Month: January 2017

Avoid Blue-Rich LED Lighting

As Dodgeville (and many other towns and cities) are planning to replace their streetlights with LED luminaires, it is imperative that we use LEDs with a CCT (correlated color temperature) of 3000 K or less (Jin et al. 2015).  This is a “warm” white light (similar to incandescent) rather than the “cold” blue-rich light often seen with LEDs.  Outdoor LED luminaires often come in at least three “flavors”: 3000K, 4000K, and 5000K.  For example, American Electric Lighting’s Autobahn Series.  5000K luminaires provide the bluest light, and should be avoided at all costs.  Of these three, 3000K would be best, and if 2700K is offered, use that.

Why does this matter?  On June 14, 2016, the American Medical Association issued guidance on this subject.

High-intensity LED lighting designs emit a large amount of blue light that appears white to the naked eye and create worse nighttime glare than conventional lighting.  Discomfort and disability from intense, blue-rich LED lighting can decrease visual acuity and safety, resulting in concerns and creating a road hazard.

The detrimental effects of high-intensity LED lighting are not limited to humans.  Excessive outdoor lighting disrupts many species that need a dark environment.  For instance, poorly designed LED lighting disorients some bird, insect, turtle and fish species, and U.S. national parks have adopted optimal lighting designs and practices that minimize the effects of light pollution on the environment.

Recognizing the detrimental effects of poorly-designed, high-intensity LED lighting, the AMA encourages communities to minimize and control blue-rich environmental lighting by using the lowest emission of blue light possible to reduce glare.  The AMA recommends an intensity threshold for optimal LED lighting that minimizes blue-rich light.  The AMA also recommends all LED lighting should be properly shielded to minimize glare and detrimental human health and environmental effects, and consideration should be given to utilize the ability of LED lighting to be dimmed for off-peak time periods.

Incidentally, for your residential lighting needs, a good local source for LED bulbs that are not blue-rich is Madison Lighting.  They have many LED bulbs in both 3000 K and 2700 K. I use 2700K bulbs exclusively in my home, and the warm white light they provide is an excellent replacement for incandescent and compact fluorescent bulbs.  Never purchase LED lighting without knowing the color temperature of the lights.

If you’re skeptical that the color temperature of LEDs is an important issue, I suggest you purchase a 2700K bulb and a 4000K or 5000K bulb with the same output lumens and compare them in your home.  I believe that you will much prefer the 2700K lighting.  If 2700K lighting is best for your home, then why should it not be best for outdoor lighting as well?

Besides, most streetlighting is currently high pressure sodium (HPS), which is inherently non-blue-rich.  You will find that 2700K LED lights offers better color rendering than HPS without the need to go to even bluer lights.

If you have ever been irritated at night by an oncoming vehicle with those awful “blue” headlights, you’ve experienced firsthand why blue-rich light in our nighttime environment must be minimized.

Why are 4000K and 5000K LED lights so prevalent?  They are easier and cheaper to manufacture, but with increased demand of 2700K and 3000K LED lights, economies of scale will reduce their cost, which today are generally slightly higher than blue-rich LEDs.

Now, a bit more about why blue light at night can be detrimental to human health, and the primary reason why the AMA issued a guidance on this subject.

In addition to image-forming rods and cones, there exist non-image-forming retinal cells in the human eye called intrinsically photosensitive retinal ganglion cells (ipRGCs) that help regulate our circadian rhythms.  Studies have shown that blue light is far more disruptive to our circadian rhythms than redder light (Lockley et al. 2003).

Now, on to the environment.  Using a clever technique that compared sky brightness at several locations on several nights both with and without snow cover, Fabio Falchi (Falchi 2011) determined that at least 60% of light going up into the night sky is direct waste lighting, and 40% or less is reflected light.  This is as good an argument as any that we still have a long way to go towards using only full-cutoff luminaires that do not produce any direct uplight.  Blue light scatters much more in the night sky than red light, and this is due to Rayleigh scattering which tells us that the amount of scattering is proportional to the inverse of the wavelength of light to the fourth power, σs ∝ 1 / λ4.  This also explains why the daytime sky is blue.

Bluer wavelengths of light thus increase artificial sky glow to a much greater extent than redder wavelengths do.  Not only is an increase in blue light bad for astronomy, but its impact on the natural world is likely to be adverse as well.

Falchi recommends a total ban of wavelengths shorter than 540 nm for nighttime lighting, both outdoor and indoor.  He goes on to say that, at the very least, no more light shortward of 540 nm should be allowed than that currently emitted by high pressure sodium lamps, lumen for lumen.

