Visible light transmittance & reflectance are calculated from spectral transmittance & reflectance in the 380 nm – 780 nm range with weighted averaging.

The spectral distribution of illuminant D65 (representing natural daylight) and the spectral luminous efficiency of a standard observer (representing human eyes’ sensitivity to different colors) are used as the weights.

The visible light transmittance of a glass affects the access to natural daylight and external view. Glasses with high visible light transmittance allow more natural daylight in indoor space and a better view of external scenery. However, if the visible light transmittance is too high, there are some adverse effects, including daylight glare and excessive solar heat gain through glasses.

If the front side visible light reflectance of a glass is too high, the glass may reflect an excessive amount of light and cause glare to occupants in the neighboring buildings or road users nearby. In Singapore, glasses with more than 20% front side visible light reflectance are not allowed.

If the back side visible light reflectance of a glass is too high, the visible light reflected by the glass back side may exceed the visible light transmitted from the external environment, when it is dark outside (e.g. at nighttime or when raining). In such scenarios, the glasses look like mirrors and it may cause certain discomfort to the occupants.

The ranges of visible light transmittance & reflectance are 0 – 1 (in decimal) or 0% – 100% (in percentage).

The visible light transmittance of an uncoated clear glass is around 0.90 (90%) and the visible light reflectance of it is around 0.08 (8%, for both front and back sides).

In general, the visible light transmittance of other glass types (e.g. tinted or low-e coated) is lower and the visible light reflectance of them is higher.

Visible light transmittance is also called visible transmittance (e.g. NFRC 300).

Visible light transmittance & reflectance are also called light transmittance & reflectance (e.g. ISO 9050 & EN 410).

Solar energy transmittance & reflectance are calculated from the spectral transmittance & reflectance in the 300 nm – 2500 nm range with weighted averaging.

The spectral distribution of solar radiation is used as the weights.

Solar energy transmittance & reflectance are the intermediate results in SHGC calculation. In practice, they are not directly used by industry, as SHGC is a better indicator of a glass’s solar heat gain performance.

Nevertheless, solar energy transmittance & reflectance are useful in understanding a glass’s solar heat gain mechanism.

As explained in the SHGC section, SHGC consists of two components, primary solar heat gain and secondary solar heat gain:

• The primary solar heat gain component is equal to the solar energy transmittance;
• The secondary solar heat gain component is equal to the solar energy absorptance multiplied by an inward flowing fraction.

A glass with low solar energy transmittance indicates its primary solar heat gain component is small, but it does not necessarily indicate its SHGC is low. As its solar energy absorptance could be high (solar energy absorptance = 1 – solar energy transmittance – solar energy reflectance), its SHGC could be still high due to the high secondary solar heat gain component.

An extreme case example is a black color opaque glass. Its solar energy transmittance is 0, but its solar energy absorptance is high (e.g. 0.80). In this case, the SHGC of such an opaque glass could be higher than 0.30.

The ranges of solar energy transmittance & reflectance are 0 – 1 (in decimal) or 0% – 100% (in percentage).

The solar energy transmittance of an uncoated clear glass is around 0.80 (80%) and the solar energy reflectance of it is around 0.07 (7%, for both front and back sides).

In general, the solar energy transmittance of other glass types (e.g. tinted or low-e coated) is lower and the solar energy reflectance of them is higher.

Solar energy transmittance & reflectance are also called solar direct transmittance & reflectance (e.g. ISO 9050 & EN 410).

Solar energy transmittance & reflectance are also called solar transmittance & reflectance in some technical documents.

Emissivity is calculated from spectral reflectance in the 5 µm – 50 µm range with weighted averaging and with further conversion from normal emissivity to hemispherical emissivity.

The spectral distribution of blackbody emissive power is used as the weights.

There are three heat transfer modes: conduction, convection, and radiation. Emissivity is related to the radiative heat exchange ability in the far infra-red range (5 µm – 50 µm).

It is important to highlight that emissivity is not related to the radiative heat exchange in the ultraviolet (UV), visible light (VIS) and near infra-red (NIR) range (300 nm – 2500 nm). Radiative heat exchange in the UV/VIS/NIR range is characterized by solar energy transmittance & reflectance properties presented above.

