Do you test daylight reflectance at different inclination angles?

According to BCA’s requirements on daylight reflectance, there are different requirements for the following installation locations:

  • Facade
  • Low-sloped roof, with less than 20 degrees of inclination angle from the horizontal plane
  • High-sloped roof, with more than 20 degrees of inclination angle from the horizontal pane

We were asked by many customers if we test the daylight reflectance at different inclination angles, in order to meet BCA’s requirements.

The short answer is: no, we do not test the daylight reflectance at different inclination angles.

Daylight reflectance, as a material property, is independent of its inclination angle. The angle in BCA’s requirement is the installation angle, it does not mean that daylight reflectance needs to be tested at various inclination angles.

More explanations

Daylight reflectance is a material property and it does not change with its inclination angle.

For example, for a surface with 10% of total daylight reflectance, its total daylight reflectance remains 10%, when the surface is tilted.

The test angle in the laboratory is dependent on the test instrument design and it is different from the installation angle in a building project.

Online total daylight reflectance calculator: V1.0.0

We are pleased to introduce our online total daylight reflectance calculator (V1.0.0, first version). Click the screenshot below to access this online calculator.

Calculation principle

The calculator simply converts an sRGB color (common in screen displays and websites) to a CIEXYZ color, whose Y component is equivalent to the total daylight reflectance of the color (refer to this Wikipedia article for more details).

This calculator calculates total daylight reflectance only and it cannot calculate diffuse and specular daylight reflectances, as the latter two components are dependent on surface finishing, but not on surface color.

The conversion is a theoretical conversion and does not introduce conversion errors. In practice, one needs to manually match a physical color with screen displayed color. This manual process introduces some errors. Nevertheless, this online tools is still useful in estimating the total daylight reflectance of color samples.

20% total daylight reflectance

BCA requires that, for roof surfaces with greater than 20° inclination angle, the total daylight reflectance shall be less than 20% (What is daylight reflectance?). The table below lists 4 colors (grey, red, green, and blue) with close to 20% total daylight reflectance.

If your color is brighter than the 4 colors (except blue color, read the explanation below the table), it is possible that it cannot meet the 20% requirement.

Color typeColor valueTotal daylight reflectanceColor display
GreyRGB (123, 123, 123)
Hex (#7B7B7B)
0.198 (19.8%)
RedRGB (248, 0, 0)
Hex (#F80000)
0.200 (19.8%)
GreenRGB (0, 144, 0)
Hex (#009000)
0.199 (19.9%)
BlueRGB (0, 0, 255)
Hex (#0000FF)
0.072 (7.2%)

Human eyes are less sensitive to blue and red colors, but more sensitive to green and grey colors. In the table above, the total daylight reflectance of the most saturated blue color in the sRGB color space is only with 7.2% of total daylight reflectance.

Daylight reflectance of partially fritted glasses

We have a detailed article on the testing procedures of partially fritted glass optical & thermal properties. For partially fritted glasses, the daylight reflectance property concerns many customers. This article aims to present some opinions from us on the daylight reflectance of partially fritted glasses.

Partially fritted glasses: glass or non-glass material?

If you are a textualist and adhere to the texts strictly, partially fritted glasses are obviously made of glass, as the word “glass” appears in the name, and you should stop reading this post from here.

If you are not a textualist and open to some discussions, below are some explanations on the differences between glass and non-glass materials in terms of optical characteristics.

There are 3 types of material surfaces, in terms of optical characteristics:

  • With specular reflectance only
  • With mixed reflection
  • With diffuse reflection only

In our opinion, conventional glasses and glasses with ceramic frit are distinct in optical characteristics. For a partially fritted glass, it is more reasonable to classify its clear part as glass material and classify its fritted part as non-glass material.

