In this work, an optical design approach is presented to design an ultrashort throw distance projection system by combination of an off-the-shelf refractive lens and two off-axis freeform mirrors. These two freeform mirrors are used to greatly shorten the projection distance by more than three times compared to conventional (rotationally symmetric) systems, while still maintaining a good imaging quality. Firstly, a direct design method that enables the simultaneous calculation of two off-axis freeform-profile mirrors by partially coupling more than three fields is introduced. The specifications of the conventional refractive lens are taken into account during this procedure. The pupil matching principle is applied to ensure good performance between the two sub-systems. The calculated mirrors then serve as a good starting point for optimization using commercial optical design software. To step from freeform profiles to freeform surfaces, the calculated two profiles are fitted into odd polynomials to evaluate the image quality and then re-fitted into XY polynomials for further optimization. Finally, the polynomial coefficients of the two freeform mirrors are imported into the optical design program. The merit function is built from RMS spot radii over the full field, and additional constraints are made for correcting distortion. After optimization, the calculated initial design quickly converges to a well performing imaging system. As an example, an ultrashort throw distance projection lens with a large 80-inch diagonal image at 400mm throw distance is designed, analyzed and compared with literature data. The values of MTF are over 0.6 at 0.5 lp/mm and the distortion is less than 1.5%: showing a very good and well balanced imaging performance over the entire field of view.
In this paper, we propose a multi-fields direct design method aiming to calculate two freeform surfaces with an entrance pupil incorporated for wide field of view on-axis line imaging applications. Both infinite and finite conjugate objectives can be designed with this approach. Since a wide angle imaging system requires more than few discrete perfect imaging points, the multi-fields design approach is based on partial coupling of multiple fields, which guarantees a much more balanced imaging performance over the full field of view. The optical path lengths (OPLs) and image points of numerous off-axis fields are calculated during the procedure, thus very few initial parameters are needed. The procedure to calculate such a freeform lens is explained in detail. We have designed an exemplary monochromatic single lens to demonstrate the functionality of the design method. A rotationally symmetric counterpart following the same specifications is compared in terms of RMS spot radius to demonstrate the clear benefit that freeform lens brings to on-axis line imaging systems. In addition, a practical achromatic wide angle objective is designed by combining our multi-fields design method with classic optical design strategies, serving as a very good starting point for further optimization in a commercial optical design program. The results from the perspective of aberrations plots and MTF values show a very good and well balanced performance over the full field of view.
A multi-fields optical design method aiming to calculate two high-order aspheric lens profiles simultaneously with an embedded entrance pupil is proposed in this paper. The Simultaneous Multiple Surfaces design method in two dimensions (SMS2D) is used to provide a better understanding of how N surfaces allow perfect coupling of N ray-bundles. In contract to this perfect coupling, our multi-fields design approach is based on the partial coupling of multiple ray-bundles. This method allows calculating the Optical Path Lengths (OPL) during the process, directly building connections between different fields of view. Both infinite and finite conjugate objectives can be designed with this approach. Additional constraints like surface continuity and smoothness are taken into account to calculate two smooth and accurate surface contours. Sub-aperture sampling factor is introduced as a weighting function for different fields which allows for a very flexible performance control over a wide field of view. A RMS 2D spot size function is used to optimize the weighting factor to achieve a very well-balanced imaging performance. A wide-field objective and a moderate aperture lens are designed and analyzed to demonstrate the potential of this design method. The impact of different weighting functions for the sub-aperture sampling is evaluated accordingly. It’s shown that this design method provides an excellent starting point for further optimization of the surfaces coefficients and initial design parameters: resulting in a very good and well-balanced imaging performance over the entire field of view.
In this work, a multifields optical design method aiming to calculate two high-order aspheric lens profiles with an embedded entrance pupil is proposed. This direct design algorithm is capable of partially coupling more than three ray bundles that enter the same pupil with only two surfaces. Both infinite and finite conjugate objectives can be designed with this approach. Additional constraints such as surface continuity and smoothness are taken into account to calculate smooth and accurate surface contours described by point clouds. The calculated points are then fitted with rotationally symmetric functions commonly used in optical design tools. A presented subaperture sampling strategy that introduces a weighting function for different fields allows for a very well-balanced imaging performance over a wide field of view (FOV). As an example, a ±45 degf/7.5 wide-angle objective is designed and analyzed to demonstrate the potential of this design method. It provides an excellent starting point for further optimization of the surfaces’ coefficients and initial design parameters, resulting in a very good and well-balanced imaging performance over the entire FOV.
