In another thread, c (Bill Ferris) wrote in message ... that:

This is the physical process that begins the
transition from daytime (photopic) to nightime (scotopic) vision.
The impact of pupil dilation on light-gathering is pretty minimal
in the whole vast scheme of things. A fully dilated pupil collects
~16 times more light that a constricted 2-mm pupil. But the fully
dark-adapted eye enjoys improved sensitivity by a factor of several
thousand over the photopic eye. Dark adaptation is a chemical
process . . .

While the relative ability to see brightness at night when measured
using a daylight reference standard does involve a change in
brightness ranging in the thousands of times, I tend to measure night
vision using my initial night vision after I first arrive at the
parking lot or roadside observing site. I compare that to my night
vision an hour or so later, after dark adaptation.

That change involves only an intensification of light by a factor of 6
to 16 - not thousands as measured from a daylight reference.

Figure 2.4(a) in Clark (1990, at p. 10) shows the increase in night
(scotopic) vision, due to photochemical dark adaptation, from an
initial 19 magnitudes per square arc second (mpsas) to 24 mpsas, or a
change of about 5 mpsas.

Since mpsas inherently is a magnitude scale, the basic rule of thumb
that 5 magnitudes equals a brightness change of 100x applies. This can
be seen in Clark's Table 2.2 at p. 7, which converts mpsas to candelas
per meter squared to measure brightness:

=============================
Table 1 - Sky brightness (in mpsas and candelas)

m_v_mpsas cd / m^2 Change relative to 19 mpsas
18 0.00711090 0.4
19 0.00283090 1 "Good" sky nightbrightness
20 0.00112700 2.5
21 0.00044867 6.3 Approx. limit of outdoor
22 0.00017862 16 background sky brightness
23 0.00007111 40
24 0.00002831 100 Limit of telescopic sky brightness
25 0.00001127 251 visible in eyepiece by the human eye

=============================

From Clark's Table 2.2, the change in one's ability to see your
parking lot environment with the naked eye, under "good" to
"excellent" skies (mpsas 18 to 21), might increase at most by a factor
of 6 to 16 - not thousands.

Your "organic night vision goggles" photochemically intensify light by
a factor of 6 to 16 times after standing in dark outside for a
half-hour.

Nonetheless, the "parking lot" 6x-16x intensification of night vision
always impresses me. The effect after standing outside for a couple of
hours is dramatic, going from ability to see people as darkened
sillouettes to being able to read lettering on their jackets.

- Canopus

Clark, R.N. Visual Astronomy of the Deep Sky. Cambridge Univ. Press.
1990.