A second problem inherent in LEDs that manufacturers have had to address is the relatively poor power quality generated by the Switch Mode Power Supplies (SMPSs) they use to power their lights on AC power. With Leading Power Factors around 0.54 (Litepanel 1x1s) and high harmonic distortion (THD upwards of 68.1%) the AC power supplies of LEDs have generally drawn a very distorted current that is significantly phase-shifted with respect to the sinusoidal voltage waveform. As such, the AC power supplies of LEDs have had an adverse effect on power quality similar to that of CFLs (use this link for more details.)
Finally, another drawback to LED fixtures until now is that the "light panel" design creates a light that is hard to control and falls off very rapidly. These characteristics have made LED light panels only suitable as Key sources in documentary interview set-ups where the Keys are typically positioned close to the interview subject. In that capacity LED light panels (with heavy diffusion) can generate a wonderful soft light that wraps around the interview subject without wilting them. However, in dramatic set lighting, where Key sources must be capable of throwing a distance, LED light panels have only limited applications as fill sources. The broad soft light they put out drops off too rapidly, and is too difficult to control, for them to be effective as a Key or Backlight source in dramatic set lighting. For LEDs to be widely used, manufacturers had overcome these shortcomings - their poor color rendering, poor power quality, and lack of versatility - to previous LED light designs. As I said at the outset, progress has been made on many of these fronts in the fixtures being introduced at NAB this year. Particularly in the case of the Litepanel Sola 6 Fresnel and the Arri L7 Fresnel.
The Litepanels Sola 6 LED Fresnel
While the Sola 6 Fresnel is definitely a step in the right direction in developing a production LED, far as I am concerned, Litepanels has not completely overcome all of the shortcomings outlined above with the Sola fixtures. Litepanels doesn’t give CRI ratings for the Sola Fresnels on their website, but when asked they say the CRI is in the 80s – which is still rather anemic compared to continuous spectrum light sources like quartz and HMI lights. The Power Factor of the Sola has been improved. But, at .85 it could stand further improvement (a Power Factor Corrected HMI has a Power Factor of .98 or near unity power.) They claim the 75W Sola 6 has the output equivalent to a 650W Tungsten, but comparing the photo-metrics published on their website to those of an Arri 650 Fresnel, the Arri has nearly three times the output of the Sola. And, while the Sola 6 has an impressive spot to flood range (10 to 70 degrees), spot/flood capability is not the only characteristic that makes a Fresnel light versatile. Of equal importance is the ability to render clearly defined shadows and cuts. The ability of Fresnels to render crisp shadows make them ideal for creating gobo effects like window or branch-a-loris patterns. The ability of Fresnels to render clearly defined cuts enables their light to be precisely cut to set pieces and talent. And, simply by adding one of a variety of diffusion material you can vary the softness of a Fresnel’s output. These are the characteristics of traditional Fresnels that make them extremely versatile, that the Sola “Fresnels” have not been able to emulate.
The Arri L7 LED Fresnel
Arri, on the other hand, may have finally engineered in their new 200W L7 LED Fresnel a fixture that combines the cool-burning, energy-efficient advantages of LED illumination with the controllable versatility of traditional Fresnel fixtures.
As you can see in the pictures above, that compare the output of the L7 Fresnel to an Arri ST-1 Quartz Fresnel, the L7 Fresnel has clear and defined shadow rendering capability like that of the ST-1 Quartz Fresnel. And, as the pictures below demonstrate, the L7 Fresnel has a spot to flood range similar to that of the ST-1 Quartz Fresnel and excellent field homogeneity in both flood and spot.
And, just like the ST-1 Quartz Fresnel (pictured below), the beam of the L7 Fresnel (pictured above) is easily controlled with barndoors - enabling the light to be precisely cut to set pieces and talent (see far right photos above & below.) And, given the discernable amount of light the L-Series Fresnel prototypes threw in a show demonstration video from IBEC last fall, on what appears to be a 6x6 Ultrabounce rigged 20’ overhead, and under the high ambient light levels of the show hall, it seems the production model L7 Fresnel has more than enough output to waste some to diffusion and color gel if one so desires (a shortcoming to most LED panels is that they have barely enough.)
