by Din » Mon Sep 04, 2017 8:59 am
Yes, it's possible. But, you have to reconstruct a real image, as opposed to a virtual image.
When you look at a real object in the real world, light emanates from the object, ie the light diverges outward from the object to your eyes. When you look at a mirror image of some object, light appears to diverge from behind the mirror, therefore the object appears to be behind the mirror. However, there is no actual object behind the mirror. Divergent light from the object hits the mirror, is redirected to your eyes, and continues to diverge. Your eyes "backtraces" the (redirected) divergent light to a point behind the mirror, and your brain assumes that, because light is diverging from some point behind the mirror, there really is an object behind the mirror. The light from the mirror causes a Virtual Image."Virtual" because there is no actual object behind the mirror from which light is actually diverging.
When you place a (converging) lens, ie a magnifying glass, between your eyes and an object, the (magnified) image appears to be coming from behind the lens. This is because, if you placed the object at some point less than the focal length of the lens, the light from the object diverges towards the lens, and continues to diverge. Again, as in the mirror example, your eyes "backtraces" the divergent light to some point behind the lens, and assumes there is a (magnified) object somewhere behind the lens. The lens creates a virtual image. However, if you placed the lens at some point greater than the focal length, the diverging light from the object hits the lens, then converges, and diverges again. So, the light comes to a real point, then diverges from that real point. Since the light is actually diverging again from a real point in space, there actually is an image of the object, as if it were the real object. This is called a Real Image, "real" because there really is divergent light from some point in space. A real image can be projected on some surface. So, if you placed a lens against a window, where there was a tree some distance away, you can place a piece of paper at the focal length of the lens, and you'll see a clear focused and projected image of the tree.
In a hologram, you record divergent light from an object, ie the light from the object diverges towards the hologram plate. You're actually recording the divergent wavefront itself. Now, there are two ways you can reconstruct the hologram. One way is to reconstruct it with the exact shape and position of the original reference. If you do this, you reconstruct the original divergent light. The image is now virtual, and cannot be projected. But, if you reverse the hologram, so you reconstruct with the exact opposite of the original reference beam, the hologram reconstructs the opposite of the original divergent light, it reconstructs convergent light. Now, the image is real, and can be projected on some surface.
So, to keep things simple, let's say you make a laser transmission hologram of a porcelain cat. You place the cat at,say, 8in (20cm) from the plate, and reference it in same direction as the cat, ie the ref and object light hits the same side of the plate. Now, you develop the hologram, replace it, and re-illuminate the plate. You'll see the porcelain cat behind the plate. This is the virtual image. If you now flip the plate (side-to-side for a side referenced ref, top top-to-bottom for a top ref) and re-illuminate the plate again. Now the cat appears in front of the plate, This is a real image. This real image can be projected on a wall. As Petr says, if the original object 8in from the plate, the real object focuses at 8in away. But, this is possible only if you laser illuminate. If you use white light, each wavelength component of the white light will create it's own image, slightly displaced, so you'll get many, many cats, all displaced slightly, creating a colour blur.
If you take the hologram of the cat, and re-illuminate it from the opposite side (the opposite of the original ref is called the 'conjugate' of the ref), but with an unexpanded laser beam, a raw laser directly from the laser, then you'll get a flat real image of the cat that can be projected at any distance, but magnified. This is because you're only reconstructing a tiny portion of the hologram, the area of the laser beam, and you see one single 2D perspective of the image.
Yes, it's possible. But, you have to reconstruct a real image, as opposed to a virtual image.
When you look at a real object in the real world, light emanates from the object, ie the light diverges outward from the object to your eyes. When you look at a mirror image of some object, light appears to diverge from behind the mirror, therefore the object appears to be behind the mirror. However, there is no actual object behind the mirror. Divergent light from the object hits the mirror, is redirected to your eyes, and continues to diverge. Your eyes "backtraces" the (redirected) divergent light to a point behind the mirror, and your brain assumes that, because light is diverging from some point behind the mirror, there really is an object behind the mirror. The light from the mirror causes a Virtual Image."Virtual" because there is no actual object behind the mirror from which light is actually diverging.
When you place a (converging) lens, ie a magnifying glass, between your eyes and an object, the (magnified) image appears to be coming from behind the lens. This is because, if you placed the object at some point less than the focal length of the lens, the light from the object diverges towards the lens, and continues to diverge. Again, as in the mirror example, your eyes "backtraces" the divergent light to some point behind the lens, and assumes there is a (magnified) object somewhere behind the lens. The lens creates a virtual image. However, if you placed the lens at some point greater than the focal length, the diverging light from the object hits the lens, then converges, and diverges again. So, the light comes to a real point, then diverges from that real point. Since the light is actually diverging again from a real point in space, there actually is an image of the object, as if it were the real object. This is called a Real Image, "real" because there really is divergent light from some point in space. A real image can be projected on some surface. So, if you placed a lens against a window, where there was a tree some distance away, you can place a piece of paper at the focal length of the lens, and you'll see a clear focused and projected image of the tree.
In a hologram, you record divergent light from an object, ie the light from the object diverges towards the hologram plate. You're actually recording the divergent wavefront itself. Now, there are two ways you can reconstruct the hologram. One way is to reconstruct it with the exact shape and position of the original reference. If you do this, you reconstruct the original divergent light. The image is now virtual, and cannot be projected. But, if you reverse the hologram, so you reconstruct with the exact opposite of the original reference beam, the hologram reconstructs the opposite of the original divergent light, it reconstructs convergent light. Now, the image is real, and can be projected on some surface.
So, to keep things simple, let's say you make a laser transmission hologram of a porcelain cat. You place the cat at,say, 8in (20cm) from the plate, and reference it in same direction as the cat, ie the ref and object light hits the same side of the plate. Now, you develop the hologram, replace it, and re-illuminate the plate. You'll see the porcelain cat behind the plate. This is the virtual image. If you now flip the plate (side-to-side for a side referenced ref, top top-to-bottom for a top ref) and re-illuminate the plate again. Now the cat appears in front of the plate, This is a real image. This real image can be projected on a wall. As Petr says, if the original object 8in from the plate, the real object focuses at 8in away. But, this is possible only if you laser illuminate. If you use white light, each wavelength component of the white light will create it's own image, slightly displaced, so you'll get many, many cats, all displaced slightly, creating a colour blur.
If you take the hologram of the cat, and re-illuminate it from the opposite side (the opposite of the original ref is called the 'conjugate' of the ref), but with an unexpanded laser beam, a raw laser directly from the laser, then you'll get a flat real image of the cat that can be projected at any distance, but magnified. This is because you're only reconstructing a tiny portion of the hologram, the area of the laser beam, and you see one single 2D perspective of the image.