An Optical Longpass Filter(OLPF) is an optical component that transmits lights that is longer than designated wavelengths light (e.g. 650nm) while blocking shorter wavelengths. In generally, there are three key parameter indicators for an Longpass Filter:
● Cut-off wavelength: refers to the designated intermediated wavelength (e.g. 650nm) at transition from transmission to reflective zone, and measure at 50% transmittance.
● Average Transmittance(Tavg): refers to the average transmittance of the transmission zone(e.g. Tavg=90% or 95%). The higher the transmittance, the better transmission.
●Average Blocking rate: Typically refers to the average transmittance in blocking area. However, the blocking efficiency is often expressed in terms of Optical Density, where OD = -log(T). Thereinto, T represents the average transmittance of the filter in the effective blocking zone. For example, a blocking rate of 1% average transmittance corresponds to OD2. The larger the OD value is, the better reflection or absorption of unwanted light.
Picture 1: Spectral Curve sample – 650nm Longpass Filter
Normally, there are two types of Longpass Filters: interference and absorption type. The company can provide interference cut-off type, absorption type, and also acrylic (PMMA) type Longpass Filters. Particularly, the interference type filter adopts advanced vacuum coating technology, featuring high transmittance, deep cut-off depth, and good film uniformity. For reference, the measured spectral curves of some typical Longpass filters are showed in picture 2.
Picture 2: Spectral Curve of Interference Cutoff Longpass Filters
Additionally, when selecting an Longpass Filter, also need to consideration of the glass substrate. In generally, substrates include ultra-white glass, quartz glass, high-borosilicate glass, and optical sapphire.However, these materials differ significantly in terms of light transmission, temperature resistance, hardness, and cost. For a detailed comparison of the characteristics of different filter materials, please contact our sales representative.
Picture 3: Optical Glass
The main application scenarios of Longpass Filters include fluorescence biological detection, spectral analysis and optical detection, laser systems and protection, infrared imaging and night vision security, industrial vision and optical detection, etc. For longpass filters in different application scenarios, please refer to the link.