The Effect of Pre-Exposure in Spectrum Photography

The effect of pre-exposure on Eastman 40, 50, I–G, I–F, and Cramer Hi-Speed plates was investigated and was found to be similar on all five. The optimum pre-exposure density in every case, as was to be expected, was equal to the density at the beginning of the straight line portion of the D-log I curve. To eliminate the Herschel effect, it was found necessary to pre-expose with a light source having an effective wave-length longer than the wave-length of any portion of the spectrum which was to be used for density measurements. A definite fading of the latent image amounting to a density decrease of 0.07 at a density level of 0.8 was observed in plates which were developed 12 hours after exposure. This fading ceased, in the cases studied, after about twenty hours.