Design patent infringement isn’t based on exact copying. Rather the test is if an “ordinary observer” will see “substantial similarity”.
Determining if a particular design of interest does, or does not, infringe upon another design patent is an interesting gray area of intellectual property law.
According to the 2008 Federal Circuit ruling in “Egyptian Goddess, Inc. v. Swisa, Inc.” 543 F.3d 665: to infringe, a given design does not have to be an exact copy of another design patent. Rather, the question is one of “substantial similarity” under the “ordinary observer” test.
Great — what the heck is this, and how does this test determine design patent infringement?
The underlying idea is that customers looking to purchase a design patented product “Y” should not be deceived by similar looking product “X”.
So as a practical matter, you should run this test using ordinary observers. Realize that if you personally are involved, it may be difficult for you to be fully objective. In this case, you are probably not a suitable “ordinary observer”. Instead, determine if disinterested outsiders see “substantial similarity“.
Although sometimes the differences between a given design and a particular design patent may be so great that no further comparisons are necessary, sometimes the designs are close. Here a “three-way visual comparison test” can be useful. This test can be done by making a composite illustration showing the patented design on one side, the “accused design” (i.e. the design being investigated) in the middle, and other designs representing the closest prior art on the other side.
Essentially it is a pattern recognition problem. The idea is to use the prior art examples to instruct the ordinary observers as to how much variation is typical in this field. Then the ordinary observer can determine if the design of interest is overly close to the design patent of interest, or not.
This sort of approach was used to compare handheld blender designs in Braun Inc. V Dynamics Corp. 975 F.2d 815, in 1992. As you can see in the above image, the “accused design” was visually much closer to the patented design than it was to the closest prior art. The court determined that the “accused design” was in fact infringing.
The best times to consider these issues are before you start producing and selling a new design. So if you have a new design that you are worried about, get the opinion of some neutral outsiders ASAP. Indeed, consider running some focus groups as appropriate. If things look too close, consider making some further design changes, and trying again.