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 images is actually closer to what you're familiar with from cinemas. Even though manufacturers make their projectors in different ways, the basic concept of HDR remains the same.
A common doubt among 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 limitations 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 greater brightness and contrast, allowing the image to be shown more clearly, especially in terms of showing light and dark details and the color rendering of the image.
In theory, however, HDR standards are set for displays like TVs. The nature of projectors was not taken into count when developing HDR conventions. For example, the brightness defined for HDR is absolute brightness, but the brightness for projectors varies according to projection size, distance, environment, and screen material. So, owing to physical differences, the brightness of a TV set may be higher than that of a projector in general.
Therefore, projectors should have their unique calibration process and their own HDR definition, which should be different from that of TVs. 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 varies. Moreover, since the way projectors generate images is different, most projectors are not as bright as TV sets. Many manufacturers are now trying to create cinema projectors that meet HDR standards to present higher image quality.
There are two key points to showing an HDR image: high dynamic range and wide color gamut. High dynamic range 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 a wider color gamut than SDR, or standard dynamic range: 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, the 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 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 the projecting lens. Depending on what the scene needs, it can reduce the light that passes through, making the dark area of an image appear darker. When making a projector, manufacturers have to decide how the variation of brightness in HDR should be translated into the corresponding color gradation of an image based on the capabilities of the projector.
When presenting HDR on projectors, it is important to have better control over the brightness of darker details. For example, under the circumstances that the maximum brightness remains unchanged, we can use the Dynamic Iris to control light intensity and thus darken the darker areas. This will create more detailed light and dark gradations, more contrast, and therefore present an 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 vary 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 represent 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 150 nits into a 100 nit image, while still maintaining light and dark gradations? It takes the experience and expertise of 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, as each of them have to be HDR-compliant 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 of them to compare and contrast the image. Sometimes even professionals could miss it. When it comes to projectors, more factors can affect viewing quality, including the environment, the distance from the projector to the screen, the viewing angle, the materials of the screen, or even the 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 values will have greater maximum brightness while a screen with low gain values will make the image darker. However, it does not mean ‘the brighter, the better’ here. The brighter the projected image, the more dark details are omitted, failing to present the most original image.
The size of the screen also matters. Most screens on 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 looks brighter in the middle, while a screen with low gain shows better dark details. Considering common environments 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 depends 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 values to brighten the image for a better viewing experience.
So, compared to other devices, HDR projectors give a viewing experience closer to that of the cinema. The projected contents display colors that human eyes see, images that directors want to show, and give audiences a most comfortable and authentic viewing experience.