[Physics Class Notes] on Luminance Pdf for Exam

Luminance is the apparent brightness of an object that appears to the human eye. A pattern of varying luminance can be seen, when we look at the world. Luminance is the luminous intensity per unit area projected in a given direction. Luminance describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. Luminance is the intensity that is measured on a surface at a specific position. The SI unit is candela per square meter, which is still sometimes called a nit. The color of the Luminance is a certain color that can be defined by hue (0o – 360°), saturation (0% – 100%) and lightness (0% – 100%). On the other hand, luminance is a measure to describe the perceived brightness of a color (Encyclopædia Britannica: “luminance, or visually perceived brightness”).

In other words, the intensity of the emitted light from a surface per unit area in a particular direction can be considered for luminances definition. It provides the amount of light that passes through, is reflected, and falls within a given solid angle. Luminous Flux the measure of the total light output of a luminous source.

Factors on which the luminance of the surface depends is as follows:

1. Nature of the surface.

2. The incident of the Luminous flux takes place in the unit area of the surface.

Light Meter

A device used to measure the amount of light is known as Light Meter. Light meter is categorized into two types known as, reflected light meter and incident light meter.

The light that is reflected and needs to be photographed has been calculated by a reflected light meter. The amount of light that falls on the subject by using the integrated sphere can be calculated by an incident meter.

Luminance Formula

The explanation of the formula of luminance with notations is as follows:

L = [K_{m}] [int] [L_{elambda}] V ([lambda]) [Delta] [lambda]

Where,

1. L is known as luminance

2. [K_{m}] is the maximum luminance efficiency

3. [L_{elambda}] is the spectral radiance

4. V ([lambda]) is the luminance efficiency function

5. [lambda] is the wavelength

6. [Delta] [lambda] = That is the wavelength interval over which the integration is taking place.

Uses of Light Meter

1. Light meters are also used in measuring illumination in the interior and switch off the output of the fitting light. This increases the efficiency of the system by reducing or lowering the burden of energy building.

2. Photography is one of the major uses of a light meter. Appropriate exposure of light in photography has been found with it. A light meter has an analog or digital computer distinctively that helps the photographer to determine the speed of shutter and f-number that needs to be selected for best exposure under a particular lighting situation.

3. Cinematography and science design are the other situations where the light meter has its application. These are also used in the areas of lighting to ensure minimum wastage of light, plant growing to ensure appropriate light levels and light pollution outdoors.

Fun Facts

1. Luminous Flux is light energy per unit of time. It can be measured in lumens. The lumen refers only to the visible power, since “light” is visible energy.

2. Luminance is the luminous intensity per unit area projected in a given direction. The SI unit of luminance is candela per square meter, which is still sometimes called a nit.

3. Luminous intensity is the luminous flux per solid angle emitted or reflected from a point. The unit of this is the lumen per steradian, or candela (cd).

4. Most luminance photometers’ have special optics that allow the user to view the source and bring the projected area into focus. Any luminous flux that leaves the source – as defined by the projected area – and passes through the lens will also pass through the Aperture. That luminous flux will enter the photodetector and permit a luminance measurement. What is being measured is power – the rate at which energy is being transferred from source to detector – but there can be no power without energy.