Control method of light field display

A control method of a light field display is provided. A control unit inputs a focal length signal to a zoom lens, so that the zoom lens is periodically switched among corresponding specific focal lengths. The control unit inputs a corresponding display signal to a display unit of a display module according to one of the specific focal lengths, and the display module generates one of image lights, the image lights respectively have different imaging distances corresponding to the specific focal lengths after passing through the zoom lens. The control unit inputs a corresponding response time signal to the display module according to one of the specific focal lengths, the one of image lights emitted by the display module passes through the zoom lens within one of response times, and the light field display projects the one of image lights to form an image at the corresponding imaging distance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of China application serial no. 202111587855.5, filed on Dec. 23, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The invention relates to control method, and particularly relates to a control method of a light field display.

Description of Related Art

Light field near eye display (LFNED) is one of current display technologies capable of solving vergence accommodation conflict (VAC), which may be divided into two frameworks of time multiplexing and spatial multiplexing. The time multiplexing framework uses a zoom lens element to repeatedly change a position of a virtual image within a short period of time, so that a human eye has a multi-depth perception, where the zoom lens element of the time multiplexing framework may include, for example, a liquid crystal tunable lens, a fluid-based tunable lens with constant volume, a fluid-based tunable lens with variable volume, or a fully elastomeric tunable lens, etc. The spatial multiplexing framework uses a microlens array to project a corresponding parallax image on the display panel. In a current light field near eye display of the spatial multiplexing framework, the microlens array is placed on an OLED display to generate a light field image. The microlens array projects light field sub-images of the display panel to a retina of the user, so that the user may view a virtual light field image composed of a plurality of sub-images stacked with each other. Therefore, imaging quality of the entire microlens array may directly affect an effect of the light field image.

In addition, in the light field near eye display of the time multiplexing framework, control of the zoom lens element is mainly triggered by a clock circuit of a main control board, so that the zoom lens element and image transmission are synchronized (SYNC). Where, the main control board is controlled by current or voltage input according to a type of the zoom lens element. Moreover, a diopter change of the zoom lens element requires a response time, and the input of different step functions and changing frequencies may all affect a time length that the diopter of the zoom lens element enters a steady state after zooming. Therefore, most of the literatures in the past tried to solve this problem by means of optimization control. Empirically, an actual output signal is quite different from an ideal step function output, so that a complicated optimization control method is required to generate an output close to the ideal step function output. However, it is almost impossible to achieve a perfect step function output, and this is only for simple step functions. Secondly, a zooming power of most zoom lens elements will decrease significantly along with increase of an operating frequency, which greatly reduces an image formation range of an optical system.

SUMMARY

The invention is directed to a control method of a light field display, where the control method is relatively simple.

Other objects and advantages of the invention may be further illustrated by the technical features broadly embodied and described as follows.

In order to achieve one or a portion of or all of the objects or other objects, an embodiment of the invention provides a control method of a light field display. The light field display includes a display module, a zoom lens, and a control unit. The display module includes a light source module and a display unit. The control method includes following steps. The control unit is used to input a focal length signal to the zoom lens, so that the zoom lens is periodically switched among a plurality of corresponding specific focal lengths. The control unit is used to input a corresponding display signal to the display unit of the display module according to one of the plurality of specific focal lengths, so that the display module generates one of a plurality of image lights, where the plurality of image lights respectively have different imaging distances corresponding to the plurality of specific focal lengths after passing through the zoom lens. The control unit is used to input a corresponding response time signal to the display module according to one of the plurality of specific focal lengths, so that the one of the plurality of image lights emitted by the display module passes through the zoom lens within one of a plurality of response time, so that the light field display projects the one of the plurality of image lights to form an image at the corresponding imaging distance.

Based on the above description, in an embodiment of the invention, according to the control method of the light field display, the zoom lens is periodically switched among a plurality of corresponding specific focal lengths, and a time during which each image light passes through the zoom lens is controlled according to the response time corresponding to the specific focal length. Therefore, compared to using a step function to control the image output, the control method of the embodiment of the invention uses a simple periodic function without being limited to a time length of a steady state of zoom lens control, so that the control method is simpler, and an actual imaging position of the image light is close to a theoretical value.

