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Monday, March 23, 2015

••◊ Review of the Sekonic C-700 Spectrometer

Sekonic sent me a demo unit of the new C-700 Spectrometer.  This meter is meant to help cinematographers, photographers, and gaffers understand where lighting issues occur and help correct them.  What it doesn't do is act as a scientific development tool with download-able spectrum spreadsheets and CIExyz plots, which is more a task for the UPRTek MK350N.  That said, what the Sekonic meter does it does much better than any other color meter tool I've seen before.


The meter is laid out like you would expect from a Sekonic product with the rotating integrating sphere at the top of the unit.  Like their new exposure meters the C-700 features a touch screen that will feel familiar to anyone with a smart phone.  On the left side of the unit there are two buttons: On and Memory.  To turn off the meter you have to press the On button for more than one second.  The memory button can save a measurement to a named file if you press it directly after taking a measurement.  This allows you to save the measurement data to a PC later via the C-700 Utility software.

As with any of the Sekonic meters, the measurement button is on the right side; appropriate for right-hander's thumbs. (second picture below)

As you may notice in the third picture below, the integrating sphere has three click-stop settings: dark calibration (bottom), ambient light (middle), and bright light/flash (top).  When you turn on the meter it has to be set in the dark calibration click stop so the meter can calibrate.  In most cases you'll use the ambient light setting when taking measurements.  If you use the bright setting with video lights it will take significantly longer to do the measurement.  The bright setting is mainly for flash photography.

Speaking of flash photography, there are two ports on the bottom of the unit.  The PC sync port can be used to take measurements with photographic flashes and the USB port is used to connect to a PC and download spectral measurements, perform firmware updates, and change meter settings.

The "hidden" black button at the bottom front of the unit is the home button, just like on a smart phone.





There is a wide range of lighting analysis tools built into the C-700 with the main focus being to help you diagnose color problems and correct them.  I didn't show the home screen below, but you'll be able to access any of these tools by tapping on the appropriate icon on the home screen.  The icon is shown the upper right corner of each of the tools.

In the first picture below you'll see a text analysis of the light.  Here I measured a light that measured a CCT of 3247K, 121 foot-candles, and a color rendering average of 95.8.  This was measured from late day golden sun entering my living room window.

In the second picture below from the spectrum analysis tool you see most of the same data, but with the spectrum plotted.  This can tell you about any problems due to missing parts of the spectrum, as common with fluorescent and LED lights.

The third picture shows how you can actually compare spectrums from a previously saved measurement.  So if you really want to fine tune a gel or match light intensity you can with this tool.

The fourth picture below shows the color rendering rating of 15 colors.  This measurement is generally associated with a CRI rating for the light.  Because this graph shows the rendering rating of each of the colors it's easy to spot problems with the color rendering of a light fixture.  We'll see an example of that later in this article.

As I mentioned previously, the real power of this meter is the ability to find and correct lighting problems.  The fifth and sixth picture below show the tools for adding a camera filter or lighting filter, respectively.  In the camera filter correction tool you can see that the light is measuring 3247K and I input a target color temperature of 5000K.  The meter is recommending that I use a Lee 80C and 82A camera correction filters to get the color temperature back to 5000K in the camera.  This would have been a more common method in the days of shooting actual film that is only available in "tungsten" or "daylight" balance. 

Likewise, in the lighting filter tool I measured a CCT of 5548K and set a target CCT of 3200K.  The tool is recommending that I use a Lee 3/4 CTO to get the light to be 3200K - which makes sense.  No more second guessing or messing around the experimental filter and gel combinations.  That's the huge advantage of this meter over an old fashioned color meter that only gives you CCT and +/-green ratings.

The final picture in this series shows the color shift of the samples relative to the target.  This helps you easily identify green/magenta shifts in the light.  The blue/amber axis basically correlates to the CCT difference, so you get that information from all the tools.  In this example it's saying that the target is much, much more blue (5000K) than the measurement (3247K) and has a magenta bias.