The L7 line of LED Fresnels introduced at NAB consists of three models: the L7-D, L7-T and L7-C. All share the same basic housing and the same 7" Fresnel lens, and all have output comparable in intensity to a conventional 1K Fresnel. They differ in terms of color temperature, with the D model outputting a daylight-equivalent 5600 K, the T model a tungsten-equivalent 3200 K, and the top-of-the-range C model offering total color control. In my opinion, the Arri L7s finally deliver the true-to-life color rendition, previously achievable only with full-spectrum tungsten sources. By color blending with a multi-emitter LED engine, the L7 is able to overcome the generally poor color rendering capabilities of other LED fixtures. Both the 3200 K and 5600 K color temperature models offer a CRI and CQS greater than 90 so skin tones, costumes and scenery will finally appear life-like under LED light. The L7-C’s fully tuneable white light can be adjusted for different skin tones, camera sensors and mixed-light environments, while specific color shades can be matched through full gamut color mixing. Unlike other LED fixtures, this level of color control does not involve compromising the quality of the light field: the L-Series is unique in combining uniform light and single shadow rendition with absolute control of color attributes.
Split Macbeth chart: each color patch shows the visible effects of studio tungsten light in the top half of the patch, and a representative multi-emitter LED lighting instrument in the bottom half. Note: this is not the L7 but results typical of the multi-emitter LED approach
An added 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 with age. This approach assures consistent realistic color rendition throughout the fixture’s life.
There are two alternative cooling systems: one passive and the other active. The passive cooling system was designed for broadcast studios. It incorporates no moving parts or fans and is therefore completely silent. The active cooling system was designed to provide a more compact and lightweight option for location work. It uses an extremely quiet (<20 dB) fan and weighs 10lbs less than the studio version.
The location fixture carries an IP54 rating for weather resistance which means that it is protected from falling rain and splashing water, and that the internal electronics, optics and LEDs are protected from dust, dirt and humidity – making it a very robust fixture that will stand up to the rigors of location production.
All the L7s feature Power Factor Correction with a near unity Power Factor of .91. Which means that the 200W fixtures will draw no more than 1.98A at 120V (220W) and cause virtually no Harmonic Distortion. Since it creates virtually no line noise, you will be able to power nine 200W L7s on the 20A circuit of a portable generator without a problem. And since the L7-T has an output comparable to a 1k Quartz Fresnel, and the L7-D has an output comparable to a 575W HMI Fresnel, the L7 series takes what you can do with a portable generator to a new level. For example, with the enhanced 7500W output of our modified Honda EU6500is, you will be able to operate a lighting package consisting of a 30 L7s. I think you would have to agree that is an incredible step-up in production capability.
At a price around $2’500, the L7s are one of the more expensive LED fixtures at NAB. But, to assure that they are not quickly rendered obsolete by the rapid advances being made in LED chip efficiency, Arri has designed them to be an expansible platform that can incorporate future developments in LED technology. Not only, do the heads allow for the incorporation of more efficient LED chips when they become available, but the light engine is also fully upgradeable, ensuring that the fixtures can take advantage of technology advances as they happen. To accommodate future control protocols (such as ANC), their firmware is also upgradeable through the USB port on the rear of each unit. They will also be compatible with planned future optic accessories that will expand the L-Series versatility. Able to incorporate future developments in LED technology, the expansible platform of the L7s ensures that they will have a long useable life and so will assure a return on investment in them. Given the rapid pace of LED chip development, I can’t think of another LED fixture that won’t be obsolete in a year or two. The scheduled release for the L7 LED units is September 2011.
With the same clear and defined shadow rendering, excellent field homogeneity, and smooth continuous flood to spot focus as a 1kw Quarts Fresnel, these first production models of the L-Series LED Fresnels may well be the first true LED Fresnel lights (use this link for more details.) For more detailed information on the Power Quality generated by LED power supplies and to see a demonstration video of the new Arri L-Series LED Fresnels, use this link to an article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it I cover some of the basic electrical engineering principles behind LED drivers, the harmonic distortion they can generate, and how it can adversely affect generators. The article and the demonstration video are available online at http://www.screenlig...generators.html.
Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston