Re: [MF 3D] MF viewer lighting

[Richard Rylander <rlrylander@xxxxxxx>]

Greg Erker wrote:

>
>   Would scuffing the lens to diffuse the
> light work as well or would you lose too much
> light internally.
> ...
>
>   So you have 8 LEDs at about 3V and 25mA (guessing).
> So this is 0.6W (ignoring losses in the dropping
> resistors or regulator). Quite a bit lower that say
> the Redbutton with 2.5V 0.8A halogen.
>
>   How would you compare the brightness? Do have
> any way to measure the EV into the eyepiece to
> compare to my viewer brightness measurments?
>

You do lose more light if the lens is just roughened, how much is "too much"
is tough to say.  If the LEDs are in a perfectly reflecting chamber where the
only way for light to escape is through a scattering but non-absorbing
diffuser, it wouldn't matter.  Real mirrors, on the other hand, reflect ~90%
if you're lucky, and diffusers also absorb, so getting as much light as
possible from the LED chip directly to the diffuser is important.

The current through the LEDs is a little less than 20mA (total current for
all 8 is 150mA, power is about 0.45W).  Aiming an exposure meter (set at ISO
100) into the eyepiece of the viewer gives me a reading of about 10EV.  I
have a Realist Red Button viewer with the original yellowed reflector
replaced by white matte photographic paper that has been "fixed" (probably
the whitest paper you can find with optical brightners), and a halogen bulb
rated 600mA @ 2.33V.  I use alkaline "D" cells with a [switchable] series
diode to drop the voltage to about the right range when the batteries are
fresh, extending the bulb life.  With slightly used batteries, I measure
585mA @ 2.15V, and the exposure meter sees 9.6EV.

This is something of an apples/oranges comparison, but basically I'm getting
about the same brightness with a lot less power using LEDs for the
backlight.  Given the price of white LEDs, you get more brightness/$ with
CCFL - the reason I went that route for the MF viewer.

Richard Rylander