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Re: Measurement Geometry
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Re: Measurement Geometry


  • Subject: Re: Measurement Geometry
  • From: Ken Fleisher <email@hidden>
  • Date: Thu, 16 Feb 2006 14:42:30 -0500

On Feb 16, 2006, at 1:05 PM, Steve Kale wrote:

Thanks for that. I'm not sure, though, how it addressed the question of why
an integrating sphere spectrophotometer is better for measuring colour
samples on canvas or other course materials.

Think about what happens when you take a light and skim it across the surface of an oil painting. The light exaggerates the texture in the painting by creating shadows from one side. If you've ever done copy photography, you will know that you usually try to set up two banks of lights, one on each side, that are about the same intensity. The idea is to eliminate the effect of shadows created from texture.


So this relates to measurement geometry because a 45/0 geometry is similar in concept to skimming a single light across an oil painting from one side only. So you can see that if your light/sensor are placed slightly differently, the sensor may have a different response because what is being illuminated varies due to the changing texture.

With d/0, you have diffuse illumination, so the sample is evenly illuminated from all sides, similar to the banks of light in copy photography. the effect of shadows is cancelled out because of the ring of illumination. In addition, the integrating sphere works in a way that your sensor is not pointed directly at the sample, but rather the light reflecting off of your sample is bounced all around the inside of the sphere thus averaging, or integrating, the color. The sensor measures off of an inside surface of the sphere. This is how effects of texture are minimized when measuring with an integrating sphere.

At this point, I feel I must state more clearly that I'm not actually suggesting using integrating spheres for measuring targets printed on canvas or other textured materials. For one thing, I don't think there are any automated integrating spheres that can do the job like a spectroscan or eye-one with the new iO. Instead, you can use these 45/0 devices if you make many measurements and average them. This should produce reasonable results for these types of materials. As I stated before, make sure that after you read the target in once, you "remove" it from the table, then replace it before taking another measurement. You might also try to not center the registration exactly the same way each time on purpose to be sure you are taking readings from multiple locations of each patch.

I've been typing this in a bit of a rush, so I hope I've been clear.

Ken Fleisher

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