Light Meter Basics
A light meter is a device used to measure the level of visible light in a particular area. Light meters are used by growers to ensure that their plants receive the optimal level of light. They are also used by lighting designers to give a room enough light without wasting energy on excessive lighting. One of the most well-known uses of light meters is in photography. At a given shutter speed, a picture needs to receive just enough light to come out well. A light meter allows a photographer to adjust the shutter speed and other photographic settings so that the picture will come out right.
Extinction Meters
The oldest light meters were extinction meters. They used a series of progressively darker light filters, each labeled with a letter or number. A photographer would look through the filters and see which was the darkest one which still let light through. He would then look up the label from that filter in a chart, which would tell him set up his photographic equipment for any shutter speed he might want to use.
Problems with Extinction Meters
The problem with this meter is that the sensitivity of the human eye to light varies from moment to moment and from person to person. If a photographer is in an area with a lot of glare or has just looked at a bright light, she may not be able to see light through a filter that would otherwise look transparent. Conversely, a photographer in a dark area might be able to see a little light through a dark filter because his pupils have dilated in response to the darkness. Photographers needed a better meter--one that gave a precise objective measure of light intensity.
Modern Meters
Modern meters are electronic. They usually use a CdS cell mounted in the front to detect the level of light. A CdS cell is a resistor, a type of semiconductor that resists the flow of current. When the meter is turned on, light flows through the CdS cell into a circuit that measures its resistance. When light hits the CdS cell, it excites the electrons in the cell, making it easier for electric current to flow through the cell and thereby lowering the resistance. A small computer measures the decrease in resistance and uses this to calculate the precise amount of light that is hitting the cell. This gives a precise measurement of the light level in a particular location instead of an approximate level based on human observation.
Tags: light through, shutter speed, enough light, Extinction Meters, level light, light meters
