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Figuring Out the Temperature of a Light Source in Photography
Color temperature is a strange, non-intuitive part of photography. Until you pick up a camera, take a photo, and edit it on a computer, you might never notice it. That's because your eyes do a great job of adjusting to the dominant light source, and unless a light falls far outside the norm your eyes will perceive it as pretty normal.
The problem is that different light sources cast different colors on your subject. The sun is a different color from your flash which is a different color from a tungsten spotlight. When you take a picture with your digital camera, the image processor more or less does what your eyes do. It takes a best guess at the "temperature" or color of the light, and then it uses that to reproduce the colors in the photo the way your eyes see it.
When done properly, everything looks great. When it isn't, everything looks a bit off. Or, if the camera guesses really wrong, it can look really off.
Take the photo to the right, for example. It's pretty close to the proper color temperature, but it's a little bit off. The camera guessed that the light source was slightly warmer than it actually was, and thus the entire scene looks a little more yellow than it really should.
This is most noticeable when you look at the background. This should appear white, but it has a slightly yellowish hue to it.
Measuring Color Temperature in Adobe Photoshop Lightroom
If you capture your images in RAW, then you can do a lot of editing after the fact to adjust the white balance of a photo and correct for the color temperature of the light. Any RAW editing software will allow you to do this, like Adobe Camera Raw or Canon's Digital Photo Professional. As an example, let's take a look at how to measure color temperature in Adobe Photoshop Lightroom, my editing software of choice.
Once you've imported your RAW image into Lightroom, go to the Develop module. One of the first things you'll see in the right-hand panel is a section for white balance (see the screenshot to the right). This well tell you the current color temperature that is being applied, and you can use the drop down menu to select from a series of presets.
You want to click on the eye-dropper. Then, hover the eye-dropper over a neutral part of the photo. This means something that should have no color. Gray is ideal, but white areas work as well. In this case, I hovered over the white background. What comes up is a magnified view of the image and the color of the specific pixels.
If these pixels are not neutral, you know the white balance is off. In this case, they look a little tan. To fix this, click on the point. Lightroom will then apply a new white balance setting to make it neutral, thus correcting the entire image.
When you look at the white balance area of the panel, you'll see a new measurement. In my case, Lightroom adjusted the white balance from 3400K to 2850K. This made the entire image slightly more cool, or blue, and it removed the tinge from the background.
Using a Gray Card to Measure Color Temperature
If it's so easy to just change the white balance in Lightroom, why do you need to measure color temperature anyway? Why can't you just use the eye dropper to fix your photos?
Well, sometimes a photo isn't so simple. If you have multiple light sources, like an ambient light level and a set of strobes or flashes, these may well be different temperatures. Trying to adjust an image with conflicting light sources is not as easy to do in Lightroom. It can be done in Photoshop, but it's a lot of work.
Instead, you'll want to measure the temperature of the white source and then adjust them all using gels. One solution is to use a light meter that measures color temperature, but these are expensive. For the mere mortrals (i.e. non professional, hobbyists and/or enthusiasts), there's a simpler solution - a gray card.
The process is simple. Take a picture of the gray card, import it into Lightroom (or your editing software of choice), and then use the eye-dropper to adjust the color temperature. This will give you a reading.
Let's say you are going to take pictures in a room, and you'll be mixing the ambient light with your speedlight. First, take a photo of the gray card under ambient light without the flash, and measure the color of that light. Then, take a photo of the gray card using the flash, and measure the color of that light.
If the speedlight is "warmer" - i.e. it has a higher temperature - then apply a blue gel to make it cooler. If the speedlight is cooler, than apply an orange gel or CTO gel to do the opposite.
The goal is to match the temperature of the two light sources, so that everything looks normal, even, and natural in your final picture.
Final Note on Editing Software
To do this, you'll need some editing software.
The screenshots and specific instructions included in this hub refer to Adobe Photoshop Lightroom 4. It's an excellent piece of software, and it is a must have for anyone that is trying to organize a large collection of photos.
If you don't want to invest in this software, you can opt for the free route. Canon offers Digital Photography Professional with all of its dSLRs, and this will allow you to do the white balance adjustments and measurements discussed above. Read this article to find out how to download and install Canon's Digital Photography Professional. This is ok if you have a small number of photos to work with.