Light Pollution Filters: Effective Solution Or Marketing Gimmick?

Light pollution filters are tools used in astrophotography to reduce light pollution in photographs. They are also used by amateur astronomers to improve visibility. The effectiveness of light pollution filters depends on the type of light pollution, the type of filter, and the type of camera used. While some sources claim that light pollution filters are ineffective or produce undesirable results, others assert that they are essential for basic astrophotography in light-polluted areas. The best way to determine the effectiveness of a light pollution filter is to review photographs taken by others using similar equipment in comparable light pollution conditions.

Light pollution filters are ineffective against LED lights

Light pollution filters are designed to reduce light pollution, particularly from sodium and mercury vapour lamps, which emit light at specific wavelengths. However, with the increasing adoption of LED lighting, the effectiveness of these filters has been called into question.

LED lights emit light across a broader spectrum, including blue frequencies that scatter more and contribute to light pollution. Unlike sodium and mercury vapour lights, which have primary spectrum wavelengths, white LEDs emit across all frequencies, making it challenging to block or filter their light effectively. This shift from orange to whiter hues in street lighting has led to concerns about the diminishing effectiveness of light pollution filters.

While some filters, like the Optolong L-Pro, have been designed to mitigate light pollution from LED street lights, their effectiveness is limited. The L-Pro filter, for instance, targets specific light pollution wavelengths associated with mercury and sodium vapour lamps but struggles against the broader spectrum of LED lights. As a result, it may produce images with a noticeable blue colour cast, requiring additional image processing to achieve a "near normal" colour balance.

Furthermore, the added brightness of LEDs often outweighs the benefits of light-cut-off shielding, as the light still spreads across a wide area. This makes it particularly challenging for astronomers to block the night sky-polluting effects of LED lighting. The intense white light emitted by most LEDs also reduces the effectiveness of light pollution filters, rendering them almost irrelevant.

In conclusion, while light pollution filters can provide some relief in certain situations, they are generally ineffective against LED lights due to the broad spectrum of light emitted by LEDs and the limitations of current filtering technology. To address this issue, organisations like the IDA are advocating for the adoption of Amber LED street lighting, which is less detrimental to nature, humans, and astronomy.

Filters are useful for astrophotography

Light pollution is a serious issue for astrophotographers, and light pollution filters can be a good way to combat this. They can help reduce the orange/yellow glow of light pollution and improve contrast, allowing you to capture the colours and light emitted by your astrophotography subject. However, not everyone agrees that using a light pollution filter is the best way to do astrophotography.

There are two main types of light pollution filters for colour cameras: broadband filters (broad-spectrum, multi-broadband) and multi-narrowband (often dual-narrowband) filters. Broadband filters are effective for capturing galaxies, reflection nebulae, dark nebulae, and natural star colours. Narrowband filters are best used on emission nebulae and supernova remnants, and they are especially useful in light-polluted areas. The type of camera you use will determine the right light pollution filter for you. For example, a stock DSLR camera will be much less effective when paired with a multi-narrowband filter.

There are also clip-in style filters, which are a better placement for filtration in the optical train and are more affordable than large threaded filters. The Optolong L-Pro Clip-In Filter is a good option, but it is only compatible with certain camera bodies.

Light pollution filters are also becoming less effective due to the increasing prevalence of LED lighting, which emits light across the entire spectrum. In addition, filters can introduce a colour cast of their own, which can be difficult to remove during post-processing. Therefore, it is important to consider the level of light pollution in your area and whether a filter is necessary, or if it would be better to invest in travelling to a darker location for astrophotography.

Filters are not useful for stargazing

Light pollution filters are not useful for stargazing. They are mainly designed to improve astrophotography by reducing the effects of light pollution. While they can help capture impressive deep-sky images, they are not meant for viewing stars. Their passbands are meant to reduce other light, providing better contrast on nebulae.

Light pollution filters selectively block specific wavelengths of light in the visible spectrum, particularly those associated with artificial light sources such as street lamps. They do not work effectively on LED lighting, which emits light over the entire spectrum. Instead, narrowband filters that transmit H-alpha light are recommended for use with LED lighting.

