Purkinje effect

With reference to "Intro to LP I - Sky glow" 's Purkinje effect

The Purkinje effect is the tendency for the peak sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.

This effect introduces a difference in color contrast under different levels of illumination. For instance, in bright sunlight, geranium flowers appear bright red against the dull green of their leaves, but in the same scene viewed at dusk, the contrast is reversed, with the petals appearing a dull red and the leaves appearing bright green.

In visual astronomy, the Purkinje shift can affect visual estimates of variable stars when using comparison stars of different colors, especially if one of the stars is red. The effect occurs because the color-sensitive cones in the retina are most sensitive to yellow light, whereas the rods, which are more light-sensitive (and thus more important in low light) but which do not distinguish colours, respond best to green-blue light. This is why we become virtually color-blind under low levels of illumination, for instance moonlight.

The Purkinje effect occurs at the transition between primary use of the photopic (cone- based) and scotopic (rod-based) systems: as intensity dims, the rods take over, and before color disappears completely, it shifts towards the rods' top sensitivity. The insensitivity of rods to long-wavelength light is related to the use of red lights under certain special circumstances - for example, on the bridges of submarines, in research laboratories, or during naked-eye astronomy. Under most circumstances, either the photopic system or scotopic system is active, not both. Under low light levels, the cones are insensitive and do not function. Under high light levels, the rods are saturated, and do not function. (Eg. Cats have great night vision but they are not sensitive to colours; because they have more rods than cones in their eyes)

Under conditions where it is desirable to have both systems active, red lights provide a solution. Submarines are dimly lit to conserve energy, but the bridge must be lit to allow crew members to read instrument panels. By using red lights, the cones can receive enough light to provide photopic vision (namely the high-acuity vision required for reading; albeit under red light the photopic vision will be monochromatic). Because the rods are not saturated by bright light and are not sensitive to long-wavelength red light, however, the crew member remains dark adapted. If the crew member left the bridge for some dimly lit part of the ship, rather than being functionally blind (as would be the case had the bridge been illuminated by full spectrum light), the scotopic system is fully dark adapted and able to provide high-sensitivity vision.

Red lights are also often used in research settings. Many research animals (such as rats and mice) have only scotopic vision - they do not have cone photoreceptors. By using red lights, the animal subjects remain "in the dark" (the active period for nocturnal animals), but the human researchers, who have one kind of cone that is sensitive to long wavelengths, are able to read instruments or perform procedures that would be impractical even with a fully dark adapted (but low acuity) scotopic vision.

For the same reason, zoo displays of nocturnal animals often are illuminated with red light. Red lights are also used as safelights in darkrooms: many photographic papers are engineered to be insensitive to red light.

INTRODUCTION TO LIGHT POLLUTION III - clutter

As a continuation to our previous post!


We proudly present once again!:

*CLUTTER*

Clutter refers to the excessive groupings of lights. Groupings of lights can potentially cause accidents to occur. For example, clutter is particularly obvious on roads where the street lights are badly designed or where brightly lit advertising surrounds the roadways. Depending on the motive of the person or organization who installed the lights, their placement and design may even be intended to distract drivers which contribute to accidents.
Figure 5 Clutter of lights reflected on glassClutter is also a hazard in the aviation environment, if aviation safety lighting must compete for pilot attention with non-relevant lighting. For instance, runway lighting may be confused with an array of suburban commercial lighting and aircraft collision avoidance lights may be confused with ground lights.

INTRODUCTION TO LIGHT POLLUTION II - glare

As a continuation to our previous post:


We proudly present:

*GLARE* (- . -)

Often after a bright camera flashes in your eye, do you feel this uncomfortable sensation, where your eyes is trying to recover from that blotches of dark areas in your vision? That is a result of BLINDING GLARE!

Glare is usually the result of excessive contrast between bright and dark areas in the field of view. For example, glare can be associated with directly viewing the filament of an unshielded or badly shielded light. Light shining into the eyes of pedestrians and drivers can obscure night vision for up to an hour after exposure. Caused by high contrast between light and dark areas, glare can also make it difficult for the human eye to adjust to the differences in brightness.

Glare is particularly an issue in road safety, as bright and/or badly shielded lights around roads may contribute to accidents as it might unexpectedly, partially blind drivers and pedestrians.

Glare can also result in reduced contrast, due to light scattering in the eye by excessive brightness, or to reflection of light from dark areas in the field of vision, with luminance similar to the background luminance. This kind of glare is a particular instance of disability glare, called veiling glare.

Glare can be categorized into different types. One such classification is described in a book by Bob Mizon, coordinator for the British Astronomical Association's Campaign for Dark Skies. According to this classification:

Blinding Glare describes effects such as that caused by staring into the Sun. It is completely blinding and leaves temporary or permanent vision deficiencies.

Disability Glare describes effects such as being blinded by an oncoming cars lights, or light scattering in fog or in the eye reduces contrast, as well as reflections from print and other dark areas that render them bright, with significant reduction in sight capabilities.

Discomfort Glare does not typically cause a dangerous situation in itself, and is annoying and irritating at best. It can potentially cause fatigue if experienced over extended periods, and lead to more serious depression, lethargy, mentally-stressful disorders.

Due to our uploader's LIMITED tech-savvy capability, the pictures turned out like that. (look upwards) SORRY! x(

INTRODUCTION TO LIGHT POLLUTION I - sky glow

For starters, it is very important that we thoroughly understand how Light pollution affects us and also others in our daily life.

A survey by the International Dark Sky Association shows that every 3 out of 5 people who resides in cosmopolitan cities such as New York, Shanghai, Tokyo, Hong Kong and the crux is that, these people are not even aware of it! Or, they just dismiss it as a trivial affair.

Therefore this page will be dedicated to explaining exactly what Light pollution is about, what causes it, how to calculate light, how fast it travels.

1. What is Light Pollution?
Light pollution is basically: Excess obstrusive man-made light which causes illumination of the night sky with artificial light against the natural background radiation.
Usually classified into Sky Glow/Urban Sky glow, glare, clutter, light-trespass and over-illumination
1. Sky Glow


Sky glow refers to the "glow" effect that can be seen over populated areas. It is the combination of all light reflected from all sources of illumination escaping up into the sky, including light that are badly-directed, being redirected by the atmosphere back toward the ground.

This redirection is due to the wavelength of the light when the air is very clear (with very little aerosols). "Rayleigh scattering" dominates in such clear air, making the sky appear blue in the daytime. When there is significant aerosol which is typical of most modern conditions that are moe or less polluted.

The redirected light has less dependence on wavelength, making a whiter daytime sky. Due to this Rayleigh effect, and because of the eye's increased sensitivity to white or blue-rich light sources when adapted to very low light levels (Purkinje effect/ Dark adaptation), white or blue-rich light contributes significantly more to sky-glow than an equal amount of yellow light.



Sky glow is of particular irritation to astronomers, as it reduces contrast in the night sky to the extent where it may even become impossible to see any but the brightest stars. Taking away our "starry starry nights".
This pair of pictures shows us the effects of skyglow. The first picture (left) with its starry starry sky is Ontario on 14/08/03 the night of the East Coast USA Blackout. The second picture (right) is also Ontario on 15/08/03 the night after the EC USA Blackout where the lights are back on again and there werent any stars visible.