These factors are determinating the quality of the image projected
Brightness and color display
Probably the most important thing about projectors is the quality of the image projected. But what makes a good, clear and crisp image? Of course brightness is an important factor. The more lumens the brighter the projection will be. But lumens are measuring a white light being projected on the screen and does not reflect the brightness of other colours being projected. So it is a standardized measure but it does not fully take into account the brightness of your content you will project. More lumens are important if you want to use the projector under daylight conditions. In a dark room you could have a good and bright projection even with less lumen. But brightness is not the only determinant factor. Maybe even more important is the colour display. In some cases increasing brightness leads to a reduced colour space which makes colours pale and unnatural looking. Good Projectors are using color wheels with good color filterr in order to see colours as they are meant to be in your video or picture.
Resolution and contrast
Another factor is the resolution. The higher resolution the more detailed the presentation of the image will be since the image is composed by more pixels and for that reason it seems sharper. The contrast also is an important measure indicating the difference between the brightest white light and the darkest black light. The higher the contrast the better and brighter the projection will be perceived. So it is important to have all these factors in mind – looking only at the lumens for example you may have a bright but poor image.