This should get you started in answering the question but it is not everything about the signs:
SECTION 1006
MEANS OF EGRESS ILLUMINATION
1006.1 Illumination required. The means of egress, including
the exit discharge, shall be illuminated at all times the building
space served by the means of egress is occupied.
Exceptions:
1. Occupancies in Group U.
2. Aisle accessways in Group A.
3. Dwelling units and sleeping units in Groups R-1, R-2
and R-3.
4. Sleeping units of Group I occupancies.
1006.2 Illumination level. The means of egress illumination
level shall not be less than 1 foot-candle (11 lux) at the walking
surface level.
Exception: For auditoriums, theaters, concert or opera halls
and similar assembly occupancies, the illumination at the
walking surface level is permitted to be reduced during performances
to not less than 0.2 foot-candle (2.15 lux), provided
that the required illumination is automatically
restored upon activation of a premises? fire alarm system
where such system is provided.
1006.3 Illumination emergency power. The power supply for
means of egress illumination shall normally be provided by the
premises? electrical supply.
In the event of power supply failure, an emergency electrical
system shall automatically illuminate the following areas:
1. Aisles and unenclosed egress stairways in rooms and
spaces that require two or more means of egress.
2. Corridors, exit enclosures and exit passageways in buildings
required to have two or more exits.
3. Exterior egress components at other than the level of exit
discharge until exit discharge is accomplished for buildings
required to have two or more exits.
4. Interior exit discharge elements, as permitted in Section
1024.1, in buildings required to have two or more exits.
5. Exterior landings, as required by Section 1008.1.5, for
exit discharge doorways in buildings required to have
two or more exits.
The emergency power system shall provide power for a
duration of not less than 90 minutes and shall consist of storage
batteries, unit equipment or an on-site generator. The installation
of the emergency power system shall be in accordance
with Section 2702.
1006.4 Performance of system. Emergency lighting facilities
shall be arranged to provide initial illumination that is at least
an average of 1 foot-candle (11 lux) and a minimum at any
point of 0.1 foot-candle (1 lux) measured along the path of
egress at floor level. Illumination levels shall be permitted to
decline to 0.6 foot-candle (6 lux) average and a minimum at
any point of 0.06 foot-candle (0.6 lux) at the end of the emergency
lighting time duration. A maximum-to-minimum illumination
uniformity ratio of 40 to 1 shall not be exceeded.