Zoom Lens Design In Zemax

[Kennedy Tadder]

Inrecent times, military demands for the thermal imaging camera have been increasing. Thermal imaging system does not require illumination to operate. In addition to their ability to form images through most forms like smoke and dust, civilian demands also have been increasing in the medical, industry, security and other fields.

Although thermal imaging systems are intended for military or security applications, it can be used for temperature measurement; however, they are not optimized for this purpose. The main aim is to detect, recognize or identify targets at long ranges by their shape; thus, resolution and sensitivity are favored over radiometric accuracy [1]. In some optical systems that have multiple fields of view, zoom lens with variable magnification and different resolving power had greatly improved the detection efficiency.

Concerning optical transmittance of IR materials, most optical glasses do not transmit above 2.5 μm. Certain special types of optical glasses can transmit up to 4 μm. The most common types of IR transmitting materials are germanium, zinc sulfide, zinc selenide and GASIR5 with transmission band between 8 and 12 μm. So, we have limited selection among these materials.

Zooming definition is one in which the focal length (and thus angle of view) can be varied by changing component positions while image position being maintained at a fixed plane. The image plane can be fixed either by optical or mechanical compensation.

The optical compensation is only guaranteed for short focal length, whereas for longer focal length, mechanical compensation must be adopted. The mechanical compensation components may be either negative focal length or positive focal length [10].

In order to design IR zoom lens for long-range detection with long focal length and a large relative aperture, the spherical aberration, the chromatism and secondary spectrum are serious aberration and difficult to correct. Therefore, a binary surface (aspheric plus diffractive terms) is adopted to balance these aberrations.

According to the parameters from Table 1, consult to the lens library and the patents, finally a four-component type zoom structure with negative compensated element is determined as the initial structure of the system [13, 14, 16].

The optimized zoom lens is shown in Fig. 1. It consists of four elements: the prefixed element, the zoom element, the compensation element and the back fixed element. The whole system is composed of only 4 lenses with overall length 200 mm.

The spacing value(s) of every component at different focal length positions in which the sum of the space values have a constant value of 156.183 mm to keep image plane at same position as shown in Table 5.

This system consists of 4 lenses. Only one diffractive lens on L1 and one conical surface on L2 are used to balance the wave-front aberration. The design data for the diffractive surface are shown in Table 6.

For a diffractive lens, the diffraction efficiency with respect to wavelength needs to be considered when evaluating the imaging performance. As the diffractive lens are very weakly powered. In addition to that, the wavelength-to-zone period ratio is very small across the entire lens, so an approximate expression for the polychromatic integrated efficiency ranging from \(\lambda_min\) to \(\lambda_max\) is given by [8]

The zoom curves that are shown in Fig. 3 clearly have no inflection points. While the focal length changes the whole curve is smooth, so the motion of the variable component and compensation component can by driven by cam in this infrared optical zoom system.

The MTF curves for the designed zoom system at different zoom positions are shown in Fig. 4 (a-e). MTF is an important parameter which reflects the imaging performance. As shown, its performance is nearly diffraction-limited at each focal length position such that the system is perfect image quality.

The spot diagrams for different zoom positions with different field angles after ray tracing are shown in Fig. 5 (a-e); the spot diagram makes use of the intensity of light spot, where the RMS radius at all positions is less than one detector pixel size 25 μm.

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Optical performance or image quality is quantified by several metrics: surrounding energy, modulation transfer function, Strehl ratio, ghost reflection control and pupil performance (size, location and aberration control).

Design constraints include: realistic lens element center and edge thickness, minimum and maximum air spaces between lenses, maximum restrictions on entry and exit angles, physically achievable glass refractive index and dispersion properties.

The manufacturing costs and delivery schedules, price of an optical glass block of certain dimensions can vary by a factor of fifty or more, depending on the size, type of glass, homogeneity index, quality and availability, being BK7, generally the cheaper.

The costs for larger and/or thicker optical blanks of a given material, above 100-150mm, generally increase faster than the physical volume due to the increase in white annealing time required to achieve an acceptable homogeneity index and internal level voltage birefringence across the blank volume.

The availability of glass blocks is determined by the frequency with which a particular type of glass is made by a particular manufacturer and can seriously affect the cost of manufacture and the schedule.

The lenses are designed first using the properties of the average index of refraction and dispersion, Abbe number, published in the glass manufacturer's catalog and through the calculation of glasses.

The properties of real glass blocks vary in relation to this ideal, the values of the refractive index can vary by up to 0.0003 or more of the values in the catalog, and the dispersion can vary slightly.

Lens mold manufacturing process: The ingredients of the glass batch for a desired type of glass are mixed in a powdered state, the powder mixture is melted in a furnace, the fluid is further mixed, while melted to maximize the homogeneity of the batch, poured into empty spaces of lenses and annealed according to time-temperature schemes.

The blank glass pedigree, or fusion data, can be determined for a given glass batch by making small precision prisms from various locations in the batch and measuring its refractive index on a spectrometer.

Delivery schedules are impacted by the availability of blank glass and mirrors and purchase terms, the amount of tools a workshop must manufacture before starting a project, the manufacturing tolerances of the parts, the complexity of any optical coatings that must be applied to finished parts, additional complexities in the assembly or gluing of lens elements to the cells and the general assembly of the lens system and any post-assembly alignment and quality control tests and necessary tools.

A multi-configuration lens corrected over a wide spectral band and field of view over a range of focal lengths and a realistic temperature range can have a complex design volume with more than one hundred dimensions.

SLCL-Doublet lenses are formed by a symmetrical liquid core cylindrical lens (SLCL) filled with variable refractive index (IR) liquid and a double cylindrical lens capable of weakening spherical aberration.

Calculated at 75% of the total aperture, the SLCL-Doublet's quadratic midpoint radius (RMS) is always less than 7m over the entire focal length range, and the peak-to-valley wavefront error remains below λ/4 limit when the focal length varies from 62.373mm to 65.814mm, within which the lenses approach the diffraction limit, demonstrating an improvement in optical performance compared to previously designed liquid-core cylindrical lenses.

In comparison to the traditional mechanical zoom lens, the liquid zoom lens adjusts the optical power of the system by changing the curvature of the lens or the refractive index (IR) of the middle of the lens.

Spherical aberration is the influencing factor that limits the optical performance of zoom lenses with adjustable focus, such as the introduction of aspherical surfaces and the introduction of biconvex microlenses with differential thickness of the elastic membranes.

These zoom lenses are used to measure the IR of liquid, due to the spatial resolution capacity of IR achieved when replacing the sphere used by a cylinder, the diffusion process and the diffusion coefficient of binary liquid can be performed.

The PWC method is one used to solve the initial structural parameters of lenses in the field of optical design, where P and W, as functions of the aperture angles of the primary aberration coefficient and C is the primary chromatic aberration of the coefficient.

To decrease the manufacturing cost, the symmetric liquid core cylindrical lens (SLCL) used as the front liquid core of the DLCL is chosen as the initial lens in this project, which is a zoom lens that varies with the RI of the liquid filled in the lens.

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