Invisible Glass Photography

By Sean Michael Ragan
Category: Science, Photography & Video
Difficulty: Moderate

Recently, I needed a special photograph for my Soda Can Label Embossing project. I wanted to show a long embossed label arranged in a spiral, so that both the front and the back of the label were visible, in order to show off both the clear embossed text on the front, and the soda can graphics on the back. And I wanted a wide, short, even spiral that would fit nicely in a 4:3 aspect ratio.

The natural curliness of the aluminum, however, made it very hard to get the spiral I wanted without wrapping the label around some kind of round support. But then the support, even if transparent, made it hard to see the backside of the label clearly. If I had to use a support form, I wanted it to be truly invisible, as if the label were floating freely in space.

There is a classic physics demonstration, sometimes disguised as a bit of stage magic, in which pieces of glass are made to disappear by immersion in a liquid that has the same refractive index as the glass itself. I had a Pyrex glass beaker of about the diameter of the spiral I wanted, and I decided to experiment with the oil method for making the beaker invisible to get the shot. It’s a bit messy, and there are some things I’ll do differently next time, but it works.

Parts
/
Tools

Vegetable oil, Wesson brand (2 gal) or baby oil, or an appropriate mixture of mineral oils, or dilute Karo syrup, or other fluid having the same refractive index as your support. For Pyrex glass, this is about 1.47.
Subject to be photographed (1)
Tape, clear, Scotch (1) for mounting subject to support

Container such as a cheap "starter" aquarium
Digital camera and appropriate lights, backdrops, and other accessories for staging your photograph
Glass support other glasses will have different indices of refraction, and won't work with Wesson oil

Steps

Step #1: Dry prep

Clean the glass support and the outer container spotless.
Mount the specimen to the glass support. I used Scotch tape for this purpose, which held up quite well against the oil. Its refractive index is not the same as the glass and the oil, so the tape itself is visible in photographs. But it was not hard to remove in post-processing.
Position the glass support, with mounted specimen, inside the outer container.

Conclusion

The photograph I eventually got using this method is still not entirely satisfactory to me, but that has more to do with how I positioned the subject on the support. The trick for using oil to make the support invisible worked great.

Next time, I would like to find an oil or other medium that is even closer to the refractive index of Pyrex. With Wesson oil, there's still a very faint blue "ghost" of the glass shape visible in the raw photographs, though this is not difficult to remove digitally. Also, I'd like to find a medium that is truly "water white" and does not require color correction from yellow. Reportedly, baby oil is an improvement on both counts.