• Question: how do holograms work??

    Asked by abdelmadjidzinet on 12 Jan 2015.
    • Photo: Tim Stephens

      Tim Stephens answered on 12 Jan 2015:

      When you compare a hologram with a regular photograph, you’re able to see the subject in three dimensions and can move your head around to see it from different perspectives.

      A regular photograph only captures information about the light that travels from the subject to the camera at a single viewing point, so you only ever see one perspective of the subject, no matter how you move your head around.

      A hologram captures information about all the light that comes from the subject from lots of different viewing points, so you end up being able to recreate the light field that originally came from the subject. This means that you can move your head around and see the subject from different perspectives.

      The way that holograms do this is by capturing something called the _phase_ of the light. Phase is actually information about the time difference between when light was reflected from different parts of the subject. When you shine light back through your hologram, you can use it to recreate the rays travelling in different directions as they’re reflected from the subject — this is what allows you to move your head about and see different views of the image.

    • Photo: Andy Hearn

      Andy Hearn answered on 22 Jan 2015:

      …. I’d like to expand on Tim’s answer a tad:

      What I still find fascinating about holograms; if you have a hologram of your face and break it in half, you won’t have holograms of the top and bottom of your face, but *two* smaller whole faces! 😮 (They will be slightly “out of focus” compared to the original hologram though).

      Break each piece in half, and you get four whole faces, and they will be noticeably blurry. Keep on breaking and we will reach a point where each piece shows just a meaningless face-shaped splodge.

      For me, it kind of helps to think of a hologram in this way (other people have different analogies) :

      – A spotlight shining on a hologram is hitting it like a series of bolts (imagine a laser gun in a game shooting way too thick lasers)

      – The surface of the hologram splits the bolts into thinner parts, and then bounces them back at different speeds, so some parts will catch up with a part of a previous bolt, messing up its angle slightly.

      – So the viewer (you and me) get many parts of the original light arriving at our eyes as at different speeds and angles, forming a ‘shape’, thus the hologram image!

      Back to the broken up hologram, each piece doesn’t have enough surface to bounce back the ‘bolts’ to interfere the previous bolts correctly, so the image becomes more blurry.