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Luminance is the apparent brightness of an object that is perceived by the human eye.
- It is defined as the intensity of light emitted from a surface per unit area in a particular direction.
- It describes the amount of light that is reflected from, passes through, or is emitted from a particular area and falls within the given solid angle.
- The SI unit of Luminance is candela per square meter (cd/m2).
- The luminance of a surface depends on the nature of the surface and the luminous flux incident on the unit area of the surface.
- Illuminance is the intensity or photoelectric effect measured on a surface at a certain position.
- The measure of the total light emitted from a luminous source is known as luminous flux.
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Key Terms: Luminance, Light meter, Ray optics, Solid angle, Visible Light, Luminous source, Intensity of light, Light source, Photoelectric effect, Luminous flux
What is Luminance?
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When a part of incident light strikes a surface and is reflected, the human eye perceives that surface as a light source.
- The luminance of an object is its apparent brightness as perceived by the human eye.
- We can observe a pattern of varying luminance when we the things around us.
- In other words, the light intensity per unit area projected in a particular direction is defined as luminance.
- It describes how much light travels through, is reflected, and falls within a particular solid angle.
- The SI unit of Luminance is candela per square meter (cd/m2).
- The luminous flux is the measure of the total light output of a luminous source.
The factors on which the luminance of the surface depends are
- Nature of the surface
- The luminance flux incident on a unit area of the surface.
The luminance of a surface can be calculated as
L = dl/dScosθ
Where
- dl: Intensity of light
- dS: Area of the light source
The other unit of luminance is nit.
- It is commonly used in display technology and is a non-SI unit of luminance.
- One nit is equal to one candela per square meter.
- The Luminance color is a specific color that may be described by hue (0° - 360°), saturation (0 percent - 100 percent), and brightness (0 percent - 100 percent).
- Luminance, on the other hand, is a metric used to characterize a color's perceived brightness.
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Luminance Formula
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The formula of luminance is given by
\(L=K_m\int{L_{e\lambda}V(\lambda)\nabla\lambda}\)
Where,
- L is known as luminance
- Km is the maximum luminance efficiency
- Leλ is the spectral radiance
- V (λ) is the luminance efficiency function
- λ is the wavelength
- Δλ is the wavelength interval over which the integration is taking place.
Light Meter
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A Light Meter is a device that measures the amount of light.
- Light meters are classified into two types: reflected light meters and incident light meters.
- A reflected light meter estimates the amount of light that is reflected and must be photographed.
- An incidence meter can calculate the quantity of light that falls on the topic when the integrated sphere is used.
Uses of Light Meter
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The following are the uses of light meter
- Light meters are used to measure interior lighting and to turn off the output of the fitting light.
- This improves system efficiency by decreasing or lessening the load of energy building.
- One of the most common applications for a light meter is photography.
- Appropriate light exposure in photography has been discovered using it.
- A light meter features an analog or digital computer that helps the photographer decide the shutter speed and f-number that should be used for optimal exposure in a given lighting scenario.
- Other applications for the light meter include cinematography and science design.
- These are also employed in lighting to guarantee minimal light waste, plant growth to ensure optimum light levels, and outdoor light pollution.
Things to Remember
- The Luminous Flux is the amount of light energy per unit of time. It is measured in lumens.
- The light intensity per unit area projected in a particular direction is defined as luminance.
- Candela per square meter is the SI unit of Luminance.
- A nit is a non-SI unit of luminnace.
- The luminous intensity is the amount of light emitted or reflected from a point per solid angle.
- The lumen per steradian, or candela (cd), is the unit of measurement of luminous intensity.
- The intensity of light in a particular area is measured by a device known as a Light meter (or Lux meter).
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Previous Year Questions
- A plano convex lens fits exactly into a plano concave lens. Their plane surfaces… [NEET 2013]
- A ray is incident at an angle of incidence i on one surface of a small… [NEET 2020]
- A ray of light travelling in a transparent medium of refractive index μ… [NEET 2010]
- A rod of length 10cm lies along the principal axis of a concave mirror… [NEET 2012]
- A thin prism having refracting angle 10∘ is made of glass of refractive index… [NEET 2017]
- A thin prism of angle 15∘ made of glass of refractive index… [NEET 2011]
- An air bubble in a glass slab with refractive index… [NEET 2016]
- An astronomical refracting telescope will have large angular magnification… [NEET 2018]
- An astronomical telescope has objective and eyepiece of… [NEET 2016]
- When light passes form one medium to another… [KCET 1996]
- Optical fiber works on the principle of… [KCET 1996]
- Which of the following is true for rays coming from infinity… [DUET 2006]
- A point source of light S, placed at a distance L in front of the centre of… [DUET 2000]
- A girl of height 150cm with her eye level at 140cm stands in front of plane… [KCET 2019]
Sample Questions
Ques. How do we calculate the luminance ratio? (2 marks)
Ans. We compute it using the following formula:
LR = L’max/ L’min
Where,
L’max = brightness + the reflected ambient light.
L’min = black brightness + the reflected ambient light.
Ques. What exactly is the difference between illumination and luminance? (2 marks)
Ans. The brightness is the amount of light emitted by a unit surface area in a certain direction. The quantity of light that shines on a surface is referred to as its brightness.
Ques. How many lumens is 1 lux? (1 marks)
Ans. 1 lux = 1 lumen per sq meter.
Ques. What is a luminance meter? (2 marks)
Ans. A luminance meter is a photometric instrument that measures luminance in a certain direction and with a particular solid angle.
