LED light engine. Whereas lamp-based projectors produce white light and then split that light into primary colors through the use of either prisms or a spinning color wheel, the HD91 uses three colored sets of light emitting diodes (LEDs) to produce red, green, and blue light separately. This light is flashed onto a single DLP chip sequentially, approximating the action of a color wheel with RGB segments.
The primary advantages of an LED light engine are longevity, uniformity, and cost. LEDs are solid-state, unlike traditional lamps, and so they last much longer. Whereas a traditional arc lamp might last up to 5,000 hours, the HD91's LEDs are estimated to run for 20,000 hours.
Finally, with regard to cost: arc lamp replacements for home theater projectors can cost several hundred dollars each. With an LED projector, you never have to replace a lamp, so ongoing maintenance costs are lower if you put a lot of hours on your projector.
LED Brightness. The HD91 does not have an "eco-mode", but instead it has an LED Brightness control that allows LED light output to be adjusted between 100% and 50% at 5% intervals. This level of control is unmatched in any other home theater projector currently available.
So what's the benefit? Say you want to use the HD91 with a 100" 1.3 gain screen. Cinema mode at full power produces 517 lumens, which works out to 22.7 foot Lamberts -- slightly too high. Simply reduce LED Brightness to 70%, thereby dropping light output to 361 lumens and screen brightness to 15.9 foot Lamberts exactly. This kind of fine-grained control helps to pair the projector to your room and your screen more precisely than a typical eco-mode on a conventional lamp.
UltraDetail. Optoma's PureEngine system includes several features designed to improve the viewing experience, the first of which is UltraDetail. This enhances the appearance of fine detail by selectively sharpening certain parts of the image, similar to the smart sharpening systems on other home theater projectors. UltraDetail has two settings: "On" creates subtle enhancement, while "HD+" is a more exaggerated effect. We left UltraDetail set to "On" most of the time, only switching to "HD+" for selected content on a case-by-case basis. The system can be disabled if you don't like the effect.
PureMotion. This is the HD91's Frame interpolation system. By inserting interstitial frames in the video stream, it adds smoothness to motion and reduces judder in film. PureMotion has three settings. We used "Low" for most film content, though the digital video effect was sometimes visible, so some folks will prefer to leave the system off. "High" is best reserved for video and live-action content, where it completely eliminates judder.
DynamicBlack. Instead of a variable iris, the HD91 has DynamicBlack which changes the brightness of the LEDs in response to the content on screen, reducing light output in dark scenes and boosting it in bright ones. There are three available settings that differ in where they set the LED power floor. DB 1 varies power between 100% and 13%, DB 2 varies between 100% and 5%, and DB 3 varies between 100% and 0%. If you're not a fan of the system, you can also manually set LED power between 50% and 100%.
Color management. The HD91's color controls are second to none. In addition to the standard RGB gain and bias adjustments, the projector includes a full color management system that is designed for ease of use. If you have a color meter and some spare time, you will find it easy to bring the HD91 into line with the Rec. 709 color gamut and 6500K color temperature standards.
Anamorphic support. Anamorphic stretch mode allows you to use the HD91 with an external anamorphic lens for native display of 2.4:1 Cinemascope films. What's more, the projector has an internal setting for both mobile and fixed anamorphic lenses, making it easier to properly scale the image when you want to watch non-cinemascope content.
Dual 12V triggers. The projector also includes two configurable 12V triggers which can be used to trigger any number of external accessory devices. Examples include a motorized lens sled for an anamorphic lens, a motorized drop-down projector screen, or a masking system on a fixed-frame screen.
Full HD 3D. The HD91 is one of the first LED projectors to include full HD 3D compatibility. The projector is compatible with both DLP Link glasses and Optoma radio-frequency glasses, though the latter require an external emitter. Neither system showed significant ghosting or jitter, though as mentioned earlier light output can be a concern.