Maximizing Environmental Dynamic Range
 Multiple screens illustrate the need for careful environmental dynamic range calibrations. |
WHEN EVALUATING PROJECTION products, emphasis is often placed on key specifications, such as brightness (lumens) and contrast (black level). These specifications present performance parameters that relate to the dynamic range the projector is capable of producing in a “black room” environment. Although vitally important, these specs are but a component of the equation necessary to define the actual dynamic range the system will produce when installed in a real-world venue.
Determining environmental dynamic range can't be done without a clear understanding of the application and the customer's objectives. Factors such as the viewing environment, ambient light, screen size, type and gain, even source material, all work in unison to create what our eyes finally see — the dynamic range of the image that is actually being produced in the environment.
Defining Terms
Let's first begin with some definitions. The term contrast is used to define the dynamic range of an image from white to black. In simple terms, the commonly referenced on-to-off contrast ratio is calculated by dividing the maximum light output measured on a full field of white by the minimum light output measured on a full field of black. Various metrics of light measurement can be used to make this calculation, including foot-lamberts (Ft.L), lux, and lumens. For purposes of our discussion here, we will reference foot-lamberts, a metric that defines the amount of light actually reflected by a surface.
Lumens are a metric that describe the brightness power of a light-emitting source. A lumen spec does not define how much light is actually reflecting off of a screen surface. Thus, a lumen spec alone does not tell us much about the brightness we will actually view in the integrated application. For example, 1000 lumens projected onto a 6'×8' screen will look far brighter than 1000 lumens projected onto a 9'×12' screen. Therefore, it's crucial that we use a metric that describes what our eyes actually see — the light emitted by the screen.
Ambient light is commonly acknowledged as all light in the projection venue, other than the light emitted by the projector. Windows, spotlights, can lights, camera lights, and light-reflecting surfaces in the venue all serve as good examples of sources of ambient light. For purposes of understanding onscreen dynamic range, we will further define ambient light in terms of light reflected by the projection screen other than the light emitted by the projector. It is the screen-reflected ambient light that compromises image black level and environmental dynamic range.
It should also be noted that sources of ambient light not reflected by the screen might also impact the viewer's perception of the projected imagery. For example, spotlights or windows positioned behind the screen that are in the audience's field of vision act as a disturbing bright point reference. If left unmanaged, they will reduce the perceived dynamic range and visual impact of the projected image.
Figuring Things Out
Although the quality of all projected content improves as the environmental contrast ratio increases, certain source material demands contrast ratios of specific levels in order to assure the integrity of the information being displayed. For example, the contrast requirements of a PowerPoint slide viewed in a conference room setting will certainly be less rigorous than the contrast performance necessary for the proper viewing of video, HD, and/or film-originated content in a screening-room environment.
As an application example, viewing material in a theatrically dark environment, such as a screening room, requires enough projector lumens to produce 12Ft.L to 16Ft.L onscreen. Calculating the number of projector lumens required to achieve 16Ft.L on a screen is a fairly straightforward process. Simply multiply the total square footage of the screen by 16 (Ft.L) and divide by the screen gain. (But be sure to calculate the square footage of the screen based on the native aspect ratio of the projector, keeping screen width constant.)
So, if we have an XGA projector (4×3 native aspect ratio) projecting on a 16'×12' front screen (also 4×3 aspect ratio) with a unity gain (1), and we apply this formula, we calculate the need to project 3072 lumens in order to reflect 16Ft.L off that screen surface. Here's the math: 16 (target Ft.L) × (16'×12' screen size) × 1 (screen gain) = 3,072 (required lumens).
Thus, if we install our 16'×12' screen in a theatrically dark venue, a projector producing 3072 lumens and 1,000:1 on-to-off contrast will produce 16Ft.L on that screen. The black level would be just .016Ft.L (16 divided by 1,000). Given the theatrically dark venue, the resulting imagery will be pleasing to view. The reason is that in such a controlled environment, the deep black levels produced by the projector are preserved on the screen, resulting in the viewer's perception of exceptional image dynamic range.
If we now introduce ambient lighting to the viewing environment such that .5-Ft.L of ambient light is reflected by the screen, our environment that once produced 1,000:1 contrast in the dark theater would suddenly produce a contrast ratio of just over 31:1 [16Ft.L / (.5Ft.L + .016Ft.L)]. The onscreen black level, which our eyes reference as the baseline to total dynamic range, becomes elevated and the image appears washed out.
If the source of the ambient light can't be controlled or removed — barring the use of a high-gain screen that enhances black level, or converting the application to rear projection — the only way to increase the dynamic range of the onscreen image is to increase the peak brightness of the image. That can be achieved by either making the image smaller or by cranking up the lumens. Utilizing a projector with a higher contrast ratio will not make a significant difference toward improving the dynamic range of an image in an elevated ambient light environment.
So how many more lumens do we need? Again, it depends on the material being viewed, as well as the viewing environment. In the example above, doubling the projector lumens to 6,142 would achieve 32Ft.L onscreen. Assuming the projector's contrast ratio is still 1,000:1, the projector's black level would also increase to .032Ft.L. Add that to the .5-Ft.L of ambient light being reflected by the screen, and we now calculate a total system/environment contrast ratio of just over 60:1 (32Ft.L /.532Ft.L). Not nearly enough for film-originated content as it was meant to be screened, but more than enough for a PowerPoint presentation, and possibly for non-critical video.
Moral of the Story
The point is this: High-contrast projectors produce great dynamic range in applications with extreme control over ambient lighting. If your application benefits from such an environment, and your source material demands exceptional dynamic range, the projector's contrast specification is vitally important and will have a tremendous impact in creating killer imagery.
In applications where ambient light striking the screen is unavoidable, increased projector lumens and/or screen gain will punch through the ambient light being reflected by the screen in order to create an environmental dynamic range that our eyes perceive as good contrast. If ambient light is a factor in your application, projector lumens become the primary performance factor to maximize onscreen dynamic range.
Remember, all manufacturers specify projector brightness based on the performance of a new lamp. As projection lamps age, light output falls, generally following a typical lumen maintenance curve that lands at 50% of original light output at the end of lamp life. Thus, when specifying the projector, select a unit with lumen performance that is 30% to 40% greater than what you actually need. The unit may be a bit brighter than necessary when the lamp is young, (many projection systems offer a lamp-low capability that allows the user to reduce lamp output), but as the lamp ages and lumens disappear, you will attain the target light output you defined as necessary to cut through ambient light and produce good environmental dynamic range.
Above all, it's important to remember it's not the specifications of any one product that assure projected imagery will be pleasing to view. The key is first understanding the application requirements and environmental influences, such as ambient light, that will impact the onscreen image. Armed with that information, we can easily define the system performance specifications we need to achieve to produce imagery that demonstrates exceptional environmental dynamic range. Once those parameters are defined, they serve to guide the selection of every system component.
Mike Levi is president of Digital Projection's North American operations in Kennesaw, Ga., and is a veteran of the premium large-screen display market.