may dalawang klaseng lumens:
first yung regular "lumen" the scientific term used in measuring light output.
second yung ANSI lumen, a spec defined by the American National Standards Institute, specifically for use in the light output for video applications. This is the spec used by the much more expensive Acer projector you mentioned.
So that Unic projector is actually really really dim. Hindi pwede sa classroom application. 1500 lumens is just as powerful as a 100 watt incandescent bulb, which is not powerful at all for projection purposes. mas maliwanag pa ang OHP. Even with the lights off and thick curtains down, talagang mahina yan.
On the other hand the Acer one is capable of doing a classroom session even during daytime, but it would be easily wash out. it can however manage a 40 student classroom during an overcast day, or with the help of light curtains. It would be great with smaller 20 student classrooms, and it's really sharp when the lights are out (we had one of those for our org).