4K TV is equivelent to Bay Bridge Lights. I can prove it.

My recent post on the new Bay Bridge Lights had several nice comments.   The lights are a new enhancement to our landmark San Francisco Bay Bridge, and have been it's been very entertaining to watch the modulated light patterns each evening.  One colleague on Linked In said (presumably tongue in cheek) "who in their right mind would want 4K in their living room when they can have 0.025K out their front window and get it commercial free!"

That got me thinking, and it turns out, it's a good point.   By some measures, the bridge and your new Ultra HD television provide the same visual performance!

Coming soon to your living room:  Ultra high definition TV,
with four times the resolution of 1080p.

In a previous post, I babbled on about the inevitability of 4K television, more accurately termed Ultra High Definition, with the "UHDTV" terminology likely driven by lawyers rather than engineers.  This new standard was prolific at the CES show in January, and several models will become available for purchase over the next few months, with prices likely to start below $4,000 for a 55-inch model.  UHDTV has four times the spatial resolution of the 1080p high definition that you are now used to.  Can you really tell the difference?  The answer is absolutely yes -- but only with proper content and viewing conditions.

Human vision has a remarkable ability to resolve detail.  The fovea of the human eye (the part of the retina that captures details in the center of your field of view) contains about 140,000 sensor cells per square millimeter.  This means that if two objects are projected with a separation distance of more than 4 microns on the fovea, a human with a normal visual acuity (20/20) can resolve them.  On the object side, this corresponds to 0.2mm at a distance of 1 meter, equaling one minute of arc.   This is the reason that the Snellen eye chart has special font characters, called optotypes, made from lines that are exactly thick enough to subtend an angle of one arc minute when viewed from 20-feet away.  That's where the term 20/20 comes from.  The optotype font is five line-widths high (notice the funny looking E at the top?).   As with all things, humans are a diverse lot, with many variables, so this is just a ballpark characterization.  Actual results may vary.

The Snellen eye chart Optotypes (line 8) subtend an
angle of five arc minutes vertically, when viewed from 20 feet

Based on this understanding of human vision, the International Telecommunications Union sector on Radiocommunication (ITU-R) recently issued a report on the present state of Ultra High Definition TV.  It concludes that the optimal viewing distance for UHDTV (3,840 x 2,160 resolution) is 1.5 times the screen height.   A 55-inch television, measured as diagonal screen size, has a picture that is about 27inchs, or 68.5cm high.  One and a half times this height means the optimum viewing distance is just a tiny bit over 1 meter.  Further away, a viewer with 20/20 vision would not benefit from such high detail, so why bother with the nearly 8 million pixel display?   Closer, and an astute viewer would start to see individual pixels, breaking the magic spell of Downton Abby's storyline with distracting dots.

Now, lets turn our attention to the Bay Bridge Lights, which use 250,000 LED lights grouped in 25,000 nodes, which we will consider pixels.  After some digging, I was able to determine the pixel spacing is 12-inches vertically.  But this is not a normal display -- the horizontal dot pitch is 30 feet, since the lights are affixed to the 306 northern support cable bands securing the bridge deck (roadway) to the huge suspension cables spanning over the towers.  In TV terms, this would imply a pixel width ratio of 30:1, far from the 1:1 "square pixel" that we are accustomed to.   As computer graphics wizard Alvy Ray Smith so eloquently puts it: "A pixel is not a little square".   This proves him right -- in this case, a pixel is really a huge rectangle!   (Yes, I know this is exactly the opposite of Alvy's point, so you don't need to remind me.  I just couldn't resist...)


The Bay Bridge has 612 cable bands supporting the deck from the suspension cable,
each with for wire ropes, for a total of over 42 miles of rope used in the 9,260-foot western span.

So, based on normal human visual acuity of 20/20, what is the optimal viewing distance of the bridge lights -- with a focus on the vertical resolution of pixels 12-inches apart?   The answer is one kilometer (actually, 0.9873 km, but who's counting).  That is to say, the vertical resolution perceived by a viewer 1 meter away from a 55-inch UHDTV (4k) television is the same as the vertical resolution perceived by a viewer 1 kilometer away from the bridge.  

Next, I had to find such a viewing location to check it out.  The Bay Lights Web site includes a map with recommend viewing locations.  Among the spot most used for photographs is Pier 7, just North of the Ferry Building.  Google maps teaches us that the end of Pier 7 is one kilometer from the second tower (W2 pier) of the bridge.   

It was a clear, cool evening here in San Francisco, so I paid a visit to Pier 7 this evening.  When viewing the bridge lights from that location, the vertical rows appear almost continuous, without visible pixelation.  This seems to reinforce the theory in calculating the optimal viewing distance.   Viewing the bridge from a closer location, such as Pier 1, could allow viewers to see individual lights more prominently.  Viewing from further away, perhaps from Coit tower or Pier 39, would look great, but not benefit from all 25,000 LED nodes -- the same effect would be achieve by fewer lights (and lower electric bills). 

Bay bridge lights viewed from Pier 7 achieve the limit of 20/20 visual acuity
(Photo: James Tensuan, The Chronicle)     

However, I'll admit it's not fully definitive.  First, when viewed from that distance, the lights appear to shimmer from atmospheric refraction.  Also, the pixel fill factor on the bridge is very low:  on a typical LCD panel, there is a very small space between the light emitting LCD cells, so that roughly 90% of the planar area of the panel is emitting light.  By contrast, the bridge has 1.25-inch LED nodes spaced on 12-inch centers, resulting in a very low pixel fill factor of 10%.  This probably contributes to imperfect viewing -- so it's not fully comparable to a $25,000 Ultra HD television.

