Re: [MF 3D] MF viewer lighting

[Richard Rylander <rlrylander@xxxxxxx>]

Greg Erker wrote:

  If you are measuring brightness through the
eyepiece of a viewer then the area doesn't really
matter. Assuming an ortho-ish viewer then the
light meter sees a lit area of roughtly the same
angle. So the same amount of light (say 4 LEDs)
shining through a 20x20mm mount should measure the
same as if you move it back and illuminate a 50x50mm
mount. The total light reaching your eye is the
same either way.

  Or am I wrong somehow?

You are half right - a Lambertian surface looks just as bright at any distance
so measuring the brightness doesn't depend on the diffuser area, but producing
that brightness does.  Illuminance is a function of the lighting geometry,
not the detector.  Your 'total light' argument holds only in a system
with condensers and field lenses, but not when a diffuser is in play.

Take a slide projector and a matte screen ten feet away with the viewer next to the projector.  Move the screen twenty feet away and the image will look one quarter as bright even though it has the same angular extent, same projector power.  Move just the viewer closer to the screen and it doesn't look any brighter.

More total Watts of light power are needed to produce the same brightness of a larger diffuser.  More than 4 LEDs will be needed to make a MF viewer as bright as the twin 35mm viewer.

A very good document on luminance, illuminace, irradiance, etc. is the "Light Measurement Handbook" available as a pdf document at:
 http://www.intl-light.com/handbook/

Richard Rylander