All typical commercial solar panels can be tested, including panels greater than 1.2 m x 2.4 m in size. The results reported are the same as our standard daylight reflectance testing service (sample report). Due to the extra effort in handling such large and heavy samples, some large-size sample surcharge is applicable.
Shown below is an example result set, with this ± 0.001 discrepancy:
Total daylight reflectance = 0.216 (21.6%)
Diffuse daylight reflectance = 0.200 (20.0%)
Specular daylight reflectance = 0.015 (1.5%)
Some customers expect that the specular daylight reflectance should be 0.216 – 0.200 = 0.016 (1.6%) and suspect that there is a typo in the test report.
Our daylight reflectance test reports are generated by in-house software automatically. The ±0.001 discrepancy is not due to a calculation mistake or a typo, but due to the rounding operations, as illustrated in the table below:
Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres
ASTM E903 is for the testing of solar absorptance, reflectance, and transmittance, but not daylight reflectance directly. The procedures of spectral reflectance measurement with integrating spheres are described in ASTM E903 in detail, but it is still insufficient to get the daylight reflectance results with ASTM E903 only.
Therefore, at OTM, ASTM E903 is supplemented with other test methods in daylight reflectance testing:
According to BCA’s requirements on daylight reflectance, there are different requirements for the following installation locations:
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.
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.
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.
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.
Total daylight reflectance
RGB (123, 123, 123) Hex (#7B7B7B)
RGB (248, 0, 0) Hex (#F80000)
RGB (0, 144, 0) Hex (#009000)
RGB (0, 0, 255) Hex (#0000FF)
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.
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.
Specular reflection only
With specular reflection only
Diffuse reflection is negligible
For such materials:
Diffuse reflectance = 0%
Total reflectance = specular reflectance
Materials with mirror finish
Metallic coating on glasses
With both specular reflection and diffuse reflection
Both components are not negligible
For such materials:
Total reflectance = diffuse reflectance + specular reflectance
Most general facade and roof materials with certain glossiness
Glasses with ceramic frit
Diffuse reflection only
With diffuse reflection only
Specular reflection is negligible
For such materials:
Specular reflectance = 0%
Total reflectance = diffuse reflectance
Materials with matt and rough surfaces: e.g. roof tiles, rough granites
The information presented above is our opinion. It is not reviewed, agreed, or approved by any external parties.
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:
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).