The emissivity of an uncoated glass surface is around 0.84. The emissivities of most natural materials are around 0.85 – 0.95.  The radiative heat exchange between an uncoated glass surface and the surroundings (or adjacent glass surfaces) is very strong.

For glass surfaces coated with low-emissivity coating (or low-e coating in short), the emissivity could be significantly lowered (for example, 0.02). For such low-e coated glasses, the radiative heat exchange with the surroundings (or adjacent glass surfaces) is significantly reduced, and hence they are with better insulation performance (and, in turn, lower U-value and SHGC).

The range of emissivity is 0 – 1 (in decimal). Normally, emissivity is not expressed in percentage.

The emissivity of an uncoated glass is around 0.84.

The emissivity of a low-e coated glass can be as low as 0.01.

For soft low-e coatings, the emissivity is dependent on the number of silver layers, with quad-silver coating performing the best in terms of emissivity.

The emissivity of hard low-e coatings (typical range: 0.15 – 0.30) is higher than that of soft low-e coatings (typical range: 0.01 – 0.10).

Emissivity is also called emittance (e.g. NFRC 301).

For each glass layer, the spectral transmittance & reflectance are directly measured by instruments.

For glazing systems with multiple glass layers (e.g. double glazing units, DGUs), the spectral transmittance & reflectance are calculated from the spectral transmittance & reflectance of the consisted layers.

Spectral transmittance & reflectance are the raw data for all further optical & thermal property calculations. In practice, they are not directly used in architectural applications.

Spectral transmittance & reflectance in the 300 nm – 2500 nm range are related to solar radiation. The spectrum range 300 nm – 2500 nm can be further divided into the ultraviolet (UV) range (300 – 380 nm), the visible light (VIS) range, and the near infra-red (NIR) range (780 nm – 2500 nm).

Spectral reflectance in the 5 µm – 50 µm range is related to room temperature surface radiative heat exchange. Glasses are opaque in this wavelength range, i.e. spectral transmittance = 0.

In the 300 nm – 2500 nm range, the spectral transmittance & reflectance curves of uncoated clear glasses are in general flat.

For low-e coated glasses, the spectral transmittance is low in the NIR range and high in the VIS range; the spectral reflectance of the low-e coated side is high in the NIR range and low in the VIS range.

No additional information

UV transmittance is calculated from spectral transmittance in the 300 nm – 380 nm range with weighted averaging.

The spectral distribution of solar radiation in the UV range (300 nm – 380 nm) is used as the weights.

UV rejection is simply calculated as: UV rejection = 1 – UV transmittance.

There are 3 types of UV radiations based on the UV radiation spectrum range:

• UVA: 315 nm – 400 nm
• UVB: 280 nm – 315 nm
• UVC: 100 nm – 280 nm

As a large portion of sun’s UV radiation is absorbed by the atmosphere, on the earth’s surface, most UV radiation is UVA radiation, with a small amount of UVB radiation and almost no UVC radiation. In practice, only UV radiation in the 300 nm – 380 nm range is considered in glass UV transmittance (or UV rejection) evaluation and it accounts for around 5% of the overall solar radiation in the 300 nm – 2500 nm range.

Exposure to excessive UV radiation in sunlight may cause skin and eye injury and increase the risk of skin cancer. Glass with low UV transmittance (or high UV rejection) is preferred for the benefit of human health.

The ranges of UV transmittance & rejection are 0 – 1 (in decimal) or 0% – 100% (in percentage).

The UV transmittance of an uncoated clear glass is around 0.6 (60%) and the UV rejection of it is around 0.4 (40%).

Many PVB interlayer or window film products can effectively block UV radiation, with less than 0.01 (1%) of UV transmittance or greater than 0.99 (99%) of UV rejection.

UV rejection is not defined in a standard. Please use it with caution.

Note: there is no consensus on the calculation method of IR transmittance & IR rejection. The information below is for reference only.

It is assumed that IR transmittance could be calculated from spectral transmittance in the 780 nm – 2500 nm range with weighted averaging.

The spectral distribution of solar radiation in the near infra-red range (780 nm – 2500 nm) could be used as the weights.

IR rejection can be simply calculated as: IR rejection = 1 – IR transmittance.

