What It Does
Intensifier filters: Pumping Up The Color
A color intensifier filter such as the Hoya Intensifier increases the color saturation in the red-orange area of the spectrum while not adding an over-all color shift to the photo. The most common time to use a filter like this is in autumn when the leaves are changing color. The Intensifier filter will increase the color saturation of all the red and red-orange leaves for fall foliage photos that really pop with color.
But this filter has other uses as well. One that has caught on strong in the last few years is in astrophotography. The Intensifier actually acts to block an area of the spectrum that is common for normal street lamps. These street lamps are the largest contributor to light pollution in the night sky. Using an Intensifier filter will yield a more natural, less brownish cast to night-sky photos saving time and aggravation in post processing later.
How It Works
Intensifier filters use a specialized optical glass known as “didymium” glass. This glass enhances one area of the color spectrum by suppressing the colors around it. This has the effect of increasing the color saturation of the color of the filter is formulated to intensify.
Insert transmission curve diagram to demonstrate
Originally developed to block a yellowish light of approximately 589nm which was emitted by hot sodium in molten glass. This is about the same wavelength that common Sodium vapor street lights emit. This filter experienced a rebirth in the 1990s as a fall-color intensifier. With the advent of digital cameras interest in the filter trailed off and but around 2014 it experienced a resurgence as an astrophotography filter.