The IRIX 95mm Edge Light Pollution Filter costs about USD 179.00, which is a lot of money for a specific purpose filter. (It’s also available in 67mm, 72mm, 77mm, and 82mm diameters.) While no filter can remove all light pollution, I wanted to see if this light pollution filter could help with my Milky Way photography.
Most filters that are described as light pollution filters, the Irix filters included, remove the wavelength of 589 nm, that of Sodium Vapor lights used for street lights. This type of artificial lighting creates a yellow haze in night images.
It is worth noting that many lighting types now being used by municipalities donʼt have the same colors as Sodium lights, and a filter like this would not be useful for them.
Sources of the Light Pollution
Light pollution is any light that is produced by humans in the night time environment. We all know large cities will create a large glow on the horizon of your image.
In the article, “Light Pollution Filter Mega-Comparison by B&H Photo Video,” Kirk Keyes discusses the light emitted by the most popular exterior lighting types and briefly discusses types of filters.
Light pollution works the same way as your camera’s flash; the brightness is a function of distance squared. Not all light pollution sources are large cities. A small, all-night convenience store 500 yards away could cause more problems for your image than a small town 10 miles away.
Does This Filter Work?
The simple answer is yes.
On the night of August 5th, 2019, while in Maine, I made a side-by-side comparison of the Milky Way without and then without the IRIX filter.
The moonset was at 10:58 pm, but the humidity was 92% that night, so the moonlight stayed visible for awhile. High tide was at 3:57 am, so I needed to finish taking all pictures by 12:30 am. If I didn’t, I wouldn’t be able to walk back along the beach. I would have to try and find my way through briers in the dark. Plus, I had one more location I wanted to photograph after these test images. All of this is to explain the bright moonglow low on the right side of the photos.
The two images below are the last without the IRIX filter, taken at 11:11:43 pm, and the first with it, at 11:16:17 pm. I chose these so the brightness of the moon glow could be as close as possible.
All of these images used the same camera settings, and the only change done in Lightroom was lens correction.
- Sony A7III
- IRIX 15mm Blackstone f/2.4
- ISO 6,400
- 15 sec
Right away, there is a very noticeable overall color difference between the two images. The image on the right, with the filter, has a much bluer colorcast.
Since the photos are in RAW, the white balance can still be edited. But I have found that it takes much less work to adjust the shots with the light pollution filter than without it.
How Much Light is Lost?
Not much overall light is lost. According to the transmittance graph from the IRIX website (include later in this article), about 95% of most colors of light are allowed to pass. But how does that translate to real use in my pictures?
The histograms for the top half of the image, the part without a direct view of the light pollution and the moon glow, are very similar. While the blue channels are almost identical, with the histogram of the image taken without the filter is just very slightly brighter.
Both the red and green channels, while being the same shape in both images, have very different brightnesses. The histogram of the image with the filter is shifted to the left about 1/3 of the width of the peak. There is less yellow in the top half of the photo as a result.
The histograms for the bottom half of the image, with the city lights, light pollution, and moon glow, are noticeably different.
The blue channels again are almost identical, with the histogram for the image without the filter being just slightly brighter. There is more of a difference between the red and green channels between the two than there was for the top half of the image.
Without the filter, the red and green channels are brighter with more data to the right, and the peak is further right. The image with the light pollution filter has darker red and greens. Overall, the IRIX Edge filter does block a small amount of light based on the histograms above.
Why I Chose My Location
I choose this side of the island for two reasons.
First was that this location has beaches allowing me to walk there when the tide is low to a little over half tide. The front side of the island has no beaches, and even in daylight, it is hard to walk the seaweed-covered rocks. The rocks and seaweed don’t leave a place to set up the camera.
The second reason I chose the backside of the island is that it would block a lot of the “local” light pollution from Yarmouth and Falmouth Maine. On that side of the island, there are about 3 miles of open water to the mainland. At night the front of the cabin can be lit by cars coming down the road to one of the campgrounds.
