Can I estimate a light source’s color temperature with a DSLR without a gray card?
Asked 11/8/2014
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I tried estimating the correlated color temperature (CCT) of household lights by photographing a white sheet of paper lit by one source at a time, then using Lightroom’s white balance eyedropper on the paper. My results were inconsistent: a Nikon SB-600 flash measured around 7400K, while an LED spot measured about 2800K and fluorescent bulbs about 2400–2650K. A studio bulb rated at 5400K later measured about 5500K with the same method.
Why might a flash read so much cooler than expected, and is there a cleaner way to estimate a light source’s color temperature without buying a dedicated gray card?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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You can use any non-reflective object that is truly white or neutral gray and be sure to expose so than none of the color channels are fully saturated (blown out). It also might help to switch your lens to manual focus and defocus so as to blur the target to average out any variances that could be caused by curvature of your target and varying light sources in different directions. Some gray microfiber cleaning cloths are close enough. So is a square cut from a piece of neutral gray construction paper with a matte finish. Most white papers have a slight bluish tint to them and don't do as well as a good gray paper will. At one time the blank pages near the end of Canon EOS Instruction manuals were supposed to be a correctly balance color of white. I've used them a time or two with good results. Just be sure to underexpose!
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
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Yes, you can estimate it, but your target matters a lot. A plain white sheet of paper is often a poor reference because many papers have a slight blue tint or optical brighteners, which can push the reading cooler and help explain an unexpectedly high value like 7400K.
A better approach is to use any matte, neutral target that is truly gray or white, not reflective, and not blown out in any channel. Good improvised options can include a neutral gray paper or a gray microfiber cloth if it’s actually color-neutral.
For more reliable results:
- use a matte neutral target
- avoid reflections and mixed light
- expose so no RGB channel clips
- slightly defocus to average texture and uneven illumination
Lightroom’s WB eyedropper can give a useful approximation, but it is not a precision color meter, especially with non-neutral targets. So yes, there is a cleaner method without a gray card: use another neutral, non-reflective object and careful exposure. Your studio-bulb result suggests the basic technique can work when the reference is closer to neutral.
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