The hottest reflective optical elements help multi

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Reflective optical elements help multispectral imaging system

the traditional optical system design usually uses the refraction characteristics of the lens, but for the applications that need to realize multispectral imaging in a wide range of laser wavelengths, the lens based system is obviously incompetent. In contrast, the reflective optical system can not only solve the problem of high-power multispectral transmission, but also effectively reduce the complexity of the system and save costs

in the application of laser focusing and multispectral imaging, the surface design of refractive lens faces two major challenges: one of the challenges is scattering, which is also the inherent characteristic of each medium. The wavelength determines the speed of light beam passing through the lens. Scattering results in different focal positions of light with different wavelengths when focusing. Composite lens can solve this problem - by choosing different materials to make a lens, because each material has different optical properties, it can balance the scattering between each other

however, this method can only work in limited bands, usually in some special bands, such as visible band, near-infrared band and short wave infrared band (SWIR). The wider the bandwidth, the more components are required. In addition to taking more time and cost, the optical transmission range of this refraction method will also be limited by the chemical properties and inherent characteristics of various lenses

therefore, this leads to the second disadvantage of the refraction system: absorption. In high-power laser focusing system, even a small part of the light energy is absorbed, which may cause serious damage to the lens. The traditional solution is to choose lens materials and coating to improve the transmittance at the corresponding laser wavelength

unfortunately, when multispectral imaging and high-power lasers are required at the same time, these two solutions cannot complement each other. In the refraction system, due to the broadening of the band, the problem is that the high transmittance performance of the coating is greatly reduced for a batch of products in the process of mass production. Therefore, for a traditional lens system, it is necessary to limit either the spectral range or the laser power, which cannot be met at the same time

multi lens demand

one application that highlights the potential cost of this dilemma is to detect and repair flat panel displays in the production process. It can be introduced in detail to search for relevant information in this station. See the light to scan the whole flat plate surface to find out defects. Once the defects are identified, the high-power laser beam (usually output by 1064nm nd:yag laser) will be aligned with the defect parts for melting. For a single optical system to serve this application, it must have high transmittance in the infrared and visible bands. Moreover, the system must focus the infrared light and visible light at the same point, so as to ensure that the laser beam can be reliably projected to the defect

even if a suitable material can be found, it can provide excellent transmission performance in such a wide range of wavebands. Such a complex and demanding optical system will also be very expensive. Therefore, the solution that has been implemented is to use two sets of composite objective lenses. The first group of composite objective lenses is used for visible light scanning, which is usually combined with the transmission of He Ne laser spot through the system for collimation purposes. When the He-Ne laser spot points to the defect, the electric console of the lens system will move here, and then use near-infrared light to replace the visible light path and remove the laser

because of the need for multiple lens groups, electric adjustment tables, etc., this scheme is not only expensive, but also very expensive to maintain. Near infrared lens and visible lens cannot focus on the same plane at the same time, so the alignment of the system is very important to ensure the normal operation of laser ablation

if a reflective optical system is used, only a few optical elements are required to meet the application requirements, because the reflective system is not constrained by wavelength. The reflection system will not produce scattering; Focusing is only related to the geometric shape, so there is no need to use a variety of components for correction. The reflected light path passing through the system (depending on the reflectivity of the mirror) is related to the wavelength, but the wavelength has little effect on it. The spectral range of metal reflective film is 10 μ m、20 μ M, etc., different materials have different reflectivity, which allows the mirror to process from ultraviolet (150nm) to short wave infrared (20 μ m) Wave band light. If extremely high reflectivity is required to avoid the influence of the internal heat source of the laser, a special laser film is required to improve the reflectivity at the laser wavelength, and the performance of the system will not be affected in other wave bands

a typical reflection system has only a few optical elements: a primary mirror is used to focus, and a secondary mirror turns the optical path to a more favorable position. The secondary reflector is fixed by a "tripod" and the entire optical system is fixed by a lens barrel (see Figure 1). Because this reflection system simplifies the optical path, compared with the refraction system, it can greatly reduce the cost and make the structure more compact and firm. This is because the optical axis of the system changes through the reflection of the mirror, thereby reducing the length of the system

Figure 1: wide area spectral imaging can be carried out with only two mirrors, and there is no color difference

for example, in panel inspection and maintenance, the advantages of the reflection system can be well reflected: only one lens can replace the two lenses of the refraction system, and there is no need to install electric shift table devices, which further reduces the volume, reduces the cost, and improves the production capacity, Because it is no longer necessary to convert the objective lens in use. This simplified optical design also means that the system adjustment becomes simpler, and the return on investment is much higher than that of the refraction system

design considerations

in multispectral applications, reflective optical systems are easier to design and implement than refractive optical systems; However, the following factors must be considered when selecting reflective objective lens: the first is to choose a fixed system or an adjustable system. The fixed version objective lens has been adjusted to its best state when it is produced in the factory. This version does not need to be recalibrated even under vibration and accidental collision (see Figure 2). Moreover, since the fixed version of the objective lens is installed more firmly and reliably than the adjustable version, even if it falls from the appropriate position or falls, it will not affect its use

Figure 2: the fixed version reflective objective lens is used for focusing, and the high-power blue laser beam applied to biomedicine can simplify the system,

make the system stronger and more compact

the fixed version lens is very suitable for industrial applications with harsh environments, and does not need to be adjusted during installation. In order to specify this kind of objective, the developer must know the variation distance - either infinite or special tube length, as well as the thickness of the cover glass. Once arranged, there is no need to adjust in the future

the adjusting version of the reflecting objective can be adjusted by one or two parameters: the length of the lens barrel can be adjusted, or the spherical aberration caused by the change of the thickness of the cover glass can be corrected (see Figure 3). Usually, it is only adjusted when the lens itself changes, but it also needs to be adjusted when the distance between the primary mirror and the secondary mirror changes. Manufacturers usually adjust the optical performance to the best state before shipment, with adjustment instructions and instructions for special parts, such as adjusting pictures. Users can install it in the system, which can be adjusted by the length of the lens barrel and the thickness of the cover glass, and then fixed

Figure 3: the adjustable reflection objective lens can enable developers and applications to meet special needs by adjusting the light path

when the adjustable lens needs to be adjusted regularly, it can be adjusted through multiple structures and has various functions. This kind of lens is available from stock. For example, it can be used in conjunction with the thickness of the lens barrel and cover glass to be installed in the position control mode before the fatigue test of parts with a length less than the typical value. It can meet the special needs of users without adding other accessories and additional costs

whether the fixed version or the adjusted version, there are some key points to pay attention to when purchasing the reflective objective lens. The most important one is the shadow size brought by the secondary mirror and the tripod for imaging. The secondary reflector is arranged on the main optical path, which will inevitably block part of the beam from passing through the system and reduce the beam quality. The three legs of the tripod can cause diffraction effects, which will be more serious in the future when the four leg straight travel design is adopted. Four leg symmetrical design, each foot will lead to diffraction effect, and will be superimposed on each other. The three leg bending design will avoid the above defects and make the light transmission more uniform

another factor to consider when using a reflecting objective is laser power. This kind of reflective objective adopts standard metal film, which has a wide range of applications, but it is not the best choice for high-power output lasers. When used with high-power laser, the damage threshold of the film needs to be considered. Some manufacturers can customize according to the requirements of users

reflective objective lens can be used in a variety of applications, such as laser annealing or drilling, which can combine visible light wavelength with infrared power transmission. Using an optical system, the wavelength can be measured from 190nm to 1 μ M thickness of the film. As long as it is within the response range of the sensor, the imaging system can cover various wavelength ranges without adding any optical elements. (end)

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