Ask S&V: Light Bright
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
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.
I have searched extensively for an HDMI splitter that will let me connect a high dynamic range source to both my new Dolby Vision–capable Vizio TV and my legacy Anthem receiver (for lossless audio).
Oppo’s UDP-203 Ultra HD Blu-ray player has dual HDMI outputs and supports Dolby Vision. It also has an HDMI input that would enable passthrough of another Hdr-capable source like a Chromecast Ultra, Roku Ultra, Fire TV, or Apple TV 4K. Here’s my question: Is there a less expensive option than the Oppo that would deliver the same result?
Andy Lutz / Houston, TX
The Hdfury Vertex ($349) is the only HDMI splitter I know of that claims to pass through Ultra HD and HDR formats (including Dolby Vision) to a TV while simultaneously passing lossless audio to a receiver with HDMI 1.4 inputs (the HDMI version your receiver presumably has). While I haven’t personally tested one, a Sound & Vision reader recently reached out to report success using the Vertex in a similar configuration to yours.
Another option is LG’S UP-970, an Ultra HD Blu-ray player that costs less than $200 and is expected to provide Dolby Vision support pending a longawaited firmware update. The UP-970 also provides dual HDMI outputs, which would let you make separate connections to both your TV and receiver.
If you need to hook up an additional high dynamic range source to the player, Oppo’s UDP-203 ($549) and UDP-205 are the only Ultra HD Blu-ray players that feature an Hdr-capable HDMI input. (Another model, the Cambridge Audio CXUHD, has an HDMI input, but that connection doesn’t support HDR. See our review in this issue.) The HDMI input on both
Oppo players only supports passthrough of Hdr10-format signals and not
Dolby Vision, however, so you should take that limitation into account before buying.
Why are different units of brightness used for different display technologies?