FND Series Reflective Neutral Density Filter

Product Description

• The reflective type is a neutral filter used to reduce the light intensity of high-energy lasers or wide-wavelength spectral regions;
• The transmittance is subdivided into many types, which can be reduced to various light intensities; moreover, combining several filters can perform fine light intensity adjustment;
• Since the untransmitted light is not absorbed by the glass, there is no need to worry about the risk of glass damage due to high temperature;
• When the laser beam is incident vertically on the filter, the light reflected by the filter will return to the laser light source; such returned light will make the vibration of the laser unstable. Please use the filter with a slight tilt to prevent the reflected light from returning to the laser light source;

There are many types of filters, and their characteristics are also divided in detail. Skilled use of these characteristics can improve the accuracy of optical experiments. The filter has a simple structure and is easy to replace. Prepare several filters, and the optimal optical conditions can be created by replacing the filters.

Product Parameters	Parameter Information
Material	K7
Coating	Chromium
Applicable Wavelength	400~700nm
Parallelism	<1′
Surface Figure Accuracy	λ/4
Surface Quality	40-20
Effective Diameter	90% of the outer diameter

 

Model	Parameters
FND-C1-001	Ø25.4*1.0mm; Transmittance: 0.01%; Average Transmittance: 0.01±0.003%;
FND-C1-001	Ø25.4*1.0mm; Transmittance: 0.01%; Average Transmittance: 0.01±0.003%;
FND-C1-01	Ø25.4*1.0mm; Transmittance: 0.1%; Average Transmittance: 0.1±0.01%;
FND-C1-01	Ø25.4*1.0mm; Transmittance: 0.1%; Average Transmittance: 0.1±0.01%;
FND-C1-1	Ø25.4*1.0mm; Transmittance: 1%; Average Transmittance: 1±0.3%;
FND-C1-1	Ø25.4*1.0mm; Transmittance: 1%; Average Transmittance: 1±0.3%;
FND-C1-10	Ø25.4*1.0mm; Transmittance: 10%; Average Transmittance: 10±2%;
FND-C1-10	Ø25.4*1.0mm; Transmittance: 10%; Average Transmittance: 10±2%;

Technical Description

– Neutral Density Filter Series

Neutral density filters are divided into two types: absorptive filters and interference reflective filters. The filter has a simple structure and is easy to replace. Prepare several filters, and the optimal optical conditions can be created by replacing the filters.

A metal film is plated on one side of the filter to maintain approximately equal energy transmittance in a wide wavelength band from the visible light region to the near-infrared light region. It works best in the 400nm~700nm band. Films with different optical densities (OD), including 0.1, 0.3, 0.5, 1.0, 2.0, 3.0, etc., are plated on optical glass or quartz substrates. Neutral density filters block optical energy through optical density (OD); the higher the optical density (OD) value, the lower the transmittance. Conversely, the lower the optical density (OD) value, the higher the transmittance.

– Differences Between Absorptive and Reflective Neutral Density Filters

The biggest technical feature of the absorptive neutral density filter is that the optical glass substrate of this filter contains optical absorbing substances, which absorb some specific wavelengths.

Reflective neutral density filters adopt the principle of thin film interference, transmitting part of the light and reflecting the other part. On a certain substrate, metal-dielectric-metal films or all-dielectric films with certain thicknesses and alternating high or low refractive indices are formed by vacuum coating.

– Filters are mainly classified by spectral band, spectral characteristics, film material, application characteristics, etc.

Spectral band: Ultraviolet filter, visible filter, infrared filter;

Spectral characteristics: Bandpass filter, cut-off filter, beam splitting filter, neutral density filter, reflective filter;

Film material: Soft film filter, hard film filter;

(Hard film filters refer not only to the hardness of the film, but more importantly to their laser damage threshold, so they are widely used in laser systems, while soft film filters are mainly used in biochemical analyzers)

Bandpass type: Light of the selected band passes through, and light outside the passband is cut off. Its main optical indicators are central wavelength (CWL) and full width at half maximum (FWHM). Divided into narrowband and broadband;

Short-wave pass type (also called low-pass): Light shorter than the selected wavelength passes through, and light longer than this wavelength is cut off. For example, infrared cut-off filter;

Long-wave pass type (also called high-pass): Light longer than the selected wavelength passes through, and light shorter than this wavelength is cut off. For example, infrared transmission filter;

– Differences Between Absorptive and Reflective Neutral Density Filters

The biggest technical feature of the absorptive neutral density filter is that the optical glass substrate of this filter contains optical absorbing substances, which absorb some specific wavelengths.

Reflective neutral density filters adopt the principle of thin film interference, transmitting part of the light and reflecting the other part. On a certain substrate, metal-dielectric-metal films or all-dielectric films with certain thicknesses and alternating high or low refractive indices are formed by vacuum coating.

– Basic Principle of Optical Coating

Optical thin film technology is generally used to control the reflectivity and transmittance of the substrate to the incident beam. Coating is a method of plating a layer of transparent dielectric film or a layer of metal film on the surface of the material by physical or chemical means. The purpose is to change the reflection and transmission characteristics of the material surface to meet different needs. To eliminate the reflection loss on the surface of optical components and improve imaging quality, coating one or more layers of transparent dielectric films is called an anti-reflection film or anti-reflective coating. With the development of laser technology, different requirements for the reflectivity and transmittance of the film layer have promoted the development of multi-layer high-reflection films and broadband anti-reflection films. For various application needs, polarizing reflective films, color beam splitting films, cold light films and interference filters are manufactured using high-reflection films.

After the surface of the optical component is coated, light undergoes multiple reflections and transmissions between the film layers to form multi-beam interference. By controlling the refractive index and thickness of the film layer, different intensity distributions can be obtained, which is the basic principle of interference coating.

(1) The reflective type is a neutral filter used to reduce the light intensity of high-energy lasers or wide-wavelength spectral regions. Compared with absorptive neutral density filters, its characteristics are less related to wavelength and more optically neutral. It does not affect the color of light. It should be noted that when multiple reflective neutral density filters are used together, multiple reflections will occur between the mirrors. It is mainly used in industrial laser processing and other instrumentation equipment. It is a light attenuation effect formed by coating a chromium film, and the light intensity is controlled by the thickness of the chromium film.

(2) Multiple specifications are available

Some thin lenses have rough and yellowish edges. Since edging of ultra-thin lenses will affect the service life and performance of the lenses, edging is not performed. This does not hinder the normal use of the lenses and is not a quality issue.

• Non-edged lens

No edging, slightly yellowish edges, which is a normal phenomenon and does not affect use; durable and sturdy.

• Edged lens

The front edges and sharp corners are all ground off, with a good hand feel; durable and sturdy.

(3) Application Diagram

Assembly

Application Examples