The calculation method of IR transmittance and rejection is not defined in a standard or technical document, to the best knowledge of the author. They are declared by many window film manufacturers, supposedly with in-house calculation methods.

IR transmittance & rejection are useful window film optical performance indicators to professionals, but they may confuse general consumers.

The solar heat gain performance of a window film product can be solely represented by its SHGC. SHGC is a well-defined thermal properties and it can be verified by third-party labs.

The IR transmittance & rejection are redundant information to consumers and are often misleading:

• A window film with low IR transmittance (or high IR rejection) is not necessarily with low SHGC (as its transmittance in the visible light range and its absorptance in the near infra-red range could be high).
• IR transmittance & rejection results cannot be verified by third-party labs.

No typical values.

Both IR transmittance & rejection are not defined in a standard. There is also no consensus on the calculation method. Please use them with caution.

Solar energy absorptance, front = 1 – solar energy transmittance – solar energy reflectance, front

Solar energy absorptance, back = 1 – solar energy transmittance – solar energy reflectance, back

Refer to the solar energy transmittance & reflectance section for the calculations of solar energy transmittance & reflectance.

Solar energy absorptance is related to the secondary heat gain component of SHGC.

Refer to the SHGC section for more information.

The range of solar energy absorptance is 0 – 1 (in decimal) or 0% – 100% (in percentage).

The solar energy absorptance of an uncoated clear glass is around 0.15 (15%).

Solar energy absorptance is also called solar direct absorptance (e.g. ISO 9050 & EN 410).

Solar energy absorptance is also called solar absorptance in some technical documents.

Transmitted & reflected colors are calculated from the spectral transmittance & reflectance in the 380 nm – 780 nm range.

In practice, the reflected color viewed from the indoor side is affected by the background object color on the outdoor side and the reflected object color on the indoor side.

The front side reflected color represents the appearance of a glass viewed from the outdoor side. It is often used to access glass color and color uniformity, as they are important to the aesthetics of building façades.

No typical values.

No additional information.

There are 8 test colors and there are a few steps to calculate the general color rendering index of a glass:

1. For each test color, its colors rendered in daylight and rendered in daylight transmitted through the glass are calculated separately.
2. For each test color, the color difference between the two colors rendered is calculated.
3. For each test color, the specific color rendering index is calculated from the color difference.
4. The specific color rendering indices of the 8 test colors are averaged to get the general color rendering index.

The color of an object is dependent on its reflectance and also the spectrum of light source. For example, a white color surface in natural daylight is rendered slightly red in red light source.

The spectrum of daylight transmitted through a glass is different from the original daylight and the colors of objects indoors are therefore rendered differently.

For example, a white color surface behind a blue color glass may be rendered slightly blue.

Glasses with high general color rendering index are preferred when object color appearance is important.

The maximum value of general color rendering index is 100, for glasses that are completely neutral (with a flat spectral transmittance curve in the visible light spectrum).

The general color rendering index of an uncoated clear glass is around 98.

No additional information.

CIE damage factor is calculated from spectral transmittance in the 300 nm – 600 nm range with weighted averaging.

The spectral distribution of solar radiation in the 300 nm – 600 nm range and the CIE damage function are used as the weights.

Solar radiation in the 300 nm – 600 nm may cause color fading of indoor artworks or furnishings. Glasses with low CIE damage factor are preferred for artwork conversation buildings (e.g. a museum), to protect the artworks from color fading.

The range of CIE damage factor is 0 – 1 (in decimal) or 0% – 100% (in percentage).

The CIE damage factor of an uncoated clear glass is around 0.75 (75%).

No additional information.

Skin damage factor is calculated from spectral transmittance in the 300 nm – 400 nm range with weighted averaging.

The spectral distribution of solar radiation in the 300 nm – 400 nm range and the CIE erythemal effectiveness spectrum are used as the weights.

Solar radiation in the 300 nm – 400 nm may cause erythema. Glasses with low skin damage factor are preferred for human skin protection.

The range of skin damage factor is 0 – 1 (in decimal) or 0% – 100% (in percentage).

The skin damage factor of an uncoated clear glass is around 0.15 (15%).

No additional information.

Glare reduction = (visible light transmittance of a glass without window film – visible light transmittance of a glass with window film) / visible light transmittance of a glass without window film.

