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Old November 28th, 2009, 09:05 AM   #1
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How can I test/measure CRI?

Is there a way to do this using simple materials and procedures with either a video camera or SLR?
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Old November 28th, 2009, 03:29 PM   #2
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You could probably do it this way:

Shoot a color chart under direct sunlight with your camera at the built-in daylight setting. This is your reference. Use a grey card to set exposure (50 IRE), then shoot the color chart.

This first step is important because hue and saturation is highly dependent upon exposure.

To compare the rendition qualities of an unknown light source, set your camera to the same built-in daylight setting, set your exposure with the grey card (50 IRE), and shoot the same color chart.

By comparing the color chart images you can see where your light source may be deficient. You won't be able to get a CRI number but you'll be able to see how it performs compared to daylight, which what a CRI rating is supposed to provide. CRI is a quantified rating. What you do with the camera is a bit more subjective, although you can easily get color values off the images of the chart.

Be aware of something called metamerism. It's how certain materials will change hue under lights with different spectral distributions. What looks blue under daylight might look purple under tungsten, for example.

And your camera may not provide completely accurate measurements. However it will provide you with an excellent idea of how the light will interact with your particular camera.
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Old November 28th, 2009, 04:36 PM   #3
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I asked this question a long time ago, even though I was sure I knew the answer, I was correct in my thoughts. Use a spectrum analyzer.

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Old November 29th, 2009, 01:33 AM   #4
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Quote:
Originally Posted by Bill Hamell View Post
I asked this question a long time ago, even though I was sure I knew the answer, I was correct in my thoughts. Use a spectrum analyzer.

Bill
I actually have one of those, too, and it certainly makes it easier. You can see the spectral qualities of the light and immediately understand why colors look the way they do.

However, they're expensive. And the one I got, which was actually designed to profile printers and sample materials for color, is pretty cumbersome. It also needs to be attached to a computer.
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