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How the world has changed and how LED is the way forward.


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The only concern I've ever had with Rotolight is... well. I wrote it up here.

 

The other problem is that they are so expensive for a 40W light. Effective, no doubt, and I'm not overlooking the difficulties of manufacturing small volumes. But the price on this stuff has to come way, way down. This consideration is also directed at Zylight, whose product is very lovely but considerably too much money compared to equivalent HMI.

 

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There is a lot of blind optimism in these comments. A close look at the improvement in lumens per watt efficiency of LEDs reveals that it, in fact, comes at the expense of color rendering. For instance, the “progress” in the Cree Xlamp line from the MT-G to the new MT-G2 demonstrates this clearly. According to Cree’s website the MT-G2 “pushes performance limits to redefine lumen levels and efficacy (it is 25% brighter then the MT-G).” But when you compare the spectral output of the two Cree Xlamp emitters side by side you see that the greater lumen output of the MT-G2 comes at the expense of color rendering capability. The reason for this is that there are limitations inherent in the remote phosphor technology that the Cree XLamps use. And, regardless of blind optimism, these limitations make it unlikely that we will ever see a Phosphor White LED with as continuous a color spectrum as we get from Tungsten.

 

In the remote phosphor approach that Cree takes to white light with their Xlamp LEDs, their designers manipulate the color spectrum put out by a blue InGaN LED to a desired "white light" by applying phosphor layers of distinct colors that are activated by the LED’s “Pump” color (450nm in the case of blue InGaN LEDs) to extend the color spectrum by a process called a “Stokes Shift.” Depending on the chemistry of the phosphors used, the color balance of light generated by remote phosphor technology can approximate daylight, or be stretched to approximate 3200k tungsten color balance. The “5500K” white LEDs use semi-transparent phosphors so the blue "pump" color comes through. In contrast to 5500K LEDs, the warmer “3200K” white LEDs have to use more phosphors and so are more opaque to the pump color, and are therefore much lower in efficiency.

 

There are several inherent limitations to the “Stokes Shift” process by which a portion of the “pump” color is transformed from shorter wavelengths to longer. First, it works in only one direction – that is why Phosphor White LEDs don’t emit color wavelengths shorter than their pump color and why Phosphor White LEDS, compared to continuous light sources, have no output at wavelengths shorter than about 425nm (which is why violet colors don't render well under them.)

 

Cree_MT-G_Spectrum.jpg

Spectral output of the Cree XLamp MT-G easyWhite LED.

 

 

The second inherent shortcoming to this approach to generating “tungsten” white light from an LED is that the Stokes Shift process reduces the total lumen output, so there is an inevitable tradeoff in lumen output and broader spectrum warm white LEDs. For that reason LED designers have to cut the high frequency output in the high 600 nm range. This tradeoff in lumen output is clearly evident if you compare the spectral output of the older Cree XLamp MT-G to the newer MT-G2. As you can see by their spectral distribution graphs above and below, the 25% greater output of the newer MT-G2 is at the expense of longer wavelength colors. The high frequency cut-off in the Cree MT-G comes at about 615nm. In order to gain more lumens, Cree dopes the MT-G2 less heavily (so that more light from the emitter comes through) and as a result the high frequency cut-off comes sooner at 600nm and the long wavelength colors of the MT-G2 drop off much quicker than in the MT-G. Since, it is the long wavelength colors that make flesh-tones “vibrant,” the additional output of the MT-G2 comes at the expense of flatter, paler flesh-tones. And, the more appreciable drop-off of longer color wavelengths in the MT-G2 results in pinks, reds, oranges, and other long wavelength colors looking duller as well.

 

Cree_MT-G2_Spectrum.jpg

Spectral output of the Cree XLamp MT-G2 easyWhite LED.

 

Even after its’ “improvement” the color output of the MT-G2 is marginally better than the Phosphor White LEDs used in Litepanels and does not compare to that of a Tungsten filament. The MT-G2 still doesn’t generate much light in the medium blue-cyan-turquoise range from about 465-510nm. And with its’ long-wavelength cutoff now at about 600 nm where a tungsten filament continues to generate light all the way out, it is even less capable of rendering long wavelength colors (pinks, reds, oranges) then the MT-G compared to a tungsten light.

 

LED_Model_Comp.jpg

Left: Tungsten source, Right: White Phosphor LED source.

 

This inability of Phoshpor White LEDs in general to render color is very visible in tests recently performed by The Academy of Motion Picture Arts and Sciences (AMPAS) as part of their “Solid State Lighting Project Technical Assessment.” In one (above) a model was photographed wearing a dress that had a number of different blue/cyan tints. Footage was shot with both a true tungsten source and a White Phosphor LED source. The tungsten-lit footage displayed all of the subtle differences in blue/cyan tones in the fabric, while the LED-lit footage, lacking cyan output, showed just a nice blue dress, without the same richness of hue. Since the light doesn’t put out much cyan, the camera/film simply can’t record it because those wavelengths are not reflected by the dress. The same holds true of flesh tones illuminated by Phosphor White LED light. As is also evident in the pictures above, skin tones don’t reproduce well under LED lights because of the steep drop off of high frequency colors (above the 600nm cut off) such as pinks, reds, oranges, and other long wave-length colors use this link for the AMPAS test results for set dressing and make up under LED light sources.) Given the unavoidable energy loss in the Stokes shift process demonstrated in the “improvement” in the Cree Xlamps, it is doubtful we will ever see a significant improvement in the color rendering capability of Phosphor White LEDs.