Ques. What is the SI unit of luminance? (1 mark)
Ans. The SI unit of Luminance is candela per square meter (cd/m2).
Ques. What is luminous flux? (2 marks)
Ans. The apparent power of light is measured by luminous flux or luminous power. It differs from radiant flux, which is a measure of the total power of electromagnetic radiation, in that luminous flux is adjusted to reflect the human eye's variable sensitivity to different wavelengths of light.
Ques. Define 1 lumen. (2 marks)
Ans. The luminous flux of light produced by a light source emitting 1 cd of luminous intensity over a solid angle of 1 sr is defined as one lumen.
Ques. What is the photoelectric effect? (2 marks)
Ans. The photoelectric effect is a phenomenon that occurs when electromagnetic radiation, such as light, strikes the material and emits electrons. The emitted electrons are known as photoelectrons.
Ques. What are the applications of luminance? (3 marks)
Ans. It has a wide range of applications in everyday life. In general, brightness is most exposed by the whiteness of the screen background it is projected on. Overexposure to it might cause deadly vision problems. As a result, its intensity is regulated in the vast majority of applications where it is used. Aside from photography, we also utilize it in TV, computers, and other screen projectors.
Ques. What are the advantages of Luminance? (5 marks)
Ans. In everyday life, a light meter may be useful in a variety of situations. Furthermore, because this is associated with luminance, which is visible light, it is used in a variety of disciplines where light is a key component.
It may be used to calculate illuminance. The intensity of light that falls on a surface at a given point is referred to as illuminance. Furthermore, this is used in measuring interiors and fitting light outputs. As a result, it decreases the building's energy consumption by boosting efficiency.
In photography, light meters aid in assessing exposure. Furthermore, this contains a digital or analog computer that indicates the shutter speed and f-speed. These two elements are critical for photographers in balancing the proper exposure. It is used in areas such as filmmaking to produce more intense illumination. We also utilize it in regions where there is very little light waste. In scientific design, we utilize it to grow plants that require the proper illumination. This is also done to avoid pollution from outside illumination.
Ques. What is luminance? (5 marks)
Ans. Luminance is defined as the physical stimulation amount that reflects the quantity experienced by humans, as well as the strength of the light emanating from the light source. When you gaze at a luminous surface using binocular vision from a certain observation point, this is the emission from the region and the injection of luminance flux into the observation point, and the unit is expressed as cd/m-2.
The calculation is as follows: L = dl/dScosθ
where,
dl: Intensity of light
dS: Area of the light source
In the following statement of the strength of light, there are two terms named luminance and illuminance, and the distinction between the two is described in brief below. Illuminance is the quantity of light that enters the target surface. It has been altered based on distance, the area of the light source, and human perception.
Ques. Write the uses of the light meter. (4 marks)
Ans. Light meters are also used to measure interior lighting and to turn off the output of the fitting light. This improves system efficiency by decreasing or lessening the load of energy building.
One of the most common applications for a light meter is photography. Appropriate light exposure in photography has been discovered using it. A light meter features an analog or digital computer that helps the photographer decide the shutter speed and f-number that should be used for the optimal exposure in a given lighting scenario.
Other applications for the light meter include cinematography and science design. These are also employed in lighting to guarantee minimal light waste, plant growth to ensure optimum light levels, and outdoor light pollution.
Ques. What is the color of luminance and the unit of luminosity? (4 marks)
Ans. The Luminance color is a specific color that may be described by hue (0° - 360°), saturation (0 percent - 100 percent), and brightness (0 percent - 100 percent ). Luminance, on the other hand, is a metric used to characterize a color's perceived brightness.
Luminosity, in astronomy, is the total amount of electromagnetic energy emitted per unit of time by a star, galaxies, or other astronomical objects. The SI unit of measuring luminosity is joules per second or watts. The values of luminosity are given with regard to the luminosity of the Sun in astronomy.
Ques. A luminescent object is placed at a depth of “d” in an optically denser medium of refractive index μ. Prove that the radius “r” of the base of the cone of light from the object that can emerge out from the surface is \(r = \frac{d}{\sqrt{\mu ^2 -1}}\) (4 marks)
Ans. The cone of light is formed due to the total internal reflection,
Thus, for TIR,
sin ic = \(\frac{1}{\mu}\)
Again, from the figure, tan ic = r/d
\(\frac{sin i_c}{\sqrt{1 - sin^2i_c}} = \frac{r}{d}\)
\(r = \frac{d}{\sqrt{\mu ^2 -1}}\)
Ques. Yellow light (λ = 6000Å) illuminates a single slit of width 1 x 10-4 m. Find out:
(i) when the diffraction pattern is viewed on a screen kept 1.5 m away from the slit, the distance between the two dark lines on either side of the central maximum;
(ii) the angular spread of the first diffraction minimum. (Comptt. All India 2012)
Ans: (i) Distance between two dark lines, on either side of the central maxima = 2 λD/a
side of eternal maxiua = 2 \(\frac{\lambda D}{a}\)
= \(\frac{2 \times 6000 \times 10^{-10} \times 1.5}{1 \times 10^{-4}}\) = 18000 x 10-6
= 18 x 10-3 = 18 mm
(ii) Angular spread of the first diffraction minimum on either side:
= θ = \(\frac{\lambda}{a}\) = \(\frac{ 6 \times 10^{-7}}{1 \times 10^{-4}}\)= 6 x 10-3 radians
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