POV Effect: The Magic of Afterimages in Electronic Displays
An Alternative for 3D POV Displays
Introduction to Persistence of Vision (POV)
The human eye is a fascinating instrument. One of its most intriguing aspects is the phenomenon of 'Persistence of Vision' (POV), or 'afterimage formation'. This means that our brain continues to perceive an image for a brief moment, even after the light signal that created the image has disappeared. This effect is the basis of how we experience moving images, from old film projectors to modern screens.
By cleverly applying this principle, we can create stunning visual illusions with electronics. Imagine a row of LEDs that moves quickly and is switched on and off at precisely the right moment. The brain 'stitches' these successive light points together, making us see a static or even moving image, even though the image is not physically present as a whole in reality.
Various Applications of the POV Effect
The POV effect has been utilized in various types of displays for decades:
7-segment displays: Although less complex, these displays also indirectly use POV. The segments light up quickly one after another, but due to the speed, we see a complete digit.
2D POV displays: These are the most common POV projects in electronics.
Television screens: Traditional televisions (especially older CRT screens) also effectively used the POV effect. An electron beam 'scans' the screen line by line, and due to the speed of this scanning and the afterglow of the phosphor, we see a stable, complete image.
A typical example is a single row of LEDs that moves linearly or circularly. By activating the LEDs at the right moment, a two-dimensional image is created in the air. The Adafruit Motorized POV LED Display [2] is an excellent example of such a project, where two strips of DotStar LEDs rotate to display bitmap images.
"3D Holographic" fans: These popular devices, often incorrectly referred to as "3D holographics" [1], are in fact advanced 2D POV displays. They consist of one or more arms with LEDs that rotate at high speed. The result is an impressive 2D image that appears to float in space, mimicking a holographic effect, but technically not a true hologram.
Expansion to 3D POV Displays
The ideas behind 2D POV displays have been further expanded in an attempt to make three-dimensional models visible. This leads to several approaches:
Volumetric POV displays: Some projects attempt to create a 'truer' 3D effect by using multiple LED strips or layers that move in a 3D structure. An example of this is the PropHelix 3D POV Display [3], which rotates a helical arrangement of LED boards to generate a volumetric image. Another impressive project even shows real-time 3D video by rotating an LED array that visualizes data from a depth camera [5].
Rotating 2D plates: A variant is the rapid rotation of a 2D display. This can work but introduces significant mechanical challenges, such as air resistance and the need for a robust construction. A double-sided display could increase effectiveness, but air resistance remains a factor.
Static 3D LED cubes: A fundamentally different, and often more expensive, approach is a completely static 3D matrix of LEDs, such as the Velleman 3D LED Cube 5x5x5 [6]. Here, nothing needs to rotate; each LED represents a voxel (volume pixel) and can be individually controlled to form a 3D image. The disadvantage is the enormous number of LEDs and the complexity of their control.
A New Approach to 3D Perception
Most of the aforementioned 3D POV displays actually require a 2D arrangement of LEDs (or multiple 1D arrays that together form a 2D plane) that rotates. Although they provide an impressive 3D effect where the model appears to stand in space and the surroundings are relatively transparent, there is an inherent disadvantage: the necessity of multiple LEDs in two directions (a 2D plane) to build the 3D image.
The approach described below proposes an alternative method for perceiving a 3D model, which does not create a transparent environment around the model (although this may still be subject to investigation). The fundamental insight is that for 3D perception with our two eyes, it is not necessary to have LEDs in 2D directions to perceive depth!
If we can ensure that LEDs light up at the correct moments along the 'light fields' [7] towards our eyes, our brain can still form a correct perception of a 3D model. This concept utilizes the principle of stereoscopy, where each eye sees a slightly different image, which is interpreted as depth by the brain.
The Concept: Two 1D LED Arrays for Stereoscopic Vision
The idea is to use a separate 1D LED array for each eye. When these two arrays rotate together, each array creates a 2D image for the respective eye. The crucial point is that each eye sees a slightly different image content, which is essential for depth perception. However, this is difficult if the LED arrays are simply placed next to each other, as both eyes would then perceive both arrays.
A simple solution like alternately lighting up the arrays does not work, because both eyes would still see the lit array (unless active 3D glasses with LCD shutters are used, similar to 3D televisions).