 


I'm not going to dwell over all the settings since that's essentially a user manual.  There are three pages of settings, but I want to cover the two most basic features.  First, you can change the LCD back light brightness.  Under normal video lighting I found the "Normal" setting to be adequate.  However if you're out in the sun you won't be able to see the display with this setting.  You'll need to use the "Bright" setting.  Obviously you want to keep the back light setting as low as possible.  The meter is powered off of 2 AA batteries.

The second feature is that you can choose between Lee and Fujifilm camera filters when the meter recommends correction filters.  Having this database built into the meter makes the meter far more advanced than other options.




Here's a measurement example from 3pm sun just outside my house.  I had to take a picture of the meter in the shade since the LCD wouldn't photograph well in the sun.  The color temperature of 5700K and Ra of 98 seems about right. 

The three graphs below are all output from the included C-700 Utility software.  The first graph shows that my target (5000K) is more amber than what I measured (5700K) - makes sense.  The next graph shows the actual spectrum of the sunshine.  The last graph shows how well the sun renders all 15 colors in the CRI plus special colors.  A very high quality light indeed!





Here's another example from early afternoon (2pm) shade.  Ambient sky light is very, very blue and in this case it's more than 9000K, but still is able to render colors well with an Ra of 98+.  The second graph below shows again that the target (5000K) is much more amber and slightly more green than the ambient sky light measurement.




In this last example I wanted to show an LED bulb, since using LED lights has been somewhat problematic for cinematographers.  Instead of showing a picture of the meter here I wanted to show a screen shot of the C-700 Utility software while viewing the "text" tab.  The LED bulb in my living room is rated for 3000K and a claimed CRI of 92.  In the Text tab we can see that the CCT is measuring 2983K, which correlates well.  However the Ra rating is about 83, which is more like what I would expect from a cheap LED bulb. 

The first graph shows that with a CCT of 2900K we aren't seeing any significant green or magenta bias, which is surprising given that this is a common problem with cheap LED bulbs.

The spectrum shows this light as a common blue LED with a phosphor coating, just as we would expect.  Notice how cyan and deep red are missing from the spectrum, unlike a real tungsten bulb.

Now this is where things get interesting.  A lot of lighting vendors make claims about the wonderful nature of "their" LED technology (everyone purchases from the same 4-5 companies).  The last graph below shows the individual rendering ratings for each of the 15 colors.  Notice how the red patch is 21.5?  You sure wouldn't want to do any product shots for a red Ferrari under these lights.  The color of the car would be WAY off.  Notice how violet has a rating of 65.2?  That means no photography of Barney dinosaur toys under these lights.  Using this graph alone you can see where color rendering problems are going to occur.  Even with orange at 77.1 you probably wouldn't want to use these lights for a cooking show that involved carrots and oranges.   ...You get the drift.


 



I'm dreading giving this meter back to Sekonic. It's a good tool to have in my bag for fixing problems.  I'll be directing a video for Video Gear showing the differences in video lights using this meter, so stay tuned for that on their blog.  I hope this overview of features was helpful.  If you have any questions please feel free to leave a comment.

2 comments:

Georges Oger said...

Hi,
Thank You for you this interesting review.
After testing this Sekonic C700, did you find the color temperature analysis was always accurate ?
THis is the main reason I would be interested to purchase such a tool: to get an accurate Color Temperature analysis.
Best Regards
Georges (france)

S. Allman said...

I don't have a more expensive tool to compare the meter to. I can say that having a spectrometer instead of a tri-color sensor *should*, in theory, make the color temperature measurement more accurate. I was able to match the color temperature of fluorescent to plasma with this meter and the camera image looked the lighting sources matched perfectly. So it's absolutely a useful tool for matching different sources in my experience. The color temperature measurements were reasonably consistent from measurement to measurement, so repeatability seems to be good as well.

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