GB2623941A

GB2623941A - System and method for colour field-sequential display

Info

Publication number: GB2623941A
Application number: GB2213852.3A
Authority: GB
Inventors: Brannan Mark; P Kuske Samuel; Newman Alfred
Original assignee: Vividq Ltd
Current assignee: Vividq Ltd
Priority date: 2022-09-22
Filing date: 2022-09-22
Publication date: 2024-05-08
Also published as: GB202213852D0

Abstract

A display system 100 comprises a spatial light modulator 104 illuminated by an illumination system 102, and an achromatic filter 108 which is switchable between transmitting light and blocking light, and which has a rise time to transition from a blocking state to a transmissive state. Colour fields are transmitted through the achromatic filter to form a field-sequential colour display where the order of respective colour fields is based on a relative intensity for a desired white balance. At least one of the colour fields is transmitted while the achromatic filter is transitioning from the blocking state to the transmissive state. The colour fields may be transmitted in an order of blue, green, and red. The blue field may be transmitted while the achromatic filter is transitioning from blocking to transmissive and another field transmitted during a fall time from transmissive to blocking. The system may be a holographic display system and the achromatic filter may have switchable regions which are controlled to reduce quantisation noise. The system may be used in an augmented reality display system, a virtual reality display system, or a head-up display system. Methods of generating and displaying field-sequential colour images are also claimed.

Description

SYSTEM AND METHOD FOR COLOUR FIELD-SEQUENTIAL DISPLAY

Technical Field

The present invention relates to display systems and methods of operating such systems, and more specifically to colour field-sequential display systems, including holographic display systems

Background

Field-sequential display systems transmit individual colour fields in sequence, using persistence of vision to form a perceived colour image when viewed. Field sequential displays can be used in projection displays, such as displays using a Digital Micromirror Device (DMD) as a modulator. Field sequential display is also common for holographic display systems, where holograms of each colour field are transmitted sequentially rather than simultaneously. Some holographic di splay systems use a DIM as a Spatial Light Modulator (SLM).

Holographic display systems provide a number of advantages over non-holographic displays. A holographic display preserves phase information in the displayed image so that a viewer can naturally focus at the perceived distance, rather than having a fixed focal plane. A viewer can choose to focus on part of the image, and those parts of the image too far, or too close, to be in focus exhibit defocus.

Regardless of whether a display system is holographic or not, field-sequential display systems require fast switching speeds between each field because multiple fields are required for each frame of the display. A 60 Hz frame rate when used with a three-colour component field sequential display requires an effective refresh rate of 180 Hz, for example. In some cases, such as high-performance PC gaming, frame rates in excess of 200 Hz are required, giving an effective refresh rate of 600 Hz for a three-colour field-sequential display.

It would be desirable to increase the rate at which frames can be displayed with

field-sequential displays.

Summary

The present disclosure makes use of an achromatic filter, also referred to as a shutter, that has a finite rise time to become transmissive, in a field-sequential display. However, rather than simply trying to make the filter switch more quickly, the present disclosure makes use of the change in transmissivity over the rise time together with a particular order of displaying the colour fields to allow faster display of colour-fields. The method has a further advantage that the filter can assist with white balance. The order in which the fields are displayed is based on the relative intensity of each field for the desired white balance. For example, the field requiring the lowest intensity may be transmitted while the transmissivity of the filter is at its lowest. This can allow the transition of the filter from the blocking to transmitting to contribute to white balance instead of, or in addition to, control of the intensity of the illumination source.

For a typical white balance with red, green and blue fields, the blue field has the lowest intensity, followed by the green field and then the red field. By transmitting the blue, green and red fields in that order, and at least the blue field while the filter is transitioning (during the rise time) the blue field is attenuated more than the green and red fields. In this way, operation speeds can be improved because it is not necessary to wait for the filter to become substantially transmissive before beginning transmission of the colour fields. Furthermore, the particular order means that the filter assists with white balance. It will be appreciated that other orders may be used for different colour fields and that the disclosure is not limited only to field-sequential colours having three colours. More colours may be used to improve colour fidelity, for example. According to a first aspect of the present invention, there is provided a display system comprising: an illumination system; a spatial light modulator illuminated by the illumination system; and an achromatic filter. The achromatic filter is switchable between transmitting light and blocking light and has a rise time to transition from blocking to transmitting. The illumination system and the spatial light modulator are configured to transmit respective colour fields to form a field-sequential colour display. The order of the respective colour fields is based on the relative intensity for a desired white balance and at least one field is transmitted while the achromatic filter is transitioning from blocking to transmitting. In other words, at least one field is transmitted during the rise time of the achromatic filter.

