Correct Answer: D. Candela
Candela is the SI base unit for luminous intensity — the brightness of light emitted from a point source in a specific direction. It is defined as the luminous intensity of a source that emits monochromatic radiation of frequency 540 × 10¹² Hz with a radiant intensity of 1/683 watt per steradian. In the SI system, candela (cd) is one of the seven base units, alongside metre, kilogram, second, ampere, kelvin, and mole. The key discriminator is that candela measures the intensity of light from a point source, not the illumination on a surface or the total luminous flux. This distinction is critical in occupational health assessments (e.g., workplace lighting standards per Indian factories act), clinical settings (operating theatre illumination), and environmental health monitoring. Candela is used to specify the brightness of light sources themselves — bulbs, lamps, or natural sources — whereas other photometric units measure derived quantities like illumination or total light output.
Why the other options are wrong
A. Lux — Lux is the SI unit of illuminance (light falling on a surface), not luminous intensity. It measures lumens per square metre (lm/m²). This is the trap — lux is commonly used in Indian workplace lighting standards and clinical guidelines, making it a plausible distractor. However, lux measures the effect of light on a surface, not the brightness of the source itself. B. Lambert — Lambert is a non-SI unit of luminance (brightness of a surface as perceived from a direction), not luminous intensity. It is rarely used in modern Indian medical or occupational health practice and is not an SI unit. This is a historical distractor that tests whether students confuse luminance with luminous intensity. C. Lumen — Lumen is the SI unit of luminous flux (total light output from a source in all directions), not luminous intensity. While lumen is commonly referenced in Indian lighting specifications and bulb ratings, it measures total light emission, not the intensity in a specific direction. Candela is intensity; lumen is total flux.
High-Yield Facts
- Candela (cd) is the SI base unit for luminous intensity from a point source in a specific direction.
- Lux (lx) = lumens per square metre; measures illuminance on a surface (Indian factories act specifies minimum lux for workplaces).
- Lumen (lm) = total luminous flux from a source; candela × solid angle (steradians).
- Luminance (brightness of a surface) is measured in candela per square metre (cd/m²), not lambert.
- Operating theatre illumination in India is specified in lux (typically 500–1000 lx), but the light source brightness is rated in candela.
Mnemonics
CILF Rule Candela = intensity (point source), Illuminance = lux (on surface), Luminous flux = lumen (total), Luminance = cd/m² (surface brightness). Use when distinguishing photometric units in occupational health. Source vs. Effect Candela describes the source (how bright the bulb is). Lux describes the effect (how bright the room is). When asked about point source brightness, think candela.
NBE Trap
NBE pairs lux with workplace lighting standards (common in Indian occupational health) to distract from candela. Students who recall "lux is used in factories act" may choose lux, forgetting that lux measures illumination on a surface, not the intensity of the source itself.
Clinical Pearl
In Indian operating theatres, lighting is specified as 500–1000 lux on the surgical field (illuminance), but the theatre lights themselves are rated in candela (intensity). A surgeon asking "how bright is this light source?" needs candela; asking "is this room bright enough?" needs lux. This distinction matters in occupational health audits and clinical safety standards.
_Reference: Park's Textbook of Preventive and Social Medicine, Ch. 10 (Environmental Health); Harrison's Principles of Internal Medicine, Ch. 1 (SI Units)_