Illumination is the density of light projected onto a screen or other surface. Projector light output is measured in ANSI (American National Standards Institute) Lumens, or simply lumens. Higher the value, brighter the light output will be. In general, when choosing between projectors with similar specifications, the ones with a higher lumen rating will cost more.

General guidelines: Projectors of less than 800 lumens are good for small groups in conference rooms where the lights are off, or the room is dimly lit. Projectors in the 800 to 1500-lumens range are best for moderate sized conference rooms or classrooms. Lights should be lowered somewhat for best image quality, but ambient light can be high enough to allow people to see one another and interact.

The advantage of a bright projector is that it can be used with the room lights on so people can interact with one another and take notes.

Digital projectors are the modern replacement to more traditional transparency projectors (OHPs). Transparency projectors work by shining light through a semi-transparent material, such as slides or film. Digital projectors project a digital image directly from a computer or other digital devices through a lens on to a screen. They are of three types - DLP (Digital Light Processing), LCD (Liquid Crystal Display) and LCOS (Liquid Crystal On Silicon).

There is no easy answer to this question of what type of projection systems to purchase. LCDs tend to produce more highly saturated colors and sharper images; most DLP projectors offer deeper blacks and higher contrast. Depending on the resolution and size of your image, you may find LCD screens too pixilated, meaning that you notice each individual point of light. However if you tend to display very detailed images, you may find the softness of DLP projectors to be a problem. As a newer technology, DLP projectors were initially more expensive than similar LCD models, but that difference has largely disappeared. It is recommended that you compare different projection systems with the same brightness and resolution to see which type of projection is best for you.