The difference between Inherent Optical Properties (IOPs) and Apparent Optical Properties (AOPs) lies in how they relate to the properties of a medium and how light interacts with it. These two types of optical properties are used in oceanography, remote sensing, and environmental sciences to describe the behavior of light in water or other media.
1. Inherent Optical Properties (IOPs)
Definition:
Inherent Optical Properties are the fundamental properties of a medium that describe how light is absorbed, scattered, and transmitted within that medium. They are independent of the viewing geometry and how the light interacts with the surroundings.
Key Characteristics:
- Intrinsically Linked to the Medium: IOPs are intrinsic to the medium itself (such as water, air, or biological materials in the medium), and they depend on the composition and physical state of the medium.
- Not Affected by Observation Conditions: These properties do not depend on the angle or position of observation; they are measured in a controlled, direct environment.
- Primary IOPs:
- Absorption Coefficient (a): Describes how much light is absorbed per unit distance as it travels through the medium.
- Scattering Coefficient (b): Describes how much light is scattered per unit distance by particles within the medium.
- Backscattering Coefficient (bb): Specific type of scattering that refers to light scattered in the backward direction.
Formula:
Absorption and scattering coefficients are often used to model the attenuation of light in the medium.
Example:
- In water, the absorption of light by chlorophyll (in plants or algae) and scattering due to suspended particles are examples of inherent optical properties.
Applications:
- Used in studies of water quality, remote sensing, and understanding the physical properties of various media.
2. Apparent Optical Properties (AOPs)
Definition:
Apparent Optical Properties are properties that depend on the viewing geometry—i.e., the direction from which the light is observed. These properties describe how light appears when observed from a specific angle or at the water’s surface, taking into account reflection, transmission, and scattering.
Key Characteristics:
- Dependent on Observation Geometry: AOPs change based on the position of the observer, the angle of incident light, and the path through which the light travels.
- Describe Observable Light: These properties describe the light that is observed from a certain angle after it has interacted with the medium, and they are influenced by how light is scattered or absorbed by the medium.
- Primary AOPs:
- Remote Sensing Reflectance (Rrs): The ratio of the reflected radiance to the incident irradiance, representing how much light is reflected from the water surface in remote sensing.
- Water-Leaving Radiance (Lw): The radiance leaving the water and heading toward the sensor or observer.
- Diffuse Attenuation Coefficient (Kd): Describes the rate at which light is attenuated (both absorbed and scattered) as it travels through the water.
Formula:
AOPs like remote sensing reflectance are typically calculated as:
Rrs=LwEincidentRrs = \frac{Lw}{E_{\text{incident}}}
Where:
- RrsRrs is the remote sensing reflectance
- LwLw is the water-leaving radiance
- EincidentE_{\text{incident}} is the incident irradiance
Example:
- The brightness of the ocean surface observed from a satellite or the intensity of light seen from underwater is an example of apparent optical properties.
Applications:
- AOPs are primarily used in remote sensing for observing the Earth’s surface, such as monitoring ocean color, vegetation, and water quality.
Key Differences Between IOPs and AOPs
| Aspect | Inherent Optical Properties (IOPs) | Apparent Optical Properties (AOPs) |
| Definition | Describes the inherent properties of a medium that govern light absorption, scattering, and transmission | Describes properties of the medium that are dependent on the viewing geometry and how light appears to an observer |
| Dependence | Independent of viewing geometry | Dependent on viewing geometry and observation angle |
| Measurement | Measured directly in a controlled environment | Measured through remote sensing or observations from a specific angle |
| Primary Properties | Absorption coefficient, scattering coefficient, backscattering coefficient | Remote sensing reflectance, water-leaving radiance, diffuse attenuation coefficient |
| Example | Absorption of light by particles in water, scattering by algae or suspended matter | Brightness of water surface from a satellite, light intensity from underwater |
| Applications | Water quality monitoring, material property analysis | Remote sensing, ocean color studies, environmental monitoring |
In summary:
- IOPs describe the physical properties of the medium, such as how light is absorbed and scattered inside it.
- AOPs describe how light appears when observed from a specific angle, reflecting the medium’s optical effects but also how it interacts with the observation geometry.