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Brightness requirements are one of the key considerations to account for when designing any projection system.
The terms projector brightness and image brightness are used throughout this paper. Here is how they are defined for the sake of this paper:
The term projector can often have limiting connotations. Classically, a projector is a system used only to display a video or image on a wall, and while this is still the case, projection systems can be utilized to display any form of visuals or infographics on virtually any display surface. These can be used for smart home displays, digital signage, laser TVs, and many other potential applications. DLP technology enables projection systems of many sizes for use in countless applications, in addition to supporting bright, vivid displays for classically defined projectors.
The two of the most frequently asked questions from customers who are new to TI DLP technology are: “How much brightness is needed for my application,” and “how bright can the projection system get?” In other words:
The first question is often asked by product developers for whom image quality is the highest priority, whereas the second question is often asked by product developers who have certain technical constraints, most frequently power, size, and cost, which will ultimately constrain the maximum brightness of the projection system.
Answering the two most frequently asked questions proposed in Section 1 depends on the following variables:
Ultimately, necessary brightness is a subjective assessment. This paper will focus specifically on how a projection system’s brightness requirement is impacted by ambient light level and screen size.
Ambient light levels vary significantly across different use environments. Consider for example the ambient light level of a movie theater compared to that of a typical office environment. Because the ambient light level in a movie theater is both well controlled and very low, the cinema industry standard for image brightness is relatively low (~50 nits).
Based upon empirical testing by TI, the estimated minimum image brightness for different ambient light environments is summarized in Table 3-1.
Ambient Lighting Environment | |||||
---|---|---|---|---|---|
Dark Room | Dim Room | Lit Room | Bright Room | Outdoors | |
Example environment | A room at night with all lights turned off | A room with soft lighting at night | A well-lit office conference room with no daylight (Home Theater) | Well-lit room with windows and indirect daylight (TV) | Indirect sunlight |
Suggested image brightness | 50 nits | 100 nits | 200 nits | 300-400 nits | 600+ nits |
Figure 4-1 provides suggested projection brightness (lumens) as a function of diagonal image size and image brightness (nits) levels from Table 3-1. Additionally, Figure 4-1 references the DLP chip class (categorized by the diagonal of the DLP chip micromirror array, called a digital micromirror device or DMD, measured in inches) associated with each brightness level. You can learn more about the DLP product portfolio here.
These calculations(4) assume a projection surface with a reflectivity of 80%. When projecting onto a non-ideal surface, the actual lumens desired may be different than those shown above. A typical white wall has a reflectivity of 80% but colored paints can reduce this number. High gain screens are designed to increase reflection in a particular viewing direction and can be used to increase image brightness without increasing projection brightness.
How bright can a DLP projection system get? The short answer is very bright. One of the core advantages of DLP technology is its high optical efficiency, enabling bright projection systems with low power consumption and compact size. If limitations on size and power are not a factor, you can create displays with over 10,000 lumens like the ones used at major sporting events during player introductions or halftime shows.
However, for this paper, the scope of the question will be limited to "What is the maximum brightness of a DLP projection solution for my size and power requirements?" The answer is “it depends” – there are tradeoffs that can be made at a system design level which depend upon the product design priorities. To make a brighter projection solution, it is necessary to also increase one of (or a combination of) these variables:
Variable | Contribution | Limitation | ||
---|---|---|---|---|
Illumination source output capability | Amount of light that can be generated | Source and DMD etendue(5) | ||
Optics size | Amount of light that can be collected | Size and cost | ||
DMD chip size | Amount of light that can be reflected | Size and cost | ||
Illumination source drive power | Illumination source brightness level | Thermal limit and cost of power design | ||
Illumination thermal solution | Amount of heat dissipated from the illumination source | Size and cost |
For a production-ready optical engine, the first three variables in Table 5-1 will be constant.
The last two variables in Table 5-1 (illumination source drive power and the illumination thermal solution) can vary depending on the design requirements of the final product. For example, a given optical module can achieve different brightness outputs depending on the current supplied to the LEDs or lasers. Lamps have a fixed power input and output, but can often be switched for higher power bulbs.
DLP technology is flexible and can be used with any illumination source. There are three main types of illumination typically used in DLP projection systems: lamp, LED, and laser. Lamp sources offer a cost effective solution; LED and laser sources offer high efficiency, solid state illumination. Lamp illumination is often used in classroom, conference room, and home theater projectors. LED illumination is often found in small, battery powered projection systems. Laser illumination is used to reduce size and increase brightness of products ranging from portable displays to high brightness, large venue projections.
For LEDs and lasers, increasing the input power to the illumination source will cause them to output more light, but also causes more heat to be generated by the illumination system. This necessitates a more efficient thermal system to keep the illumination sources at their recommended operating temperatures. Therefore, as brightness increases, so does the illumination device temperature, which in turn drives an increase in the complexity of the illumination thermal system. It is worth noting that higher illumination source output will also result in a higher heat load on the DMD. Therefore, at some point, the size of the DMD thermal solution may need to be increased as well.
Additionally, it is important to keep in mind that as the power to the illumination source increases, so does brightness, but at a decreasing rate, resulting in increased heat generation. Although illumination power and brightness varies for different illumination types, all illumination sources will have an optimal brightness range in which they can operate with maximum efficiency. The thermal management solution must be carefully designed to remove the necessary heat from the system while minimizing the impact to the product’s size and cost.