THE IDEA BEHID THE TEST

How does the camera chip react on LED - compare to natural light sources?
 
Are there any color difference in a RAW file that comes from a LED source versus natural light sources as Daylight or Tungsten?  Does the limited color spectrum in the LED give the human skin a kind of «plastic / digital»  surface in post production?

By John Christian Rosenlund, Cinematographer, FNF: 

 

How does the camera chip react on LED - compare to natural light sources?
 
Are there any color difference in a RAW file that comes from a LED source versus natural light sources as Daylight or Tungsten?  Does the limited color spectrum in the LED give the human skin a kind of «plastic / digital»  surface in post production? 


 

Why another LED test?

Previous LED tests have in common that they have been measured directly from the lamp and generally only with CRI values. Our focus have been to test how the light react on the human skin and examine how the colors are received in the camera file. 

 

Based on experience from my last feature where I got the oportunity to test a new type of LED, I have the impression that the Deep Blue LED (that emits light from 380 Nanometer, and using three phosphors) penetrate the skin differently than two phosphor blue 430 Nm  LED. 

 

In my opinion, the visual result for Deep Blue LED gives a «healthier» look on the skin, than the blue chip LED, widely used in the industry. This is my experience only, and what I have measured. What we do know is that Light that goes below 430 Nanometer start to penetrate the dead skin surface before it bounce back to the camera. Blue LED at 430 Nm bounce of the dead skin surface only, before it reach the Camera sensor. I have found no literature or technical information about this related to cinematography. Our LED test, and specially your opinion, after examin the Arri Raw images, will hopefully give us new and more exact answers. 

Some questions where we would like to get better answers:
  • What happens after the light has moved from the lamp through the skin and then reflected back to the camera chip? 

  • What happen with the surface of the skin tone when you colorgrade a constant image, lit by the different light sources - from real Daylight, Candle -light to LED ? 

  • Are there any differences in the reflected colors from the skin and direct light from the light source?

  • Are there opportunities or limitations in the various LED sources up against classic sources such as HMI / Tungsten / Daylight and Candle light?

   
 

We selected 9 different types of Bi Color LED. 
  • Measureed the light both directly from the source and reflected colors from the skin. 

  • The values are  presented in both CRI and TLCI (Television Lighting Consistency Index 2012)

We measured  the quality of color on the different sources only. 

We have tried to achieve a constant quality of softness for the different light sources. 

We used a color-neutral diffusion and bounced light to keep a constant softness on the different sources. 


 

Equipment: 

The test was shot on Arri Alexa with a Angenieux Optimo 24-290mm T4-5.6 exept for the Candle light test where we used a Cooke 5/i 40 mm at T1.4 for Wide shot and Cooke 5/i 100mm for the Close Up.

We did not use any filters.  We recorded the test in both  RAW 16:9 and ProRes 4:4:4:4. 
 

The same setup for all light-sources : 
  • A close-up of a dark-skinned and fair-skinned actor with a grey and color chart between them. 

  • A medium Two shots of a dark-skinned and fair-skinned actor with a grey and color chart between them. 

 

DAYLIGHT

We used the  row of windows at Storyline Camera Rental as the natural light source. (Covered  the inner-walls with black velvet /molton to get rid of reflected light from the inside)

Some selected LED was also tested up againt real Daylight (LED from left side of the face) 

 

TUNGSTEN 3200K.

The test was  done in a "black room" with similar layout as the daylight set-up.

 

CANDLE LIGHT

The test was done in a "black room". Similar layout as the tungsten / daylight - setup. 

 

HMI 5600K.

The test was done in a "black room" with similar layout as the daylight set-up. 

 

Measure presision and quality

To be able to measure high presision light quality, our Sprectormeter have been calibratet up against a known and traceable reference light sourch. Our Stellarnet spectrometer was calibrated to a 'Bentham Instruments' halogen lamp, traceable to the NPL (National Physical Laboratory) standarder. 

The measurement was preformed by Per Bøhler, one of the initiators of the TLCI (Television Lighting Consistency Index 2012)

Initiator of the test, Cinematographer John Christian Rosenlund, is founder and part-owner of one of the test vendors, Softlights. Per Bøhler (former chief engineer at NRK) and FNF have assured neutrality and reliability of the test. 

LED up against real Daylight.