1800MP projector very dim

We have an 1800MP projector that is very dim when used with an Elmo or laptop.  The laptop's desktop when projected onto the screen appears very dark and the teacher must turn the lights off in order for the students to see what is being projected clearly.  We have put a new bulb in the unit but this did not help.  Is it possible for the lenses to become dirty over time?  I do see some dust accumulation in the focusing lenses.  Is there anywhere else along the light pathway that can become dirty enough to darken the projected image?  The projector is not set to "Econo mode" or anything like that.  We've simply run out of ideas on what to do with this projector.

Thanks in advance.