Clear Object on White
When photographing a transparent subject on a light background, success will come from doing two things right: lighting the background and defining the edges. My preferred method with a tabletop setup is to use black glass as the foundation, with the table placed at least six feet from the white paper background. This way, I can light the background separately from the subject without worrying about spill and the mirror-like black plexi surface will reflect the bright background quite well.
I place a softbox below table height and centered on the background in order to evenly illuminate the white paper (or white wall) that will become the white background. Alternatively, two softboxes on either side of the table can also be used to evenly illuminate the background—although this method requires more attention to spill and flare, so flags may need to be positioned between these lights and the tabletop set. The benefit of one light below table height is flare is not an issue.
With the background illuminated, adjust your camera settings (in manual exposure mode, especially if you’re working with strobes) in conjunction with the light’s output and position until you’ve created the background you want. (Meaning, for those uninitiated: To make the background brighter, increase the light’s power, move it closer or open up the aperture.) This could be pure white, or a hot spot that fades to a darker tone or really anything you like—so long as it’s generally nice and bright. Through the viewfinder, you’ll now see the foundation of your image is there. The only thing missing is the clear subject, which probably isn’t yet well defined against the bright background.
To define the edges of the clear object, I don’t use lights. I use black flags or black foamcore sheets, usually about 2×3 feet in size for a subject smaller than a bread box. These act as negative fill. They reflect in the edges of the clear subject to effectively trace a dark line against the bright background. The position of those flags—how close to the subject and how far forward or behind—dictates the thickness and intensity of the outline. Sometimes, an additional flag placed above the subject is necessary to continue the outline across the top of the object. In the end, a single light is all it takes to make this shot work.
Clear Object on Black
The good thing about photographing a transparent subject on a black background is that you don’t have to worry about lighting the background; you only have to make it dark. You could do this with a black seamless paper or black velvet drape positioned several feet from the tabletop in order to create a dark black background, and then position softboxes behind and to the sides of the subject, directed at the subject, in order to create the white highlight edges. But I’ve been leaning toward an even simpler method of achieving the same result, and it doesn’t take much space at all. It uses a flag to create a silhouette behind the subject that’s sure to go dark black.
Again with a black glass tabletop (although black velvet works well too), I can use the same setup as I used for the white background shot. But instead of aiming the softbox at the background, I can position it directly behind the subject, pointed at the subject and right into the camera. It’s best if this softbox is fairly large—say 36 inches or more in the long dimension—and positioned within a foot or two of the subject. This softbox will create the white outline edges on the clear object, but first we have to turn the background black. To do this, I place a black flag or foamcore sheet directly between the softbox and the subject so that only a thin strip of softbox is visible on each side of the subject. This way, the background will be pure black (if exposed correctly, that is) and you’ll create two thin strip lights behind the subject that will translate into those bright white edges. A white reflector can be positioned above the subject if highlights are missing atop the subject.
This method is equally simple, allowing for some fairly refined lighting with only a single source. The key is to ensure you don’t overexpose and make the black background dark or, even worse, middle gray. With the illumination coming from the exposed strips of softbox, it won’t take much to set off the subject with white highlights against the black-silhouetted background.
Whether shooting on black or white, I find that the most important post processing adjustment is to contrast—ensuring deep blacks and bright whites—because after all, there shouldn’t be many tones in between. Spotting and retouching of highly visible little blemishes and dust may be necessary as well, but in the end the success of these shots is largely a function of knowing how to light them. Remembering that you don’t light a clear object—you light what is seen through it—is a great place to start.