References
Falchi, F. 2011, MNRAS, 412, 33
Falchi, F. 2016, The World Atlas of Light Pollution, p. 44
Jin, H., Jin, S., Chen, L., et al. 2015, IEEE Photonics Journal. 7(6), 1-9
Lockley, S. W., et al. 2003, J Clin Endocrinol Metab. 88(9), 4502–5

Polarization of Starlight

The space between stars is not a perfect vacuum. It contains gas molecules and dust grains, although they are few and far between by any terrestrial standard. In the presence of a magnetic field, many types of interstellar dust grains line up in a way that is reminiscent of iron filings near a bar magnet. When light from a star passes through a region of space with magnetically-aligned dust grains (though in this case the short axis of the dust grains aligns with the local magnetic field), light with the electric field vector perpendicular to the long axis of the grains is less likely to be absorbed by the grains than light whose electric field vector is parallel to the long axis of the grains. This causes the light passing through such regions of space to become slightly polarized, and the polarization of starlight is something we can measure easily here on Earth. In this way, the strength and orientation of invisible interstellar or circumstellar magnetic fields can be determined at a distance.

Various astrophysical processes result in polarized electromagnetic radiation.  The differential absorption already mentioned polarizes the light from all stars to one degree or another.  Only the Sun—which is vastly nearer—offers us almost completely unpolarized light. Scattering of light off of interstellar clouds and planetary surfaces also results in polarization.  Finally, both synchrotron and cyclotron emission produce a characteristic polarization.

The polarization of starlight can be measured by the use of a polarimeter attached to the telescope.  Unlike standard photometry, polarization is simpler to measure with ground-based telescopes because the measurements are relative rather than absolute and, under normal circumstances, the Earth’s atmosphere does not affect the polarization state of incoming light.  Care must be taken, however, to ensure that the telescope itself does not create instrumental polarization due to oblique reflections.  Placing the polarimeter at the unfolded Cassegrain focus is one desirable configuration (Hough 2006).

References
Hough, J. 2006, A&G, 47, 3.31

Eridanus Delights

The sixth largest constellation in the sky stretches from near Rigel on the west side of Orion down to 1st-magnitude lucida Achernar (declination -57°), a star that rotates so rapidly that its polar diameter is not even ¾ its equatorial diameter (Domiciano de Souza et al. 2014).  Achernar (α Eri) is appropriately named.  It means “The End of the River” in Arabic.

Eridanus, the River, contains two very special, easily seen, stars. 40 Eridani (also known as Keid and Omicron2 Eridani), a visual triple star system (magnitudes 4.4, 9.5, and 11.2) just 16.3 light years away, presents the most easily observed white dwarf star, 9.5-magnitude 40 Eri B, visible in any telescope.

A little further west we can find 3.7-magnitude Epsilon Eridani, the nearest star beyond the Alpha Centauri system thought to harbor one or more planets. Compared to our Sun, ε Eri is cooler (K2V), much younger (200-800 Myr), and somewhat metal-deficient (74% solar), and it is just 10.5 light years away. This youthful star still sports a dusty disk between radii 35 and 75 AU (Greaves et al. 1998), inside of which its putative planet, Epsilon Eridani b—at least 0.6 to 0.9 Jupiter masses—travels around the star in a highly elliptical orbit, completing one revolution every 6.85 to 7.26 years. At periastron, Epsilon Eridani b lies between 1.0 and 2.1 AU from its parent star, and at apastron, its distance is 4.9 to 5.8 AU (Mizuki et al. 2016). However, the existence of this or any other planets in the system is still far from certain, primarily due to the high level of photospheric activity that is difficult to disentangle from the radial velocity signals of any possible orbiting planets (Giguere et al. 2016).

References
Domiciano de Souza, A., Kervella, P., et al. 2014, A&A, 569, A10
Giguere, M. J., Fischer, D. A., et al. 2016, ApJ, 824, 150
Greaves, J. S., Holland, W. S., et al. 1998, ApJL, 506, L133
Mizuki, T., Yamada, T., et al. 2016, A&A, 595, A79

A Space Shuttle Remembrance

On Tuesday, December 19, 2006, I witnessed a delightful event: the Space Shuttle Discovery and the International Space Station traveling together through the western sky, only about 1° apart.

Around 6:34 p.m., I spotted a -1 magnitude International Space Station (ISS) traveling NE above the western horizon. It quickly became apparent that there was a +1 magnitude point of light moving right along with the ISS, leading it by about one degree. It was the Space Shuttle Discovery, which had undocked from the ISS just 2h25m earlier (4:09 p.m.)!

I quickly surmised that Discovery must have fired retrorockets to put some distance between it and the ISS by lowering Discovery‘s altitude. Since Discovery was at a lower altitude, it had been orbiting faster, which is why it was leading the ISS by about a degree. As the pair approached the constellation Lyra, further evidence of Discovery‘s lower altitude occurred when it disappeared into the shadow of the Earth several degrees further west of where the ISS disappeared a few seconds later.