These codes provide complete information on emergency lighting requirements; however, a good introduction is found in NFPA 101, Section 220.127.116.11-18.104.22.168.3:
"Emergency illumination shall be provided for a minimum of 1½ hours in the event of failure of normal lighting. Emergency lighting facilities shall be arranged to provide initial illumination that is not less than an average of 1 ft-candle (10.8 lux) and, at any point, not less than 0.1 ft-candle (1.1 lux), measured along the path of egress at floor level. Illumination levels shall be permitted to decline to not less than an average of 0.6 ft-candle (6.5 lux) and, at any point, not less than 0.06 ft-candle (0.65 lux) at the end of 1½ hours. The maximum-to-minimum illumination shall not exceed a ratio 40 to 1."
It is important to remember that code requirements reflect minimum standards and, therefore, are generally considered only a starting point in designing emergency lighting systems. Additional emergency lighting, beyond minimums, is warranted in some facilities, depending upon facility use and other relevant factors. Hospitals and nursing homes, for example, are excellent candidates for additional emergency lighting. The same is true of manufacturing and production areas, which often present numerous potential safety hazards even under the best of conditions.