Final answer:
The question involves the subject of Physics as it refers to illuminance levels measured in foot-candles or lux. It is associated with lighting standards and recommendations for various settings, which are not directly answered by the options provided.
Step-by-step explanation:
The question refers to the illuminance level of 50 foot candles (540 lux), which is associated with lighting standards in various settings. However, none of the options directly relate to standard lighting requirements or recommendations for these specific illumination levels.
To clarify illuminance briefly, it is the amount of light (luminous flux) falling on a surface area, and it is measured in lux (lx), or in the US customary system, foot candles. The illuminance is calculated using the formula: I = P / (4πr²), where I is the illuminance in lux, P is the luminous flux in lumens (lm), and r is the distance from the light source to the surface in meters.
Guidelines and standards for illumination, such as those provided by the Occupational Safety and Health Administration (OSHA), suggest optimal lighting levels for various activities. For example, OSHA recommends 500 lux for desktop lighting in office settings. If a lighting fixture provides insufficient illumination, it may need to be adjusted in height to meet these recommendations, using principles such as the inverse square law to determine the appropriate height adjustment for achieving the correct illuminance.