Long Pass Filter (LP Filter for short) is a core component of precision optical interference type, with the core function of wavelength selective separation. It precisely allows long-wavelength light to pass through and blocks short-wavelength light, and is widely used in high-end optical systems such as medical, laser, and industrial imaging.
I. Core Functions
The core function is to achieve high proportion transmission of long-wavelength light (such as infrared light) and efficient blocking of short-wavelength light (such as ultraviolet/short-wave visible light), without affecting the intensity and propagation direction of the transmitted light. The core performance highlights "high transmission, high cut-off, and steep transition", with the transmittance in the transmission zone being ≥85% (up to over 95% for high-end models), and the cut-off depth in the blocking zone being OD3 to OD6, which can precisely ensure the imaging quality and signal purity of the optical system.
II. Working Principle
Based on the interference effect of multi-layer optical films, it differs from the traditional color glass absorption filter. By depositing alternating high and low refractive index hard films on the optical substrate surface, the short-wavelength light undergoes destructive interference and is blocked, while the long-wavelength light undergoes constructive interference and passes through smoothly. It has the advantages of temperature stability, no energy loss, and long service life, and can be flexibly customized for cutoff wavelengths across the entire spectrum from ultraviolet to infrared.
III. Manufacturing Process
1.The process adopts a combination of precision optical coating and cold working techniques, adhering to optical-grade standards throughout. The core procedures are divided into four steps: Substrate selection and pretreatment K9 glass, fused quartz, sapphire and other substrates suitable for different scenarios are selected. After cleaning, polishing and drying, the substrates are ensured to be free of impurities and have a high degree of smoothness, laying a foundation for coating.
2. Core Coating Process
Two processes, namely ion beam sputtering (the top choice for high-end applications, featuring high precision and deep cut-off and electron gun evaporation (a general-purpose mass production method, efficient and economical), are adopted to deposit multi-layer films in a vacuum environment. Real-time monitoring is carried out to ensure the accuracy of the film parameters.
3. Post-processing Optimization
The stress of the film layer is eliminated through annealing, followed by secondary cleaning and inspection. Then, precise edge grinding and chamfering are carried out to enhance the product's stability and assembly compatibility.
4. Finished Product Inspection and Screening
Core parameters such as transmittance, cut-off depth and surface shape accuracy are detected through precision equipment like spectrometers and interferometers. After ensuring that the products meet the standards, they are cleaned and packaged.
IV. Application Fields
1.Medical optics
It is used in fluorescence microscopes, medical imaging, and biochemical detection equipment to block excitation light and allow fluorescence signals to pass through, thereby enhancing the accuracy of detection and imaging.
2. Laser System
Adapted for industrial lasers and lidars, it filters the pump light and stray light, ensuring the purity and stability of the laser.
3. Industrial Imaging and Machine Vision
It is used in industrial cameras and inspection equipment to filter out short-wave stray light, enhancing the contrast of imaging and the accuracy of detection.
4. Security Monitoring
It is compatible with infrared monitoring and night vision devices, filters visible light, and achieves night vision imaging without red light at night.
5. Spectroscopic Instruments and Scientific Research
It is used in spectrometers and astronomical equipment to achieve wavelength separation, facilitating scientific research and material composition detection.
6. On-board Optics
Adapt to in-vehicle infrared cameras and LiDAR, filter out stray light, and enhance the stability of autonomous driving sensors.
Ultraopto’s Long Pass Filter is a high-performance edge pass optical filter designed to transmit long-wavelength light while providing deep optical density rejection for short-wavelength radiation, serving as an essential component in infrared (IR) imaging, fluorescence spectroscopy, laser systems and thermal imaging equipment. Crafted from premium optical-grade glass, fused silica and ultra-thin PMMA/acrylic substrates, our Long Pass Filters deliver an average transmittance of over 80% in the target long-wavelength transmission band (visible to far-IR), with a sharp cut-on edge and strict cut-on wavelength tolerance of ±2% to ensure precise spectral separation and minimal short-wavelength light leakage. The rejection region features an average optical density (OD) of ≥4, and up to OD6 for high-performance models, efficiently blocking unwanted UV and visible light and maximizing the detection sensitivity of long-wavelength signals in optical and thermal imaging systems.
Manufactured with advanced high-vacuum ion beam sputtering coating technology and precision substrate processing, our Long Pass Filters achieve superior surface quality with a 60-40 scratch-dig rating and a clear aperture of over 90%, with scratch-resistant and durable coatings that minimize light scattering and reflection loss for optimal optical performance. We offer both standard thick glass and ultra-thin (≤0.3mm) flexible designs, with the ultra-thin version ideal for weight-sensitive and compact optical setups such as portable IR sensors, miniature medical diagnostics and wearable optical devices. The ultra-thin filters can be directly bonded to other optical components with acrylic-based adhesives, enabling seamless integration into miniaturized optical systems, and feature excellent environmental stability with coatings resistant to temperature changes, humidity and industrial field wear. Custom cut-on wavelengths are available across the visible, NIR and mid-IR spectral ranges (400nm-5000nm), with customizable filter sizes, shapes and mounting options to match specific application requirements, including large-aperture designs for IR imaging systems and small-aperture designs for fiber optic sensors.
Our Long Pass Filters are widely applied in a broad range of professional optical and industrial fields, including IR imaging and thermal imaging for blocking visible light and transmitting IR radiation to achieve high-contrast thermal imaging of objects and environments, and fluorescence spectroscopy for filtering out short-wavelength excitation light and transmitting long-wavelength fluorescence signals for high-sensitivity biological and material analysis. In laser systems, the filters are used for beam cleanup, stray light reduction and IR laser transmission, while in biomedical imaging and diagnostic equipment, they enable high-contrast IR imaging of biological tissues and medical samples. They are also essential in environmental monitoring, remote sensing and aerospace optical sensors for IR spectral detection and measurement, in industrial inspection equipment for non-destructive testing and thermal analysis, and in display technology and machine vision systems for selective long-wavelength transmission and noise reduction.
As a leading manufacturer of precision optical filters, Ultraopto offers fully customized Long Pass Filter solutions to meet your unique spectral and application requirements. Our professional R&D and engineering team can tailor cut-on wavelengths, transmission/rejection performance, substrate materials (optical glass, fused silica, PMMA/acrylic) and filter dimensions, with ultra-strict tolerance control for high-precision optical systems such as IR astronomy and high-power laser systems. We also provide custom anti-reflection (AR) coatings on the back surface to eliminate ghost images and interference effects caused by back surface reflection, and hard coatings for enhanced scratch resistance and durability in industrial environments.
All custom Long Pass Filter orders undergo multi-stage strict optical performance testing and quality inspection, including transmittance/absorbance measurement, cut-on wavelength verification and environmental stability testing, with a detailed quality inspection report provided to ensure the final product fully complies with your design specifications and application needs. Backed by advanced optical coating technology, precision substrate processing experience and a complete quality control system, Ultraopto ensures stable batch supply capacity and consistent product quality, providing global customers with high-quality Long Pass Filter products and one-stop optical filter solutions. For more detailed technical parameters, product quotation information or custom design solutions for our Ultraopto Long Pass Filter, please contact our professional sales and technical team through the official website, and we will provide you with efficient, personalized and professional optical solutions to support the development and optimization of your high-precision optical systems.
