In the microscope camera, industrial CCD/CMOS camera parameter table, we often see a label: spectral response 380-650nm (with infrared cut-off filter). Many customers will wonder: what exactly is this little filter? What effect does it have on imaging? When do you need to dismantle it? This article is for your detailed infrared cut-off filter (IR-CUT, IRCF).
Infrared cut-off filters, referred to as IR-CUT for short in English, are also often referred to as infrared cut-off filters and heat mirrors. It is a piece of optical glass with a multilayer optical film coated on the surface, generally installed between the lens and the CMOS/CCD photosensitive chip. Its optical properties are clear: it selectively transmits visible light, blocking near-infrared light above 700nm. The visible light range is about 380-650nm, that is, red, orange, yellow, green, blue, and purple light that can be perceived by the human eye; near-infrared light above 700nm is invisible to the human eye, but CCD and CMOS photosensitive chips are naturally very sensitive to this light.
The photosensitive chip itself does not distinguish between visible light and infrared light, and infrared light will also be converted into electrical signals to participate in imaging. If there is no infrared cut-off filter and a large amount of near-infrared light enters the sensor, the color picture will be reddish, purplish, and the overall color deviation will be serious. The color of the specimen captured by the camera is inconsistent with the color directly observed by our naked eyes under the microscope. After installing the infrared cut-off filter, the near-infrared is intercepted, the camera only collects visible light signals, and the color reproduction is accurate, which is very suitable for biological slices, stained samples, and conventional metallographic bright field observation.
Optical lenses are designed according to the optical path of visible light. The refractive indices of visible and near-infrared light are different, and the imaging focal plane positions of the two are not the same. When there is no IR-CUT, the infrared light will be imaged in another focal plane, superimposed on the normal image, resulting in decreased contrast, blurred edges, fog, and reduced image clarity. By blocking the infrared light, the infrared cut-off filter ensures that all the light participating in the imaging is concentrated on the same focal plane, and the image is clean and sharp.
The near-infrared light itself carries heat, and continuous irradiation of the photosensitive chip will bring temperature rise, increase the dark current of the camera, and eventually generate noise and spots. The infrared cut-off filter blocks the infrared thermal radiation and reduces the heat of the sensor, which is conducive to obtaining a low-noise, clean image.
Corresponding to your camera parameters: labeled "380-650nm (with infrared cut-off filter)", the reverse understanding: After removing the infrared cut-off filter, the spectral response range of the camera extends to the infrared band, which can receive near-infrared signals and become a full-spectrum camera.
Preserve the infrared cut-off filter (standard for conventional color cameras) Applicable scenarios: ordinary biological microscope bright field observation, stained specimen photography, color metallography, appearance detection, scenarios requiring true color reproduction. Advantages: Accurate color, transparent picture, consistent with human vision; Disadvantages: Can not receive infrared signals, can not be used for infrared detection.
Removal of the infrared cut-off filter (full-spectrum camera) Applicable scenarios: fluorescence microscope imaging, near-infrared flaw detection, infrared transmission detection, special fluorescence dye acquisition; advantages: It can receive near-infrared light above 700nm to capture fluorescence signals; disadvantages: The picture will be severely color skewed in color mode, generally used with monochromatic mode.
Note: Infrared cut-off filter ≠ Fluorescence excitation filter/emission filter. IR-CUT is only responsible for blocking the infrared; the excitation and emission filters of the fluorescence system are a set of independent filters used to screen fluorescent signals at specific wavelengths, and the functions of the two cannot be substituted for each other.
Q1: Do black and white industrial cameras also need infrared cut-off filters? A: Black and white cameras do not pursue color. Many black and white cameras do not have IR-CUT at the factory, which can receive visible light + infrared at the same time to improve sensitivity. Only a few black and white detection scenes with extremely high requirements for stray light control are optional.
Q2: Will the infrared cut-off filter affect brightness? A: The qualified infrared cut-off filter has a high transmittance of visible light, a small loss of visible light, and almost no brightness attenuation to the naked eye; it only specializes in reflecting/absorbing the infrared band.
Q3: Can I disassemble the IR-CUT myself? A: It is not recommended to disassemble it by yourself. The filter is inside the camera. It is easy to disassemble and install photosensitive chips, leaving dust and oil stains, which directly causes image black spots. It needs to be replaced by the manufacturer in a dust-free environment.
The infrared cut-off filter is an inconspicuous but vital optical accessory in color microscopic cameras, color industrial cameras. It essentially simulates the human eye's photosensitive properties: blocking near-infrared light that is invisible to the human eye, but sensitive to sensors, in exchange for accurate colors, clear and clean images. Keep a simple principle in mind when selecting a model: to be true to color, keep the IR-CUT; to do fluorescence, infrared imaging, choose to remove the full spectrum version of the infrared cut-off filter.
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