A surface with specular reflection only
Specular reflection only

Optical characteristics:

  • With specular reflection only
  • Diffuse reflection is negligible

For such materials:

  • Diffuse reflectance = 0%
  • Total reflectance = specular reflectance

Examples:

  • Conventional glasses
  • Materials with mirror finish
  • Metallic coating on glasses
A surface with mixed reflection
Mixed reflection

Optical characteristics:

  • With both specular reflection and diffuse reflection
  • Both components are not negligible

For such materials:

  • Total reflectance = diffuse reflectance + specular reflectance

Examples:

  • Most general facade and roof materials with certain glossiness
  • Glasses with ceramic frit
A surface with diffuse reflection only
Diffuse reflection only

Optical characteristics:

  • With diffuse reflection only
  • Specular reflection is negligible

For such materials:

  • Specular reflectance = 0%
  • Total reflectance = diffuse reflectance

Examples:

  • Materials with matt and rough surfaces: e.g. roof tiles, rough granites

Disclaimer

  • The information presented above is our opinion. It is not reviewed, agreed, or approved by any external parties.

Daylight reflectance, visible light reflectance, luminous reflectance, and light reflectance value: are they the same?

They are the same in physical meaning: all of them are quantities representing the fraction of visible light reflected by a surface.

For general applications, the results are equivalent. For example: 0.50 (50%) of daylight reflectance = 0.50 (50%) of visible light reflectance = 0.50 (50%) of luminous reflectance = 50 of light reflectance value (LRV).

In practice, there are some subtle differences in the test results, due to the different test methods used. Below are the practices implemented in our lab:

Most of the time, the results obtained with different methods are very close (typically less than ±0.01 of variation).

Are daylight reflectance and solar reflectance the same?

Both quantities are about the reflectance of material surfaces. However, they are different.

  • Daylight reflectance is about the reflectance of a surface to visible light.
  • Solar reflectance is about the reflectance of a surface to solar energy.

Shown below is the solar radiation spectrum (red color part is for the sunlight at sea level). The solar radiation consists of 3 parts: 1) ultraviolet (UV) radiation; 2) visible light and 3) infrared (IR) radiation.

  • Daylight reflectance is in terms of visible light only.
  • Solar reflectance is in terms of UV radiation, visible light, and IR radiation, i.e. the entire solar radiation spectrum.

Additionally, in the lab, the daylight reflectance test and solar reflectance test are two different tests:

  • Daylight reflectance test: only the spectral reflectance in the 380 nm – 780 nm range (the visible light range) is measured. Typically, we also separate the total, diffuse and specular components of it.
  • Solar reflectance test: the spectral reflectance in the 300 nm – 2500 nm range (the entire solar spectrum, including UV radiation, visible light, and IR radiation) is measured. Typically, we also measure the material thermal emittance and calculate the solar reflectance index (SRI).

New release of lab profile 2020 edition

We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.

What are in the laboratory profile?

  • A comprehensive overview of the laboratory
  • Detailed introduction of 10 popular tests
  • 14 informative technical insights articles

You may download a soft copy of our lab profile.

Daylight reflectance, color and gloss

Color and gloss are commonly used quality check quantities by material suppliers and construction companies, whereas daylight reflectance are relatively new to the industry. This article aims to explain the relationship between the three quantities.

Daylight reflectance and color

The total and diffuse daylight reflectance is directly related to color. Materials with light color are with high total or diffuse daylight reflectance; materials with dark color are with low total or diffuse daylight reflectance. Our eyes cannot quantify daylight reflectance, but they can assess daylight reflectance qualitatively.

Most color instruments with integrating sphere geometry can measure color in the SCI (specular component included) mode or the SCE (specular component excluded) mode. Color measured in the SCI mode is equivalent to the total daylight reflectance; color measured in the SCE mode is equivalent to the diffuse daylight reflectance.

The closest color scale is Yxy scale. Of course, due to various technical constraints, color measurement instruments cannot accurately determine daylight reflectance, but the results are sufficient for internal use.

Daylight reflectance and gloss

The specular daylight reflectance is directly related to gloss. Materials with high gloss are with high specular daylight reflectance; materials with low gloss are with low specular daylight reflectance. Our eye can qualitatively assess if a surface is with high gloss or with low gloss.

Gloss meters can measure material gloss level. However, the relationship between gloss meter readings and specular daylight reflectance is qualitative. It is not possible to quantitatively coorelate them.

Notes to applications

For projects where low specular daylight reflectance is required, materials with low gloss level are preferred.

For projects where low total daylight reflectance is required, materials with dark color are preferred.