A non-SI unit called 'nit' is the unit of which of the following photometric quantities used to measure a multitude of light intensity?
Luminance
Understanding Photometric Quantities and Units
Photometry is the science of measuring light in terms of its perceived brightness to the human eye. Various quantities are used in photometry to describe different aspects of how light behaves and interacts with surfaces. These photometric quantities have specific units, both in the standard SI system and sometimes in non-SI units.
What is the Unit 'Nit'?
The question asks about the non-SI unit called 'nit'. The 'nit' is a unit commonly used to measure a specific photometric quantity related to light intensity perceived from a surface. It represents candelas per square meter.
Let's examine the photometric quantities listed in the options and determine which one uses 'nit' as a unit.
- Luminous exposure: This quantity measures the amount of light energy falling on a surface over a period of time. Its SI unit is lux-second ($\text{lx} \cdot \text{s}$). It is related to the integral of illuminance over time.
- Luminance: This quantity measures the intensity of light emitted or reflected by a surface in a given direction, per unit area and per unit solid angle. It describes how bright a surface appears to the human eye. The SI unit for luminance is candela per square meter ($\text{cd/m}^2$). The non-SI unit 'nit' is equivalent to one candela per square meter ($\text{1 nit} = 1 \text{ cd/m}^2$).
- Luminosity: While 'luminosity' is a term sometimes used colloquially to mean brightness, in a precise photometric or radiometric context, it might refer to luminous efficacy (lumens per watt) or total luminous flux (lumens). It is not a standard photometric quantity with 'nit' as its unit.
- Luminous emittance (also known as luminous exitance): This quantity measures the total luminous flux emitted, reflected, or transmitted by a surface per unit area. Its SI unit is lumen per square meter ($\text{lm/m}^2$), which is equivalent to lux ($\text{lx}$). It describes the total light leaving a surface, regardless of direction.
Based on the definitions, the unit 'nit' is specifically associated with Luminance.
Comparing Photometric Quantities and Units
Here is a summary of the quantities and their units:
| Photometric Quantity | SI Unit | Non-SI Unit (where applicable) | Description |
|---|---|---|---|
| Luminous exposure | lux-second ($\text{lx} \cdot \text{s}$) | - | Amount of light energy per unit area over time |
| Luminance | candela per square meter ($\text{cd/m}^2$) | nit (1 nit = 1 $\text{cd/m}^2$) | Perceived brightness of a surface |
| Luminosity | - | - | Related to luminous efficacy or total flux (less common specific unit) |
| Luminous emittance | lumen per square meter ($\text{lm/m}^2$) or lux ($\text{lx}$) | - | Total light leaving a surface per unit area |
Therefore, the non-SI unit 'nit' is the unit of Luminance.
Revision Table: Photometric Units
| Unit Name | Quantity Measured | Type (SI/Non-SI) |
|---|---|---|
| nit | Luminance | Non-SI |
| candela per square meter ($\text{cd/m}^2$) | Luminance | SI |
| lux-second ($\text{lx} \cdot \text{s}$) | Luminous exposure | SI |
| lumen per square meter ($\text{lm/m}^2$) or lux ($\text{lx}$) | Luminous emittance (exitance) | SI |
| candela ($\text{cd}$) | Luminous intensity | SI (Base Unit) |
| lumen ($\text{lm}$) | Luminous flux | SI Derived |
Additional Information on Luminance and Nit
Luminance is a critical measurement in fields like display technology (TVs, monitors, phone screens), lighting design, and aviation displays because it directly relates to how bright a surface appears to the human observer under specific viewing conditions. A higher luminance value means the surface appears brighter. The unit 'nit' is widely used in the display industry to specify screen brightness. For example, a smartphone screen might have a peak brightness of 500 nits, while an HDR TV might reach 1000 nits or more.
Understanding luminance is essential because it accounts for both the amount of light emitted or reflected and the area from which it originates, as well as the direction of view. This makes it different from luminous flux (total light output) or illuminance (light falling on a surface).
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