Luminance or Illuminance ?

Started Mar 28, 2013 | Discussions thread
GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

The definitions I have seen distinguish between the brightness of a surface (luminance) and the amount of light incident on a surface (illuminance). I'm not sure how widely recognised this distinction is, but using google produces some confirmation:

http://www.jimonlight.com/2008/12/18/lighting-101-luminance-vs-illuminance/

http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml

Both very useful.   luminance is a measure of the "brightness" of light that you "see" versus illuminance which is a measure of the intensity of light falling on your retina.  Thanks Tom.

I also went here:  http://www.cambridgeincolour.com/tutorials/camera-metering.htm   and now realize that dependence on evaluative in-camera metering can lead to significant issues.

For my purposes, using spot metering, I think that the following is true:

Luminance is what my eyes "see".  It is a measure of apparent "brightness" (intensity) of the visual field.  It is what the front of the objective lens "sees".  Luminous intensity/area, measured in Cd/meter^2, energy per area.

Illuminance is what is falling on my retina. It is a measure of illumination on an area of a sensor. It is what causes electrons to be produced in photosites.  In general higher Illuminance leads to more electrons ... up to a point. Luminous flux/area, measured in lux/meter^2, also energy/area

So when I am spot metering, looking at a scene with an EVF, I am getting an impression of luminance of (parts of) the scene, both visually and with the histogram.  But since both the (jpeg) image and the histogram in the EVF are generated by illuminance of the sensor I am really measuring illuminance of (parts of) the sensor.

(I suppose that if I were to use an OVF, I would get a real, actual measure of luminance, affected only by the optics.)

Bottom Line (I think):

When spot metering with the live view EVF using the histogram I am surveying the illuminance on the sensor to give me an idea of the range of luminance of the scene.

Do I have it?

t

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