No wonder Tungsten and Actual Daylight Look Better! Good news for LEDs

J

jcs

Veteran
I was curious why daylight and tungsten was easier to deal with, especially for skin tones. Looking at the spectral distribution of light, tungsten and real daylight provide a continuous spectrum of light, while fluorescent/CFL produce highly spiked light- not continuous. The good news is that the latest LED lights come very close to tungsten/daylight for continuous spectral light. I've got some low-cost CFLs in 2x3 and 2x2 light rigs: I'm going to replace the CFLs with LED bulbs at around 5000K (probably Cree) to match the other ~5000K LED lights I use (currently Z96, YN300, YN600, and F&V R-300). An LED spot in a clamp-on work light will also be handy.


http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html
http://www.popularmechanics.com/tec...scent-vs-led-ultimate-light-bulb-test#slide-1 (see the spectral charts)


(CFL can work OK, but I had to use a tuned camera color profile and make sure WB was set carefully, otherwise trying to fix in post was time consuming).
 
jcs said:
The Cree's flicker when recording at 240fps (Sony FS700). Looks like they flicker at 120Hz: http://www.candlepowerforums.com/vb/showthread.php?361354-CREE-BULBS-AND-FLICKER . I didn't see any flicker at 24fps (120/5 = 24). Not clear if any incandescent replacement LEDs won't flicker (perhaps non-dimmable versions won't flicker? LEDs don't inherently flicker, guessing it's the PWM AC-DC circuit).
Click to expand...

The flickering or strobing is caused by the PWM ( Pulse Width Modulation) circuit scheme of the LED driver. There are LED drivers that do not use PWM but use variable current for different dim levels.. It is really referred as constant current in that it does not pulse the led at various time "on" durations. At lower dim levels, in PWM, the time "on" duration is shorter which leaves the time "off" duration longer hence the noticeable strobing as opposed to the full bright dim level where the time "on" is longer than the time "off" duration which leaves little time in the "off" state to notice the flicker. Most leds are dimmed this way. At certain shutter speeds of the camera, stroboing may occur even at a higher fps rate. All PWM schemes are not the same and the modulation frequency varies also from mfr to mfr. The led emitter by itself does not cause the flickering/strobing. I mainly use CREE high output leds in my lighting designs with custom CC drivers that do not flicker at any frame rate or dim levels. The CURRENT that goes through the leds is changed for various dim levels and not the FREQUENCY.
The CREE leds used in the example for the christmas tree lights above are not in the same class as the lighting power class ones made by cree. The christmas lights most likely use a simple DC rectifier that filters out the negative half wave of the AC component that is fed to the leds. The 1/2 wave AC is still 60 HZ and hence the noticeable flicker.
 
Last edited:
jcs said:
The good news is that the latest LED lights come very close to tungsten/daylight for continuous spectral light.
Click to expand...

If you're happy with them, fine. But don't confuse LEDs with continuous spectrum lighting. LEDs have much more in common with HMI and fluorescent than they do with incandescent. LEDs produce photons at their band-gap frequency. You get "full spectrum lighting" from LEDs by either mixing frequencies (R, G, and B LEDs) or by picking a frequency that can excite a range of phosphors, just like a fluorescent light. The closest we've gotten to "full spectrum" from LEDs is with remote phosphor technology, which is indeed quite promising.

The typical "full spectrum" fluorescent used for video will have six or more phosphors, and some filtering. There are spikes, but they are largely tamed. The spikes that are left tend to be very narrow and the "gaps" between the spikes are largely filled. This comes at a cost; the efficiency is typically somewhat lower which is the cost of taming that massive green mercury spike. But a good fluorescent bulb / ballast meant for color correct work can be quite good indeed -- look at the Kino Flo True Match bulbs. I'm just sayin' that this is a far cry from cheap big-box-store fluorescent bulbs that are typically found in commercial buildings.

Perhaps your best bet for "portable sunlight" is FIPEL, if they can produce enough lumens to be practical for video.
 
Bruce- remote phosphor looks promising- another brand promoted by Vincent Laforet: http://www.bbslighting.com/products/area-48-soft-remote-phosphor . At $2000+ per panel, they're something to consider if the projects make it worth it.
I like to test thoroughly before spending more to make sure other cost effective options can be ruled out.

Some of the LEDs have a decent continuous spectrum (Eco Smart Bright White with a 90 CRI):
lightbulb-wars-14-0911-xln-89449162.jpg

Compared to an Ecosmart True Color CFL (88 CRI):
lightbulb-wars-12-0911-xln-56112789.jpg
As I read elsewhere, a higher CRI (88 in this case) doesn't necessarily make a great light.

The LED above looks pretty decent compared to the remote phosphor graphs:
http://www.lamptech.co.uk/Spec Sheets/LEDi 230-12K58RP830-B22d Philips.htm
http://www.cineolighting.com/index.php/pages/product_hs/108

Incandescent is still better, with 99.7 CRI and very smooth spectral gradation (Philips Soft White bulb):
lightbulb-wars-02-0911-xln-81349517.jpg
(they of course use a lot more energy, get hot, and don't last as long).

Not clear what LEDs are in the consumer Cree bulbs, however the spectral graph looks similar to the above LED graph (a smaller blue lobe and a larger red lobe): http://www.cree.com/~/media/Files/C...d Modules/XLamp/Data and Binning/XLampXTE.pdf
 
You must log in or register to reply here.
Back
Top