 |
|
|
|
|
|
|
|
|
 |
 |
 |
 |
 |
 |
 |
 |
|
 |
||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||||
 |
 |
Artificial Light Sources 1)While the spectral distribution of radiation of natural light sources is an approximately continuous sequence, artificial light sources often exhibit irregularities in emission. These irregularities are not necessarily perceived by the human eye since it is fairly flexible in adapting to the prevailing illumination and still sees relatively strong color tones as white when no means of comparison is available. This is not the case with film, it reacts fairly sensitively to aberrations in color. One is therefore well-advised to acquaint oneself with the properties of artiticial light, especially as the types of lamps in use are becoming more and more numerous. One has to be particularly careful with gas discharge lamps such as fluorescent lamps. Incandescent lamps
Incandescent lamps are temperature radiators at color temperatures between 2900 K and 2400 K, depending on output. High color temperature has high wattage and low color temperature a low wattage. The filament of incandescent lamps is made of metals with very high melting points ( osmium, tungsten ). Inert gases such as argon and crypton are used as fill-gases. If incandescent lamps are operated with over- and undervoltage, the color temperature is changed and so is their service life. Approximately 90% of the radiation output from incandescent lamps lies in the infrared range with maximum radiation of 1000 to 2000 nm. The blue content of radiation in light from an incandescent lamp is on the other hand very low.
As the name indicates, halogen lamps contain lorine, chlorine, bromine or iodine. Halogen lamps emit a light of 3200 K to 3400 K. They are ideal for some types of artificial light films, but too blue for artificial light films adapted for higher color temperatures. It is imprtant to take note of the color balance of each film, there are considerable differences of up to 3 decamireds among artificial light films. The advantages of halogen lamps in the burning area of light sources have led to numerous variations. Nowadays, there are metal-vapor lamps with a daylight characteristic of 6000 K.
Photoflood lamps are a type of incandescent lamp with the very high color temperature of 3200 K. The normal designation Nitraphot indicates the fill-gas used ( nitrogen ). Their relatively short life of between 10 and 100 hours is a result of its high output. Most artificial lightfilms are adapted to the light of photoflood lamps so that normally no filter is required when using ehtm.
The xenon high pressure lamp produces light with a very similar radiation to daylight ( 6000 K ). Radiation output is very high. For this reason, xenon light is very often used in studio spotlights. They are five times as efficient for work with daylight film than photoflood lamps. Heat production is also lower for the same output. Smaller lamps should also be of interest for the home studio as this light only requires a very weakacting filter for correction of illumination ( elimination of ultraviolet ).
Fluorescent lamps are lamps in which a gas is excited to produce self-radiation in the ultraviolet range. UV radiation in its turn excites a coating on the inner side of the glass tube causing it to light up. Because of their frequent use, they are an important source of light for photography. Fluorescent lamps are not temperature radiators; the spectral distribution of energy is of a different kind for every make and sometimes more, sometimes less irregular. For this reason, shots taken with fluorescent lighting often have a color cast which can be of all shades depending on the type of lamp. This color cast is particularly unpleasant since it dominates the whole shot, super-imposing all colors. An improvement can only be achieved if one knows the type of lamp involved. If this is not possible, the only thing one can do is to make a test shot. Our color correction filters can then be used to remedy the situation in each case. Conversion filters are only suitable in rare cases. The problem of color cast in shots taken in fluorescent light is becoming less and less important since manufacturers are producing better and better fluorescent coatings.
The mercury maximum pressure lamp has a similar construction and function as the xenon lamp. Its flux is also similar. The appropriate light source when a high level of UV radiation is required. The lamp is also admirably suited to excite fluoresscence. The mercury high pressure lamp is often used for street ligthing. One recognizes them by their dull blue light immediately after being switched on. The mercury-vapor lamps are not temperature radiators, correction is carried out by the B+W UV protection filter.
On the main arterial roads out of big cities, especially at cross-roads and pedestrian crossings, the yellowish-orange light from sodium-vapor lamps is used in most cases. The contrast to mercury-vapor lmaps with its greenish-blue light is abvious to all. These lamps radiate light within a narrowly limited spectrum or monochrome light. In such cases, it is very difficult to achieve a change by means of filters.
|
|
|
FON : +49 ( 0 ) 671 601-0 ....... FAX : +49 ( 0 ) 671 601-302 ....... E-mail : filter@schneiderkreuznach.com |  |
