Definitions of Key Terms/ Basic Characteristic

What are VLT, IRR, UVR and TSER?

When you want to install solar protection films, consumers should know definitions of key terms and understand what manufacturers post for their films. It will make consumers choose the rightest film as they want for worth the money they paid and less confusion of many brands of films that having a lot of advertisings or propagandas in markets. So a suitable decision method of window film selection with the important key terms will be the rightest way in nowadays. Because these parameters are scientific numbers that tell to their characteristics, or we may say “Quality you can prove”. Therefore important customers will not be deceived anymore.

VLT = Visible Light Transmission

VLT means visible light that transmit through film and display as percentage. For example, VLT = 40% means the film let visible light pass through the film 40%. The more VLT, the film is more transparent or The less VLT, the film is darker.

In general, VLT is opposite to darkness of film because VLT is percentage of visible light transmission but darkness of film is percentage of visible light rejection. However windows film communication of film shops at Thailand in the past often were darkness than VLT. For example, film 40%, 60% or 80% usually mean films that have darkness of 40%, 60% or 80% respectively. And that time, film shops have no transmission meter or not to show customers film specification for getting understand. Films with 40% darkness may have VLT from 60%-40%, or film with 80% darkness may have VLT from 20% to 5%. Sales just brought some samples and customers chose the darkness they preferred. And the most customers made decision to get darker film for heat reduction. Or if they were afraid about too dark at night time, they would decrease a little darkness of film and agreed to get a bit hotter. Nowadays, decision of window film selection from estimation of film darkness is not suitable at all, because of these reasons.

1. Heat rejection is not considered from darkness or VLT anymore. There are factors related to heat rejection such as IRR, UVR, TSER etc. They will be mentioned later.

2. Clarity from inside or outside which relate to privacy feeling or in the other hands, feeling to show off or let people to see the beautiful of car interior. This point is considered from VLT directly.

3. Customers are able to choose more transparent film without hot feeling at day time. Because they can choose the more transparent film with high IRR which can reject the most infrared radiation (IR). So customers still feel cool even they use more transparent films.

4. With newest of technology, car window films are able to adjust VLT or darkness automatically like transition lens of sun glasses (Photochromic function). Customers will get dark films at day time and get more transparent at night time. They will feel cool and have best visibility in every situations. And moreover, this type of film will conserve your vehicle energy.

IRR = Infra Red Rejection

IRR means infrared rejection of film that display as percentage. For example, IRR = 60% means the film can reject infrared radiation of 60%. Then the higher IRR, the better IRR rejection. Generally, infrared radiation from solar is the main radiation causing heat. If we measure power per square meter of sunlight at ground level and sun is on 12 o’clock position, composition of 1,000 W/m2 of sunlight will be infrared ray of 520 - 550 Watt, visible light of 420 - 430 Watt and ultra violet ray of 30 - 50 Watt. [refer https://en.wikipedia.org/wiki/Sunlight] or written as percentage and ration of infrared (IR) : visible light (VL) : ultraviolet (UV) is 52-55% IR : 42-43% VL : 3-5% UV. And we usually use the approximation of composition of solar as 53% IR : 44% VL : 3% UV.

It can be seen that the most energy (53%) of solar is infrared ray which is electromagnetic wave mainly causing heat. We will find that infrared called as “Heating ray”. Many people may hear that remote controller use infrared light for communication. Actually, infrared light has wider application than this such as infrared camera for night vision or infrared camera which sensor from heat wave or infrared ray of living things or in many drying industries infrared light bulbs are used as heat generating source instead of old heat wires. Therefore infrared ray really is “Heating ray”.

UVR = Ultra Violet Rejection

UVR means ultra violet rejection of film that display as percentage. For example, UVR = 70% means the film can reject ultra violet radiation of 70%. Then the higher UVR, the better UVR rejection. Generally ultra violet radiation is ray hurting skin and eyes and fading/deteriorate of car seat, cushion, fabrics, furnishings, accessories and console. More information about ultra violet radiation can be found at “What is UV400?”

TSER = Total Solar Energy Rejection

TSER means a number informing to rejection of total solar energy. This parameter is able to calculated approximately by weighted average of %VLT, %IRR and %UVR measured. As mentioned above, solar energy is approximately composed of visible light (VL) 44%, infrared (IR) 53% and ultra violet (UV) 3%

Definitions of Key Terms of film you should know (additional).

% Solar Transmittance (T-sol)

The ratio of the amount of total solar energy in the full solar wavelength range (300-2,500 nanometers) that is allowed to pass directly through a glazing system (e.g., a film/glass combination) to the amount of total solar energy falling on that glazing system. Value is usually expressed as a percent.

