It is easy to assume that the projector lamp from one might work in the other, but they will not. They both use projector lamps that are very similar. The HD39Darbee and HD28DSE share a casing and can be mixed up easily. This situation applies to many other models. The Serial number is your best way to determine the model. This is because they use the same physical casing for multiple models. Every Optoma projector has a serial # listed on the casing. This is where the serial number comes in helpful.