# QotW: 3D vision and broken sunglasses

How can you make a normal TV look 3D?
02 March 2021

## Question

Pavel got in touch to ask "Take sunglasses and remove one lens. Watch a normal television film with one eye darkened by a sunglass lens, and the other free. The film will appear in 3D. Can someone explain to me, how does this work?" After dusting off her sunglasses, Eva Higginbotham put the question to 3D vision expert Andrew Glennerster from the University of Reading...

Eva Higginbotham's been casting her eye about for the answer to listener Pavel's question...

Eva - A curious question indeed, but it turns out that everyone with normal vision should be able to experience this. I dug out my dusty sunglasses and put the question to 3D vision expert Andrew Glennerster from the University of Reading.

Andrew - Pavel asks about a very interesting phenomenon that was discovered and published almost exactly 100 years ago by Carl Pulfrich. He was one of the world's leading experts on stereoscopic, otherwise known as 3D, displays at the time. Ironically, he was blind in one eye so he could not see the effect himself.

Eva - In the cinema, 3D movies work by sending a different image to each eye by a combination of a clever filming technique and the 3D glasses you wear. But this phenomenon is a little different, and you can try it out yourself at home without any fancy kit, as Andrew explains.

Andrew - If you put a dark filter over one eye (eg using one lens of a pair of glasses, as Pavel describes) this slows down the processing of the image in that eye. So, when the brain compares the images in the two eyes, it is using a delayed version from the dimmed eye. That means that if an object is moving from left to right and the left image is dimmed, the rays of light that the brain receives are the same as if the object really was further away.

Eva - If the object moves in the other direction, the depth effect is reversed, and the faster the motion the greater the effect.

Andrew - For example, this means that a pendulum that’s really swinging in front of you in a flat plane will appear to move along an oval-shaped path in depth, that is, it’s coming nearer to you as it swings to the left, and further away from you as it swings to the right. But it will reverse direction and move the opposite way around the path if you swap the filter and put it over the other eye.

Eva - And it works on more interesting objects than just a pendulum - it was used in a Diet Coke ad in 1989, and Doctor Who used it extensively in their 1993 charity special Dimensions in Time.

Andrew - So, Pavel is describing an effect that has generated a lot of scientific interest; it even has clinical implications for recent ophthalmic therapies. But, as you can see, wearing a dark lens is no substitute for the 3-D glasses you use in the cinema: you wouldn't want to watch a film in which people walking left to right were close and people walking the other direction were far away!

Eva - Thanks Andrew! And if you want to give it a go for yourself, you can grab your sunglasses, head to youtube and look up ‘Pulfrich effect’ spelled the German way with an R I C H at the end,  where there are lots of videos designed just for that. Next week, we’re considering this question from Michael

Michael - Why can’t batteries such as AA and AAA size be recharged? What’s the difference between regular batteries and rechargeable especially lithium versions? Is this a “Big Battery” conspiracy to sell  more batteries, or are their valid reasons?