Luminance Questions

Luminance can be defined as the apparent brightness of any object that is visible to the human eye. And also, it can be described as the photoelectric effect or intensity calculated on a surface at a specific position. In other words, luminance is the intensity of the radiated light from the surface per unit area within a particular direction. The luminance of any surface depends on the nature of that surface, and in addition, it depends on the amount of luminous flux incident on the unit area of the surface. Estimating the total light output of a luminance source is known as luminous flux.

If the L= Luminance, K_m = Maximum luminance efficiency, V(λ) = Luminous efficiency function, L_eλ = Spectral radiance, λ = Wavelength and Δλ = Wavelength interval over the integration that is taking place, then luminous can be expressed as,

The SI unit of luminance is cd/m² (candela per square metre).

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Important Questions with Answers

1. What is meant by light metre?

An instrument used to calculate or measure the amount of light is known as a light metre.

2. What are the uses of a light metre?

• The light metre is useful for cinematography and science design. Light metres are also used to ensure minimal waste, plant growth to ensure appropriate light levels, and light pollution outside.
• Besides, light metres are used to measure interior illumination and turn off fitting lights. The system becomes more efficient by reducing or lowering the energy-building burden.
• A light metre is most commonly used in photography. In photography, it is used to determine the proper exposure to light. Under particular lighting conditions, a light metre consists of an analogue or digital computer to help the photographer determine the shutter speed and f-number; it must be chosen for the best exposure.

3. What is the photoelectric effect? Explain.

Electrically charged particles are discharged from or within a material that absorbs electromagnetic radiation. The photoelectric effect can be defined as a metal plate that is often described as ejecting electrons when light falls on it.

4. Which factors affect surface luminance?

The surface’s luminance is dependent on the following parameters.

• It depends on the surface’s nature.
• It depends on the luminous incident flux on a unit area of the surface.

5. What is the SI unit of luminous?

1. candela
2. cd/cm²
3. cd/m²
4. cd/mm

Answer : c. cd/m²

Explanation: The SI unit of luminance is cd/m² (candela per square metre).

6. Calculating the total light output of a luminance source is known as illuminance.

1. True
2. False

Answer: b. False

Explanation: Measuring the total light output of a luminance source is known as luminous flux.

7. What is the importance of luminance?

Luminance and contrast are essential to the visual system’s functioning. There can be no vision without light, that is, luminance = 0, and no temporal patterns can be seen without contrast. The ability to react to luminance is a necessary – first step in seeing, allowing all other visual processes to take place.

8. What exactly is the difference between lightness and luminance?

Lightness is a prediction of how an illustrated colour would appear to a viewer in colourimetry and colour appearance models. While luminance is a linear measurement of light, lightness is a linear prediction of how humans experience that light.

9. Are stars luminous or non-luminous?

Stars are luminous because they radiate light.

10. Which of the following colours has the highest luminance?

1. White
2. Yellow
3. Blue
4. None of the above

Answer: b. Yellow

Explanation: Yellow has the maximum luminance, almost 94%, and is only 6% away from pure white. Blue has less than half the brightness of yellow, which is 44%.

Practice Questions

1. State and explain the formula of luminance.
2. What is illuminance?
3. What is luminance?
4. What is the difference between luminescent and luminous?
5. Are luminosity and brightness the same?

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