Poor vision is frequently caused by the condition wherein one’s retina is no longer located in the lens’s focal plane. This displacement can be either in front of the focal plane (hyperopia), or behind it (myopia). For this project, I wanted to show how the location of the retina could affect the focus of an image and how corrective lenses are used to compensate for these conditions.

Check out this riveting – riveting! – video of the Sliding Retina.

I needed to accomplish the following things:

1) I needed to make a “retina” that could move. Similar exhibits frequently have a retina that can be moved by hand. Whenever possible, I like to take any manual manipulation of a display out of the visitors’ hands (literally). (Given the opportunity, visitors – children especially – will put a great deal of physical stress on any moving parts.) 2) I needed to make a way for the visitor to insert corrective lenses in to the optical path. Again, similar exhibits simply have a set of “glasses” tethered to the display so that the visitor can put them in the proper place. This almost never works. The visitors most often look through the glasses themselves and cannot seem to figure out how they fit into the display. 3) I needed to provide an image displayed on the retina that could be manipulated. 4) I needed to guide the visitor through the display as much as possible insofar as the device is not really intuitive.

Here’s how I did it:

1) To make the retina move, I simply made a tiny sled attached to a lead screw. When the arrow buttons at the top of the display (arrow shaped buttons visible in the picture) are pushed, the retina will move backward and forward, simulating the displacement of the retina seen in hyperopia and myopia. I put limit switches at the end of the range of motion that turn off the motor when the limit of travel is reached.
2) I abandoned the uses of the fake eyeglasses mentioned above. Rather, to provide for the insertion of the corrective lenses into the light path, I put the lenses into a disk that can be spun using a handle at the front of the display. While the purpose of the disk is not immediately apparent, museum visitors tend to randomly turn, press, flip, slide, or otherwise manipulate any sort of mechanism that presents itself. I hoped that this tendency would lead visitors to discover the disk’s purpose: if turned left, the myopia lens correction is inserted, if turned to the right, the hyperopia lens is inserted, and, in the vertical position, a lensless hole is provided for the light to pass through.
3) The image on the retina is generated by a puck-style stage light hidden in the cabinet. The light passes through a small cut-out of a smiley face on the front of the cabinet, through a lens in the eye model on the top of the cabinet, through the visitor-selected corrective lens, and finally onto the retina. Finding a light powerful yet small enough was a real problem. I finally settled on the halogen stage light. However, it was so hot that it melted the plastic that held the smiley face image. I had to put a muffin fan in the cabinet which draws air directly across the front of the bulb and keeps the image from melting.
Smiley Face in Focus

Smiley Face in Focus

Smiley Face out of Focus

Smiley Face out of Focus

4) Since the puck light used to illuminate the retina was so hot, I decided to put it on a timer. So I put a large button on the front of the cabinet right in the area where the explanatory graphics are located. To call attention to the light, I illuminated the button itself. But since it is somewhat counterintuitive what to do after the light is activated, I needed to call attention to the retina control buttons. I wired the lights so that when the retina light is activated, that button goes dark, but the retina control buttons suddenly illuminate. This tends to draw the visitors’ attention to those buttons, giving a hint as to what they should do next. When the time is up, the retina control buttons go dark, the retina illuminator button lights, and the retina image fades away, ready for the next visitor.