LED lights and green shift

E

eco_bach

Well-known member
Hi
I own several Z96 LEDs and just picked up a 500LED from Konova.
Haven't noticed any color shifts at all with the z96's, but with the larger Konova, my first tests show a distinct green cast.

Will try further tests to confirm my custom color balance is accurate as possible, but was wondering if anyone else has experienced this with LED's?

Are some LED's more susceptible to this green shift?
 
Yes, some leds are more susceptible to green shift or as some will call it a greenish cast. Leds come in many flavors from low cost mass produced and little if any quality control check to the high output, high efficiency and high CRI varieties along with a large menu of everything between the two extremes. Manufacturers of led light units are there to make money of course and there are really few choices to make as far as the overall plan of attack. They can make them really cheap with a really cheap price offering, better with a mid range price and extra features, good with high output and desirable features along with a decent light quality and finally go all out and make a pro unit designed with light quality/high CRI/ even beam spread/robust build and pro powering options. Most first time buyers of led lights (and this is my opinion) are "had" with the usual overrated specs and are impressed with the multi teeney weenie leds and of course the affordable low price with total disregard to eveness of beam spread, color rendition and build quality. I often hear them say that the greenish cast cam be corrected with a proper white balance or in post editing. True, there are units that have multi small leds with great color qualities without green spikes but be prepared to pay a premium. I have seen led units from the same manufacturer and the same model light exhibit different color casts on side by side comparisons, and those were not models from different dates of manufacture. When leds are manufactured ( the led emitters themselves) they are put in what is called led "binning" categories which denotes their output class, color temperature in chart ansi grid placement, and other photometric classification based on design parameters. If led emitters come from the same batch and bin, well you have a match array or group of leds that will perform in a very identical manner. Most affordable led light units out there do not have that strict tightness of quality control and therefore production units vary in color and performance as the leds come from not only different batch production but from different sources as well. A led emitter manufacturing company that produces millions if not billions of tiny leds has a huge surplus of greenish cast ones will eventually source them out for a dime a dozen to be eventually used in them el cheapo units to "it's too good to be true price" buyers.
 
True. They tend to follow the cool blue trend for obvious reasons. Cool blue are the leds of choice when all you want is the highest light output as the bluish ones give just that. Looks good on output lux specs and you can always gel them with amber/orangeish CC lters to get that blue out. But doing so will surely intoduce the sickly yellowish green tint that is next to impossible to get rid of (yellow and blue mix out to green). Even trying to white balance in cam will have you repeating the WB over and over again. Idealy they ( the mfrs.) should really get the neutral or warm binned leds to begin with and CCing them to cooler blue will be a cinch without introducing off color casts. That is why the high CTI (color temp. index.) numbers usually come in the warmer tint flavors rather than the cool blues as they do not have any unwanted tint/spike flavoring.
 
Norman99 said:
… When leds are manufactured ( the led emitters themselves) they are put in what is called led "binning" categories which denotes their output class, color temperature in chart ansi grid placement, and other photometric classification based on design parameters… there are units that have multi small leds with great color qualities without green spikes but be prepared to pay a premium.
Click to expand...

While the better LED light manufacturers rely on such careful binning practices to provide as accurate and consistent color performance as they can; this approach to white light with LEDs has an inherent limitation: as you can see from the spectral power distribution graphs below, regardless of how tight the bin parameters are set, by their nature the spectral distribution of even high CRI White LEDs is less than optimum for motion picture lighting applications.

LED_Comp_Sprectrum.jpg

A spectral power distribution of a lamp indicates how much energy is present in each part of the spectrum. Natural sunlight and incandescent lamps have a continuous spectrum. Given how they produce white light, even high CRI White LEDS have a discontinuous spectral quality that is unlike that of natural daylight, HMI, or incandescent light. In the case of the 3200K White LEDs above, the phosphors added shape the spectral distribution by enhancing certain colors in the spectrum to simulate the spectral distribution of incandescent light. As a result, the spectral distribution of White LEDs resembles a series of peaks and valleys. If you look at the spectral power distribution of even a high CRI white LED (above left) you will notice a big spike at about 465nm (the blue LED) and a broader bump between 500 and 700nm produced by the phosphors. Even though the spectral power distribution has these peaks and valleys, the human eye perceives the light as white light.

While the discontinuous spectral distribution of high CRI White LEDS (far left) may appear white to the eye, and the color of objects illuminated by it appear natural to the eye, to film emulsions and digital imaging systems designed to reproduce accurate color under continuous spectrum light sources (like daylight or incandescent lamps), the color of the same objects will appear unnatural on screen. That is, the hue of an object being illuminated by this "apparent white light" can be drastically different than expected when it appears on the screen. For example, below is a "Macbeth chart" contrasting the resulting effect upon different color swatches of studio tungsten light and a representative high CRI White LED lighting instrument.

