Main exposure equation
(camera settings based on scene Luminance and film speed):

(N^2)/t = L*S/K

N is F-numbers
t is exposure time
L is average scene luminance
S is ISO speed
K is calibration constant


Per Lambert's Cosine Law if we have a perfect diffuser, then:

Reflectance R is:

R = pi*L/E = pi * K/C

L is luminance
E is illuminance.

K is the calibration constant for a reflected meter and
C is calibration constant for an incident meter

For an 18% gray card:

R = .18
if we set E to 1.0 (for 100%) then:

L = .18 / pi

So, the reflected measurement in cd/m^2 should be

The incident lux multiplied by .18/pi (or .0573).

Sekonic's Calibration Constants are:

K = 12.5

Lumisphere C = 340
Flat Diffuser C = 250

This means that:

With sphere retracted ("flat"): pi*12.5/250 = .157
With sphere out: pi*12.5/340 = .115
So, incident and spot should agree on either a 15.7% gray card or
a 11.5% gray card (but NOT an 18% gray card!), depending on sphere position.



A 15.7% gray card is only 2 tenths of a stop darker than an 18% one and I use the
exposure read-out of the spot meter (i.e. its read-out in f/stop and shutter) much more
than I use its absolute read-out (i.e. read-out in cd/m^2 or footlamberts). So the best
calibration method for my own personal usage is:

First, retract the sphere and calibrate the incident meter to spec.

Second, do NOT calibrate the spot meter to spec. Instead, calibrate it so that its
exposure read-out (f/stop and shutter) match the incident read-out for a nearly-
collimated light falling on an 18% gray card.

By so doing, I accept a 2-tenths stop miscalibration in the spot meter's absolute reading
in exchange for effectively changing its calibration constant so that spot and incident
readings agree for 18% gray instead of 15.7% gray. This is more practical for my personal
working style, since I rarely meter in cd/m^2, but I often meter in exposure read-out, and
have already conditioned myself to think of 18% gray as "neutral."