Please consider this as you shop for your next television at Best Buy.   If you plan to watch your UHDTV from more than five feet away, you'd better spring for the 84-inch model to achieve that optimal viewing distance.

Bay Bridge Lights - a 25 KiloPixel, two mile display

I’m very fortunate to live in the Bay Area,  with a decent view of the marvelous Bay Bridge from our house on a hill in San Francisco.  The Bay Bridge plays second fiddle to the Golden Gate bridge, but it is still an icon, connecting us with Oakland, not to mention the rest of the country.  Tonight was a very special occasion, with the launch of Bay Lights  the installation art project you’ve probably heard about.   Unfortunately, it was drizzly and overcast, so our view was limited.  But I watched the inauguration via web streaming with my family.  The telecast featured Mayor Ed Lee and serial politician Gavin Newsom.  While it was a nice event, the video streaming video quality sucked… but that’s a topic for a future post.



Artist Leo Villareal controls the Bay Lights from his laptop.  (Photo credit: Cy Musiker/KQED)

Being involved in display technology, I was naturally curious about the engineering behind Bay Lights.  The project was initiated by artist Leo Villareal , who (according to his web site) “is known internationally for his light sculptures and site-specific architectural works.”  Leo unquestionably did an amazing job at fundraising, and navigating a complex bureaucracy in order to attach 100,000 feet of cable to the bridge.  Before I forget, they still need $2 million bucks.  You can donate here.   

The overall system – over two miles long – provides and amazing showcase of geometric illuminated patterns.   The entire North side of the bridge appears to shimmer  with dynamic and unpredictable movement.  Villareal, whose office is right down the street from our house, programmed algorithms in the display drivers for the lighting sequence, which makes for an impressive site.   But I wondered about the display technology used. 

It took some digging, but I found that Bay Lights is based on LED technology from Philips Color Kinetics.  A team of engineers in Boston formed Color Kinetics in 1997, and it was acquired by Philips five years ago.  The Bay Lights project uses a product called eW Flex SLX.   125,000 white LED’s were used in the installation, grouped into 25,000 nodes, each strapped to the vertical rods serving as the suspension structure for the bridge.  The nodes measure about an inch and a quarter wide, and are spaced one foot apart.    Overall, the press reports say this covers a span of two miles wide and 500 feet high, on four towers.   Nearly 4.5 miles (24,000 feet) of LED light strips were installed.

The Color Kinetics architecture groups 50 nodes  into a string driven by one controller.  Each node is illuminated on an 8-bit (255 level) brightness through pulse width modulation.   This means that the LED is quickly switched on and off to create the perception of different light levels.  For example, a 50% duty cycle will cause the appearance of half brightness. 

 Control signalling is made over data connections on CAT 5 cable, using a proprietary protocol at 500 kbps.  The control data is processed by hubs, probably in the configuration shown here.

One of Color Kinetics proprietary technologies is the Chromastic chip, a low-cost and lower power consumption LED driver included in each node.  The Chromasitc is typically used for RGB color control of LED architectural lighting, but for the Bay Bridge it provided a convenient means for  Villareal to provide the 8-bit brightness control.

For the past month, I’ve occasionally seen tests of the lighting system in the middle of the night from the bedroom window. When I posted reports, some of my Facebook friends have questioned the environmental aspects of lighting up these zillion LED clusters, just for fun. Let’s explore this.

The Philips eW Flex SLX modules use for the bridge are emit white light at 4,200 degrees Kelvin correlated color temperature, and hit a maximum of 16.2 lumens per five-LED node. Each node consumes up to one Watt at full brightness, which means the luminous efficacy is 16.2 lm/W.

The Philips press release states that “the new LED lighting system uses 85 percent less energy than traditional lighting technologies” 

So I wondered:  what are “traditional lighting technologies” for an installation art light show??   It seems the luminous efficacy of a typical incandescent bulb is 16 lumens per watt, virtually identical toeW Flex SLX modules.  So this would NOT explain an energy cost savings of 85%.  What’s up with that? 

With 25,000 one-watt modules, the Bay Lights consumes 25 kilowatts of power a full illuminance.  The organizers say that the lights will be turned on from “dusk until 2:00am”.  Looking at sunset times, that equates to 7 hours 15 minutes per day on average, or a total of about 180 kW/hours per day.  A recent report  says that electricity costs in San Francisco are currently more than 60% higher than the national average, at 21.2 cents per kilowatt hour.  So if all these nodes were turned on, that’s over $14,000 in electrical consumption per year.  The organizers claim that power costs are $11,000 per year, so we’re in the right ball park  --- it’s doesn't seem like too much.

You may have also been wondering about total luminance flux.  With 25,000 modules at 16.2 lumens, that comes to  at total of over 400,000 lumens  -- equivilent to around 20 typical movie theater projectors.   (Except, of course, we're talking about a 10,000-foot by  500-foot irregular shaped "screen" which is emitting, not reflecting light.)

If you're interested in longevity, the Bay Lights project says they are committed to having this art installation functional for "at least" two years.  But the good news: the LED nodes are rated at about 40,000 hours, which should keep them running for 15 years, assuming dusk to 2am operations -- just over 7 hours -- each day.   I would guess that the atmospheric conditions and salt water will greatly shorten the time before something fails.

Bottom line:  this is a cool art project, but a bit expensive at $8 million capital cost.   The $11,000 per yer in electrical costs is at least particially subsidized by solar credits.  Pundits are predicting tourism benefits of nearly $100 million.   So... why not!   400,000 lumens of visual art is kind of nice, in our fair city.