The selenium photometer is the best as this does not require external batteries but uses a photoelectric cell. This is funneled through various resistances and shunts and displays on an analog meter numbers called EV- exposure values. Meter cycles left to right. One sets on a dial the film speed being used ( before any photo session) I use ASA 400 , and then turns the superimposed analog dials to line the needles up- and reads off the exposure values in F-stops and shutter speeds- usually- and this is the nice touch- there is more than one possible combination. In effect a form of circular slide rule. This modern type of photometer ( a term serious shutterbugs prefer to Light meter- as it is used for Photography)- was essentially perfected as far back as l947. It is moderately compact, can fit in a normal pocket, and uses Photovoltaic ( which generate their own juice) cells, unlilke the Cadmium cells which require an external voltaic battery to provide juice- the Cadmium Photometers alter the resistance- thus need outside juice, the Seleniums generate their own- lower maintenance costs. They are hard to get but useful and scientific items! The Slide Rule Museums has a series of patent diagrams on photometers of the wheel-dial or analog type- of which I am familiar. Properly maintained these things hold up.
The exposure is controlled by the camera's light meter
Any film camera with a light meter, and every digital camera, has a sensor that will do that.
If you mean a light meter that is used in a photographic studio, then it measures the light that hits the 'subject' from the studio lighting, the light meter then tells you what you need to set your camera to, for the correct exposure.
To use exposures, it is best to know what they are. Exposures are combinations of film speed, shutter speed, and aperture. To use them best, a photographer should use the results (via the camera's light meter) to see if the combinations will create the desired light in camera's sensor (digital) or film.
The type of light meter that measures the light intensity in pictures is a foot-candle meter. They help in setting the camera right and help in calculating the best lighting.
Use a phone app like light meter which uses the camera to give a luminosity level.
Scientist use a light meter to show how light in shining on a surface !
A light meter is more precise and versatile than the built-in exposure meter on typical SLR/DSLR/digital cameras..Once the type and speed of the film (in the case of older SLR camera films, or the ASA setting on your camera) are entered or set and matched on both camera and meter, you can use the meter to measure the strength of the light source shining on the subject of your photograph or the amount of light and/or shadow that is reflected by the subject (i.e., a person) to determine the shutter speed and aperture (lens opening) settings to get a properly exposed photograph (not so dark that you cannot discern an image or so overexposed that the necessary details are "washed out," except in certain situations like shooting a silhouette of a person or object against a lighter background).
Typically, a light meter. However, some may require the use of a color temperature meter as well.
Electromagnetic radiation is just light. You can measure light with a light meter, which tells you intensity, or a camera sensitive to different frequencies. You can also use a magnetometer (Gaussmeter) to measure the B field component or a voltmeter to measure the E field component.
the voltmeter use a SLR camera because this is the latest camera and the good light and good shot of a picture
It is a light meter reading of a small centralized area within the camera's field of view. I don't know what the angle would be for various cameras that have a version of the feature, but I can say that some old hand held spot meters have a measuring angle of 1 degree in a field of view of about 35 degrees. In many cases, the spot meter area is marked within the view finder in some way. Correct use of a spot meter (or for that matter, pretty much any camera light meter) requires a good understanding of metering concepts and the image medium, whether it is film or digital.