Why Is Brightness Measured Differently on Different Displays?

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Q Why are different units of brightness measurement used for different display technologies? For example, I’ve seen lumens used for home theater projectors, foot-lamberts for movie theaters, and nits for high dynamic range-capable TVs. Wouldn’t it be less confusing for the average consumer to lump everything together as a single measurement?
Nathan Robertson / via e-mail

A It would be less confusing for average consumers if there were a single light measurement unit that could be used to compare video displays. Unfortunately, light measurement is a complex topic, and one that can’t be lumped into a one-size-fits-all spec. Let’s break it down.

Brightness is a term that describes not light output, but the subjective impression of light. And since that impression can be altered by many factors — ambient room light, for example — it can’t be translated into a quantitative unit. Lumens, on the other hand, is the SI (International System of Units, aka the metric system) unit of measure for luminous flux, or the perceived power of light. Lumens measurements are weighted to account for the eye’s sensitivity to light, and they quantify the total power of light in all directions. While projector manufacturers typically cite lumens measurements for their products, a more common application of lumens ratings is for LED lamps and bulbs.

Foot-lamberts is a measurement unit that, as you’ve noted, is used for movie theaters and quantifies luminance reflected off of a projection screen. Sound & Vision typically cites foot-lamberts measurements in our reviews of projectors and flat-panel LCD and OLED displays. The SMPTE (Society of Motion Picture & Television Engineers) recommendation for movie theaters is 16 ft-L — and that’s with the projector measured open gate, with no film running to reduce the light output of the projector’s lamp. New laser-based digital projection systems used in theaters are capable of greater light output than the SMPTE recommended spec — up to 31 ft-L for 2D sources in the case of Dolby Cinema theaters.

Finally, there’s nits, which is equivalent to cd/m², the SI unit of measure for luminous intensity (1 nit is equal to 1 cd/m²). Similar to luminous flux, luminous intensity is weighted to account for the sensitivity of the human eye, but it differs in that the measurement is directional rather than omnidirectional. Why are nits used to quantify the light output of high dynamic range displays? A likely reason is that HDR specifications are based on the reference flat-panel display monitors that are used to master content, as opposed to projection systems that incorporate screens.

WildGuy's picture

i didn't understand completely but its interesting to know.

Thomas J. Norton's picture
I'd add here that for our purposes nits and foot-lamberts are the same thing. They're simply based on different standards. A foot-lambert (ft-L) is equivalent to an English measurement like an inch, while a nit is equivalent to a metric standard, like a centimeter. Nits (candelas per square meter, or cd/m2) can easily be converted to foot-lamberts (1 ft-L=3.43 nits, and numerous on-line converters are available). For clarity, I've personally been using both in my reviews where you might see something like "137 nits (40 ft-L)."

Eventually I'll likely drop the ft-L reference altogether, as the use of nits is rapidly becoming the standard—except in the commercial movie industry where ft-L is still in common usage. Even referring to the open gate standard for movie projectors is now anachronistic (but not the related 16 ft-L number, although few theaters have ever consistently met that standard). Few movie theaters today still use film projectors, and with digital projection "open-gate" is meaningless.

Lumens is a very different standard, as mentioned in the above response. The only place you'll likely find it in our universe is in video projector specs. Even there it's largely irrelevant, not because it's a useless number but because different projector manufacturers use different techniques to measure it. Comparing the lumens ratings of projectors from different manufacturers is therefore dubious at best. You might be able to rely on the lumens ratings of different projectors from the same manufacturer (though take them with a grain of salt). But between different manufacturers, forget it.