Refer to the visible light transmittance & reflectance section for the calculation of visible light transmittance.

Daylight glare can be reduced by reducing the visible light transmittance of a glass.

However, there is no simple linear relationship between visible light transmittance and daylight glare.

No typical values.

Glare reduction is not defined in a standard. Its physical meaning is not solid. Please use it with caution.

There are 3 heat transfer modes: conduction, convection and radiation.

U-value is calculated from the conductive, convective and radiative thermal resistances of all possible heat transfer paths through a glazing system.

U-value indicates the thermal insulation performance of a glass.

For glasses with low U-value, the heat transfer through the glass due to indoor/outdoor temperature difference is smaller. Glass U-value is important in winter conditions, due to the large indoor/outdoor temperature difference in winter.

U-value is a nighttime thermal property. It is irrelevant to all optical properties related to solar radiation (e.g. solar energy transmittance & reflectance). It is strongly related to glass surface emissivity, gas type and gap thickness (if the glass is an insulating glazing unit, IGU).

We have an online glass U-value calculator for you to experiment various glass configurations.

U-value is also dependent on the environmental conditions, mainly the wind or airflow speed, as higher wind or airflow speed introduces stronger convective heat transfer.

The default U-value in most standards is the winter condition U-value. It is also possible to calculate the summer condition U-value or the U-value under a specific condition.

The unit of U-value is W/(m²K) in SI units or Btu/(hr·ft²·°F) in imperial units. The relationship between the SI unit and the imperial unit is:  1 Btu/(hr·ft²·°F) = 5.678 W/(m²K), with an easy-to-remember conversion factor 5.678.

The U-value of a single layer uncoated glass is around 5.5 W/(m²K), of a hard low-e coated single layer glass is around 3.5 – 4 W/(m²K), of a DGU with soft low-e coating is around 1.5 – 1.8 W/(m²K).

We have an online glass U-value calculator for you to experiment various glass configurations.

The full name of U-value is thermal transmittance, though this full name is rarely used in usual communications.

U-value is also called U-factor (typically expressed in the imperial unit) in the US (e.g. NFRC 100).

As shown above, SHGC consists of two components:

• Primary solar heat gain: the solar heat directly transmitted through a glass in its original solar radiation form.
• Secondary solar heat gain: the solar heat absorbed by a glass and further transferred to the indoor space as heat and via all 3 heat transfer modes (conduction, convection and radiation).

The primary solar heat gain component is just the solar energy transmittance of the glass.

The secondary solar heat gain component is calculated as the solar energy absorptance of the glass multiplied by its inward flowing fraction. The solar energy absorptance is calculated with the simple relationship: solar energy absorptance = 1 – solar energy transmittance – solar energy reflectance. The inward flowing fraction can be calculated in a similar approach for U-value calculation, with the 3 heat transfer modes (conduction, convection, and radiation) considered.

SHGC indicates the solar heat gain performance of a glass.

For glasses with low SHGC, the amount of solar heat transferred to the indoor space is less. Glasses with low SHGC are preferred in summer conditions, as the cooling load due to solar heat gain could be reduced; glasses with high SHGC are preferred in winter conditions, as the heating load could be partially offset by the solar heat gain.

SHGC is a daytime thermal property. It is dependent on both optical properties related to solar radiation (e.g. solar energy transmittance & reflectance) and optical properties related to room temperature surface radiative heat exchange (e.g. emissivity).

It is important not to confuse SHGC with solar energy transmittance. Solar energy transmittance is the primary solar heat gain component of SHGC only. The SHGC of a glass is always greater than its solar energy transmittance.

Glasses with low SHGC are preferred in summer conditions. There are 3 possible ways to reduce glass SHGC:

• Reduce glass solar energy transmittance: the primary solar gain component of SHGC is lower;
• Increase glass solar energy reflectance: the solar energy absorptance of the glass (and hence the second solar heat gain component of SHGC) is lower, due to the relationship: solar absorptance = 1 – solar energy transmittance – solar energy reflectance.
• Improve glass insulation: the inward flowing fraction of the glass (and hence the second solar heat gain component of SHGC) is lower.

For example, the SHGC a DGU glass can be lowered slightly by changing the gas type from air to argon, though argon gas does not change glass optical properties.