DESCRIPTION OF THE EMBODIMENTS

FIG.1is a flowchart of a control method of a light field display according to an embodiment of the invention.FIG.2is a block diagram of a light field display according to an embodiment of the invention. Referring toFIG.1andFIG.2, an embodiment of the invention provides a control method of a light field display10. The light field display10includes a display module100, a zoom lens200and a control unit300. The display module100includes a light source module110and a display unit120.

In the embodiment, the light source module110is configured to provide illumination light I, and the illumination light I is incident to the display unit120. The light source module110is, for example, one or a plurality of laser diodes (LD), light-emitting diodes (LED) or other suitable light sources. The illumination light I is, for example, red light, green light, blue light, or other suitable color light or a combination thereof. The display unit120is located on a transmission path of the illumination light I, and is configured to convert the illumination light I into image light IL. The display unit120is, for example, a spatial light modulator such as a digital micro-mirror device (DMD), a liquid-crystal-on-silicon panel (LCOS panel), or a liquid crystal panel (LCD). In addition, the zoom lens200is, for example, a liquid crystal lens or a liquid lens.

In the embodiment, the control unit300includes, for example, a microcontroller unit (MCU), a single-chip microcontroller, a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a programmable controller, a programmable logic device (PLD) or other similar devices or combinations of these devices, which is not limited by the invention. Moreover, in an embodiment, each function of the control unit300may be implemented as a plurality of program codes. These program codes may be stored in a memory, and the control unit300may execute the program codes. Alternatively, in an embodiment, each function of the control unit300may be implemented as one or a plurality of circuits. Implementations of the functions of the control unit300by means of software or hardware are not limited by the invention.

In an embodiment, the control method includes following steps. In step S100, the control unit300is used to input a focal length signal FS to the zoom lens200, so that the zoom lens200is periodically switched among a plurality of corresponding specific focal lengths. In step S120, according to one of the plurality of specific focal lengths, the control unit300is used to input a corresponding display signal DS to the display unit120of the display module100, and the display module100generates one of a plurality of image lights IL, where the plurality of image lights IL respectively have different imaging distances corresponding to the plurality of specific focal lengths after passing through the zoom lens200. In step S140, according to one of the plurality of specific focal lengths, the control unit300is used to input a corresponding response time signal TS to the display module100, and one of the plurality of image lights IL emitted by the display module100passes through the zoom lens200within one of a plurality of response time, so that the light field display10projects one of the plurality of image lights IL to form an image at a corresponding imaging distance.

The following will describe in detail how to use a periodic function to determine the response time corresponding to different specific focal lengths, so that the control method of an embodiment of the invention may output image light close to a theoretical imaging position.

FIG.3is a schematic diagram of different specific focal lengths and corresponding preferable imaging distance ranges thereof according to a control method of a light field display of an embodiment of the invention. Referring toFIG.3,FIG.3illustrates that the zoom lens200may be switched among a plurality of specific focal lengths L1, L2, L3and L4. Where, the different specific focal lengths L1, L2, L3and L4have different functional forms in a modulation transfer function (MTF). Generally, the larger a modulus of a vertical axis is, the better the image quality or image contrast is. When the modulus is greater than or equal to 0.3, a blur/clarity degree of the image light IL after passing through the zoom lens corresponding to the specific focal length has made it impossible for a human eye to judge a difference between different moduli. Namely, the specific modulus greater than or equal to 0.3 and smaller than or equal to 1.0 determines a preferable imaging distance range of each of the specific focal lengths L1, L2, L3and L4, where the preferable imaging distance range represents the nearest position and the farthest position of the image that is clear and acceptable to the human eye relative to the zoom lens200. Taking the specific focal length L3as an example, two intersection points between the specific focal length L3and a straight line M with the modulus of 0.3 determine the preferable imaging distance range of the specific focal length L3on an imaging distance axis, i.e., a range between a nearest imaging position L3−and a farthest imaging positions L3+. Moreover, since the intersection point between the specific focal length L4and the straight line M with the modulus of 0.3 only forms the nearest imaging position, the farthest imaging position corresponding to the specific focal length L4may be defined by the maximum focal length of the zoom lens200.

