How Does Luminance Affect Light Perception?
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.
Image will be uploaded soon
Factors on which the luminance of the surface depends is as follows:
Nature of the surface.
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 = \[K_{m}\] \[\int\] \[L_{e\lambda}\] V (\[\lambda\]) \[\Delta\] \[\lambda\]
Where,
L is known as luminance
\[K_{m}\] is the maximum luminance efficiency
\[L_{e\lambda}\] is the spectral radiance
V (\[\lambda\]) is the luminance efficiency function
\[\lambda\] is the wavelength
\[\Delta\] \[\lambda\] = That is the wavelength interval over which the integration is taking place.
Uses of Light Meter
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.
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.
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
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.
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.
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).
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.
FAQs on Luminance: Meaning, Formula, and Real-World Uses
1. What is luminance and what does it measure?
Luminance is a photometric measure that quantifies the perceived brightness of a surface from a particular angle. It describes the amount of light that is emitted, passed through, or reflected by a surface and falls on the eye. Essentially, it measures how bright a surface appears to a human observer, making it a crucial concept in display technology and lighting design.
2. What is the main difference between luminance and illuminance?
The key difference lies in what is being measured:
- Luminance measures the light leaving or emitting from a surface in a specific direction. It is what makes a surface appear bright. Think of the brightness of a TV screen.
- Illuminance measures the amount of light falling onto or illuminating a surface. It describes how well-lit an object is. For example, the amount of light from a lamp hitting your desk is illuminance.
In short, illuminance is the cause, and luminance is the effect we perceive as brightness.
3. What is the SI unit of luminance and how is it defined?
The SI unit of luminance is the candela per square metre (cd/m²). This unit is also commonly referred to as a 'nit'. It is defined as the luminous intensity (in candelas) of a light source in a given direction, divided by the projected area of that source as viewed from that direction. It effectively represents the concentration of light energy leaving a surface.
4. Why is luminance a key metric for display devices like TVs and smartphones?
Luminance is a critical performance metric for displays because it directly relates to the perceived brightness and picture quality. A higher luminance value (measured in nits or cd/m²) means the screen can produce a brighter image, which is essential for:
- Visibility in bright environments: A high-luminance screen remains visible and clear even under direct sunlight.
- HDR (High Dynamic Range) content: HDR requires high peak luminance to reproduce bright highlights and create a more realistic and impactful image.
- Contrast Ratio: The luminance of the brightest white and darkest black determines the display's contrast, affecting image depth and detail.
5. How does the concept of luminance in physics differ from radiance?
Luminance and radiance both describe light emitting from a surface, but they differ based on the observer. Radiance is an objective, radiometric quantity that measures the total amount of electromagnetic energy emitted per unit area, regardless of whether a human can see it. In contrast, luminance is a photometric quantity that is weighted according to the human eye's sensitivity to different wavelengths of light (the luminosity function). Therefore, luminance represents the perceived brightness to a human, while radiance represents the total physical power of the light.
6. Can a matte and a glossy surface of the same colour have different luminance values?
Yes, absolutely. Even if they are the same colour and illuminated by the same light source, their luminance will differ based on the viewing angle. A glossy surface reflects light specularly (like a mirror), creating a very high luminance in one specific direction (the 'glare' spot) and lower luminance elsewhere. A matte surface reflects light diffusely, scattering it more evenly in all directions. This results in a more uniform luminance from any viewing angle, but it will be lower than the peak luminance of the glossy surface's glare spot.