The range of SHGC is 0 – 1 (in decimal). Normally, SHGC is not expressed in percentage.

The SHGC of an uncoated clear glass is around 0.8 – 0.9.

In general, the SHGC of other glass types (e.g. tinted or low-e coated) is lower.

SHGC is also called total solar energy transmittance (TSER) or solar factor (e.g. ISO 9050 & EN 410). The acronym “g-value” is also often used in some technical documents.

Shading coefficient = SHGC / 0.87 and it is just a simple conversion.

Refer to the SHGC section for the calculation of SHGC.

The physical meaning of shading coefficient is the ratio of the solar heat gain of a glass to that of a 3 mm clear glass, with a nominal SHGC of 0.87.

Shading coefficient is a meaningful glass thermal property in the old days (e.g. 1970s). Before technologies such as low-e coating or insulating glazing were widely used, the practical way to reduce glass solar heat gain is to apply shading, e.g. adding an overhang or louver. The benchmark solar heat gain performance is a clear glass.  A shading device applies additional shading, characterized by its shade coefficient.

There is also a simple model to combine the shading effects of multiple shading devices together. For example, the shading coefficient of a glass is 0.8 (SC₁); the shading coefficient of an external overhang is 0.7 (SC₂); the shading coefficient of an internal blind is 0.3 (SC₃). The overall shading coefficient of the combined system is then SC = SC₁ × SC₂ × SC₃ = 0.8 × 0.7 × 0.3 = 0.168. Shading coefficient is meaningful and convenient in modeling shading devices, though the model is highly simplified.

Shading coefficient is redundant in modeling glasses, as glass shading coefficient cannot be directly calculated and it must be converted from SHGC.

As SHGC has clearer physical meaning and can be directly calculated, SHGC is the primary property of glass solar heat gain used in this article.

As shading coefficient = SHGC / 0.87, the range of shading coefficient is 0 – 1.15 (in decimal). Normally, shading coefficient is not expressed in percentage.

The shading coefficient of an uncoated single layer glass is around 1.

In general, the shading coefficient of other glass types (e.g. tinted or low-e coated) is lower.

No additional information.

TSER = 1 – SHGC

Refer to the SHGC section for the calculation of SHGC.

It is a non-standard property mainly used by window film manufacturers.

As TSER = 1 – SHGC, refer to the SHGC section for more explanations.

As TSER = 1 – SHGC, refer to the SHGC section for some typical values of SHGC.

TSER is not defined in a standard. Please use it with caution.

LSG ratio = visible light transmittance / SHGC

Refer to the visible light transmittance and SHGC sections for the calculations of visible light transmittance and SHGC.

LSG ratio is a type of gain-to-pain ratio.

Natural daylight (proportional to visible light transmittance) is the gain, as it reduces the energy consumption due to artificial lighting.

Solar heat gain (proportional to SHGC) is the pain, as it increases the energy consumption due to more cooling load in summer.

For daylighting applications, glasses with high LSG ratio are preferred.

The LSG ratio of an uncoated clear glass is slightly higher than 1.

The LSG ratio of some DGU glasses with soft low-e coating could be higher than 2.

LSG ratio is not defined in a standard. Please use it with caution.

RHG = shading coefficient × 630 W/m² + summer condition U-value × 7.8 °C (in SI units)

RHG = shading coefficient × 200 Btu/(h·ft²) + summer condition U-value × 14 °F (in imperial units)

Refer to the shading coefficient and U-value sections for the calculations of shading coefficient and U-value.

In summer condition daytime, there is both solar heat gain and heat gain due to indoor/outdoor temperature difference.

It is a bit inconvenient to evaluate glass thermal performance in summer conditions. For example, a glass may be with lower SHGC but higher U-value than another glass.

With RHG, it is convenient to use a single value to evaluate glass thermal performance in summer conditions. The weights (630 W/m² for shading coefficient and 7.8 °C for summer condition U-value) are fixed for a standard summer daytime condition.

The unit of RHG is W/m² (in SI units).

The RHG of an uncoated clear glass is around 600 W/m².

In general, the RHG of other glass types (e.g. tinted or low-e coated) is lower.

RHG is not defined in a standard. Please use it with caution.