Re: Lacrosse 5: analysis of brightness

From: Thierry Marais (
Date: Fri Oct 21 2005 - 16:23:50 EDT

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    Hi Ted and list,
    Your brightness analysis is very interesting.
    > In the event of a consensus that Lacrosse 5 is less red, could that explain its
    > greater apparent brightness? 
    In my humble opinion, yes.
    The single fact that we are able to distinguish the usual orange/yellow colour
    for the Lacrosses means we are in photopic vision, the one used in daylight ;
    and not the scotopic vision, used for dark scenes and colourless. The eye
    photopic response peaks at 0.55 um (micrometers), that is yellow (rather on 
    the green side). The response falls by one half at 0.61 um, orange-red. If the
    light reflected by the Lacrosses was perfectly monochromatic at these two
    wavelenghts, our eye-guessed magnitudes would differ by 0.75 magnitudes,
    all other things unchanged (surface, reflectivity). Of course, the real spectrum
    is wide and obvioulsy presents a cutoff for short wavelenghts, but because of
    the steepness of the eye response between red and yellow, the exact location
    of this cutoff can significantly influence the estimated magnitude. But it doesn't
    mean that there is no change in the reflecting area...
    Thierry Marais.
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