As HDR technology has become very widespread, more TV sets, monitors, and projectors have the HDR badge on them. However, the truth is that the definition of HDR might vary with different display devices. Compared to TVs, the way projectors generate 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 doubt among the consumers when facing more and more home projectors in the market is if the latter can really be qualified as HDR devices. Therefore, we will start with the characteristics of the projector. Taking the technology and limitation into account, we will explore whether a projector can really be HDR. Finally, we will redefine the meaning of HDR for projectors.
HDR (High Dynamic Range) is a standard for displays that have the capability to show images with higher dynamic range, allowing the image to be shown more clearly, especially in terms of showing the light and dark details and color rendering of the image.
In theory, however, HDR standards are set 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 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 unique calibration process and their own HDR definition, which should be different from those of TV sets. As mentioned, unlike TV sets, depending on the distance between the projector and the screen and the material of the screen, the brightness of projectors vary. Moreover, since the way projectors generate image is different, most projectors are not as bright as TV sets. Many manufacturers are now trying to create cinema projectors that meet the HDR standards to present higher image quality.
There are two key points to show an HDR image: high dynamic range and wide color gamut. HDR has higher dynamic range, which allows the image to have more contrast between its light and dark areas, therefore showing more subtle details. The other important factor we should know is that HDR standards have wider gamut than SDR standards: they can present colors that exist in DCI-P3, or even Rec.2020, closer to the colors that human eyes can see.
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 thanks to the development of high dynamic range and wide color gamut.
Dynamic Iris is a technology that allows projectors to control how much light gets through its projecting lens. Depending on what the scene needs, it can reduce the light that passes through, making dark area 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, it is important to have better control over the brightness on the darker details. For example, under the circumstances that the maximum brightness remains unchanged, we can use the Dynamic Iris to control the light intensity, thus darken the darker areas. This will create more detailed light and dark gradations, more contrast, and therefore present HDR-like image.
Compared to TV sets or monitors, what is more challenging for projectors is that the light and dark areas that the audience sees varies with the size of the screen. Therefore, manufacturers have to go through some extra processes 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 more sharp while the curve for projectors should be smoother. This explains how different devices with different natures of generating images can still present complete HDR details.
Before defining how HDR projectors present HDR contents, first ensure precise tone mapping. For example, how to transform an image that has 150nits into a 100nits image, while still maintaining the light and dark gradations? It takes the experience and expertise of the color engineers 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, each of them have to be HDR in order to enjoy a true HDR experience. Most consumers are not able to identify if they are watching HDR content without having at least two monitors in front to compare and contrast the image. Sometimes even professionals could miss it. When it comes to projectors, more factors can affect the viewing quality, 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 screens in the market are between 100 inches and 120 inches. The gain value usually falls between 1.0 to 2.0, sometimes even 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.
Still, all depend on 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 gives a viewing experience closer to that in the cinema. The projected contents display colors that human eyes see, images that directors want to show, giving the audience a most comfortable and authentic viewing experience.