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    Re: SNO-T "night" sight tube?
    From: Bill Morris
    Date: 2018 Nov 22, 11:53 -0800

    Brad,

    You write that anti reflective coatings attenuate the light transmitted through a lens. You may like to reflect on the following.

    Around  8 to 10 percent of light may be lost by reflection at the surfaces of a lens, depending on the angle of incidence and the refractive index of the glass, and in a two lens system, the transmission losses may exceed 20 percent. The corresponding loss of brightness would not be perceptable (we can just about perceive a halving of brightness), but it  would result in a disproportionate loss of contrast, important for example when there is poor contrast at the horizon.

    Some of the light reflected by one surface may be reflected at a second, with a net result that it is returned in its original direction. But there is no guarantee that it will come to the same focus as the light that has not been reflected, resulting in ghost images. If it arrives nowhere near an image plane, it introduces diffuse illumination called flare.

    The coating on the SNO-T lenses appears to be purplish blue in reflected light and reduces the co-efficent of reflection to around one percent for visual work in daylight.

    Kind regards

    Bill Morris

    Pukenui

    New Zealand

       
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