Projector Brightness, A Simple Formula
More than any other component, movie projectors present the greatest number of variables to comprehend and control. A good place to begin is understanding how bright a projected image appears to an audience.
Lumens (lm) are the standard unit for projector brightness, and Foot Lamberts (ft-L) expresses projector output spread across a movie screen’s surface, adjusted for Screen Reflexivity (Gain). The formula for Foot Lamberts is Projector Lumens divided by Image Area (in square feet) multiplied by Screen Gain.
How many Foot Lamberts (ft-L) are required to produce proper cinematic playback? The Society of Motion Picture & Television Engineers (SMPTE) says 12 ft-L to 22 ft-L, and the “academy standard” is 14 foot lamberts in a darkened theater. However, outdoor operators need to see brightness a little differently.
Outdoor movie events are almost always located in the vicinity of street/security lights, electric signage, passing vehicles, traffic signals or under bright moonlight. These ambient light sources reduce on-screen brightness, which is why recommended luminance for projectors used outdoors is bumped up to 16 ft-L.
Digital Projector Resolution
At typical seating distances, a commercial-quality projector with 1920×1080 (HD) resolution provides impressive detail. Increasing resolution to 3840×2160 (4K/UHD) on a gigantic movie screen does not guarantee even a minor improvement.
Here’s why: HD or UHD, whichever projector produces more shades of contrast and the greatest number of color hues will make objects look more real with greater depth and detail. In other words, more pixels are not always the answer.
Projector Contrast and Dynamic Range
Contrast is the range of subtle differences in luminance between the brightest white and the darkest black a projector can produce. It is perceived as shadow detail and object dimensionality.
Contrast ratio is the brightness of an all white image divided by the light reflected from an all black image. Bigger is better. A 5000:1 contrast ratio means its white output is 5000 times brighter than its black image. Ambient light reduces movie screen contrast in any real-world setting, an especially relevant issue in outdoor movie projection.
When purchasing a projector, knowing these measurement methods will help sort out contrast claims:
ANSI measures a static 16-block checkerboard pattern, 8 black blocks on screen simultaneously with 8 white blocks.
- ANSI ratios are consistently lower than other methods because adjacent black block readings are compromised by white-block illumination scattered in the image engine and lens
- Hardest to manipulate
- Typically hundreds to one (n00:1)
Full On/Off (FOFO) is the brightness of a full screen of white then, in a separate moment, a full screen of black
- Full-off black can be made blacker by cutting lamp output or closing an auto iris microseconds before restoring full-on white at full power
- Tiny reductions of black levels not perceived by most people can cut a contrast by a factor of ten
- FOFO ratios are typically thousands to one (n000:1)
Dynamic Contrast Ratio measures the ratio between brightest and darkest shades while a picture is changed over time
- As with FOFO, a higher ratio is possible by dimming lamp power or closing an auto iris between frames or scenes, not a bad thing when applied to overall dark or bright scenes
- For an outdoor movie projector, 20,000:1 is a minimum dynamic contrast ratio, and higher-lumen units report ratios up to 3,000,000:1
Manufacturers’ contrast claims can be terribly misleading when comparing projectors. Here’s why:
- ANSI contrast ratios are the most truthful, yet they are rarely reported. If disclosed, an ANSI rating of 300:1 would be average, 700:1 would be very good, and 1000:1 would be extraordinary.
- Projectors specifying both Full-On/Full-Off and Dynamic Contrast ratios can be compared, although same-frame specs may be vastly different due to the imaging engine and lens performance
- A few makers publish Dynamic, Full On/Off (without dynamic output changes) and ANSI contrast specs. Kudos for disclosure.
- Projectors with higher lumen output are capable of greater contrast ratios, logical since the brightest white and darkest black are what is tested
Contrast ratios are easy to manipulate and overstate. The brightest-darkest comparison does not accurately express the number of shades a projector will render in a static frame or sequentially. Viewing two projectors side by side is the ultimate test where image detail and object depth are strong cues.
Projector Light Sources
Lamps versus Lasers
Six characteristics differentiate lamps and solid-state lasers
- Lamp brightness is 3,000 to 30,000 lumens for units in production for outdoor use. Lasers can pump out up to 75,000 lumens.
- Laser light is scattered less, so colors appear brighter and more accurate. The image is sharper regardless of distance from screen. More uniform luminance and greater control of brightness levels is also perceived as increased contrast.
