Re: Colored sparks (photography) (fwd)

---------- Forwarded message ----------
Date: Mon, 15 Dec 1997 06:57:36 -0500 (EST)
From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Colored sparks (photography)

Hello All,
Gary said:
>I shot a roll of film on my coil.  While most of the shots were the
>normal time exposures of the sparks, I was especially anxious to see
>photos of the brilliant yellow sparks I had produced with table salt.
>Upon getting the prints back, the normal shots came out super, but I was
>utterly disappointed in the yellow spark shots.  The arcs were all
>properly exposed, but their color showed absolutely nothing different
>from the normal sparks.  It's like I was photographing ghosts!  How could
>visual and photographic perception be so different?
>FWIW, I was using ASA200 Kodacolor print film, between /f2.0 and /f2.8.
>Between 5-8 second exposure times produced some real nice shots.
        Sparks emit considerable u.v. as well as a broad spectrum of colors.
Normally, a u.v. filter and the coatings on the lenses blocks most of the
hard u.v., but some of the longer u.v. wavelengths get through as well as
quite a bit of the blue end of the spectrum.  Meanwhile, the human eye is
most sensitive in the yellow-green end of the spectrum, and we see blue
relatively poorly, and don't perceive u.v. at all.
        To complicate matters further, the film has dye layers in it to
achieve the broad sensitivity to colors, and these layers have different
spectral sensitivities.  You might experiment with a video camera instead,
using a series of color filters to see what wavelengths are being emitted.
Mark S. Rzeszotarski, Ph.D.