The effectiveness of a light pollution filter also depends on the imaging location and the types of projects being photographed. For example, multi-narrowband filters are extremely effective for photographing emission nebulae, planetary nebulae, and supernova remnants from heavy light pollution. However, they are not a good match for galaxies or other broadband targets.

Additionally, the quality of the images produced with light pollution filters can be misleading. A master of image processing may be able to make a terrible image look good, while someone with basic image processing skills and a better light pollution filter may struggle to produce a good-looking image. Therefore, comparing the performance of light pollution filters based on end-result images may not be an accurate representation of their effectiveness.

In conclusion, light pollution filters are not useful for stargazing as they are designed specifically for astrophotography. Their effectiveness varies depending on the lighting conditions, the type of camera used, and the targets being photographed.

Filters block specific parts of the wavelength

Light pollution filters selectively block specific wavelengths of light in the visible spectrum, typically associated with artificial light sources such as street lamps. The primary wavelength blocked falls in the brown-orange range, produced by sodium vapour lamps, which emit light at around 589 nm. Low-pressure sodium vapour lamps emit light at 589.3 nm, with only 15% transmittance through the filter. Mercury vapour lamps are more challenging to filter, as they emit light across many wavelengths, appearing blue (435.8 nm), green (546.1 nm), or yellow-orange (578.2 nm). LED lighting, which emits light across the entire spectrum, is also difficult to filter and has rendered light pollution filters less effective.

The L-Pro filter is a light pollution filter that targets the main light pollution wavelengths, removing airglow and mercury/sodium vapour lamp emissions while retaining normal colours. It has a very interesting bandpass selection, with a wavy transmission line featuring five humps. Each of the four major dips is a known source of artificial light pollution that the filter has selectively blocked. The filter darkens the image and adds a slight blue colour cast.

The UHC filter is another light pollution filter that drastically darkens the image and adds a purple colour cast, as it only allows red and blue light to pass through. The filtered images appear unnatural but can be improved through editing.

Infrared cut-off filters are another example of wavelength-selective filters. They transmit visible wavelengths between 400 and 700 nm while blocking higher wavelengths in the infrared region (700 to 2500+ nm). These filters are used to protect infrared-sensitive sensors from infrared wavelengths.

Wavelength-selective liquid crystal tunable filters (LCTFs) are constructed from a stack of fixed filters and linear polarizers. The spectral region passed by LCTFs depends on the choice of polarizers, optical coatings, and liquid crystal characteristics. LCTFs typically perform well in the 400 to 700 nm region.

Filters are of limited use due to LED lights

Light pollution filters are of limited use due to the increasing prevalence of LED lights. LED lights emit light across the entire spectrum, and this includes wavelengths that traditional light pollution filters are designed to block. As a result, these filters are becoming less effective at reducing light pollution.

The effectiveness of light pollution filters depends on the type of lighting in a given location. Traditional light sources, such as mercury vapour or sodium lamps, emit light within specific wavelength ranges that filters can block out. However, with the transition to LED lighting in many cities and neighbourhoods, light pollution filters are becoming less useful.

LED lighting blankets the entire visible spectrum with no gaps, unlike the light emitted by traditional light sources. This means that light pollution filters, which are designed to block specific wavelength ranges, are unable to effectively reduce the light pollution caused by LEDs. The shift from traditional lighting to LED lighting is resulting in a noticeable change in the colour of the night sky, from an orange hue to a whiter hue, as LEDs emit a "cool white" light.

While some filters, such as the Optolong L-Pro, have been designed to mitigate the effects of LED lighting, they may not be effective in areas with a high density of LED lights. In addition, these filters can affect the colour balance of images, requiring careful image processing to achieve a "near-normal" colour balance. Therefore, for those interested in astrophotography, it may be necessary to travel to darker locations to capture images of the night sky without the interference of light pollution.

It is worth noting that light pollution filters are not intended for viewing stars, as stars emit light within the wavelength ranges that these filters block. Instead, light pollution filters are designed to improve the visibility of nebulae by reducing light pollution and providing better contrast. However, with the increasing prevalence of LED lighting, even nebulae may become more challenging to observe without specialised filters or equipment.

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