Namely, in the embodiment, the nearest imaging position and the farthest imaging position of the specific focal length L1, L2, L3or L4corresponding to each response time are respectively the intersection points between its modulation transfer function and the straight line M formed by the specific modulus, and the preferable imaging distance range corresponding to each specific focal length L1, L2, L3or L4is defined by the minimum and maximum values of the intersection points on the imaging distance axis or by the minimum value and the maximum focal length of the zoom lens200. The specific modulus is greater than or equal to 0.3 and smaller than or equal to 1.0.

FIG.4is a curve diagram of a focal length characteristic function according to a control method of a light field display of an embodiment of the invention.FIG.5is a schematic diagram of obtaining time differences corresponding to preferable imaging distance ranges according to the focal length characteristic function in a control method of a light field display according to an embodiment of the invention. Referring toFIG.4andFIG.5, based on the nearest imaging position and the farthest imaging position corresponding to each of the specific focal lengths L1, L2, L3and L4, the control unit300controls the focal length of the zoom lens200to be changed among the different specific focal lengths L1, L2, L3and L4through the focal length signal FS. Since the step function has difficulties in signal processing, it is preferred to use a periodic function to generate the focal length signal FS. For example,FIGS.4and5illustrate that a focal length characteristic function L(T) is a sine function. In this way, the preferable imaging distance ranges (i.e., the ranges between the nearest imaging positions and the farthest imaging positions) corresponding to each of the specific focal lengths L1, L2, L3and L4respectively correspond to time differences ΔT1, ΔT2, ΔT3and ΔT4thereof under the focal length characteristic function L(T), for example, a time required for a virtual image corresponding to the specific focal length L3to move from the nearest imaging position L3−to the farthest imaging position L3+according to the focal length characteristic function L(T) is ΔT3. In addition, these time differences ΔT1, ΔT2, ΔT3and ΔT4will determine response time Δt1, Δt2, Δt3and Δt4(i.e., corresponding to the response time signal TS in step S140) of the display unit120to correspondingly emit the image light IL when the zoom lens200is at each of the specific focal lengths L1, L2, L3and L4. Although the periodic function is used to generate the focal length signal FS while it is calculated that the respective response time Δt1, Δt2, Δt3and Δt4of the display unit120to emit the image light IL are not perfect theoretical values, by using the specific modulus to limit the response time Δt1, Δt2, Δt3and Δt4, the human eye cannot judge an error in the imaging position.

Namely, in the embodiment, the control method further includes following steps: Each response time is determined according to the time difference ΔT1, ΔT2, ΔT3or ΔT4corresponding to the preferable imaging distance ranges of the focal length characteristic function L(T), and the control unit300outputs the corresponding response time signal TS to the display unit120according to the response time.

In an embodiment, the focal length characteristic function may be: L(T)=a×sin(bT+c), wherein a, b and c are characteristic parameters, and T is a time period.

In addition to determining the time difference ΔT1, ΔT2, ΔT3or ΔT4according to the focal length characteristic function L(T) and the specific modulus, the actual response time Δt1, Δt2, Δt3, Δt4corresponding to each of the specific focal lengths L1, L2, L3, L4are preferably determined by the minimum brightness and minimum contrast perceivable by human eyes. Namely, if the minimum brightness and the minimum contrast perceivable by human eyes are satisfied, the time difference ΔT1, ΔT2, ΔT3or ΔT4is equal to the response time Δt1, Δt2, Δt3or Δt4. Conversely, if the minimum brightness and the minimum contrast perceivable by human eyes are not satisfied, the response time Δt1, Δt2, Δt3or Δt4is adjusted according to the minimum brightness and the minimum contrast perceivable by human eyes.

