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Old Jun 18th 2008, 11:11 PM   #1
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Reflection Reflectance and Gray Cards

I am a little confused about the whole concept of photography Gray Cards and reflectance of a 90% white card.

I am trying to measure the reflection of certain materials, but am not sure of a good method.

The diffuse reflection is the amount of light diffused due to a rough surface.

The specular reflection is the amount of light reflected on a relatively flat microscopically non-rough surface such as a plane mirror.

However, for example, an experiment was conducted using a 18% gray card from Kodak, and using a photodiode, a parallel light source hitting the gray card had the reflection intensity measured with the photodiode. The distance from source to gray card, and card to photodiode is the same, but the photodiode is at a 45 degree angle to the graycard plane, and the source is at a 90 degree angle relative to the gray card plane.

Thus, my question is... does this 18% gray card Reflectance meaning and 90% white card reflectance really mean that 18% of light in the stated wavelength range will be reflected, and 82% will be absorbed?

As from the experiment data... the actual photodiode voltage readings gave over 20% intensity reflectance from a DIFFUSION reference.

My problem with the previous experiment:
Since the light is parallel and 90 degrees to the gray card, the reflection direction would be directly back at the light source. Thus measuring 45 degrees would not be measuring specular reflection but diffuse reflection.....

But even this diffuse reflection is already over 20%?.... is there a difference between this 18% gray, color reflection and intensity reflection?
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