Mireds and Correcting the Color Temperature of a Light

As I've recently shown in blog posts, sometimes a video light doesn't exactly have the color temperature that the manufacturer suggests it does.  Case in point, the K55 bulbs from Kino-Flo that are actually closer to 7000K than 5500K.

There's a somewhat less well known method for using filters to compensate for the color temperature of a light fixture using 'Mireds' (pronounced "my-reds").  'Mired' is an abbreviation for "micro reciprocal degree".  Cinematography.com has a nicely written posting about it.  There's also a Wikipedia page, of course.

The concept is quite simple.  Let's say you have a tungsten light, which we measured at 3092K in our last blog post, and you want to compensate that light to 5600K daylight.  The Mired shift is calculated as the difference between the reciprocal of each color temperature multiplied by 1,000,000.

So...

Tungsten = 1,000,000/3092 = 323.4

and...

5600K daylight = 1,000,000/5600 = 178.6

thus...

Mired value = 178.6 - 323.4 = -144.8

Not surprisingly, companies that make filter products, such as Lee Filters, provide Mired calculators.  When I entered in an input light of 3100K and an output of 5600K I found  the following results.  So combining a 3/4 CTB with their 1/4 CTB would actually give me close to what I want.  Using a full CTB doesn't get me quite as close.  They also smartly suggest a plus green filter in case I need to use the tungsten lamp in combination with fluorescent lights.

---

---

I would have verified this with real world measurements, but SOMEONE (you know who you are!) in the shop returned the UPRTek spectrometer this week!  I'm curious how well this calculation works with discontinuous spectrums light LED and fluorescent.  Obviously it works well for tungsten, HMI, and plasma; all of which have nice continuous spectrums and color rendering.

Let's do another example out of curiosity.  How about we turn those Kino-Flo Diva K55 bulbs to actual 5500K.  The calculator is telling me that the nearest match is 444 - Eighth C.T. Straw or 212 - L.C.T Yellow.  I would likely try the 212 filter first since that biases the light toward 5600K.

---

 

---

One thing that Mired calculation does not do for you is compensate for green/magenta shifts on LED and fluorescent lights.  That's the type of thing that can be quickly analyzed and figured out if you have a device like the UPRTek spectrometer and know how to read (x,y) coordinates.  It's not as technical as it sounds.  In fact it's pretty dead-brain simple with that spectrometer...if I still had it.

Video Lighting for High Frame Rate Video

There was a question posted on Cinematography.com recently about lighting for high frame rate video.  The forum member wanted to know which lights to use.  That got me thinking, which lights are appropriate for the FS-700 we have in rental stock?  The FS-700 can shoot anywhere from 24fps up to 960fps, but as we all know, the footage is only really usable for film/video work up to 240fps.

On the forum, M. David Mullen - who I think is going for a world record commenting on the most cinematography web sites - commented that he tends to use 5k tungsten fixtures when shooting high speed video.  I was wondering if that was overkill or was it really true that you need that sort of massive bulb?  The amount of flicker a tungsten light produces is mainly related to the thermal mass (i.e. physical size) of the bulb.  The bigger it is, the longer it takes to warm and cool between 60Hz cycles from AC wall power.  So all of you DIY-ers out there can forget about using household 100W bulbs.  You need some real video-purposed lights here.  For HMI and LED, the flicker is related to the ballast design.

I grabbed four lights off the show room floor and lit up a Macbeth chart.  The lights used were...

• Arri 300W Tungsten
• Desisti 1kW Tungsten
• Ushio Pro Panel LED
• Film Gear 575W HMI

I purposely set the Ushio LED panel at 100% on it's internal dimmer since it's output is fairly dim compared to the tungsten Fresnels I was using.  Dimming at different levels wasn't tested.  I also set the HMI ballast to flicker-free mode because that was the best performance the light could achieve for this test.  My goal wasn't to defeat the lights, just to show what they do under their best performance and what we can recommend to customers that need to shoot high-speed video.

As expected, the Arri 300W light shows significant flicker at all frame rates.  What surprised me is that the Desisti 1k didn't really show much flicker, if any.  The Ushio and Film Gear lights were both flicker free at all frame rates.

In the video below the top half of the frame is a static frame grab.  The bottom half of the frame is the continuous video capture.  I wanted to make sure there was a consistent frame of reference to see the flicker effects easier.

---

Comparison of Video Lights for High Frame Rate Video from Video Gear on Vimeo.

---

As always, if you have questions please call the shop and we'll help you pick the best lights to use for any video capture situation.  Salvador would love to talk lights with you, especially after the new Mole-Richardson LED lights came through the shop!  When the customer left with the M-R LEDs I'm pretty sure Salvador had to hold back a tear, but don't tell him I wrote that.