Shortpass Filter

A shortpass filter is a type of optical filter that transmits light with wavelengths shorter than a certain cut-off wavelength, and blocks or reflects light with longer wavelengths. It is also known as a low-pass filter or an edge filter.

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Major Specifications of Shortpass Filter

  • Cut-off wavelength: This is the wavelength at which the transmission of the filter drops to 50% of its maximum value. It defines the boundary between the transmission and rejection ranges of the filter. The cut-off wavelength is usually given by a single number, such as 600 nm
  • Transmission range: This is the range of wavelengths that are transmitted by the filter with high efficiency. The transmission range is typically shorter than the cut-off 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%
  • Rejection range: This is the range of wavelengths that are blocked or reflected by the filter with high efficiency. The rejection range is typically longer than the cut-off wavelength, and depends on the design and quality of the filter. The rejection range is usually specified by the average or maximum transmission percentage, such as 1% or 3%
  • Slope: This is the measure of how steeply the transmission of the filter changes from high to low around the cut-off wavelength. A steeper slope indicates a sharper transition between the transmission and rejection ranges, and a better performance of the filter. The slope is usually given by the width of the transition region, such as 10 nm or 25 nm
  • 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

FWHM and OD of a Shortpass Filter

FWHM stands for full width at half maximum, and it is a 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. For example, if a filter has a peak transmission of 90% at 600 nm, and it transmits 45% at 580 nm and 620 nm, then its FWHM is 620 nm - 580 nm = 40 nm. A smaller FWHM means a narrower bandwidth and a more selective filter

OD stands for optical density, and it is a measure of how much light is blocked or attenuated by the filter. It is defined as the negative logarithm of the transmission ratio, which is the ratio of the transmitted light intensity to the incident light intensity. For example, if a filter transmits 10% of the incident light, then its transmission ratio is 0.1, and its OD is -log(0.1) = 1. A higher OD means a lower transmission and a more effective filter. OD is often used to specify the blocking level of a filter in the rejection range, where the transmission is very low. For example, an OD 2 filter blocks 99% of the incident light, and an OD 6 filter blocks 99.9999% of the incident light

OD to percentage blocking

Different Substrate for Shortpass Filter

Some of the common substrates used for shortpass filters are:

  • Quartz: Quartz is a crystalline form of silica that has high transmittance in the ultraviolet (UV) and visible regions, and low thermal expansion. Quartz is suitable for filters that operate in the UV range or require high temperature stability. However, quartz is more expensive and harder to polish than glass.
    - Fused silica: Fused silica is an amorphous form of silica that has high transmittance in the UV and visible regions, and low thermal expansion. Fused silica is similar to quartz, but it has lower birefringence and higher purity. Fused silica is also suitable for filters that operate in the UV range or require high temperature stability.
  • Soda lime float glass: Soda lime float glass is a type of glass that contains sodium and calcium oxides. It has high transmittance in the visible and near-infrared (NIR) regions, and low cost. Soda lime float glass is suitable for filters that operate in the visible or NIR range or require low cost. However, soda lime float glass has low transmittance in the UV region, high thermal expansion, and low chemical resistance.
  • Borofloat or borosilicate glass: Borofloat or borosilicate glass is a type of glass that contains boron oxide. It has high transmittance in the visible and NIR regions, and moderate thermal expansion. Borofloat or borosilicate glass is suitable for filters that operate in the visible or NIR range or require moderate temperature stability. However, borofloat or borosilicate glass has low transmittance in the UV region and higher cost than soda lime float glass.
  • BK-7: BK-7 is a type of optical glass that has high transmittance in the visible and NIR regions, and good optical quality. BK-7 is suitable for filters that operate in the visible or NIR range or require good optical quality. However, BK-7 has low transmittance in the UV region, high thermal expansion, and higher cost than soda lime float glass.
  • Colored glasses: Colored glasses are types of glasses that contain elements, dyes, or other colorants that absorb certain wavelengths of light. They have variable transmittance depending on the colorant composition and thickness. Colored glasses are suitable for filters that require broad blocking ranges or simple fabrication. However, colored glasses have low transmission efficiency, broad transition regions, low surface quality, high wavefront distortion, high temperature dependence, and degradation over time.

Does shortpass filter made of color glass need coating?

A color glass filter uses substrates that incorporate elements, dyes, or other colorants that absorb desired wavelength ranges while transmitting others. The amount of absorption is dependent on the substrate thickness and the colorants it incorporates. These filters can have a wide variety of functions, such as bandpass, shortpass, longpass, or neutral density filtering. They are ideal for blocking a wide range of wavelengths, as it is difficult and expensive to achieve strong blocking over a large wavelength range using coated interference filters.
However, color glass filters also have their share of other disadvantages when compared to coated filters. Color glass may function across broad wavelength ranges, but dielectric coatings typically achieve higher transmission of passed wavelengths and sharper transitions between blocked and passed wavelengths. Color glass filters also have lower surface quality, higher wavefront distortion, and higher temperature dependence than coated filters. Moreover, color glass filters may suffer from degradation or fading over time due to exposure to high-intensity light or environmental factors.
Therefore, some types of coating may be applied to color glass filters to enhance their performance and durability. For example, anti-reflection (AR) coating may be used to reduce the reflection loss and increase the transmission efficiency of the filter. Hard coating may be used to improve the surface quality and scratch resistance of the filter. Protective coating may be used to prevent the colorant from fading or leaching due to moisture or UV radiation. However, these coatings may also increase the cost and complexity of the filter fabrication and alignment.
In summary, a shortpass filter made of color glass does not need coating to function as a basic filter, but it may need coating to achieve higher performance and quality standards for certain applications. I hope this answer helps you understand the role of coating in color glass filters.

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