The achromatic filter may be substantially achromatic across visible wavelengths, such from around 380 nm to around 700 nm.

The rise time may be characterized by a rise time constant for a substantially exponential rise, and the achromatic filter may be substantially fully transmissive after a duration of five rise time constants. Waiting for the rise time before transmitting any image fields imposes significant delays. This sets a practical limit on the number of fields that can be displayed. For example, if the rise time constant is lms, a delay of 5ms is imposed every time the filter switches during which time no fields can be transmitted. As described herein, it has been found that at least one field can be transmitted while the filter is still transitioning to a transmissive state, without waiting for the full rise time.

The filter will steadily become more transmissive over the rise time, and so it will apply more attenuation to the light the earlier it is transmitted in the rise time, or the earlier it is transmitted after "opening" the filter or starting / activating a transition to switch from blocking to transmitting. The specific order of colour fields uses this to contribute to white balance. A dual benefit of increased response time and improved white balance is achieved. It will be appreciated that other white balance controls may be used in combination with this, such as adjusting laser intensity.

In some examples, the achromatic filter comprises liquid crystal. Liquid crystal technology is commercially available and can provide achromatic (such as neutral density) filtering. In other examples, the achromatic filter may comprise electrochromic materials or a Kerr cell.

The achromatic filter may be positioned at any suitable position in the optical path to a viewer. For example, it could be positioned before or after the SLM. In some examples, the achromatic filter may be part of the illumination system.

While the disclosure is generally applicable to any field-sequential colour display, it is well suited to the commonly used red, green and blue colour fields. In an example, the respective colour fields comprise blue, green and red fields transmitted in that order through the achromatic filter, and at least the blue field is transmitted while the achromatic filter is transitioning from blocking to transmitting. This has the further advantage of blue light being required to have the lowest intensity for good white balance, when red, green and blue colour fields are used. The attenuation of the filter as it transitions from blocking to transmitting assists with the white balance. For example, substantially uniform input intensity may be used for the blue, green and red fields, with the changing attenuation of the filter over time providing the correct relative intensities for white balance.

In some examples, the blue field begins transmission 1ms or less after a start time of the filter transitioning from blocking to transmitting. The blue field may be transmitted substantially simultaneously with a start time of the filter switching from blocking to transmitting, or substantially simultaneously with a control signal for the filter to switch from blocking to transmitting, for example. This allows faster operation and increases time available for transmission of fields.

White balance may be adjusted by altering a relative timing of the fields, such as the blue, green and red fields. This can provide an alternative or additional method of white balance adjustment than adjusting, for example, the intensity of the illumination system for each particular colour.

A complete field sequence, such as all of the blue, green and red fields, may be transmitted during the rise time. In this case, the rise time of the filter has less impact on the time taken to display a field. In some examples sufficient intensity of all the fields may be transmitted entirely during the rise time, so that the filter can be deactivated sooner as well.

A complete field sequence, such as all of blue, green and red fields, may pass through the achromatic filter in less than 5 ms from a start time of the achromatic filter transitioning from blocking to transmitting. This allows faster operation, such as at least 200 full-colour field sequences per second when there are three colour fields per image (600 individual blue, green or red fields per second). Other examples may use even less time, for example less than 2.5 ms from the start time may also be sufficient, allowing operation at up to 400 full colour field sequences per second when there are three colour fields per image (1200 individual blue, green or red fields per second). The achromatic filter may have a fall time to switch from transmissive to blocking and at least one field may be transmitted for at least part of the fall time. This can assist with faster switching by using a greater proportion of the transition time of the achromatic filter. In some examples, red light is transmitted for at least part of the fall time while the filter is transitioning from transmitting to blocking. Some examples may transmit an entire field in the fall time, and possibly at least part of another field too. For example, red light may be transmitted entirely in the fall time, and green light may also then be at least partially transmitted in the fall time.

The present disclosure can provide further advantages for displays where a frame is made up of a number of regions displayed sequentially in addition to the colour field sequence discussed above. This is particularly the case with holographic displays. The field of view is relatively small, perhaps only a few millimetres on each dimension. One way to increase the field of view is to transmit a sequence of different holograms all targeting a different position. If these fields are transmitted fast enough then persistence of vision acts to combine them at the viewer's eye, so a single image with a larger field of view is perceived. Transmission of different regions within a frame may be referred to as a "spatial sequence".

When colour field-sequential display is combined with a spatial sequence, such as to expand a field of view, many fields per second must be displayed. Advances in image processing algorithms and processing resources means that calculating Computer Generated Hologram (CGH) fields at this rate is feasible, and spatial light modulators capable of performing at this rate are available. For example, a Digital Micromirror Device (DMD) can operate at 10,000 Hz. Sufficiently fast filters are more of a challenge. Mechanical filters are complicated to manufacture. Solid-state filters, such as liquid crystal-based filters, are relatively easy to manufacture but their response time is slow. The present disclosure allows relatively slower filter technologies, such as liquid crystal-based filters, to be used.

As briefly mentioned above, the disclosure may be useful for holographic displays when the achromatic filter comprises a plurality of switchable regions which are independently controlled to increase a field of view of the display system. For example, the paper "Expanded Exit-Pupil Holographic Head-Mounted Display With High-Speed Digital Micromirror Device" by Kim et al, 03 May 2018 org/10.4218/etrij 2017-01 06 discusses architectures to expand a field of view using a filter (referred to as a shutter) positioned in a Fourier plane of a holographic display system. Alternatively, or additionally, the plurality of switchable regions may be controlled to reduce quantisation noise visible in the output of the holographic display system, such as described in PCT patent application PCT/GB2022/051867 filed on 19 July 2022 and incorporated herein by reference for all purposes.

An augmented reality display system, a virtual reality display system, or a head-up display may comprise the display system described above, with or without other features also described.

In another aspect, there is provided a method of generating a field-sequential colour image. The method comprises: activating an achromatic filter to switch from a blocking to a transmissive state; and transmitting blue, green and red fields in that order through the achromatic filter. At least the blue field is transmitted during a rise time of the achromatic filter to switch from the blocking to the transmissive state.

All of blue, green and red fields may be transmitted during the rise time The blue field may begin transmission less than lms after, or substantially simultaneously with, activating the achromatic filter to switch from the blocking to the transmissive state.

Some examples may comprise adjusting white balance by changing a relative

timing of the blue, green and red fields.

A complete field sequence of blue, green and red fields may pass through the achromatic filter in less than 5 ms, or less than 2.5ms, from a start time of the achromatic filter transitioning from the blocking to the transmissive state.

At least part of the red field may be transmitted during at least part of a fall time after the achromatic filter is deactivated and is transitioning from the transmissive state to the blocking state.

The method may be a method of displaying a field-sequential hologram, and the achromatic filter comprises a plurality of regions, the method comprising sequentially activating the plurality of regions to increase a field of view of an output hologram and/or to reduce quantisation noise visible in an output hologram.

In another aspect, there is provided a method of displaying field-sequential images comprising a plurality of fields, each of the plurality of fields having a different colour, via an achromatic filter, the method comprising transmitting at least one of the plurality of fields during at least one of a rise time and a fall time of the achromatic filter, and wherein an order in which the plurality of fields is displayed is based on a desired white balance and a transmissivity of the achromatic filter over time.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings

Brief Description of the Drawings

Figure 1 shows a diagrammatic representation of a holographic display system according to an embodiment; Figure 2 is a diagrammatic representation of the transmission of blue, green and red fields in that order during a rise time of a filter region; Figure 3 depicts relative output intensities of blue, green and red light for a colour temperature of 6500K, Figure 4 depicts the relative input intensities of blue, green and red light in that order according to the example of Figure 2 to give the relative output intensities of Figure 3; Figure 5 depicts the relative input intensities of red, green, blue light in that order during a rise time of a filter region to give the output relative intensities of Figure 3; and Figure 6 depicts an example method according to an embodiment.

Detailed Description

A holographic display system according to an example is depicted in Figure 1, Figure 1 shows, in general terms, a holographic optical system 100. The system 100 comprises an illumination system or light source 102 configured to generate at least partially coherent light. The system 100 further comprises a spatial light modulator (SLM) 104 arranged to be illuminated by the at least partially coherent light. A lens 106 has a focal length, f, and is positioned one focal length from the SLM 104.

The light source 102 may, for example, comprise a laser module or an LED. The light source 102 is configured to generate at least partially coherent light at a plurality of wavelengths in sequence (corresponding to red, green and blue, for

example).

The SLM 104 may be configured to modulate at least one of the phase, amplitude, binary phase and binary amplitude of the light. The SLM 104 may be, for example, a DIVED, an LCD, an amplitude LcoS or a phase LcoS.

The system 100 further comprises a filter 108, such as a spatial filter, comprising a plurality of regions that can be independently switched between substantially blocking and substantially transmitting light. A single aperture HO of the filter is depicted corresponding to a region or portion which has been activated to transmit light. The filter 108 is positioned one focal length from the lens 106, on the opposite side of the lens 106 from the SLM 104. The controllable regions or portions can be manufactured in a variety of ways. For example, the filter 108 could be manufactured ofli quid crystal and operated to either substantially allow light to pass or substantially block light. The liquid crystal may have a high switching speed such as pi-cell or Fen-oelectric LCD (F LCD).

As shown, the SLM 104, lens 106 and filter 108 are coaxial. Other configurations may also be used, such as a folded light path that may allow a more compact display.

The SLM 104 is configured to generate a light field which is a quantised representation of a target light field, H. The target light field, H, may be determined in any suitable way, for example by applying the algorithms to determine a CGH light field in W02020/14852 tAl. The arrangement of the holographic optical system 100 is such that the Fourier transform of the light field, F(H), is formed at a plane coinciding with the position of the filter 108. This plane is the Fourier plane of the SLM 104 as imaged by the lens 106. The target light field is determined such that the Fourier transform of the target light field, F(H), substantially corresponds with the position of the transmissive filter region 110. The other portions of the filter therefore act to block generally unwanted components of the H in the Fourier plane.

The filtering of Figure 1 can be useful for applications where filtering components in the Fourier plane is advantageous. A field of view may be increased, for example, by targeting different regions of the Fourier plane with respective fields that together combine at the user's eye thought persistence of vision to represent a single frame. Further details, including additional optical layouts to position a filter in a Fourier plane, are in the paper "Expanded Exit-Pupil Holographic Head-Mounted Display with High-Speed Digital Micromirror Device" by Kim et al, 03 May 2018. Kim et at recognise that LC-shutters can be used but does not consider the switching speed, suggesting only that it is synchronised with the DATD The filtering of Figure 1 may also be useful to reduce quantisation noise introduced by the quantisation for display on the SLAT. For example, if F(H) does not overlap at least the Fourier transform of the complex conjugate of the target light field, F(H*), and the second order components in the Fourier plane of the SLM 104, then quantisation noise located in those components can be blocked by the filter 110. Further information of the use of a filter to block quantisation noise can be found in PCT/GB2022/051867 filed on 19 July 2022 and incorporated herein by reference for all purposes For clarity, Figure 1 depicts a transmissive SLM, it will be understood that the principles discussed here are not limited to this and can equally be applied to reflective SLATs. Likewise, the same principles apply to other types of modulators than a SLAT.

It will also be understood that holographic display systems may provide a filter at positions other than a Fourier plane as depicted in Figure 1. For example, it may be desired to apply spatial filtering in an image plane, perhaps to block unwanted components such as "zero-order" light which will be in a slightly different position depending on the wavelength. More generally, some examples may position the filter before the SLM or incorporate the filter into the illumination source.

As discussed above, in one example, the filter 108 has Liquid Crystal (LC) regions or apertures 110, which can be considered to function as shutters. Such LC shutters do not immediately switch between transmissive and blocking or opaque states -instead it has a rise and fall time during which light is still being transmitted but with a degree of attenuation. The transmission over time can be modelled as an asymptotic exponential rise (on opening or activation) and fall (on closing or deactivation) with associated time constants for each er, /fan). The time to reach complete transmission (on opening) or complete extinction (on closing) is approximately five times these time constants respectively due to the nature of the exponential function.