Image 7 shows my shooting location for these images. The small dotted line is approximately the path of the light pollution in my photos. That is the line to the South Portland/Cape Elizabeth area 15 miles away.
I was sitting in a Bortle Class 4 location looking towards Portland, Maine, and a Bortle Class 8+ horizon.
Image 8 shows the whole area from my image location in the top right to the primary light pollution source in the bottom left. The other large islands in between the two sites are scarcely populated and not a source of light pollution for these images.
The glow from the Portland area was quite bright that night because the humidity was 82%. All of that moisture in the air provided sources for the light to bounce off of it in all directions.
IRIX Edge Light Pollution Filter Specifications
|Available sizes||67, 72, 77, 82, 86, 95 mm|
|Thread size||Standard x 1.0|
|Blocked light wave size||589 nm|
|Frame thickness, without threads||3.5 mm|
|Anti-reflective coatings||Both sides, multi-layer|
|Special type coating||Double-sided waterproof and oil repellent NANO coating|
|Frame Material||Highly Durable Aluminum|
|Light blocking||+0.3 Stops|
The IRIX Edge Light Pollution filter comes with a rigid plastic case with foam padding. The filter case seems very strong and would provide excellent protection. A nice feature is that there are two finger indents in the foam to assist with getting the filter out of the case.
The body of the case is approximately 4.5″ square, plus about 0.75″ for the loop on the top. This a nice if you only have one or maybe two filters in your bag, but the case is too big and bulky if you have more than a couple.
Visible light is just a tiny part of the electromagnetic spectrum that ranges from violet, 380 nm, to red, 700 nm. Just adjacent to the visible spectrum are ultraviolet and infrared.
The human eye is not able to see the light in the ultraviolet and infrared parts of the spectrum and beyond, however camera sensors can. Camera manufacturers put filters in front of the sensor to block this light to prevent these frequencies from affecting your images.
The IRIX filter has a significant absorbance centered on 589.3 nm, which is the primary color emitted by Low-Pressure Sodium lamps. There is another absorbance centered at about 740 nm. That is past the boundary of red and into the infrared. This second absorbance should not have a significant impact on night images unless your camera is “astro-modified” to specifically let some of the infrared light in for deep space objects.
Learn more about astro-modified cameras and modification types here.
Lastly, there is a smaller absorbance at about 530 nm; this is a green color. There is some of this color in both Low-Pressure Sodium and High-Pressure Sodium lights, but this isn’t the primary color that is blocked by the IRIX filter.
Looking to the Future
Municipalities are moving away from these older types of lighting to newer LED lights. For each benefit of LED lights, there is a drawback to the night environment.
LED lights are brighter; the effect of that is obvious. LED lights produce a bluer, whiter light, which created issues. That is because a broad-spectrum white light is not easy to filter out in post-processing. There is also research showing the addition of blue to lighting at night may affect many animals that aren’t used to blue light at night during all phases of the moon.
LED lights are more energy-efficient, so they are environmentally beneficial in that aspect. LED chips are also directional; they need diffusers to spread the light into broad areas. Some dark-sky friendly places with LED lights are using shutters to keep the light shining only at the ground, which is a good step.
As more white-light LED lights replace Low-Pressure Sodium vapor lights, these types of filters will be less useful.
IRIX Edge Light Pollution Filter Conclusions
I chose the IRIX filter for primarily one reason; it was made by the same company as my lens. That was important to me because, with a 15 mm wide-angle lens with a 95 mm filter size, I wanted to make sure the filter and frame were thin enough to produce minimal vignetting on the edges.
While I know, the two images donʼt have identical edits and look slightly different. You will always get better photos when you go to locations that have dark skies. And most of us can go and do that once a year, maybe.
For the majority of people that live on the east or west coasts, it is very tough to find dark skies. And yes, you can spend a little extra time editing to try and remove some of the light pollution glow.
But I feel the filter gives me a better starting point. So I will continue to use my light pollution filter while shooting the night sky when the scene includes a significant light pollution source. I am happy with the money I spend on this IRIX Edge Light Pollution filter.