This article focuses on illumination. We will discuss the terms like lumen, luminous intensity, Laws of illuminance and some solved examples. The information in this article helps you extensively in your SSC JE Electrical and GATE Electrical preparation journey.

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illumination of a surface is defined as the luminous flux received by the surface per unit area. It is represented by the symbol and measured in lux (or metre candle or lumen/m^2)

Types ofilluminationin slit lamp

The above figure shows Polar curves of an incandescent lamp. If the luminous intensity is measured in a horizontal plane about a vertical axis, a curve is plotted between luminous intensity and the angular position as shown in figure (b).

Reflection of visible light allows us to view objects in the world. The study of the concept of light is connected with illumination. There are some terms and definitions, which enables us to understand illumination and electric lighting in detail. The luminous flux received by the surface per unit area is known as illumination. It is denoted by the letter 'E' and is measured in Lux.

Luminous flux as” the amount of radiant energy emitted per unit time in the form of light waves by a luminous body”. However, luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). It is represented by \(\phi\) or F and is measured in lumens.

The human eye is more sensitive to certain colours and the below figure gives the idea of sensitivity of human eye to different wavelengths between (0.004 mm to 0.00075 mm) 4,000AU to 7,500AV (Angstrom Unit) .The eye discriminates between different wavelengths in this range by the sensation of colour.

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Hence a polar curve may be defined as “a graph representing the light distribution of a lamp in a horizontal or vertical plane.”

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Types ofilluminationPDF

The quantity of light emitted by a lamp is measured in terms of its luminance flux (measured in lumens). The brightness of the beam in a specific direction is measured by the luminous intensity, which is expressed in candelas. A lamp with a single lumen bulb and optics that evenly concentrate light into a one steradian beam produces a beam with a luminous intensity of one candela. The source would have a luminous intensity of 2 candela if the optics were modified to focus the beam into 1/2 steradian. The resulting beam is brighter and narrower, but the luminous flux is constant.

Illumination lightingdesign

This article concludes all the information related to the Illumination, which helps in propelling your preparation for various AE/JE examinations. To boost your preparation, you should test yourself through a series of Mock Tests for Electrical Engineering Exams. You can check the syllabus for the AE/JE exam. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examinations, including GATE Electrical and AE/JE Electrical exam.

Uniform Diffuse Source: A uniform diffuse source is one in which the intensity per unit projected area is same from all directions.

The candle power or luminous intensity of a source is different in different directions. The average candle power of a source is the average value of its candle power in all directions. This average candle power is also known as mean spherical candle power.

Brightness or luminance may be defined as the luminous intensity of the lamp per unit projected area (apparent area) of either light source or reflecting surface. It is represented by B or L and is measured in candela/m² (or nit or stilb or lambert or metre-lambert),

The candle power of a source is defined as the number of lumens emitted by that source in a unit solid angle in a given direction.

In the experiment P5.1.2.1, an observation screen is set up parallel to the optical axis so that the path of a parallel light beam can be observed on the screen after passing through a collecting or dispersing lens. The focal length is determined directly as the distance between the lens and the focal point

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Example 2: A room 9m x 12m is illuminated by twelve 100 watt lamps. The luminous efficiency of the lamp is 30 lumens per watt and the coefficient of utilisation as 0.45 the average illumination.

It states that "the illumination of a surface is inversely proportional to the square of the distance between the surface and source provided, the light source is a point source"

A plane angle is subtended at a point in a plane by two straight converging lines. It is usually represented by the Greek letter \(\theta\) and is measured in degrees or radians.

Illuminance refers to the intensity of the photoelectric effect measured on a surface at a specific point. In simpler terms, luminance is the measure of light intensity emitted from a surface per unit area in a specific direction. It indicates the amount of light that passes through or is reflected from the surface and is contained within a given solid angle.

In the realm of illumination, several important terms are crucial for understanding lighting design and technology. To study illumination we need to know some terms and definitions. They are as follows

These techniques, deeply rooted in tradition, continue to captivate audiences with their timeless appeal. Let's explore some of these revered techniques:

Illumination is the art and science of lighting spaces to enhance visibility, aesthetics, and ambiance. Whether through natural sunlight, electric fixtures, or innovative technologies, illumination plays a crucial role in our daily lives. It can transform ordinary environments into extraordinary ones, influencing our mood, productivity, and overall well-being.

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The angle of incidence of illumination holds significant importance when considering the interaction of light rays with a surface or object. It plays a vital role in determining the direction of reflection or refraction, guided by the laws of physics and optics.

A Solid angle is subtended at a point in space by an area and is the angle enclosed in the volume formed by an infinite number of lines lying on the surface of the volume and meeting at the point. It is usually presented by the Greek letter \(\omega\) and measured in steradian.

The laws of illumination are fundamental principles that govern how light behaves in different environments. These laws include the inverse square law, Lambert's cosine law, and the law of reflection, all of which guide the design and application of lighting in various settings.

It is the luminous flux per unit solid angle in a given direction. It is represented by I and is measured in (lumen/steradian) or candela (cd). Luminous intensity formula is given as

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Luminous flux is often used as an objective measure of the useful light emitted by a light source, and is typically reported on the packaging for light bulbs, although it is not always prominent. Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and a lightbulb with a higher ratio of luminous flux to consumed power is more efficient.

Illumination on surface CD = \(\frac{flux (lm)}{areaCD(m^2)}\)= \(\frac{flux (lm)}{(areaAB/cos\theta)} =\frac{l}{d^2}\times cos\theta\)

In most of the practical types of lamps or light sources, the luminous intensity or candle power is not uniform in all directions. This is due to its unsymmetrical shape and design. It is essential to know exactly how the light is distributed and is usually given in the form of polar curves in illumination.

Example 1: When a 250V lamp takes a current of 0.8 ampere, it produces a total flux of 3,260 lumens. Calculate (i) M.S.C.P. of the lamp; and (ii) efficiency of the lamp.

The drop in luminous intensity along OC or at 0 degrees of horizontal polar curve is due to the break in the filament where the current enters and leaves. If the luminous intensity is measured in a vertical plane at various angles (or angular positions) a polar curve in the vertical plane obtained as shown in (c). The drop or depression in luminous intensity 180 degrees of the vertical polar curve is due to the position of the lamp holder.

Cosine cube (\(cos^3\theta\)) law state that the “illumination at any point on a surface is dependent on the cube of cosine of the angle between line of flux and normal at that point”

The amount of light produced by a source is referred to as illumination. Illumination is achieved through the heating effect of electric current. When a solid or vapor is heated, it emits energy into its surroundings. The unit used to measure illumination is the lux.

It states that “the illumination, E at any point surface is directly proportional to the cosine of the angle between the node at that point and the line of flux”.

The bandwidth of radiations visible to human eyes is very small i.e., from \(4.3\times 10^{14}Hz\ to\ 7.5\times10^{14}Hz\). The white sunlight is composed of different color radiations coming in the visible spectrum having wavelengths between 4,000 to 7,500 AU.

Understanding these properties is crucial in various domains, including architecture, photography, stage lighting, and interior design. Let us delve into these fundamental aspects of illumination: