Dissolve Units are used when you want to use two projectors at once for your slide presentation. Most dissolve units can fade an image to black, fade one image into another image and have flashing or overlay capabilities. They are used when you want your presentation to be continuous instead of waiting for your projector to switch from image to image. Most units have multiple speeds for blending one slide into the next, along with special effects features. You can also reverse dissolve to go back to previous slides.
Most units also have built in timers and come with a remote control. You can set the length of time you want the image to stay on screen and when to fade to the next slide. This means you can let the projectors run continuously and unattended. They are small, portable and lightweight. Dissolve units can range in price from about $160 to $700.