Head Lights and Star Lights

By Clark M. Thomas

Who hasn't noticed those horribly bright blue-white automobile head lights? They nearly drive us to crash, yet they are legal. Why? The manufacturers say they are merely the same wattage as regular, more yellow, halogen lights: so it must be OUR fault; or maybe we are just unaccustomed to superior lighting, and we need time to visually adapt to progress.

The problem with those new light sources is not their wattage. It is where they emit light on the visible spectrum. Measurements of light intensity are made in laboratories with silicon devices, which are not equally sensitive to the same frequencies as our eyes. Furthermore, our eyes have two types of sensory cells (rods and cones), with very different frequency sensitivities.

As astronomers, we well know that certain things are easy to see with the naked eye, and some things are either hard to see, or impossible. Planetary nebulae, emission nebulae, and even such exotics as the Horsehead and California nebulae, all are helped by different filters that remove competing light to leave very narrow bands of light for our eyes.

The scotopic (dark-adapted, rod-using) human eye is most sensitive to blue and green light. That is why green laser pointers work great, but red lasers operating at longer frequencies appear much less bright, even with equal output energy.

CCD chips are more sensitive toward the red end of the spectrum, which is why they shine with emission neblae, and with H-beta sources such as the Horsehead and California nebulae.

In contrast, visual OIII filters enhance planetary nebulae which are brighter toward the blue and green frequencies where the scotopic human eye is most sensitive. A CCD chip can image planetary nebulae anyway, but less efficiently.

I had already figured out in my mind the basic reason for why those bluish white car lights are so offensive. A technical article by W. J. Collins, of Collins Electro Optics LLC, in Colorado, provided me with the hard numbers. You can access this article at the following URL: http://ceoptics.com/ccd/ccd_tech_report.html It also goes into fascinating detail about the spectral frequencies of different types of galaxies, stars, etc.

It is generally agreed that the spectral range of human vision is between ~380 to ~760 nanometers (nm= billionths of a meter). At 505 nm (which is green) the minimum threshold of perceptible vision is the base ratio 1. In comparison, yellow light requires 100 times the intensity to produce the equivalent scotopic response, orange 1,000x, and red 10,000x ... but blue requires only 2x!

During photopic (cone) vision (light levels above approximately 10 LUX), the peak visual sensitivity shifts upwards to 555 nm, which is closer to yellow. It is interesting to note that those bluish white car lights, when we see them on during the day under blue skies, are not as irritating.