The solution lies in directing the light beam. A possible approach is to place an elongated lens (e.g., a plano-convex cylindrical lens) over the LED arrays. This would allow the light beam from each array to be made narrow enough to reach only the intended eye. However, such lenses may be difficult to obtain.
A more pragmatic solution can be found in a simple optical principle: the use of slits. If we create two vertical slits in a piece of paper (or other opaque material) and place a 1D LED array behind each slit, a limited angle is created within which each LED array is visible. By carefully positioning the arrays and slits, each eye can perceive its own specific image.
These 'two vertical slits, each with a 1D LED array behind it'-sets can be placed multiple times around the circumference of a cylinder (e.g., 3 or 6 sets). The more of these sets there are, the higher the potential frame rate can be, or the less quickly the cylinder needs to rotate to create a stable image. It is important that the additional sets are not visible to the eyes, except when they are in the correct position to build part of the image (via the POV effect).
See the attached slides 3D-display_setup001.pptx to show the concept
Conclusion and Advantages
This concept, is still in the idea phase and far from a detailed elaboration, offers promising advantages over the previously mentioned 3D POV displays:
- Fewer LEDs: It requires significantly fewer LEDs for comparable image quality, which reduces hardware costs and complexity.
- Simpler control: Fewer LEDs also mean much simpler control electronics, making the project more accessible.
- No significant mechanical challenges, such as air resistance as in the case of rotating 2D-displays
A useful starting point for the mechanical aspects of such a rotating display, which already includes some of the necessary considerations, is the aforementioned Adafruit Motorized POV LED Display [2].
References:
[1] https://www.youtube.com/shorts/XC9qPCvZa8A
[2] https://learn.adafruit.com/motorized-pov-led-display/overview
[3] https://www.instructables.com/PropHelix-3D-POV-Display
[4] https://www.youtube.com/shorts/RmlJm2MNTV0
[5] https://www.hackster.io/news/this-pov-display-shows-3d-video-in-real-time-1e1d88569e94
[6] https://www.elektor.com/products/velleman-3d-led-cube-5x5x5-blue
[7] https://en.wikipedia.org/wiki/Light_field
The human eye is a fascinating instrument. One of its most intriguing aspects is the phenomenon of 'Persistence of Vision' (POV), or 'afterimage formation'. This means that our brain continues to perceive an image for a brief moment, even after the light signal that created the image has disappeared. This effect is the basis of how we experience moving images, from old film projectors to modern screens.
By cleverly applying this principle, we can create stunning visual illusions with electronics. Imagine a row of LEDs that moves quickly and is switched on and off at precisely the right moment. The brain 'stitches' these successive light points together, making us see a static or even moving image, even though the image is not physically present as a whole in reality.
Various Applications of the POV Effect
The POV effect has been utilized in various types of displays for decades:
7-segment displays: Although less complex, these displays also indirectly use POV. The segments light up quickly one after another, but due to the speed, we see a complete digit.
2D POV displays: These are the most common POV projects in electronics.
Television screens: Traditional televisions (especially older CRT screens) also effectively used the POV effect. An electron beam 'scans' the screen line by line, and due to the speed of this scanning and the afterglow of the phosphor, we see a stable, complete image.
A typical example is a single row of LEDs that moves linearly or circularly. By activating the LEDs at the right moment, a two-dimensional image is created in the air. The Adafruit Motorized POV LED Display [2] is an excellent example of such a project, where two strips of DotStar LEDs rotate to display bitmap images.
"3D Holographic" fans: These popular devices, often incorrectly referred to as "3D holographics" [1], are in fact advanced 2D POV displays. They consist of one or more arms with LEDs that rotate at high speed. The result is an impressive 2D image that appears to float in space, mimicking a holographic effect, but technically not a true hologram.
Expansion to 3D POV Displays
The ideas behind 2D POV displays have been further expanded in an attempt to make three-dimensional models visible. This leads to several approaches:
Volumetric POV displays: Some projects attempt to create a 'truer' 3D effect by using multiple LED strips or layers that move in a 3D structure. An example of this is the PropHelix 3D POV Display [3], which rotates a helical arrangement of LED boards to generate a volumetric image. Another impressive project even shows real-time 3D video by rotating an LED array that visualizes data from a depth camera [5].