- Rated lifetime for lamps is 1,500 to 5,000 hours to reach 50% brightness in full-power mode, and up to 15,000 hours using Eco mode to slowly lower brightness and power by leaving the projector on for up to 30 minutes. Lasers last up to 20,000 hours before dimming to 50% of rated output.
- Lamps lose 20 to 25% of initial brightness in the first 500 to 750 hours of use, then drop gradually to 50% output at end of life. Laser projector output tends to decline at a steady rate.
- Lamp-based units currently cost less than a similarly-bright laser powered projector. However, total cost of ownership conclusively favors laser projectors above 6,000 lumens, especially when loss of brightness and image sharpness are taken into account.
Lamp based projectors are predicted to be approaching retirement, although the time horizon is uncertain.
Projector Imaging Technologies
Independent of the light source, moving images created by one of two video imaging technologies dominate big-screen outdoor movies.
Liquid Crystal Display (LCD)
In an LCD projector, light from a lamp/laser is split into red, green and blue by dichroic mirrors. These mirrors reflect specific wavelengths while allowing others to pass through, creating the three primary colors from which all colors can be created.
Each primary-color beam is directed to its own LCD imager panel, an array of cells (typically measuring 0.55 to 1-inch diagonally) controlled at the pixel level to allow more or less light to pass through – varying degrees of transparent, opaque, or translucent.
Each red/green/blue panel produces one digital image per video frame (although many imagers switch faster), and the light from all three is combined in a prism before the full-color image is projected through the main lens onto the movie screen.
Digital Light Processor (DLP)
This guide covers single-chip DLP projection, in line with the needs of the greatest number of outdoor movie operations. Three-chip DLP projectors are a major leap in cost and complexity, with entry-level brightness suited to movie screens 40’ and wider.
Light from the projector’s lamp/laser is passed through a rapidly spinning wheel of color filters with segments of red, green, and blue. Some systems add yellow, cyan or magenta, and even white (clear) segments to the wheel to improve color accuracy.
The color-filtered light beam is aimed at a chip covered by an array of microscopic mirrors corresponding to pixels. Each micro mirror is mounted on a pivoting stalk that allows it to alternate between two positions, one reflecting light through the projector’s lens and the other into a light-absorbing trap.
Individual pixels on a DLP’s Digital Micromirror Device (DMD) can cycle between “on” or “off” thousands of times during one frame of video, and the portion of time a mirror spends in the on position determines a pixel’s brightness. Because our eyes don’t react fast enough to see the flickering, the variable on/off duty cycle performs like a dimmer.
Similarly, light filtered by the spinning color wheel is directed to the DMD and onto the movie screen one color after another. This happens so fast that our brain perceives the sequence of single-color images blended together as a full-color image.
LCD/DLP Pros and Cons
LCD and DLP imaging chips can both project excellent blacks and contrast with enhancement techniques like dynamic iris, light source modulation, and by reducing the amount of stray light inside the light engine that would otherwise make its way to the screen.
Both have space between pixels which can produce a noticeable ‘screen door effect’ or SDE. With less space between pixels, DLP images are slightly smoother compared to LCD-based projectors with similar resolution.
A potential problem with DLP color wheels occurs when fast moving on-screen objects produce a perceived burst of red, green and blue known as a rainbow artifact. Newer single-chip DLP projectors have higher speed (up to 5x) six-segment color wheels, greatly reducing the likelihood that rainbow artifacts will be seen. Double data rate (DDR) DMD chips further eliminate artifacts by updating individual mirrors at a faster rate.
A final note: Side by side, DLPs tend to have slightly higher perceived contrast than LCD-based projectors.
Taking all factors into account, several manufacturers offer great value at a lower total cost of ownership for commercial-quality outdoor movie images 10′ to 40′ wide: Christie Digital, Optoma and Panasonic. The following projector models are currently recommended, sorted by brightness range:
3,500 to 4,500 Lumens
- Optoma EH335 (single chip DLP | metal halide lamp)
- Optoma EH412 (single chip DLP | metal halide lamp)
5,000 to 6,000 Lumens
- Optoma EH470 (single chip DLP | metal halide lamp)
- Optoma ZH506 (single chip DLP | laser diode)
- Panasonic PT-VMZ60U (3-LCD image | laser diode)