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Old 12th March 2013, 02:17 PM
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rich rich is offline
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Join Date: Nov 2008
Location: Shebbear, N Devon
Posts: 1,642

The point is, the incident light on the card target is not "white" but blue, and many of the longer wavelengths are missing.

The scope lenses will be designed to bring basically two bands to a common focus (achromatic) or three bands (apochromatic) and in the case of the wind free tunnel, some of these bands are actually absent or only there in very small proportion.

An object will normally only be able to reflect the light falling upon it, which is why we can't see things in their "true" colour under sodium street lights. The target card doesn't appear totally blue - or at least my brain tells me it isn't blue - so some frequencies other than blue are getting through the pigmented plastic.
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