7-2 Perambatan Cahaya pada Material: Reflektor Lambertian

Summary

In this segment of the TF-3200 building physics course, the concept of the Lambertian reflector is explored. A Lambertian surface reflects light diffusely in all directions, with the maximum intensity occurring at a zero-degree angle, i.e., directly facing the surface. The relationship between reflected light flux and intensity is discussed, including the constant luminance produced by the reflector. The course also covers how reflectance can be calculated by comparing a sample material to a known reference, demonstrating the practical application of Lambertian reflectance in determining material reflectance values.

Takeaways

• 😀 A Lambertian reflector is a surface that reflects light diffusely in all directions, with the maximum reflection intensity occurring at a zero Gamma angle (normal direction).
• 😀 The reflected light intensity at any angle between 0° and 90° follows the formula: I_gamma = I_0 * cos(gamma), meaning intensity decreases with the increase of the Gamma angle.
• 😀 Luminance produced by a Lambertian surface remains constant in all directions, since luminance is defined as the ratio of light intensity to the projection area at the same Gamma angle.
• 😀 The luminance of a Lambertian surface is independent of the observation angle, meaning it will appear equally bright regardless of where it's observed from.
• 😀 The reflected light flux (VR) can be calculated using an integral that involves the intensity of light at different angles and the space angle (D Omega), resulting in a formula that links VR to I_0.
• 😀 The integral formula for reflected light flux (VR) results in a proportional relationship between light flux and reflectance, with a factor of π.
• 😀 The luminance (L) of a Lambertian reflector can be related to its reflectance (rho) and the illumination (E) it receives using the equation: L = (rho * E) / pi.
• 😀 The reflectance of a material sample can be estimated by comparing its luminance to that of a reference material with known reflectance under the same lighting conditions.
• 😀 The reflectance of a sample material (rho_sample) can be determined by comparing the luminance of the sample to the luminance of a reference material with a known reflectance value.
• 😀 An example calculation demonstrates that if a Lambertian reflector sample has a luminance of 10 candela/m² and a measured illumination of 100 lux, its reflectance is approximately 0.314.

Q & A

What is a Lambertian reflector?

A Lambertian reflector is a surface that reflects light diffusely in all directions, with the maximum intensity occurring in the normal direction to the surface.

How does the reflected light intensity change with the angle of observation?

The intensity of reflected light decreases as the angle of observation (Gamma) increases, following the relationship I = Inol * cos(Gamma), where Inol is the maximum intensity.

Why is the luminance produced by a Lambertian surface uniform?

The luminance is uniform because it is the ratio of light intensity to the projection area, which remains constant in all directions, independent of the observation angle.

What is the significance of the equation involving reflected light flux (VR)?

The equation shows that the reflected light flux (VR) is the integral of the reflected intensity multiplied by the space angle, indicating how much light is reflected over all possible directions.

How does the relationship between reflected light flux and maximum intensity work for a Lambertian surface?

The reflected light flux (VR) is proportional to the maximum intensity (Inol) and depends on the space angle and the cosine of the observation angle, leading to a relationship that can be integrated over the angles.

How is the luminance of a Lambertian surface related to its reflectance and illumination?

The luminance (L) of a Lambertian surface is related to its reflectance (rho) and the illumination (E) it receives by the equation L = rho * E / pi.

How can the reflectance of a material sample be estimated?

The reflectance of a material sample can be estimated by comparing its luminance to a reference material with a known reflectance under the same illumination conditions.

What is the formula used to compare the reflectance of a sample and a reference material?

The formula used is: L_sample / L_reference = rho_sample / rho_reference, where L represents luminance and rho represents reflectance.

In the example provided, how is the reflectance of a Lambertian reflector estimated?

In the example, with a luminance of 10 candela per m² and an illumination of 100 lux, the reflectance is estimated to be approximately 0.314 using the given relationship.

What role does the angle of observation (Gamma) play in the reflectance of a Lambertian surface?

The angle of observation (Gamma) affects the reflected intensity, with the intensity decreasing as the angle increases from the normal direction (Gamma = 0°).