Beating the LED streetlights: IDAS light-pollution suppression LPS-D2 filter – Astronomy Now – Astronomy Now Online
An IDAS LPS-D2 filter suitable for two-inch push-fit telescope camera accessories with an M48 × 0.75 connection thread. The filter has male and female threads on either side, hence it is stackable with other filters and M48 adaptors. The LPS-D2 is also available in 52mm and Canon APS-C clip-filter formats. All images: Ade Ashford.
At a glance
Type: light-pollution suppression filter for low-/high-pressure sodium vapour and LED lights
Coating technology: Ion-Gun Assisted Deposition (IGAD)
Suitability: DSLR and astro cameras
Connection thread: M48 × 0.75 (male and female on either side, hence stackable)
Substrate thickness: 2.5mm
Diameter of filter glass: 49mm
Price: £175 (M48 and 52mm); £185 (Canon APS-C clip filter)
Manufacturer: ICAS Enterprises, Japan
Supplier: rothervalleyoptics.co.uk
Light pollution is a regrettable fact of life for most of us. By night, the sky over our cities and towns – even villages – is increasingly awash with the glare of unnecessary or misdirected artificial light. This is not only a tremendous waste of energy, but it upsets nocturnal ecosystems and harms human health, disturbed sleep patterns and the disruption of natural circadian rhythms.
For almost three decades, Tokyo-based ICAS Enterprises’ IDAS Division has been responsible for manufacturing some of the world’s most respected interference filters for suppressing light pollution for astronomers. Their LPS-D1 filter made its debut in 1991 at a time when the main sources of artificial illumination in our towns and cities were low- and high-pressure sodium vapour and mercury vapour lamps. Fortunately for astronomers, both sodium (Na) and mercury (Hg) vapour lamps share a common characteristic: they typically emit light in specific and largely narrow wavelength bands of the spectrum – so-called emission lines – that can be removed by an interference filter.
The LPS-D1 was designed for one-shot CCD/CMOS colour cameras and DSLRs to eliminate the glow from low-pressure sodium and high-pressure mercury street lights, while substantially reducing the peak intensities of high-pressure sodium light emissions. Both the IDAS D1 and P2 filters pass the desirable spectral lines of hydrogen-beta, oxygen-III, hydrogen-alpha light from nebulae, plus diatomic carbon (C2, the so-called Swan bands) from comets. The LPS-P2 is virtually identical to the D1 except for a slightly greater red sensitivity encompassing sulphur-II emissions.
The challenge posed by white light LEDs
As many of us up and down the United Kingdom and around the world are now acutely aware, the nature of street lighting is rapidly changing. The mellow yellow glow of low-pressure sodium light is being replaced with the energy-efficient yet brilliant white glare of light-emitting diodes (LEDs). I never thought that I would lament the passing of sodium street lights, but at least their light was relatively easy to mitigate. White LEDs, on the other hand, emit what is largely a continuous spectrum across a swathe of wavelengths (or colours, if you prefer) which is far harder to filter out.
If you consult the accompanying graph that shows the spectral profile of a typical white LED in blue, you will see immediately that it emits its greatest intensity of light – almost 98 per cent transmittance – in a well-defined peak at a wavelength close to 463 nanometres (nm), which is 4.63 × 10–7 metres, or 0.000463mm. Thus, the peak emission of a typical white LED is actually in the violet end of the blue region of the visible spectrum, at wavelengths that research has shown disrupts human circadian rhythms …….