.... I am also really not convinced about LED on skin tones.
There is good reason to be wary about LED's ability to render a good skin tone. The inability of Phosphor White LEDs used in the Litepanel and Coolights 1x1 arrays to render color accurately has been well established in tests recently performed by The Academy of Motion Picture Arts and Sciences (AMPAS) as part of their “Solid State Lighting Project Technical Assessment.” (see http://www.screenlig... Output AC LEDs
for details.) In one (below) 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 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. Left: Tungsten source, Right: White Phosphor LED source.
The same holds true of flesh tones illuminated by 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. As the illustration below, comparing the reflected spectral distribution of a Caucasian skin tone under theoretical pure white light (an even distribution of all wavelengths) to that of a Phosphor White LED demonstrates, absent these wavelengths the skin tones look pale under LEDs because light reflected by the skin tone is likewise absent these critical long wavelength colors. Reflected Spectral Distribution of Caucasian skin tone under theoretical White Light and Phosphor White LED Light
In the picture above illuminated by the Phosphor White LED, both the cyan/blue dress and the skin tone, don’t reproduce well because you can't get accurate color reflected from an object unless that color is in the light in the first place. In other words, if the light source doesn’t generate the color (cyan), it is not reflected by the object (the dress) and so the camera/film simply can’t record it.
Another drawback to White Phosphor LEDs is that their color output is very inconsistent. That is because their color output is effected by a number of factors: the binning and manufacturing tolerance of their blue pump, the thermal management of the fixture, the ageing of the phosphors, and even the ambient temperature. For example, a one degree shift in the junction temperature of the blue InGaN LED (pump color) in remote phosphor LEDs, will cause a +/- 2nm shift in the dominant wavelength. If compounded by the average wavelength variation of +/- 2nm of blue InGaN LEDs, a 5nm divergence from the prescribed 455nm wavelength of the pump color will create color inconsistency of 5 MacAdams ellispses. While not readily apparent to the eye, image capture systems will easily see this variation.
And, as broadcast studios lit exclusively with Phosphor White LEDs are finding out their output depreciates overtime and their color shifts much faster than the manufacturers say (see illustration below.)
For more details regarding the issues surrounding the use of LED lights in motion picture lighting see our company newsletter at http://www.screenlig... Output AC LEDs
- Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental and Sales in Boston.