Therefore, in a particular example where an LC shutter is used to generate colour-field sequential holograms at a high frame rate, it may further be necessary to sequence the colour-fields to pass through the shutter within 5 -iris, (i.e. faster than the complete rise time of the shutter, where the fall time is not included in the sequence). For example, some LC shutters, such as those commercially available from Bolder Vision Optik, Inc. of Boulder, Colorado, can achieve T -rise 10 Trate, where T rise 7-- 1 ms, which is comparable to the duration of each colour-field for standard projector framerates of 30 fps and above.

When fields are transmitted sequentially during the rise time, the attenuation of the shutter is different for each colour-field transmitted by the shutter. It has been found that selecting a particular sequence of colour-fields of blue, green and red to be transmitted by the filter is advantageous in order to achieve a good white balance at the projector's output using a uniform intensity set of colour-fields. Such a sequence is depicted in Figure 2, for an LC filter or shutter which has Tri,e = 1.5ms and Tian = 100Rs. It can be seen how the blue field is attenuated more than the green field, and the green field is attenuated more than the red field. It can also be seen how all the fields are transmitted in a time of around 2.2ms, so that the filter can be deactivated before it has become substantially fully transmissive. The fact the filter has not become fully transmissive can also be seen in Figure 2 because the transmission percentage is still some way off the asymptote (by extending the rise curve).

Achieving a good white balance requires adjusting the relative intensities of the colour-fields to imitate their relative proportions at a desired colour temperature (for example, a standard "daylight white" of 6500 K). In general, a white balanced output constructed from monochromatic RGB requires JR. > 16 > 1 Figure 3 depicts the relative output intensities for blue, green and redlight at a colour temperature of 6500K. By selecting the sequence of colour-fields such that the shutter transmits first blue, then (-Teen, and finally red light, this white-balanced relationship is approximately achieved at the shutter output when the incident or input colour-fields have nearly equal intensity. That is, the shutter itself acts like a spectral filter which aids in white balancing a uniform intensity set of colour-fields. Conversely, if a sequence was chosen where the shutter transmits first red, then green, then blue light, then the intensities of the incident colour-fields would need to be greatly different (with red requiring much greater intensity than blue order to achieve a white-balanced output after the shutter. This can be seen in Figures 4 and 5.

Figure 4 shows the relative input intensities using the colour-field sequence of Figure 2 (Blue, Green, Red) to give the relative output intensities of Figure 3. This shows how the required input power for each colours is substantially balanced. By way of comparison, Figure 5, which is not according to the present disclosure, shows the required input intensities to give the relative output intensities of Figure 3 if the colour field sequence is red, green, blue in that order transmitted during the rise time. The order of Figure 5 requires significantly different intensities between the colours, with red much more intense than blue.

As shown in Figure 4, with the colour sequence of blue, green, red, the projector's output can be white balanced using illumination sources that are nearly balanced in input intensity, which better matches off-the-shelf light sources.

While the discussion above showed all transmission during the rise time, other examples may extend the colour-field sequences into the fall time of the shutter.

Similarly, other examples may use a different filter technology, providing it has a finite rise time during which the filter is partially transmissive.

Figure 6 is a flow chart of an example method of generating a field-sequential colour hologram according to this disclosure. The method comprises sequentially activating regions of an achromatic filter to switch from a blocking to a transmissive state and transmitting blue, green and red fields in that order through each activated region. At least the blue field is transmitted during a rise time of the activated region to switch from the blocking to the transmissive state.

Each region of the filter is denoted by an integer index number, such as 1 to 8 for a filter with eight regions. At block 602, the filter region corresponding to n is activated. At blocks 604, 606 and 608 blue, green and red fields are obtained and displayed sequentially in that order during the rise time after activation of the filter region. Block 604 may be substantially simultaneous with the activation of the filter region. After all of the blue, green and red fields have been displayed the region is deactivated at block 610. Some examples may deactivate the region during the display of the red field at block 608, so that the red field is at least partially displayed during a fall time of the filter region.

At block 612, a check is made if there are further regions, if there are n is incremented at block 614 and the method returns to block 602. Otherwise, n is reset at block 616, such as reset to 1, and the method returns to block 602 to repeat the sequence. Block 616 may also involve progressing to a new frame for a moving image.

