Optical design of an ultrashort throw ratio projector with two freeform mirrors

Abstract

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.

Advances in the design and manufacturing of novel freeform optics

“…In general, the freeform surfaces used for projectors are smooth and continuous surfaces characterized by various analytical functions. The design concept of projection device with two external mirrors was given to provide ultrashort distance between the projector and the screen [53]. The throw distance for 78.3 inch screen was reduced from 200 cm to 48 cm and obtained a well-balanced image quality with low distortion.…”

Radial scan form measurement for freeform surfaces with a large curvature using stylus profilometry

“…As their application in business, education, households, and digital cinemas increase gradually, digital light processing (DLP) projectors, which can project a large screen at a short distance, have advanced rapidly in recent years due to a fast-growing consumer market. Freeform optics with a large curvature can meet the demand for a high resolution, low distortion, and wide field of view, and often act as core mirrors for short or ultra-short throw ratio projection optics [3][4][5]. Therefore, precise form measurements and form error analysis of large curvature freeform optics are vital for their application.…”