 

In order to get from an LED fixture the consistent color and quality of light that one gets from an inexpensive Tungsten Fresnel, requires a multi-color LED array and a sophisticated micro-processor controlled light engine. For example, the Arri L7-C LED Fresnel overcomes the generally poor color rendering capabilities of inexpensive LED fixtures by blending the output of different color LEDs. Another benefit to using a color blending multi-emitter LED engine is that the L7’s firmware can calibrate the blend of different color emitters to compensate for the inevitable color shift of the LEDs because of ambient temperature and age. This approach assures consistent realistic color rendition at any temperature and throughout the fixture’s life – something that “dumb” Phosphor White LED 1x1 panels cannot do. Unfortunately, the Arri L7-C costs four times what a comparably sized Tungsten Fresnel costs and, if the history of Arri HMIs are any indication, it is not likely that LED fixtures of this level of complexity will come down in price.

 

So while it might be true that LED fixtures will get marginally better than they are today, it is not likely that they will get cheaper because to get the color rendering required for photographic purposes from LEDs requires ever more sophisticated, and therefore expensive, fixtures (use this link for more details.)

 

- Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

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... imagine a 4k HMI equivalent fixture that you could plug into a wall or run off a tiny suitcase generator? That would really revolutionise things.

 

The revolution has already happened. You can plug a 4k HMI into the wall and run it off a portable generator. In fact you can now power a 12k Par from portable Honda generators (see picture below.) There are a number of 240V outlets in a typical house, office, or industrial plant in this country capable of powering a 4k HMI. The most common are air conditioner outlets, dryer outlets, range outlets, outlets for large copy machines in offices, and the outlets for motorized equipment in industrial plants.

 

 

12K_Paralleling_Sm.jpg

Two paralleled Honda EU6500s powering a 12k HMI Par.

 

 

If you look at the breaker of these circuits on the building service panel you will notice that they use two pole breakers - either 30A or 50A.. Each pole of the breaker is in a sense an independent 30A or 50A 120V circuit. That is, if you measure the voltage from each pole of the breaker to ground it will be 120 volts, and if you measure the voltage between the two poles of the breaker you will notice that it is 240 volts. The 120 volts of the two poles adds up to 240V because the 120V circuits are on opposing legs (and are therefore additive) of either a single-phase electrical service of a house, or a single-phase secondary step down transformer of a office or industrial plant. In residential settings, this is how higher voltages are supplied to household appliances like Dryers, Electric Ranges, Air Conditioners, Motors, etc. that require more power than can be reasonably supplied by a single 120V circuit. Many of these household and industrial 240V receptacles use a three wire system (no neutral) because they are designed to power single phase motors or heating elements that draw a perfectly balanced load and return no current because the single phase service legs are 180 degrees out of phase and cancel each other out. How you use a 240V circuit to power a 4k HMI depends on whether the ballast is electronic or magnetic.

 

So that they can be used in both North America and Europe, electronic 4k ballasts are designed to operate at voltages between 95-150V and 195-250V. A power factor corrected electronic ballasts operating at 240 Volts of, say, a dryer plug will draw roughly 18.4 Amps on each leg of the single phase circuit, which is well within the capacity of these circuits which are usually rated at 20, 30, or 50 Amps per leg.

 

To operate a 120V 4k magnetic ballast from a 240V circuit requires a 240v-to-120v step down transformer like the 60A Full Power Transformer/Distro we make for the Honda EU6500is generators. Like it does with the 240V output of the Honda EU6500is Generator, our 60A Transformer/Distro converts the 240 volts supplied by these industrial and household receptacles back to 120 volts in a single circuit that is the sum of the two single-phase legs of 30/50 amps each. That is how our 60A Transformer/Distro makes a 60A/120v circuit out of a “30A/240v” or a “50A/240v” circuit and is capable of powering bigger lights, like 4ks with magnetic ballasts, 5ks, or even a 6000W Six Light Mole Par, off a 6500W generator. It can also be used to power multiple 120V luminaries off of 240 Volt circuits because our Transformer/Distro automatically splits the load of whatever you plug into it evenly over the two legs of the 240V circuit so there is no neutral return. You can maximize the power you can pull from these 240 Volt receptacles if, rather then plugging an electronic ballast directly into the 240 receptacle, you plug it in through a step-down transformer and operate it at 120 Volts. Where a 4k power factor corrected 4k electronic ballast at 120V draws only 36 amps, you will still be able to power additional lights, like a 1200 Watt HMI (11 Amps) and a 800 Watt HMI (8 Amps), of the same circuit.

portableGenBook.jpg

 

For more detailed information on using 4k HMIs on standard wall outlets, I would suggest you read a white paper I wrote on the use of portable generators in motion picture production that will be available soon as an e-book from the Academy of Production Technology Press (APT.)

 

BoxBookLinkGenSetSm.jpg

 

Harry Box, author of The Set Lighting Technician’s Handbook has cited my article in the just released 4th Edition of Harry Box's “Set Lighting Technician's Handbook” and featured on the companion website “Box Book Extras." Of the article Harry Box exclaims:

 

“Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working.”

 

“Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."

 

The original white paper is still available online for free at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer,

ScreenLight & Grip,

Lighting Rental & Sales in Boston

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