A sequence of blue, green, red ha.s been discussed above because this corresponds to the relative intensity of red, green and blue components for typical white balance. Other orders may be used for different colours making up the colour field-sequential image and/or different white balances.

While the discussion above, and the example of Figure I, considered a holographic display system, the methods and systems discussed herein can be applied to any type of colour-field sequential display and is not limited to holographic displays. Furthermore, some examples may have only a single achromatic tilter/shutter, rather than the filter comprising a plurality of apertures or regions discussed in Figure I. The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMSI. A display system comprising: an illumination system; a spatial light modulator illuminated by the illumination system; and an achromatic filter which is switchable between transmitting light and blocking light and which has a rise time to transition from blocking to transmitting; wherein: the illumination system and the spatial light modulator are configured to transmit respective colour fields through the achromatic filter to form a field-sequential colour display; the order of the respective colour fields is based on a relative intensity for a desired white balance; and at least one of the respective colour fields is transmitted while the achromatic filter is transitioning from blocking to transmitting.
2. The display system of claim 1, wherein the respective colour fields comprise blue, green and red fields transmitted in that order through the achromatic filter, and at least the blue field is transmitted while the achromatic filter is transitioning from blocking to transmitting.
3. The display system of claim 1 or 2, wherein the blue field begins transmission 1ms or less after a start time of the filter transitioning from blocking to transmitting.
4. The display system of claim 1, 2 or 3, wherein white balance is adjusted byaltering a relative timing of the fields.
5. The display system of any preceding claim wherein a complete field sequence passes through the achromatic filter in less than 5 ms from a start time of the filter transitioning from blocking to transmitting.
6. The display system of any preceding claim, wherein a complete field sequence is transmitted during the rise time.
7. The display system of any preceding claim, wherein the achromatic filter has a fall time to switch from transmissive to blocking and at least one field is transmitted for at least part of the fall time.
8. The display system of any preceding claim, wherein the display system is a holographic display system.
9. The holographic display system of claim 8, wherein the achromatic filter comprises a plurality of switchable regions which are controlled to increase a field of view of the display system.
10. The holographic display system of claim 8 or 9, wherein the achromatic filter comprises a plurality of switchable regions which are controlled to reduce quantisation noise visible in the output of the holographic display system.
11. An augmented reality display system, a virtual reality display system, or a head-up display system comprising the display system of any of claims 1 to 7 or the holographic display system of any of claims 8 to 10.
12. A method of generating a field-sequential colour image, the method comprising: activating an achromatic filter to switch from a blocking to a transmissive state; transmitting blue, green and red fields in that order through the achromatic filter, wherein at least the blue field is transmitted during a rise time of the achromatic filter to switch from the blocking to the transmissive state.
13. The method of claim 12, wherein the blue field begins transmission less than lms after activating the achromatic filter to switch from the blocking to the transmissive state.
14. The method of claim 12 or 13, comprising adjusting white balance by changing a relative timing of the blue, green and red fields.
15. The method of any of claims 12 to 14, wherein a complete field sequence of blue, green and red fields passes through the achromatic filter in less than 5 ms from a start time of the achromatic filter transitioning from the blocking to the transmissive state.
16. The method of any of claims 12 to 15, wherein at least part of the red field is transmitted during at least part of a fall time after the achromatic filter is deactivated and is transitioning from the transmissive state to the blocking state
17. The method of any of claims 12 to 16, wherein the method is a method of displaying a field-sequential hologram, and the achromatic filter comprises a plurality of regions, the method comprising sequentially activating the plurality of regions toincrease a field of view of an output hologram.
18. The method of any of claims 12 to 17, wherein the method is a method of displaying a field-sequential hologram, and the achromatic filter comprises a plurality of regions, the method comprising sequentially activating the plurality of regions to reduce quantisation noise visible in an output hologram.
19. A method of displaying field-sequential images comprising a plurality of fields, each of the plurality of fields having a different colour, via an achromatic filter, the method comprising transmitting at least one of the plurality of fields during at least one of a rise time and a fall time of the achromatic filter, and wherein an order in which the plurality of fields is displayed is based on a desired white balance and a transmissivity of the achromatic filter over time.