Light output. The HD91 is rated at 1,000 ANSI lumens. After a 15-minute warm up, the projector's internal white test pattern gave a reading of 880 lumens, which was the maximum sustained output we obtained in our testing. Unlike lamp-based projectors, LED-based projectors reach their maximum brightness immediately upon startup. And whereas lamp-based models "warm up" to their full brightness over a few minutes, LED-based projectors like the HD91 actually lose brightness over the first few minutes until they stabilize.
The brightest usable image mode was the aptly named Bright mode, which measured 840 lumens. Bright mode emphasizes green, reduces contrast, and is locked to the DynamicBlack 3 power setting. It is suitable for max-brightness applications like watching sports in a room with some ambient light, though its low output means that ambient light should still be curtailed as much as possible to avoid washing out the image.
Cinema mode is the preferred operating mode for home theater use. At 517 lumens, Cinema mode offers much better color and contrast than Bright mode while still maintaining most of its light output. Photo mode, at 513 lumens, is similar to Cinema mode, though it contains small changes to gamma and color saturation that enhance the visibility of mid-tone detail. The remaining modes are Reference (408 lumens) and Film (386 lumens), both of which attempt to provide a more accurate image than Cinema mode but sacrifice light output to obtain it. Overall, we felt that the trade-off was not worth the loss in brightness, so we stuck with Cinema mode for most viewing.
The biggest brightness concern on the HD91 is the 1.9:1 zoom lens. At the telephoto end of the zoom, the lens reduces light output by 46%, bringing Cinema mode from 517 lumens to 278 lumens. Be aware of this reduction before mounting your projector.
Contrast. Out of the box, the projector's black level is a touch higher than some other home theater projectors, but some fine-tuning of the projector's gamma controls can improve black level significantly. Black level after calibration is comparable to many other home theater projectors. Contrast in any given scene, on the other hand, is particularly good. The picture showed above average pop, with great background separation and three-dimensionality.
Care should be taken when setting the projector's gamma controls, as the projector has excellent shadow detail once calibrated but doesn't look particularly impressive out of the box. Our preferred setting uses the 2.4 preset with a -3 offset. On our test unit, this gives depth to shadows without crushing detail and vastly improves the projector's contrast and shadow detail.
While DynamicBlack is primarily a brightness control, it also improves black level in dark scenes. While the choice of whether or not to use DynamicBlack is ultimately yours, we do offer this word of advice. If you plan to use DB, calibrate your projector with DB enabled. The DynamicBlack control alters the power levels of the LEDs differently, so a perfect calibration with DB turned off will undergo significant color shift if DB is then enabled later.
Color. The HD91's color controls are intuitive and comprehensive, so it is easy to calibrate the projector if you have the knowledge and equipment to do so. If you don't, you can hire a local AV professional to fine-tune your projector.
Sharpness and clarity. The HD91 has a crystal-clear HD image with plenty of detail, and that's without using the projector's UltraDetail sharpening system. With that system engaged, fine detail jumps out from both standard-def and HD content. The effect can actually be a little overwhelming if you're using a source that is already high in detail, such as The Dark Knight or Baraka on Blu-ray.
Input lag. After extensive testing, the HD91's lag performance the same in every image mode provided PureMotion is disabled. With PureMotion off, the HD91 measures 76 milliseconds of input lag, or about four and a half frames on a 60 fps signal. PureMotion increases lag to 126 milliseconds (7.5 frames).