Thanks, I guess I need to expand the web site a bit, (cough).

== Controls ==

The meter has two momentary pushbuttons on the top surface.

o The left button turns the meter on and off:  Tapping it turns
  the meter on; Holding it down for a second turns the meter off.

o The right button toggles the meter between normal mode and
  delta mode and when held down takes a reference reading for
  displaying the difference between two spots.

The meter turns itself off if no button is pressed in 5 minutes.

== Normal Mode ==

The meter 'wakes up' in normal mode and works like a digital spotmeter where
the spot measured a spot on the enlarging meter.  The number displayed shows
the quantity of light in 1/100's of a stop.  The 1/100's display eliminates
rounding errors when several measurements are used to determine paper grade,
base exposure and burn and dodge exposures.

Normal Mode measurements determine the base exposure by measuring the
shadows [low key], mid-tones [portraiture] or highlights [high key].

When used with the Darkroom Automation F-Stop timer a very simple
relationship appears:

‘Paper Speed’ = Meter Reading + Timer Setting

where Paper Speed is a constant.  Given a paper and desired tone there is a
number that is a product of time and light intensity that will produce that
tone. Paper Speed is easily determined by making a test strip with no
negative and adding the time of the strip showing the correct tone to the
number shown on the Enlarging Meter.  As we are working in stops for both
time and light intensity simple addition is all that is needed for all
exposure determinations.

When Paper Speed is known the F-Stop timer setting is simply:

 Timer Setting = Paper Speed - Meter Reading.

Any timer can be used with the meter: F-Stop charts and a spiral F-Stop
scale for clock-faced timers come with the meter.  Using charts and an
ordinary timer is the most common method for ‘F-Stop printing,’
an F-Stop timer makes the work easier and more pleasant.

== Delta Mode ==

The right button toggles the meter between Delta Mode and Normal Mode. Delta
mode shows the light intensity difference between any two spots on the
image.  The meter is placed under the part of the image to be used as the
‘reference’ or ‘base’ and the delta button is held down for a second.  The
meter display changes to 0.00 and the meter now shows the difference in
light intensity in stops.  An ‘r’ or ‘-‘ shows in the display to indicate a
delta reading.  The reference reading is remembered when the meter is turned
off.

Use Delta Mode to:

o Determine paper grade by measuring the intensity
  difference (‘delta’) between the shadows and the
  highlights or between the two most important tones
  in the image.

o Make fine corrections after making a test print by
  measuring the difference between two spots in the
  image: a spot where the print has the tone you want
  and a spot that has the tone you want to correct.
  The delta reading is the adjustment to the F-Stop
  timer/print time.

o Determine dodging and burning requirements.  Several
  methods are possible but an example is: Measure the
  foreground highlights and the highlights in the
  clouds - the delta reading is the amount to burn
  in the sky.

== Meter Calibration ==

The meter is calibrated so that full scale, 9.99, is approximately the
amount of light needed to achieve full black in two seconds on Ilford MGIV.
The other end of the scale, 0.00, is the amount of light needed to achieve
full black in 2000 seconds – 33 minutes of exposure.

All meters read the same under the same conditions.  This allows data to be
shared among photographers.

A very good one, though an 'easel densitometer' won't
read the same as an ANSI/ISO densitometer.  On the other
hand the 'density' it reads is the density that matters.
Readings are very stable, all meters are calibrated,
linearity is excellent.  And it has a delta mode for
reading intensity differences so if used as a
densitometer it is easy to zero out film base+fog and
enlarger height/f-stop.

And every EM-10 is different.  A volume control potentiometer
is used for the logarithm conversion: the graph of OD/Meter reading
is not pretty.

http://www.nolindan.com/em10graph001.jpg

.05 OD -> .15 stops

The resolution depends on the density as the readout
isn't linear.  And you have to turn the lights on
to read the dial.

For it's original purpose - to match light intensity
when making Ilfochromes it is a great little gadget.
As a densitometer, it can be done, and it is cheap,
but it was never meant to be one.