Al Grif­fin

Ask S&V: Light Bright

Sound & Vision - - CONTENTS -

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?

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