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Image brightness targets and calculating foot lamberts from projector lumens

This article outlines the image brightness standards, as measured in foot lamberts, for home theaters. It also explains how to calculate predicted foot lamberts for any given screen and projector combination.

Any display should have enough brightness for the viewing environment in which it is used. Brightness, or more correctly luminance, is a quantifiable measure of the amount of light from a display:

Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. Wikipedia

The US unit of luminance is the Foot-lambert or ft-L whereas the SI unit is Candela per Square Metre or cd/m2. 1 ft-L equals 3.426 cd/m2. The US unit is the one typically used in home theater design and calibration.

Luminance targets

Targets for luminance depend on the amount of ambient light in the viewing environment. The reason for this is that the level of ambient light affects the perceived dynamic range in the image by lowering the on/off contrast ratio. Higher luminance levels are needed to maintain good contrast performance in environments with ambient light.

In light controlled environments it is common practice to use standards derived from commercial theaters. The main target is detailed in SMPTE 196M which defines the luminance target as 16ftL (without a film in the projector, which typically equates to 14ftL with a film). The Digital Cinema System Specification has adopted this standard with their 14ftL +/-3ftL target.

For environments without ambient light control the important thing is to maintain a good contrast ratio. For movie watching a minimum contrast ratio of 1000:1 is recommended. Based on this Digital Projection, a manufacturer of projectors, recommends 40ftL for environments with some ambient light and 60ftL for high ambient light locations.

Calculating luminance

Luminance figures for flat panel displays are around 40-60ftL for Plasmas and 80-120ftL for LCDs. For this reason LCDs are often used in locations with high ambient brightness such as family rooms with lots of windows.

For projection systems we need to calculate luminance to ensure the combination of projector, screen material and screen size will provide enough light output to meet targets.

Luminance in FtL= (Illuminance in Lumens / Screen Area in Ft2) * Screen Gain

A big screen with unity or negative gain (like the wide viewing angle, acoustically transparent types we like to use) will need a high light output projector to meet brightness targets.

There are two factors which might increase the luminance target we aim for when designing a projection system for a home theater:

  • Subjective preference. Some people like an image that is brighter than commercial cinema targets.
  • Drop off in light output from bulb based projectors. Most projectors use bulbs as their light source which might lose 25-50% of their brightness over their lifespan. To counter this we might aim for an initial brightness of 28ftL and an end of life brightness of 14ftL. Many projectors have controls to allow the lamp power to be set which will allow the power to the lamp to be increased as it’s output drops.

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10 thoughts on “Image brightness targets and calculating foot lamberts from projector lumens”

  1. I intend to have my future living room (17’8”x17’2”)a double vocation beside being a living room be also a Home Theatre, intend to have JVC DLA X500 projector on a white screen(Stewart or Grandview) because I will control light with opaque drapes. Seating at 13’6” throw around 15′ screen 135” diagonal,(on 17’8” wall) I know it is exceeding a little THX norms. Will I have enough Foot Lamberts for excellent quality picture or it will be a lot better on a 120” screen? Any further comments are welcome, thank you……….

    1. Hi Marcel

      Thanks for your comment.

      Let’s review the main requirements….for fully light controlled (pitch black) HT we target 14 foot Lamberts (ftL). For a room with moderate ambient light we target 30-40ftL (which is what a Plasma TV puts out…LCDs typically put out 70-100ftL.

      You should also factor in loss of brightness over the lifespan of the bulb, so that even near the end of the lifespan you are still within targets. I normally recommend aiming for 33% more lumens than if you target 14ftL, so around 20ftL.

      Assuming full dark in the room using roller shades with blackout material and side channels to prevent light leakage to the side, and a bottom rail to prevent light leakage to the bottom… you’d need 700 lumens with a 1.3 gain 2.35:1 aspect ratio screen. That seems to be within the capabilities of the X500, though as you will note where you mount the projector does make some difference to the light output…closer to the screen provides more output as there are less losses in the lens.

      If you aimed for 40ftL (and allowing for the 33% loss over the bulb life) then you’d need 1,750 lumens with the same screen.

      Hope that helps!

      1. May I ask two questions about that luminance formula you used? First, when doing 2.35 projection by zooming the horizontal black bars off the visible screen, should the screen area in the formula refer only to the visible 2.35 picture or to the entire 1.78 output of the projector (which includes black bars)?

        Also, should the “illuminance in lumens” part of the formula be the manufacturer’s lumen specification, or should it be some lower number based on calibration settings, etc.? Calculators like the one used by Projector Central seem to apply a much lower lumen rating than the spec, but it’s unclear how they arrive at the number they do.

        I’m asking because I’d like to use a 130″ wide 2.35 screen with a DLA-X35, but I feel I may be pushing the limits if the actual lumens are much lower than spec (1300).

        1. Hi JR, thanks for reading.

          You should use the D65.REC709 calibrated lumens figure for the actual light output you will get when the projector is calibrated for HD viewing. UHD makes this more complex, as many PJs have less light output when asked to reproduce this wider color space due to losses from the filters that are inserted into the light path to allow the PJ to hit P3 color points. See our UHD 101 article for more education on UHD.

          Manufacturers do not publish calibrated lumens figures for their PJs. The best source I have found is http://www.projectorreviews.com.

          The calibrated lumens figure should be derated for the loss in illuminated pixels with any screen aspect ratio that does not match the native aspect ratio of the PJ (i.e. 2.35:1 screen, 16:9 projector) as part of the light output is lost to black bars.

          Assuming HD “widescreen” is 1920×810 then the loss is 23.7% [1920×810 = 1,555,200 and 1920×1080=2,037,600].

  2. Hello,
    I see this article is mostly applicable to projectors, but I’m wondering about LCD TVs. Are the different luminance recommendations based solely on ambient light, regardless of display type? For example, when watching ANY display in a totally dark room, is 16 ftL a better target?

    I became curious about this recently because I got a new 75″ LCD/LED, and I followed the usual recommendation of 30-40 ftL for peak white. However, when watching in darkness, I actually found this to be a little unpleasant.

    Thanks!

    1. Thanks for your comment. THX actually has different recommendations for different display types, and the luminance recommendations in this blog post are for projection systems. From discussions with THX I understand that these luminance recommendations in circulation take into account lateral viewing angles (i.e. how much of your field of vision is full with the display). Generally TVs will have much smaller lateral viewing angles than projectors, but with the new TVs such as 75″ or 85″ maybe we need to revisit these recommendations?

      1. Interesting, thanks. Someone else mentioned that the 30-40 ftL suggestion came out when the biggest TVs were only like around 30 inches. A lot of it probably boils down to preference. In total darkness, my TV seems ok even at 16 ftL, but below that it looks pretty dim.

  3. I am planning a home theater 18’x28′, 135” diagonal 16:9 screen a DLA750X projector, viewing distance 15-16 feet, projector throwing distance 16-17 feet, total light control room, will I have enough Feet Lambert to obtain very good image quality?

    Does a 4 seaters being in an angle (5 degrees) 2 seats together then a pie angle console then 2 others seats together, in that manner each seat have 2 armrest each, Is the angle fact aaaudio or video issues?

    Thank for a rapid answer…………

    1. Hi Marcel

      The DLA-X750R is the same as the RS500. You’ll get about 1500 D65.REC709 calibrated lumens from this projector at a mid-zoom position. You should be able to use this figure together with the gain for your screen to calculate on screen brightness.

      Nyal

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