Remark: The smaller this number, the cooler objects will be when directly exposed to sunlight passing through the window, since they will be exposed to less incident solar energy.

% Solar Reflectance (R-sol)

The ratio of total solar energy which is reflected outwardly by the glazing system to the amount of total solar energy falling on the glazing system. Value is usually expressed as a percent.

Remark: This number together with the T-sol determines the solar absorption value of the film. This latter value is most critical in determining what film is suitable for a given glass type & situation. Generally, the higher this number, the better.

% Solar Absorptance (A-sol)

% Solar Absorptance (A-sol): The ratio of the amount of total solar energy absorbed by a glazing system to the amount of total solar energy falling on the glazing system. Solar absorption is that portion of total solar energy neither transmitted nor reflected. Since solar transmittance and solar reflectance are measured directly, the following equation should be used in calculating solar absorption.

Solar Absorption = 1.00 - (Solar Transmittance) - (Solar Reflectance)

Remark: Generally, the lower this number, the better. This number is a critical determinant in the potential for thermal stress (how hot the glass gets). Too much solar absorption can excessively warm the glass cause window failure, either though glass breakage or seal failure. Always use the Film-to-Glass Recommendation Chart (a web app is available for specifiers) to avoid wrongly specifying a film for a given glazing system.

% Visible Light Transmittance (VLT)

The ratio of the amount of total visible solar energy (380-780 nanometers) that is allowed to pass through a glazing system to the amount of total visible solar energy falling on the glazing system. Value is usually expressed as a percent. Glare is influenced by visible light transmittance through a glazing system. Visible light accounts for about 44% of the sun's energy reaching Earth's surface. The VLT value is often weighted or measured in the area of the spectrum most easily sensed by the human eye, around 550nm.

Remark: The smaller this number, the greater the glare reduction. Of concern to many clients because while they want glare reduction, they often do not want a room "too dark."

% Visible Reflectance-exterior and interior (VLR-ext and VLR-int)

The percent of total visible light falling on a glazing system that is reflected by that system. Generally, VLR values are for exterior surfaces, those exposed to sunlight, unless otherwise specified. For dual-reflectance films, values are often given for each surface, the exterior (usually listed first in specification charts) and the interior (listed second).

Remark: A guide to how "shiny" a film looks from the exterior of a building relative to other films. Clear glass has a VLR of about 8%. And the lower the interior reflectance value, the less shiny the window will appear at night from the interior when it is very dark outside but brightly lit inside.

U-Factor: (sometimes called U-value)

The U-Factor (sometimes called the "U-value") should be understood as the overall heat transfer coefficient of the glazing system. The U-value is a measure of the heat transfer that occurs through the glazing system between its outer and inner surfaces. This value is a function of temperature, and is expressed in BTUs per square foot per hour per degree Fahrenheit ( BTU/ ft2/hr/°F or w/m2). The lower the U-value, the better the insulation qualities of the glazing system. Alternative definition: The "coefficient of heat transfer;" a measure of the ability of a material to resist heat transfer. The number is actually the number of BTUs per square foot per hour per °F of temperature difference (or w/m2 per °C) across a barrier. The lower the U-value, the slower heat moves by conduction through the material.

Others in the insulation and construction industry use the measure of "R-Value," which denotes a material's ability to act as an insulator. The higher the R-Value, the slower the heat transfer rate; it is the reciprocal of the U-Value, expressed as R = 1/U. A window with a U-value of 0.25 has an R-value of 4.0 (1 divided by 0.25). U-Value and R-Value measurements are similar—but reciprocal—in nature. They quantify the rate at which heat is transferred through a material due to temperature differences between its opposing surfaces. The window films industry uses two standards of measurement to determine U-values for glazing systems:

Shading Coefficient (SC)

The ratio of the solar heat gain through a given glazing system to the solar heat gain under the same conditions for clear, unshaded double strength window glass (DSA). Shading coefficient defines the sun control capability or efficiency of the glazing system relative to a standard window.

Solar Heat Gain Coefficient (SHGC)

Also known as the g-value, the SHGC is the fraction of incident solar radiation that actually passes through that window, including solar energy that is both directly transmitted and that which is absorbed and subsequently released inwardly by re-radiation and conduction. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat enters a room space. This number is the mathematical complement of the TSER value. In other words, the sum of the TSER (Total Solar Energy Rejection, in decimal form) of a glazing system and its SHGC value is 1;

therefore, SHGC = 1 – TSER

Thus, if the TSER of a specified film/glass combination is 58%, then 1 - .58 = .42, which is the SHGC of the window.

Note: This term is being increasingly used in the window film industry because it is a central term in the window glazing industry which does not use the term “TSER.”

Remark: This is the industry’s standard number for comparing the solar performance of a window. The lower this number, the better.