7 Replies Latest reply on Mar 24, 2016 10:55 AM by altarace

    Finding light source color temperature with neutral card

    altarace

      Hi all;

      I have an adjustable color LED strip that I'd like to get as close as possible to 6700 Kelvin. I'm hoping I can take photos of a neutral card under the light and adjust the LED color based on the temperature/RGB values I get from Lightroom (5.7.1).  Is that possible? Can someone help with the workflow on how to do that?

       

      Many thanks

        • 1. Re: Finding light source color temperature with neutral card
          Jao vdL Adobe Community Professional & MVP

          This is certainly possible but realize you will not get a a calibrated color temperature this way. Typical workflow would be to shoot raw (obviously), preferably tethered. Shoot an image, do the eyedropper color balance on the grey card and observe the resulting temperature indicated in Lightroom. Adjust your LED and repeat until you are close to your target. The problem is that the value you get will depend on the chosen profile and will depend on the specific color model used in Lightroom, which is different from other software and so the value you get is only valid within that. You could calibrate the scale by using a few known light sources and doing the same thing and then you will have created a calibration of the Lightroom scale that you could use to translate the 6700K target into the Lightroom scale.

          • 2. Re: Finding light source color temperature with neutral card
            altarace Level 1

            Thanks for the detailed reply. I followed your recommendation and used a known light source (5K CFL bulb http://chilp.it/d45e381) and LR indeed came up with a 5250K temp using the eyedropper. But when I tried the same with the LED strip (set to the white color http://chilp.it/739e8c0) the resulting temp was 50K (LR max) so I'm not sure what that means. BDW, full red LED resulted in 2K, full green in 8K, the yellow was 4.6K and anything with the blue was at 50K.  So is the blue somehow throwing LR off?

            • 3. Re: Finding light source color temperature with neutral card
              Jao vdL Adobe Community Professional & MVP

              I was afraid of that. The numbers really only make sense for a black body radiator such as a tungsten bulb or the sun that is a broad-band light source. CFL bulbs typically are well fairly well done using a series of rare-earth phosphors that are tuned to human vision and with fairly broad-band emission. If you use more monochromatic light sources such as single color LEDs you might not get correct numbers. This really depends on the wavelengths coming out of the LEDs. If the color filters on your camera are not at the same wavelengths as the LED and the three color sensitivities of the camera sensor are not the same as your eyes, you can get to a situation where your eye sees neutral colors while the camera sees something completely non-neutral. This is evident from your two raw files. Your LED illuminated card probably looked grey to the eye but to the camera it looked purple. This effect is called metameric failure and is common if your light's spectrum is very "spiky". That's probably what is happening in your case. You likely have three narrow band LEDs that you are mixing. You can't really apply the single "color temperature" black body model to this and you then get the strange results you observe. There are special high-color rendering index LEDs that have much broader spectrum light emission that have much less problems with metameric failure and that should work well for this purpose.

              • 4. Re: Finding light source color temperature with neutral card
                Jao vdL Adobe Community Professional & MVP

                BTW, the blue LED probably has a fairly large UV component. Many camera sensors are more sensitive to UV light than human vision is. You might get a bit better results with a UV filter on your camera but I would not trust any white balance value you get out of this.

                • 5. Re: Finding light source color temperature with neutral card
                  altarace Level 1

                  Thanks again. I completely forgot about that but yea, I had the Tiffen UV Filter on when I took the photos

                  • 6. Re: Finding light source color temperature with neutral card
                    ssprengel Adobe Community Professional & MVP

                    Also, LEDs are typically pulsed so the color may change if you are using a fast shutter.  Try a long shutter, maybe 1 second or longer if you can, to minimize the variation in color due to flicker, or at least take one exposure with a long shutter and one with a shorter shutter typical of what you might use when photographing in the studio to compare if the color is different.

                     

                    As far as UV affecting things, if the background you're photographing glows in UV light, this means the background is converting UV to visible light, and you may not be able to block the effects of the UV just by a UV filter since the background will have converted the UV to a different, more visible, wavelength.  You could test your background with a UV pet-spot-detector flashlight with UV LEDs to see if it glows.

                    • 7. Re: Finding light source color temperature with neutral card
                      altarace Level 1

                      Thanks, had to give this a try.

                      So no matter what f stop or exposure time I used (ranging from f2-f22 and 1/320-1") LR eyedropper still registered any LED color that incorporated the blue (including the so called white) at 50K.

                       

                      It's actually kind of cool (in a perverse way) to see the metameric failure JAo mentioned take place.