As a novel detection approach which simultaneously acquires two-dimensional visual picture and one-dimensional
spectral information, spectral imaging offers promising applications on biomedical imaging, conservation and
identification of artworks, surveillance of food safety, and so forth. A novel moderate-resolution spectral imaging system
consisting of merely two optical elements is illustrated in this paper. It can realize the function of a relay imaging system
as well as a 10nm spectral resolution spectroscopy. Compared to conventional prismatic imaging spectrometers, this
design is compact and concise with only two special curved prisms by utilizing two reflective surfaces. In contrast to
spectral imagers based on diffractive grating, the usage of compound-prism possesses characteristics of higher energy
utilization and wider free spectral range. The seidel aberration theory and dispersive principle of this special prism are
analyzed at first. According to the results, the optical system of this design is simulated, and the performance evaluation
including spot diagram, MTF and distortion, is presented. In the end, considering the difficulty and particularity of
manufacture and alignment, an available method for fabrication and measurement is proposed.
Fourier telescopy is an active unconventional imaging technique. Three or more beams from different spatially separated transmitters are pointed at a distant and faint object. The spatial Fourier spectrum of the object is carried on the reflected temporally modulated signals. The image of the target can be reconstructed from the back signals by demodulation and phase closure algorithm. The conventional demodulation processing is calculating spectrum directly by inverse Fourier transform of the signal. However spectrum estimated by inverse Fourier transform has non-negligible errors caused by frequency shift error of the Acoustic-optical modulator, the noise and the relative motion between beams and the target. An improved demodulation method based on spectrum correction of FT is proposed. The method corrects the amplitude and the phase on the demodulated frequency of the signal by which better reconstructed image can be obtained. In this paper, the effect of the frequency shift error in Fourier telescopy demodulation is investigated. The degradation of the reconstructed image is simulated. We summarize the new demodulation method based on spectrum correction and give the simulated comparison between the conventional demodulation and the developed method. The result confirms the effectiveness of the improved demodulation method.
In order to meet the needs of space borne and airborne hyperspectral imaging system for light weight, simplification and high spatial resolution, a novel design of Féry-prism hyperspectral imaging system based on Zemax multi-configuration method is presented. The novel structure is well arranged by analyzing optical monochromatic aberrations theoretically, and the optical structure of this design is concise. The fundamental of this design is Offner relay configuration, whereas the secondary mirror is replaced by Féry-prism with curved surfaces and a reflective front face. By reflection, the light beam passes through the Féry-prism twice, which promotes spectral resolution and enhances image quality at the same time. The result shows that the system can achieve light weight and simplification, compared to other hyperspectral imaging systems. Composed of merely two spherical mirrors and one achromatized Féry-prism to perform both dispersion and imaging functions, this structure is concise and compact. The average spectral resolution is 6.2nm; The MTFs for 0.45~1.00um spectral range are greater than 0.75, RMSs are less than 2.4um; The maximal smile is less than 10% pixel, while the keystones is less than 2.8% pixel; image quality approximates the diffraction limit. The design result shows that hyperspectral imaging system with one modified Féry-prism substituting the secondary mirror of Offner relay configuration is feasible from the perspective of both theory and practice, and possesses the merits of simple structure, convenient optical alignment, and good image quality, high resolution in space and spectra, adjustable dispersive nonlinearity. The system satisfies the requirements of airborne or space borne hyperspectral imaging system.
A novel moderate-resolution imaging spectrometer spreading from visible wavelength to near infrared wavelength range
with a spectral resolution of 10 nm, which combines curved prisms with the Offner configuration, is introduced.
Compared to conventional imaging spectrometers based on dispersive prism or diffractive grating, this design possesses
characteristics of small size, compact structure, low mass as well as little spectral line curve (smile) and spectral band
curve (keystone or frown). Besides, the usage of compound curved prisms with two or more different materials can
greatly reduce the nonlinearity inevitably brought by prismatic dispersion. The utilization ratio of light radiation is much
higher than imaging spectrometer of the same type based on combination of diffractive grating and concentric optics. In
this paper, the Seidel aberration theory of curved prism and the optical principles of Offner configuration are illuminated
firstly. Then the optical design layout of the spectrometer is presented, and the performance evaluation of this design,
including spot diagram and MTF, is analyzed. To step further, several types of telescope matching this system are
provided. This work provides an innovational perspective upon optical system design of airborne spectral imagers;
therefore, it can offer theoretic guide for imaging spectrometer of the same kind.