http://www.flickr.com/groups/strobist/d ... 679167124/
This thread is the epitome of misinformation, ignorance, confusion, lies and lack of industry knowledge regarding color balance and other factors, particularly wrong and libelous misrepresentations regarding AlienBees and certain European brands. So I thought I would take some time to explain it in simplified general terms photographers can understand.Color distribution:
Light is measured along two axis . . . the Yellow/Blue axis (usually express as Color Temperature) and the Red/Green axis (often referred to as Tint).
A light source that has a Color Rendition Index (CRI) of 100 (such as an ordinary tungsten lamp or pure direct sunlight) produces a smooth distribution curve containing all color wavelengths. As color temperature is raised, the shorter wavelengths (blueish) are accentuated in relation to the longer wavelengths( Reddish), but the distribution curve remains smooth, thus the light is free of tints, color spikes and other anomalies that would cause tint errors.
Xenon Flash Tubes produce a characteristic distribution of color spikes that are distributed quite evenly and thus produce a very high, but not perfect, CRI.
While a Xenon flash tube can be optimized to produce a zero R/G tint at a specific operating point, changing the power level by any means will invariably result in changes to the color temperature, tint and flash duration. This is a physically reality and cannot be avoided, regardless of the cost or design.Methods of power control in Pack and Head systems:
When flash power is adjusted by switching flash capacitors in or out of the circuit, both color temperature and tint remain relatively constant, while flash duration becomes shorter at low power (few capacitors) and longer at high power (more capacitors). This typifies the performance of basic pack and head systems.
The limitations of such systems are that power can typically only be accomplished in 1 f-stop increments, and the sharing of a capacitor bank by multiple heads results in uncontrollable changes in flash durations, ratios of power delivered to the various heads, etc. Another factor is that mixing of different head types (IE standard heads, ringflash head, short duration heads, etc, results in a completely unpredictable relationship of all parameters.
Manufacturers of pack and head systems have gone to great lengths to minimize these factors, at the cost of operating complexity and price.Power Control in MonoFlash systems:
The vast majority of monoflash units. regardless of cost or country of origin, control flash power by varying the voltage applied to a fixed bank of flash capacitors. All such systems, including AlienBees, Profoto, Elinchrom, WL Ultra and most other brands, follow the same characteristics:
Color Temperature drops at 75° to 90°K per f-stop of power reduction (typically 350 - 400° variation from full to 1/32 power).
R/G axis tints typically shift by 7 to 10 units over the same power adjustment range.
Flash duration typically lengthens by about double over the same power adjustment range.
Exceptions/modifications to these rule are found in our WLX1600/3200 models, which combine voltage variation with capacitor switching to achieve.Profoto D1 misleading marketing and specifications:
It is often stated on forums by AB detractors that "Profoto D1 maintains 5600°K +/-50° color constancy over the entire power control range." The source of this misleading information lies in Profoto's specifications of "Color Accuracy = +/-50°K" and Color temperature = 5600°K" Uninformed readers assume this to imply the DI maintains 5600°˚ +/-50°K over the entire power control range. Nothing could be farther from the truth.
In reality, the color accuracy spec pertains to the shot to shot color repeatability at a specific power setting
, and the 5600° spec also applies only to one specific power setting.
Actual testing in our labs of the D1 revealed a D1 full power color temperature of 6100°K and a minimum power (1/64 power) color temperature of 5400°K . . . a 600° actual color shift, and slightly over twice the flash duration relative to full power. Over this same range, the R/G tint shifts by about 10CC units.
By contrast, in parallel tests. AlienBees color shifted 375°K from Full to 1/32 power while the RG tint shifted by 7CC units. The flash duration doubled from full to 1/32 power.
Another note is that Profoto publishes only misleading t.5 flash duration times . . . merely specifying it as "flash duration" and omits the more meaning full t.1 duration.AB1600 VS Elinchrom RX600:
AB detractors often make the statement that Elinchrom lights produce essentially no color shift and faster flash durations than AB1600.
In the same series of tests of D1 above, AB1600 performance over the Full to 1/32 power range was compared to RX600 over the same range. The result showed the two units were virtually identical
, yielding 350 - 375°K color shift, exactly the same R/G tint values with both shifting 7CC units, the same t.5 and t.1 flash durations, and identical recycle times and identical performance when operated from our Vagabond Mini Lithium battery pack. They are virtual performance clones in these parameters,
with the RX600 offering only somewhat better power setting repeatability at the low end of the power range as a result of the digital VS analog power setting method.IGBT Controlled Flash units:
At this writing and to the best of my knowledge, there are only three IGBT controlled pro-level studio flash systems on the market (though there are rumors of a fourth special purpose version coming from Hensel.The systems are the Paul C. Buff Einstein 640 monoflash, Broncolor Scoro and Grafit pack and head systems, and somewhat obscure Photogenic Solaire monoflash.
Unlike voltage controlled or capacitor switching systems, IGBT systems utilize Insulated Gate Bipolar Transistors to interrupt the current flow through the flashtube, much like is done on a smaller scale in most speedlights. By using a sophisticated microprocessor, the applied capacitor voltage and the timing of the flash tube shutoff are established for each 1/10f power setting to allow +/-50°K color balance shift over the entire power range, an extremely wide, accurate and repeatable power control range, and exceedingly fast t.1 flash durations that shorten dramatically as the power is decreased.
Einstein has two operating modes: Constant Color and Action Mode. In Action Mode, color temperature is allowed to rise as power is reduced in order to achieve exceedingly fast t.1 time as short as 1/13,500 second, while in color mode, minimum t.1 flash duration is 1/8000 and color temperature is held to +/-50°K
Scoro and Grafit systems achieve minimum t.1 flash durations of 1/6000 to 1/7500 second and Solaire achieves 1/3500 second. See the below link for further Einstein specs.http://www.paulcbuff.com/pcb2009/e640detail.html
Further explanations can be found at http://blog.bronimaging.com/2010/01/bro ... trol-ectc/
It should be noted that absolute values of color temperature, from my experience, cannot be obtained from commercial color meters, and must be derived from actual shooting in RAW and determined by the use of an accurate, calibrated grey card target and appropriate software. Absolute color temperature readings are influenced by the room environment, RAW software used and other factors. I use WhiBal card and Adobe Bridge, in a neutral grey room
I strongly urge qualified forum members to comment on this content, and to conduct their own tests and post there findings or disagreements or further observations in this thread. Less qualified members are invited to request clarification of aspects that may be foreign or difficult to absorb.