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I want to read the ambient light falling on the subject. What do I need: a light meter or an exposure meter? Or are they the same thing?

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Light meter and exposure meter are interchangeable terms for the same instrument. We can use them two ways. A reflection reading is taken by pointing the meter at the subject. We are measuring the amount of light energy that is reflected from the subject -- it has already hit and is being reflected.

We can also measure the light before it hits the subject. We point the meter back at the camera from the subject's position. We are measuring the light that is about to hit. In the jargon of photography, this is called an incident light meter reading. The word "incident" is Old French for about to happen. The incident light meter is equipped with a translucent sphere or a concave sphere covering the entry opening. A incident meter reading is essentially the same as a refection reading provided the reflection meter is measuring at the subject plane and reading the reflected light from a gray card with an 18% reflective surface.

The light meter / exposure meter used in photographic applications outputs data unique to setting the exposure of a camera. We are talking aperture settings (f-number) and shutter speed intertwined with the sensitivity of film or digital chip (ISO) and of course the illuminate level.

There are non-photographic applications for the light meter (not exposure meter). These are used by architect and lighting technicians. They are dealing with lighting levels at the workplace etc. They use light meters that output data in terms of Standard Candles per square foot or square meter or LUX or Lumens.

It is possible to convert the data outputted by either style. Conversion charts and conversion factors exist so it is challenging but not impossible to make the conversion.

  • @ Alan , Shouldn't that be 'convex', not 'concave' ? All incident meters I've used had convex spheres . – David Barry May 11 '17 at 6:27
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    @ David Barry -- Both concave and convex translucent entrance covers work. The Weston light meters series used concave. The idea is to alter the angle of entry so the meter's angle of view is about 180 degrees. – Alan Marcus May 11 '17 at 6:43
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I want to read the ambient light falling on the subject. What do I need: a light meter or an exposure meter? Or are they the same thing?

They can be. Exposure meters are a subset of light meters that show the reading in EV (exposure value). There are also scientific/engineering light meters for measuring light intensity in lumens or footcandles, that are not exposure meters.

Most digital cameras come with a meter inside the camera that can measure the light reflected off the subject. Most digital cameras also have auto-exposure systems that can take the data from the metering sensor and use it to automatically adjust the exposure settings in the camera to get a good exposure for the amount of light sensed.

If however, as you say, you want to directly measure the amount of light falling on a subject, rather than reflected back from them, then you need an incident meter, rather than a reflective meter. There is a difference, because what color/how dark the subject is can affect the reflective reading, while it will never affect the incident reading. Incident meters must be held where the subject is to measure the light. For that reason, it must be an external meter; not the one in the camera.

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