The working principle of a long wave pass filter is indeed based on the interference effect of multi-layer dielectric films. Its core mechanism is to use the precise design of multi-layer dielectric films to achieve selective transmission of light within a specific wavelength range by utilizing the reflection and interference of light waves between the film layers, while reflecting or absorbing light of other wavelengths. The following is a detailed explanation of the specific principles and implementation methods:

1、 The physical basis of interference effect of long wave pass filter:

1. Interference conditions of light waves

When light waves propagate in multilayer dielectric films, reflection and transmission occur at the interfaces of film layers with different refractive indices. Due to the existence of optical path difference, there will be constructive interference (enhancement) or destructive interference (attenuation) between reflected light waves. By controlling the thickness and refractive index of the film layer, the filter enables specific wavelengths (long waves) of light to satisfy constructive interference conditions, thereby achieving efficient transmission; Short wave light is reflected or absorbed due to destructive interference.

2. Fabry Perot cavity effect

Multilayer dielectric film can be regarded as a superposition of multiple Fabry Perot cavities. Each cavity is composed of alternating high refractive index layers and low refractive index layers, and light is reflected multiple times inside the cavity to form multi beam interference. By adjusting the thickness of the cavity and the refractive index of the film layer, the wavelength range of transmitted light can be precisely controlled, achieving long wave pass characteristics.

2、 Design and Implementation of Multilayer Dielectric Film for Long Wave Pass Filter:

1. Selection of membrane material

High refractive index materials, such as zinc sulfide and titanium dioxide, are used to enhance reflection efficiency.

Low refractive index materials, such as silicon dioxide and magnesium fluoride, are used to form optical path differences.

Construct interference conditions by alternately stacking high and low refractive index materials.

2. Film thickness control

The thickness of each layer of film is usually designed to be 1/4 (λ/4) of the target wavelength to ensure a π phase difference when light is reflected at the interface of the film layer, thereby optimizing the interference effect.

3. The stacking effect of multilayer films

By increasing the number of film layers (such as tens or even hundreds of layers), the cut-off depth (reflectivity for short wave light) and transmittance (transmittance for long wave light) of the filter can be significantly improved.

3、 The specific function of long wave pass filter:

1. Spectral segmentation

Allow light with a wavelength greater than the cutoff wavelength (long wave) to transmit, while reflecting or absorbing light with a wavelength less than the cutoff wavelength (short wave). For example, in a fluorescence microscope, a filter can block excitation light (short wave) and only allow fluorescence signals (long wave) to pass through, thereby improving image contrast.

2. Anti reflection and high reflection design

Front surface: coated with high reflectivity film or multi-layer dielectric film combination, reflecting short wave light.

Rear surface: coated with anti reflection film (AR film) to reduce the reflection loss of long wave light and ensure efficient transmission of long wave light.

3. Environmental adaptability optimization

By sealing the packaging and selecting film materials (such as low thermal expansion coefficient materials), the impact of temperature and humidity changes on the performance of the filter is reduced, ensuring its stability in harsh environments.