 |
 |
 |
|
|
|||||||||
|
|
|
|||||||||
|
|
|
|||||||||
|
|
|
|||||||||
|
|
|
|||||||||
|
|
|
|
||||
|
|
|
|
||||
|
|
|
|
||||
|
|
|
|
||||
|
|
|
|
||||
|
|
|
|
||||
|
|
|
|
Different illuminants
Views:
Spectra of light reflected from a piece of white paper. Units of radiance in W/m2/sr/nm
Views:
Different lightbulbs create very different colors.
What we see depends on the illuminant, and iluminants vary considerably, as shown at right. For many of us, the most common type of light is the the standard tungsten light bulb (spectra at right). Halogen light and candle light are very similar, the former having a slightly steeper onset in the short-wave region, the latter with a concave, almost exponentially rising curve. Second, is a fluorescent light in an office. When electric current is passed through the gas, it produces ultraviolet light, which strikes a phorphor coating, making it fluoresce. The 'white' of a computer screen has features comparable to the spectrum shown, in that it also consists of both broad and sharp peaks, typically broad peaks at 440 and 530 nm, and sharp peaks at 620 and 710 nm. Last, is sunlight after sunset in rainy weather conditions. The light of a blue sky has a more flat spectrum across all wavelengths, whereas the light coming from grey clouds at midday is more intense in the middle and long-wave region than in short-wave. Dips occurring at 690, 730 and 760 nm are typical for all daylight spectra and indicate probably the absorption spectra of compounds such as water. An additional kind of lamp, used frequently in offices, stores, and to light public spaces, is the vapor lamp. Vapor lamps contain a gas under low pressure, but the gas glows with visible light when electricity passes though it. The color of the light depends on the gas.
The first electric lamps, arc lamps, were first developed in the early 1800s. later in the 19th century, the filament lightbulb and the vapor lamp were developed. See the sections on incandescent lightbulbs, vapor lamps, and color constancy for more information. In brief, modern electric lamps produce light in three different ways.
Following is a summary of the colors seen in the light box above.
Bulb
Purpose
In the light box above...
Daylight bulb
Imitates natural daylight
The colored walls around the bulb appear just as the would in daylight.
Regular incandescent bulb
Common household light bulb
The filament emits a yellowish-white light, and the walls have a strong yellow tint.
Mercury vapor lamp
First high intensity discharge lamps, but now mostly obsolete.
Left wall is blue, but right is blue-grey. The light has no red, so the right wall cannot reflect it. Light has some yellow wavelengths, seen on bottom wall.
Low-pressure sodium lamp
sometimes used for street lighting
The colors around this lamp show that its light is an almost pure yellow. Controls light pollution because the monochromatic light does not interfere with astronomical observation.
High-pressure sodium lamp
Street lighting in cities, sports arenas
Give most objects a similar color as daylight.