As HDR technology has become more popular, more TV sets, monitors, and projectors have the HDR badge on them. That doesn't mean that they all use the same definition of HDR though. Compared to TVs, the way projectors generate the image is actually closer to those in the cinema. Even though manufacturers make their projectors in different ways, the basic concept of HDR remains the same.
A common uncertainty among consumers when facing so many home projectors in the market is if they can really be qualified as HDR devices. In this article, we will explain the characteristics of a projector. Taking the technology and limitation into account, we will then explore whether a projector can really be HDR. We'll end with how we define HDR for projectors.
HDR (High Dynamic Range) is a standard for displays that can show images with higher dynamic range, enabling the image to be clearer, with emphasis on the light and dark details and color rendering of the image.
HDR standards were designed for displays like TV sets; the nature of projectors were not taken into count. For example, the brightness defined for HDR is absolute brightness, but the brightness for a projector varies according to projection size, distance, environment, and screen material. So, owing to the physical difference, the brightness of a TV set may be higher than a projector in general.
Therefore, projectors should have their own unique calibration process and their own HDR definition, which should be different from LCD's. The brightness of projectors depends on many variables like the distance between the projector and the screen or what the screen is made of or even the lighting engine inside the projector. Plus, TVs light the image in a completely different way. (think staring at a light bulb vs staring at the wall the light is illuminating.) Many manufacturers are now trying to create cinema projectors that meet the HDR standard to present a higher image quality.
HDR images are composed of two parts: High Dynamic Range and wide color gamut. High dynamic range just means brighter brights and darker darks therefore showing more subtle details in between. HDR standards also have wider gamut than SDR (Standard Dynamic Range) standards: they can present colors that exist in DCI-P3, or even Rec.2020, closer to the colors your eye can perceive in the world outside of screens.
For projectors, no matter if they come with SDR or HDR technology, their methods used to present light and dark details will affect the black and white colors of the image they project. In recent years, with the development of HDR technology, projectors are getting better at showing light and dark details similar to HDR images you see on a flat panel.
Dynamic Iris is a technology that allows projectors to control how much light gets through the projecting lens. Depending on what the scene needs, it can reduce the light that passes through, making dark areas of an image appear darker. When making a projector, manufacturers have to decide how the 0-1000nits variation of brightness in HDR should be translated into the corresponding color gradation of image based on the capabilities of the projector.
When presenting HDR with projectors, the Dynamic Iris allows us to have better control over the brightness on the darker details. For example, under the circumstances that the maximum brightness remains unchanged, we can control the light intensity, thus darken the darker areas. This will create more detailed light and dark gradations, more contrast, and present an HDR-like image.
When compared to TV sets or monitors, it's more challenging for projectors because the light and dark areas that the audience see varies with the size of the screen. There is a lot of adaptation and translating technology from flat panels when manufacturing HDR projectors. For example, when calibrating colors and EOTF curves (EOTF curves representing how human eyes react to brightness changes), the curve for HDR TVs can be sharper while the curve for projectors needs to be smoother or utilizing the Dynamic Iris to increase contrast.
Before defining how HDR projectors present HDR content, we needed to ensure precise tone mapping. For example, how do you transform an image that is 150nits into a 100nits image, while still maintaining the light and dark gradations? It takes the experience and expertise of a color engineer to find the best performance within the capabilities of the projector to keep all the details and tonal changes of the image.
A few key elements are needed to present a true HDR image: the display, the player, and the content need to be HDR in order to enjoy a true HDR experience. Most consumers are not able to identify HDR content without having at least two monitors in front of them to compare. Sometimes even professionals miss it. There are a lot of factors that can affect the viewing quality of a projector including the environment, the distance from the projector to the screen, the viewing angle, the materials of the screen, or even personal preferences of the audience.
The closer the projector is to the screen, the brighter the image, and vice versa. The Screen Gain value will make a difference, too. Gain value is an index measuring how reflective a screen is. A screen with high gain value will have brighter maximum brightness, while a screen with low gain value will make the image darker. However, it does not mean ‘the brighter, the better’ here. The brighter the image is projected, the more dark details are omitted, failing to present the most original image.
The size of the screen also matters. Most of the projector screens on the market are somewhere between 100 to 120 inches. The gain value usually falls between 1.0 and 2.0, sometimes higher. Usually, a screen with high gain value looks brighter in the middle, while a screen with low gain value shows better dark details, making black blacker. Considering the common environment at home, most home projector screens have a lower gain value of around 1.0 for a wider viewing angle and better dark detail performance.
Even with all these factors mentioned, the quality still depends heavily on the actual viewing environment. For example, when watching 3D movies, since the 3D image is usually darker, some consumers would choose a screen with higher gain value to brighten the image for a better viewing experience.
So, compared to other devices, HDR projectors give a viewing experience closer to your local theater. The projected contents display colors that human eyes see, images that directors want to show, giving the audience a comfortable and authentic viewing experience.