Rotating 2D plates: A variant is the rapid rotation of a 2D display. This can work but introduces significant mechanical challenges, such as air resistance and the need for a robust construction. A double-sided display could increase effectiveness, but air resistance remains a factor.
Static 3D LED cubes: A fundamentally different, and often more expensive, approach is a completely static 3D matrix of LEDs, such as the Velleman 3D LED Cube 5x5x5 [6]. Here, nothing needs to rotate; each LED represents a voxel (volume pixel) and can be individually controlled to form a 3D image. The disadvantage is the enormous number of LEDs and the complexity of their control.
A New Approach to 3D Perception
Most of the aforementioned 3D POV displays actually require a 2D arrangement of LEDs (or multiple 1D arrays that together form a 2D plane) that rotates. Although they provide an impressive 3D effect where the model appears to stand in space and the surroundings are relatively transparent, there is an inherent disadvantage: the necessity of multiple LEDs in two directions (a 2D plane) to build the 3D image.
The approach described below proposes an alternative method for perceiving a 3D model, which does not create a transparent environment around the model (although this may still be subject to investigation). The fundamental insight is that for 3D perception with our two eyes, it is not necessary to have LEDs in 2D directions to perceive depth!
If we can ensure that LEDs light up at the correct moments along the 'light fields' [7] towards our eyes, our brain can still form a correct perception of a 3D model. This concept utilizes the principle of stereoscopy, where each eye sees a slightly different image, which is interpreted as depth by the brain.
The Concept: Two 1D LED Arrays for Stereoscopic Vision
The idea is to use a separate 1D LED array for each eye. When these two arrays rotate together, each array creates a 2D image for the respective eye. The crucial point is that each eye sees a slightly different image content, which is essential for depth perception. However, this is difficult if the LED arrays are simply placed next to each other, as both eyes would then perceive both arrays.
A simple solution like alternately lighting up the arrays does not work, because both eyes would still see the lit array (unless active 3D glasses with LCD shutters are used, similar to 3D televisions).
The solution lies in directing the light beam. A possible approach is to place an elongated lens (e.g., a plano-convex cylindrical lens) over the LED arrays. This would allow the light beam from each array to be made narrow enough to reach only the intended eye. However, such lenses may be difficult to obtain.
A more pragmatic solution can be found in a simple optical principle: the use of slits. If we create two vertical slits in a piece of paper (or other opaque material) and place a 1D LED array behind each slit, a limited angle is created within which each LED array is visible. By carefully positioning the arrays and slits, each eye can perceive its own specific image.
These 'two vertical slits, each with a 1D LED array behind it'-sets can be placed multiple times around the circumference of a cylinder (e.g., 3 or 6 sets). The more of these sets there are, the higher the potential frame rate can be, or the less quickly the cylinder needs to rotate to create a stable image. It is important that the additional sets are not visible to the eyes, except when they are in the correct position to build part of the image (via the POV effect).
See the attached slides 3D-display_setup001.pptx to show the concept
Conclusion and Advantages
This concept, is still in the idea phase and far from a detailed elaboration, offers promising advantages over the previously mentioned 3D POV displays:
- Fewer LEDs: It requires significantly fewer LEDs for comparable image quality, which reduces hardware costs and complexity.
- Simpler control: Fewer LEDs also mean much simpler control electronics, making the project more accessible.
- No significant mechanical challenges, such as air resistance as in the case of rotating 2D-displays
A useful starting point for the mechanical aspects of such a rotating display, which already includes some of the necessary considerations, is the aforementioned Adafruit Motorized POV LED Display [2].
References:
[1] https://www.youtube.com/shorts/XC9qPCvZa8A
[2] https://learn.adafruit.com/motorized-pov-led-display/overview
[3] https://www.instructables.com/PropHelix-3D-POV-Display
[4] https://www.youtube.com/shorts/RmlJm2MNTV0
[5] https://www.hackster.io/news/this-pov-display-shows-3d-video-in-real-time-1e1d88569e94
[6] https://www.elektor.com/products/velleman-3d-led-cube-5x5x5-blue
[7] https://en.wikipedia.org/wiki/Light_field
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