shortpass filters to create custom bandpass filters.

Major Specifications of Longpass Filter

  • Cut-on wavelength: This is the wavelength at which the filter achieves 50% of the maximum transmission. It determines the lower limit of the passband region and the upper limit of the blocking region.
  • Transmission range: This is the wavelength range over which the filter transmits light with high efficiency. It is usually specified by the minimum and maximum wavelengths of the passband region.
  • Blocking range: This is the wavelength range over which the filter blocks light with high attenuation. It is usually specified by the minimum and maximum wavelengths of the rejection region.
  • Slope: This is the bandwidth over which the filter transitions from high blocking to high transmission. It is usually expressed as a percentage of the cut-on wavelength. A smaller slope corresponds to a sharper transition and a narrower transition region.
  • Optical density: This is a measure of light blocking ability of the filter, considering all forms of attenuation. It is usually expressed as a logarithmic scale, where OD = -log(T), where T is the transmittance. A higher OD means a lower transmittance and a higher attenuation.

Use Examples for Shortpass Filter

  • For fluorescence microscopy, longpass filters are often used as barrier or emission filters to block excitation light and pass only fluorescence emission. The cut-on wavelength of these filters should match or exceed the peak emission wavelength of the fluorophore. For example, if the fluorophore has a peak emission at 520 nm, a longpass filter with a cut-on wavelength of 530 nm or higher can be used.
  • For colorimetry, longpass filters are often used to isolate specific colors or color ranges from a white light source. The cut-on wavelength of these filters should match or exceed the dominant wavelength of the color. For example, if the color is red, a longpass filter with a cut-on wavelength of 620 nm or higher can be used.
  • For machine vision, longpass filters are often used to enhance contrast or remove unwanted glare or reflections from an image. The cut-on wavelength of these filters should match or exceed the wavelength of interest or interference. For example, if the image contains infrared information or noise, a longpass filter with a cut-on wavelength of 700 nm or higher can be used.

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