FIG.6is a curve diagram of a brightness characteristic function according to a control method of a light field display of an embodiment of the invention. A brightness characteristic function B(ΔT) shown inFIG.6is: B(ΔT)=d×eg/ΔT+h, where d, g, and h are brightness characteristic parameters, and ΔT is a time difference. Referring toFIG.6, based on the above-mentioned brightness characteristic function B(ΔT), brightness values B1, B2, B3, B4corresponding to an image received by the human eye may be calculated respectively according to each of the time differences ΔT1, ΔT2, ΔT3, ΔT4corresponding to the focal length characteristic function L(T) in these preferable imaging distance ranges. For example, InFIG.6, the brightness values corresponding to the time differences ΔT3and ΔT4are respectively B3and B4. Where, the maximum value in the brightness values B1, B2, B3and B4corresponding to the time differences ΔT1, ΔT2, ΔT3and ΔT4is defined as the maximum brightness value. Since the brightness characteristic function B(ΔT) of the embodiment is an increasing function, the brightness value calculated according to the maximum value ΔTmax of the time differences ΔT1, ΔT2, ΔT3and ΔT4may be the maximum brightness value. For example, inFIG.5, the maximum time difference calculated according to the focal length characteristic function L(T) is ΔT4, the time difference ΔT4is then the maximum value ΔTmax, and its brightness value B(ΔTmax) (equal to B4) is the maximum brightness value. Moreover, Bth on the vertical axis ofFIG.6is the minimum brightness value perceivable by human eyes. Therefore, if the brightness value corresponding to each of the time differences ΔT1, ΔT2, ΔT3, ΔT4is lower than the minimum brightness value Bth, the corresponding response time Δt1, Δt2, Δt3, or Δt4thereof may be adjusted to (calculated according to the brightness characteristic function B(ΔT)) a time corresponding to the minimum brightness value Bth or a time corresponding to the maximum brightness value B(ΔTmax). Alternatively, in an embodiment, the response time Δt1, Δt2, Δt3and Δt4must be greater than or equal to the time corresponding to the minimum brightness value Bth (calculated according to the brightness characteristic function B(ΔT)), and less than or equal to the time corresponding to the maximum brightness value B(ΔTmax).

Namely, in the embodiment, determination of each response time Δt1, Δt2, Δt3, Δt4includes the following steps: The maximum brightness value B(ΔTmax) is calculated according to the brightness characteristic function B(ΔT), where the maximum brightness value B(ΔTmax) is a brightness value of the maximum value ΔTmax in the plurality of time differences ΔT1, ΔT2, ΔT3, and ΔT4corresponding to the plurality of specific focal lengths L1, L2, L3, and L4in the brightness characteristic function B(ΔT).

FIG.7is a curve diagram of a contrast characteristic function according to a control method of a light field display of an embodiment of the invention. A contrast characteristic function C(B) inFIG.7is:

C⁡(B)=qB×σ⁢e-r⁢(lnB-μ)22⁢σ2+s,
where q, r, s, μ, and σ are brightness characteristic parameters, and B is a brightness value. Referring toFIG.7, according to the above-mentioned brightness values B1, B2, B3and B4, contrasts C1, C2, C3, and C4of the corresponding image may be respectively calculated based on the contrast characteristic function C(B). For example, the contrast of the image corresponding to the brightness values B3and B4inFIG.7are respectively C3and C4. Cth on the vertical axis ofFIG.7is the minimum contrast perceivable by human eyes. Therefore, if the contrast corresponding to the brightness value is smaller than the minimum contrast Cth, for example, the contrast C4of the image corresponding to the brightness value B4inFIG.7is smaller than the minimum contrast Cth, the brightness value thereof may be adjusted to the maximum brightness value B′ corresponding to the minimum contrast Cth, or adjusted to the above-mentioned minimum brightness value Bth or the brightness value B(ΔTmax), and then the adjusted brightness value is used to calculate a correct response time Δt of the image light IL output from the display unit120.

In brief, in the embodiment, determination of each response time Δt1, Δt2, Δt3and Δt4further includes a following step: According to the contrast characteristic function C(B), it is determined whether each response time Δt1, Δt2, Δt3, or Δt4satisfies a following first conditional function: B(ΔT)≤(ΔTmax), B(ΔT)≥Bth, and C(B (ΔT))≥Cth.

In the embodiment, determination of each response time Δt1, Δt2, Δt3, Δt4further includes a following step: If the above-mentioned first conditional function is satisfied, each response time Δt1, Δt2, Δt3, Δt4is the time difference ΔT1, ΔT2, ΔT3, ΔT4thereof.

In the embodiment, determination of each response time Δt1, Δt2, Δt3, Δt4further includes a following step: If the first conditional function is not satisfied, each response time Δt1, Δt2, Δt3, Δt4is a time value corresponding to B(ΔTmax) or Bth, and each response time Δt1, Δt2, Δt3, Δt4satisfies a following second conditional functions: B(Δt)≤(ΔTmax), B(Δt)≥Bth, and C(B (Δt))≥Cth, where Δt is each response time.

