Pepper’s Ghost

One of the most alluring pieces of equipment I have ever encountered is the phoropter, which is used by optometrists during eye exams to assess a subject’s vision and ascertain that subject’s eyeglass prescription.  The following link offers a description of phoropters, accompanied with pictures: https://en.wikipedia.org/wiki/Phoropter.  As a young child, I recall being pleasantly intrigued by watching my optometrist whirl the lenses and prisms in the device around with ease to alter my view; the swift changes seemed somewhat magical to me.  During eye exams, optometrists place a phoropter over a subject’s eyes and ask the subject to read a series of letters both close up and far away.  They iteratively change the lenses and other optics in the device to deduce the optimal prescription of the subject.  However, to my knowledge, optometrists have not yet implemented the Pepper’s Ghost illusion in conjunction with phoropters.  Since the ghost image is fainter than the object itself, a Pepper’s Ghost eye exam would be more challenging than the traditional one.  This would prove especially valuable for accessing candidates to jobs with strict vision requirements, such as astronauts.

I made a basic phoropter using two circular cut-out pieces of white cardboard and four pairs of lenses.  The lenses are displayed and labeled in the picture below.  As distinguishable from the labels, the types of lenses used were thick positive spherical, spherical meniscus, meniscus, and thin positive spherical.  An interesting aside fact I learned after reading about lenses is that plus lenses are synonymous with convex lenses, whereas minus lenses are synonymous with concave lenses.  Plus lenses are prescribed to fix farsightedness and minus lenses are prescribed to fix nearsightedness.  The two thick positive spherical lenses shown on the far left side of the image refract incident light appreciably, and thus when looking through them, the view appears quite blurry.  I was able to see clearly through the other three pairs, so the thick positive spherical lens pair served as an outlier case.

Lenses

I traced out the shape of each of the lenses onto the two cardboard wheels with a pencil, such that the center of each lens was 4 centimeters from the rim of the wheel.  Shown below is a picture of my tracings on the wheels.  Scissors were used to pierce through the center of these marked regions and cut the shapes out.  Additionally, I cut out the shape of the largest of my lenses, the spherical meniscus, in two locations on a cardboard box for viewing windows of the ghost image.

Traced Lenses

Prior to fixing the lenses in place, I cleaned them with rubbing alcohol, rinsed them off with water, and then dried them with wipers.  A hot glue gun was used to securely mount the lenses on the wheels.  A picture of the two wheels with lenses attached is given below.

Phoropter

With the phoropter constructed, I next needed to make the Pepper’s Ghost illusion and position the wheels around the viewing windows I established.  I used a cardboard box to house the object and piece of acrylic needed for the illusion.  Two small holes were cut through the box in the locations where the center of each wheel was to be placed; small holes were also cut through the center of each wheel, as seen in the above picture of the wheels with lenses glued on.  Two large screws were pushed through the small holes on the box and the wheels.  Below is a picture of the phoropter mounted to the box.

Outisde box

Duct tape held the acrylic piece steady inside the box.  The object selected to produce the ghost image was a desk clock.  A clock was chosen because it provided the subject a means of discerning information.  The visual challenge was to look through the phoropter at the ghost image and tell the time, based on the locations of the second, minute, and hour hands of the clock.  The overhead view of the components inside the box is seen below.

Overhead

In order to clearly see Pepper’s Ghost with my system, the lights were turned off and an LED light bulb was placed overhead the piece of acrylic as an illumination source.  Two shots through two different lens pairs are illustrated below.  The first is through the thick positive spherical lenses and appears blurry and unclear; the view through the right lens is the actual clock.  The second is through the spherical meniscus lenses, and the ghost image of the clock is discernible through the left lens.  The time on the actual clock is 11:01 with the red second hand on second 42; of course, since the ghost image is a reflection, it is inverted to look like the time is 12:59 with the second hand on second 18.

Blurry image

Clear image