TOPIC: The Science of Light: A Comprehensive Study of Luminous Intensity and the Candela:SELF STUDY
1. Introduction to Luminous Intensity
Luminous intensity is a fundamental physical quantity that measures the power emitted by a light source in a particular direction per unit solid angle. Unlike raw power (measured in Watts), luminous intensity is weighted according to the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit for this quantity is the Candela (cd).
2. The Historical Evolution of the Candela
Before the modern SI system, light was measured using "Standard Candles."
The Candle Flame: Originally, 1 candela was the light produced by a candle made of spermaceti wax burning at a specific rate.
The Planckian Radiator: In 1948, the definition shifted to the light emitted by a blackbody at the temperature of freezing platinum.
The Modern Definition (1979): The CGPM redefined the candela based on monochromatic radiation. Today, it is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz to be 683 lm/W.
3. The Geometry of Light: Steradians and Solid Angles
To understand the Candela, one must understand the Steradian (sr). While a radian measures an angle in 2D, a steradian measures a "cone" of light in 3D space.
4. Distinguishing the Four Pillars of Photometry
The confusion between Option (a), (b), and (c) in your original question arises because they are mathematically linked.
A. Luminous Flux (Lumen)
The Lumen is the total "amount" of visible light. If you have a lightbulb that emits 1 Candela in every direction, its total flux is 4π lumens (approx 12.57 lumens).
B. Luminous Intensity (Candela)
This is the "strength" of the light in a specific direction. A laser pointer has very low lumens (total light) but extremely high candelas in one tiny direction.
C. Illuminance (Lux)
Lux measures how much light actually hits a surface.
This is critical for interior design and workplace safety, ensuring desks are bright enough for reading.
D. Luminance (Nit)
Often confused with intensity, luminance is the "brightness" of a surface, like a phone screen or a TV. It is measured in candelas per square meter (cd/m2).
5. The Human Factor: The Photopic Curve
The reason we don't just use Watts to measure light is that the human eye is not equally sensitive to all colors. We are most sensitive to green light (555 nm). A green LED and a blue LED might use the same power (Watts), but the green one will have a much higher Candela rating because our eyes perceive it as brighter.
6. Practical Applications in Modern Industry
Automotive: Headlights are measured in candelas to ensure they throw light far enough down the road without blinding others.
Architecture: Engineers use "Candela Distribution Curves" to decide where to place lights in a stadium or office.
Electronics: Your smartphone's brightness is often rated in "Nits" (which is derived from the Candela).
Summary Checklist for Competitive Exams
Unit of Luminous Intensity: Candela (cd)
Unit of Luminous Flux: Lumen (lm)
Unit of Illuminance: Lux (lx)
Unit of Solid Angle: Steradian (sr)
Relationship: 1 cd=1 lm/sr
Section 1: Photometry & Light Units
What is the SI unit of Luminous Intensity?
(a) Lumen (b) Candela (c) Lux (d) Steradian
Ans: (b)
Which unit measures the 'Total Light Output' of a bulb?
(a) Candela (b) Lux (c) Lumen (d) Watt
Ans: (c)
Illuminance is measured in:
(a) Candela (b) Lux (c) Lumen (d) Steradian
Ans: (b)
1 Lux is equal to:
(a) $1\text{ lumen}/m^2$ (b) $1\text{ candela}/m^2$ (c) $1\text{ lumen}/sr$ (d) $1\text{ watt}/m^2$
Ans: (a)
The unit of Solid Angle is:
(a) Radian (b) Steradian (c) Degree (d) Candela
Ans: (b)
Luminous Efficiency of a light source is measured in:
(a) Lux/Watt (b) Lumen/Watt (c) Candela/Watt (d) Watt/m²
Ans: (b)
A point source emits 1 Candela. The total flux emitted is:
(a) 1 Lumen (b) $\pi$ Lumens (c) $4\pi$ Lumens (d) $2\pi$ Lumens
Ans: (c)
If distance from a source is doubled, the Illuminance (Lux) becomes:
(a) Half (b) Double (c) One-fourth (d) Four times
Ans: (c) (Inverse Square Law)
The frequency used to define 1 Candela is:
(a) $540 \times 10^{12} \text{ Hz}$ (b) $440 \times 10^{12} \text{ Hz}$ (c) $683 \text{ Hz}$ (d) $100 \times 10^9 \text{ Hz}$
Ans: (a)
Luminance (Brightness of a screen) is measured in:
(a) Lux (b) Nit ($cd/m^2$) (c) Lumen (d) Dioptre
Ans: (b)
Section 2: Fundamental SI Units & Dimensions
Which of the following is NOT a base SI unit?
(a) Meter (b) Kelvin (c) Joule (d) Ampere
Ans: (c)
The SI unit of Thermodynamic Temperature is:
(a) Celsius (b) Fahrenheit (c) Kelvin (d) Rankine
Ans: (c)
Dimension of Luminous Intensity is represented by:
(a) [L] (b) [A] (c) [J] (d) [Cd]
Ans: (c) or [J] in some conventions
Amount of substance is measured in:
(a) Kilogram (b) Mole (c) Gram (d) Pound
Ans: (b)
1 Angstrom is equal to:
(a) $10^{-10} \text{ m}$ (b) $10^{-9} \text{ m}$ (c) $10^{-8} \text{ m}$ (d) $10^{-6} \text{ m}$
Ans: (a)
Section 3: Mechanics & General Physics
The unit of Plane Angle is:
(a) Steradian (b) Radian (c) Degree (d) Minute
Ans: (b)
Which physical quantity has the unit 'Siemens'?
(a) Resistance (b) Conductance (c) Inductance (d) Capacitance
Ans: (b)
The unit of Power of a Lens is:
(a) Meter (b) Dioptre (c) Watt (d) Lumen
Ans: (b)
1 Horsepower (HP) is equal to:
(a) 746 W (b) 1000 W (c) 500 W (d) 746 kW
Ans: (a)
Which unit is used to measure Astronomical distances?
(a) Nautical mile (b) Light year (c) Weber (d) Tesla
Ans: (b)
The unit of Magnetic Flux is:
(a) Tesla (b) Weber (c) Henry (d) Faraday
Ans: (b)
The unit of Viscosity is:
(a) Poise (b) Pascal (c) Newton (d) Joule
Ans: (a)
Which quantity has the same unit as Impulse?
(a) Force (b) Momentum (c) Torque (d) Acceleration
Ans: (b)
The unit of Inductance is:
(a) Ohm (b) Henry (c) Farad (d) Tesla
Ans: (b)
Radioactivity is measured in:
(a) Curie (b) Becquerel (c) Both a & b (d) None
Ans: (c)
Join 