In the embodiment, determination of each response time Δt1, Δt2, Δt3, Δt4further includes following steps. The control unit300outputs an adjustment signal AS to the display module100according to the brightness value (according to the brightness characteristic function B(ΔT)) and the contrast (according to the contrast characteristic function C(B)) calculated according to each response time Δt1, Δt2, Δt3, Δt4, and the light source module110of the display module100adjusts the illumination light I output to the display unit120according to the adjustment signal AS or/and the display unit120of the display module100adjusts a time of outputting the image light IL according to the adjustment signal AS.

In the embodiment, the above-mentioned step of using the control unit300to input the focal length signal FS to the zoom lens200, so that the zoom lens200is periodically and continuously switched among a plurality of corresponding specific focal lengths L1, L2, L3, and L4further includes a following step. The zoom lens200periodically modulates a diopter thereof according to a sine wave driving signal (for example, the sine function shown inFIG.4), so that the plurality of image lights IL from the display module100pass through the zoom lens200at different times to have different imaging distances corresponding to the specific focal lengths L1, L2, L3, L4.

For example, a, b and c in the above Table 1 are a set of characteristic parameters of the focal length characteristic function L(T), d, g and h are a set of brightness characteristic parameters of the brightness characteristic function B(ΔT), and q, r, s, μ and σ are a set of brightness characteristic parameters of the contrast characteristic function C(B). Δtmax and Δtmin are respectively the maximum response time and the minimum response time that may be obtained according to the above-mentioned characteristic parameters under the condition of satisfying the above-mentioned first conditional function and second conditional function. Bmax and Cmax are the brightness value and contrast corresponding to Δtmax, and Bmin and Cmin are the brightness value and contrast corresponding to Δtmin. In an embodiment, Bmax may be set to the minimum brightness value Bth or the brightness value B(ΔTmax), and Δtmax and Cmax may be a time difference and contrast corresponding to the minimum brightness value Bth or the brightness value B(ΔTmax).

Table 2 shows the time differences ΔT1, ΔT2, ΔT3, ΔT4calculated according to a set of specific focal lengths L1, L2, L3, and L4based on the focal length characteristic function L(T), the brightness values B1, B2, B3, B4respectively calculated according to the time differences ΔT1, ΔT2, ΔT3, ΔT4based on the brightness characteristic function B(ΔT), and the contrasts C1, C2, C3, and C4respectively calculated according to the time differences ΔT1, ΔT2, ΔT3, and ΔT4based on the contrast characteristic function C(B). Where, the logical judgment represents whether the brightness values B1, B2, B3, B4and the contrasts C1, C2, C3, C4corresponding to each of the time differences ΔT1, ΔT2, ΔT3, ΔT4satisfy the above-mentioned first and second conditional functions. Since the brightness value B1and the contrast C1calculated based on the specific focal length L1satisfy the first conditional function and the second conditional function, the response time Δt1of the image light IL output by the display unit120corresponding to the specific focal length L1is equal to the time difference ΔT1generated according to the corresponding preferable imaging distance range thereof. Since the brightness values B2, B3, B4and the contrasts C2, C3, and C4respectively calculated based on the specific focal lengths L2, L3, and L4cannot satisfy the first conditional function and the second conditional function at the same time, the response time Δt2, Δt3, and Δt4of the specific focal lengths L2, L3, L4are set as the response time Δtmax.

In summary, in an embodiment of the invention, in an embodiment of the invention, according to the control method of the light field display, the zoom lens is periodically and continuously switched among a plurality of corresponding specific focal lengths, and a time during which each image light passes through the zoom lens is controlled according to the response time corresponding to the specific focal length when the display unit outputs the image light. Therefore, compared to using a step function to control the image output, the control method of the embodiment of the invention uses a simple periodic function without being limited to a time length of a steady state of zoom lens control, so that the control method is simpler, and an actual imaging position of the image light is close to a theoretical value as human eyes cannot judge an error value thereof.

Besides, the control method of the embodiment of the invention further optimizes the response time by using the brightness characteristic function B(ΔT), the minimum brightness value Bth perceivable by human eyes, the contrast characteristic function C(B) and the minimum contrast Cth perceivable by human eyes. Therefore, according to the control method of the embodiment of the invention, the light field display may provide the best viewing quality.