Up to now we have been talking about adaptation luminance without
saying to which luminance a human observer would adapt. It is assumed
that we will adapt to the luminance of our fixation point which
approximately covers one visual degree (
, by some
authors). Just to make things a little bit more complicated, the
adaptation luminance depends on the surrounding luminances as
well. The influence of the surrounding is not large, but if there are
some glare sources in the periphery, the veiling luminance should be
taken into consideration.
Bright glare sources in the periphery reduces contrast visibility because light scattered in the lens obscures the fovea. The influence of the veiling luminance to the adaptation luminance is well documented in the literature, and we will present here a model introduced by Moon and Spencer [MoSp45], because this is the model used by Larson et al. in [LaRP97].
Moon and Spencer proposed the next formula for the corrected
adaptation luminance 
:
where is the corrected adaptation luminance in 
, 
is
the average foveal luminance in , 
is the
luminance in the direction 
, 
is foveal half
angle, 
, and K is the constant
measured by Holloday [Holl26], 0.0096.
It is obvious from the above equation that the periphery contributes
less than 
to the adaptation luminance. If there are no bright
sources in the periphery this influence can be neglected.