When deciding which lasers to get, you need to decide the wavelength and wattage (light intensity) needed for your particular sized screen. The higher the wavelength the less ambient visible light will effect your blob tracking. 850nm and 880nm lasers are recommended over the 780nm lasers. The higher the mW rating of the laser the brighter the laser. You typically need at least 4 lasers, one in each corner of your screen. For larger setups, you will want 6-8 lasers. Consider buying an extra laser or two because the lasers can be easily damaged if you wire them incorrectly.