LED_Macbeth_Tungsten-WhPhos.jpg
Split Macbeth chart: each color patch shows the visible effects of studio tungsten light in the top half of the patch, and a representative Phosphor White LED lighting instrument in the bottom half.

A common test chart used for assessing color performance of motion picture imaging systems, the chart above would be more accurately called a "split Macbeth chart" because each color patch shows the visible effects of the two light sources – studio tungsten in the top half of the patch, and the high CRI White LED lighting instrument in the bottom half. Although your computer display is not likely to be a calibrated reference monitor, the wide variations in color patch hue caused by the discontinuous spectral distribution of the White LED lighting instrument should be readily apparent.

What accounts for these results? First, as you can see from its' spectral power distribution above, White LEDS, compared to continuous light sources, have no output at wavelengths shorter than about 425nm, which means that violet colors don't render well. Second, there is minimal output in the medium blue-cyan-turquoise range from about 465-510nm, which is why the aqua-type colors don't render well either. Lacking these complementary colors within the spectrum, skin tones and warm, amber-yellow colors don't stand out. Third, with the long-wavelengths cutoff in the high-600 nm range, pinks, reds, oranges, and other long wave-length colors tend to look a little dull under Phosphor White LEDs, compared with how they look under continuous spectrum light sources (daylight, HMI, Tungsten) which extend all the way out on the long-wavelength end. Finally, as you can see from the gray scale at the bottom, this particular Phosphor White LED Luminary has an overall magenta bias. While you can white balance out this magenta bias in digital video cameras, the camera is not able to replace the parts of the spectrum that are missing all together. Given these results, you can see that simply by nature of its' discontinuous spectral distribution, even high CRI White LEDs will never accurately reproduce colors on screen regardless of how tight the bin parameters are set in sorting.

The CRI ratings published by LED manufacturers can be misleading. Where the CRI index indicates the ability of a light source to reproduce to the eye only 8 colors faithfully (a different 8 colors are used in Europe) they should be taken with a certain amount of skeptism. In the case of LED luminary manufacturers, for instance, it is possible to tune their output to the limited color range of the CRI color scale and deliver good color rendering to the eye while delivering generally poor color reproduction on the screen.

For a cost effective alternative to high CRI Leds that delivers a full spectrum, consider Light Emitting Plasma (LEP) lamps. LEP lamps have a CRI of 94+. More important than their high CRI ratings, is the fact that LEP lamps generate light with a continuous color spectrum. If you compare the spectral power distribution graphs (below) of natural daylight and LEP lamps (available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorHigh%20Output%20LEPs) you see that, except for very brief drop outs at approximately 425 nm and again at 475 nm, the light output of LEP lamps is almost identical to natural daylight.

Plasma_vs_Mole_LED.jpg

And, as also can be seen in their spectral distribution graphs above, Plasma lamps have a much more continuous color spectrum than even the best LED luminaries on the market today. For instance, LEP lamps, unlike LED lamps, generate light at wavelengths shorter than 425nm. Unlike LED lamps, LEP lamps also output in the medium blue-cyan-turquoise range from about 465-510nm And LEP lamps extend all the way out on the long-wavelength end, well beyond the 600 nm cutoff of LEDs. Capable of a fuller spectrum colors look more vibrant under LEP light where they tend to look a little dull under LEDs.

As a continuous spectrum source, colors not only appear more natural and vibrant under LEP lamps than under LED lamps, they also reproduce more accurately on the screen since, as is also evident by their spectral distribution graph, the output of LEP lamps is almost an exact match to the spectral sensitivity of daylight film emulsions and digital sensors. Plasma lights will deliver the same true-to-life color rendition previously achievable only with full-spectrum Daylight or HMI sources. As an added bonus, color meters, like the Minolta III F, that make their calculations of the Color Temperature (CT) based on a light sources continuous spectrum, are able to generate accurate reading of the CT and Green/Magenta of LEP lamps where they are almost completely useless with LEDs.

At present there are three motion picture LEP lamp heads on the market: the Photon Beard Nova 270, the Helio 270, and Hive Lighting’s Hornet 180. All three lamp heads use the same Luxim Plasma Emitter behind Fresnel lens. The Photon Beard Nova 270 and Hive Hornet 180 can be operated on batteries at 28 Volts or off a Universal (90-305Vac, 50/60Hz) AC power supply (the power supply is separate in the case of the Photon Beard Nova 270. The Helio 270, by comparison is a stripped down, more robust location production instrument that offers a built-in 120V/60Hz AC power supply (no DC option) with near unity (.99) Power Factor. As such, the Helio 270 is nearly half the price of the other two heads.

For our company newsletter I have put together an overview of the technology and what LEP products are available for motion picture lighting (available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorHigh%20Output%20LEPs.) In this newsletter article I have tried to cut through some of the hype surrounding both LEP and LED lights.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston
 
You must log in or register to reply here.
Back
Top