Picture quality details
The 640 was easily able to fill my big 120-inch reference screen with a bright image that held up well in the dark, and even under dim lighting. Light output is actually a bit higher than the more expensive Optoma HD142x, although that unit exhibited better black levels for more overall contrast and pop, important in a dark room.
The Epson's color was decent in the best picture mode (Cinema), but fell short of some higher-end units in terms of measurements. An exceedingly bright green was the biggest color issue, but nothing else was too terrible, and watching real program material (as opposed to test patterns) I had no major complaints.
I can complain about that resolution, however. The image looked softer overall than any of the 1080p units, which was especially visible the larger the image got. Lines, in particular diagonals, showed jagged edges that crawled with movement, and the edges of test showed visible blocking. And of course the pixel grid itself was visible, especially in bright scenes, creating the impression of a screen door over the picture. (That's why they call it the "screen door effect.")
None of those issues would be a deal-breaker to less-discerning viewers, however, who would likely focus instead on the massive image. Especially if they sit far enough away or don't have perfect vision.
To arrive at all of results below I measured the Epson 640's best default picture setting, Cinema with the Auto Iris disabled (I did not perform any calibration). The exceptions are peak white luminance and derived lumens, which were measured in Dynamic mode (thanks to Chris Heinonen for the lumens calculator). All observations and measurements were taken on my reference Stewart StudioTek 130 screen.
|Black luminance (0%)||0.134||Poor|
|Peak white luminance (100%)||67||Good|
|Avg. gamma (10-100%)||1.99||Poor|
|Avg. grayscale error (10-100%)||3.744||Average|
|Dark gray error (20%)||3.966||Average|
|Bright gray error (70%)||3.315||Average|
|Avg. color error||5.651||Average|
|Percent gamut (Rec 709)||91.14||Average|
|1080p/24 Cadence (IAL)||Fail||Poor|
|Motion resolution (max)||300||Poor|
|Motion resolution (dejudder off)||N/A||N/A|
|Input lag (Game mode)||51||Average|