Sodium vapor lamps, which are used in street lights, cause much of the light pollution produced by cities. These lamps emit yellowish light with a wavelength around 589nm. Didymium is particularly good at blocking this wavelength and finds its way into many light pollution filters, including the Hoya Intensifier (also known as the Hoya Red Enhancer). Here are a few test results showing what the Hoya Intensifier can do to help reduce the effect of light pollution in your astro images.
Before and After (Raw)
Below are a couple of shots taken from the Grand Canyon with some of the darkest skies in the United States. Even here, light pollution is visible near the horizon. These shots were taken one right after the other using a Canon EOS 6D body and a Canon EF 35mm F/1.4L USM lens. Each exposure was one minute long (tracked), taken at F/2.8 and ISO1600.
Figure 1: Hoya Intensifier, Before and After
The effect is plainly obvious; the yellow gradient is much less present in the filtered version of the image. Less obvious is the filter’s overall darkening effect across the frame. In these 100% crops, notice how the dim stars in the original image are also less present.
Figure 2: Hoya Intensifier, Before and After, 100% Crops
Before and After (Exposure-Corrected)
The fact that the stars themselves are dimmer means that this filter (like all others) cuts a little bit of light from the entire spectrum, and not just the undesired light pollution. Here are a couple of white-balance-corrected flats, one without the filter and one with, taken with identical camera and lens settings. Notice how the filtered flat is darker overall than the unfiltered flat.
Figure 3: Unfiltered vs Filtered response
Doing a little math, we find that the filtered flat is 17% darker on average, or -0.26ev when compared with the unfiltered version. We can compensate for this -0.26ev difference by shooting 20% longer subs when the filter is on, or by using exposure correction in post. Here is the original unfiltered image compared with an exposure-corrected version of the filtered image. A custom white balance was derived from the filtered flat and applied to the filtered light frame as well, counteracting the natural bluish tint the filter imparts upon the final image.
Figure 4: Raw image vs Filtered with White Balance adjustments and Exposure Correction
Looking at the 100% crops, we can see that the dimmer stars are retained when we compensate for the -0.26ev loss of light when using the filter.
Figure 5: Hoya Intensifier, Before and After, 100% Crops, with WB + Exposure Correction
The Hoya Intensifier obviously improves light pollution in images when used at a Dark Sky Site. It will no doubt provide improvements to more light-polluted areas, but the effect will not be as apparent. With a -0.26ev reduction in overall light passed to the sensor, it will make whatever lens you attach it to almost a third-stop slower, and you will need to use a custom white balance to retain good color balance. So long as you can deal with the increased exposure time, this filter can increase your final image Signal-to-Noise ratio, making it easier to pull faint details out in post.