A light meter is an essential tool in photography to ensure properly exposed photographs. Certainly all digital cameras today will have them built in and use them to measure the amount of light and ensure a properly exposed photograph. This first article takes a look at lightmeters in general.
Light meters use a special cell, or photo sensor made from either selenium, silicon, or a photo resistor made from CdS - a cadmium sulfide resistor that is also found in electrics of street lights which decides if they should be turned on or off!
A photo sensor made from selenium or silicon is able to power up when light is available to them and the selenium and silicon cells generate their own electricity from the light.
With a photo sensor made from selenium of silicon, the amount of electricity produced by the cell depends upon the amount of light there is. If there is plenty of light, the cell will produce a lot of electricity and if there isn't much light then the photo sensor won't be able to produce as much electricity. The amount of electricity produced is how light is measured.
But CdS photo resistor works a little differently, it can't generate it's own electricity but instead works by varying the amount of electricity that passes through it, and this is dependant on the amount of light there is. So, if there is lots and lots of light the CdS cell creates a minimal amount of resistance and therefore conducts electricity very well. If there is not much light at all then the cell has a high resistance, meaning that it doesn't conduct electricity very well. This variation in the amount of resistance created by the CsS cell is how light is measured on a light meter.
whether it's a selenium or silicon cell generating electricity from the amount of light available, or a CdS cell controlling the amount of electricity in light, these results need to be displayed! To do this we connect the electricity to a meter read out. In it's simplest form, the result is displayed using an analogue meter; a needle over a scale, where you can watch the needle sway backwards and forwards with the varying light and rest on the given reading. More commonly nowadays light metering results are displayed on a modern hand-held digital read-out, typically a meter with all sorts of sophisticated computerised extras! And similarly with built-in camera meters the computer on board inside the camera processes the information to determine the correct exposure based on the settings you’ve chosen to work with.
There are a couple of different hand-held light meters available and each measures light in a slightly different way. These different meters will measure either reflected, or incident light. you can also get meters that can do both! For more information on how the differences of these two light meters, and how they work visit this article on the Photography site.