For those who have never heard of it, cross polarization is a rarely used (anymore) but effective technique at eliminating reflections from light sources. It is a valuable yet simple tool for any tabletop photography with non-metallic reflective objects, such as liquor bottles like the one above. Why?
The primary problem with producing a striking bottle shot is how to light the label without creating an odd reflection. Lighting a bottle is all about reflecting light (or black cards) at the edge of the glass, separating it from the background. This means all the lights for the bottle are behind the set and never illuminate the label. So the label needs to be lit with an additional light from the front, which will produce a reflection.
In the today’s digital world, typically this reflection is removed in post, or the label is not lit but airbrushed and/or cut out from another picture and dropped in. For simple bottles and labels, the additional post-production can be minimal. However, for complicated bottles/labels, advanced lighting scenarios, or images that need to be captured in a single shot, that post-production can add on significant cost and time. In these cases, cross polarization is a much better option and requires minimal setup. Here's how.
After all of the lights are in place, apply a polarizing filter on the lens and spin it so it effects the reflections of the lights for the bottle as little as possible. Next, place a sheet of polarizing gel over the light for the label and turn on the modeling lamp. Then, while looking through your viewfinder, spin the light modifier for the label. You will see the reflection progressively get dimmer until it completely disappears, which is when you stop. Now that light source is cross-polarized to the lens and will not create a reflection regardless of how bight.
Now, of course, this method does have its drawbacks. Polarizing gel is expensive, a special order item and progressively looses it effect, especially when it becomes creased or warped from heat. Also, placing a polarizing filter on the lens eats up a stop and half of light, requiring all lights to be at least 3 times brighter. For the light being cross-polarized though, it is being filtered twice and will need to be 8 times brighter.
However, if you have the strobe power, and the little extra money and patience to purchase the gels, it will certainly save time, and money, in post.